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  • 201.
    Divin, Andrey
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Khotyaintsev, Yuri V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vaivads, Andris
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Toledo-Redondo, S.
    European Space Agcy, ESAC, Sci Directorate, Madrid, Spain..
    Markidis, S.
    KTH Royal Inst Technol, Dept Computat Sci & Technol, Stockholm, Sweden..
    Lapenta, G.
    Katholieke Univ Leuven, Ctr Math Plasma Astrophys, Dept Math, Leuven, Belgium..
    Three-scale structure of diffusion region in the presence of cold ions2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 12, p. 12001-12013Article in journal (Refereed)
    Abstract [en]

    Kinetic simulations and spacecraft observations typically display the two-scale structure of collisionless diffusion region (DR), with electron and ion demagnetization scales governing the spatial extent of the DR. Recent in situ observations of the nightside magnetosphere, as well as investigation of magnetic reconnection events at the Earth's magnetopause, discovered the presence of a population of cold (tens of eV) ions of ionospheric origin. We present two-dimensional particle-in-cell simulations of collisionless magnetic reconnection in multicomponent plasma with ions consisting of hot and cold populations. We show that a new cold ion diffusion region scale is introduced in between that of hot ions and electrons. Demagnetization scale of cold ion population is several times (similar to 4-8) larger than the initial cold ion gyroradius. Cold ions are accelerated and thermalized during magnetic reconnection and form ion beams moving with velocities close to the Alfven velocity.

  • 202.
    Dogurevich, Pavel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Kinetic simulation of spherically symmetric collisionless plasma in the inner part of a cometary coma2019Student paper other, 10 credits / 15 HE creditsStudent thesis
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  • 203.
    Dokgo, Kyunghwan
    et al.
    Southwest Res Inst, San Antonio, TX 78238 USA.
    Hwang, Kyoung-Joo
    Southwest Res Inst, San Antonio, TX 78238 USA.
    Burch, James L.
    Southwest Res Inst, San Antonio, TX 78238 USA.
    Choi, Eunjin
    Southwest Res Inst, San Antonio, TX 78238 USA.
    Yoon, Peter H.
    Univ Maryland, Inst Phys Sci & Technol, College Pk, MD 20742 USA;Kyung Hee Univ, Sch Space Res, Yongin, South Korea;Korea Astron & Space Sci Inst, Daejeon, South Korea.
    Sibeck, David G.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.
    Graham, Daniel B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Swedish Inst Space Phys, Uppsala, Sweden.
    High-Frequency Wave Generation in Magnetotail Reconnection: Nonlinear Harmonics of Upper Hybrid Waves2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 14, p. 7873-7882Article in journal (Refereed)
    Abstract [en]

    MMS3 spacecraft passed the vicinity of the electron diffusion region of magnetotail reconnection on 3 July 2017, observing discrepancies between perpendicular electron bulk velocities and (E) over right arrow x (B) over right arrow drift, and agyrotropic electron crescent distributions. Analyzing linear wave dispersions, Burch et al. (2019, https://doi.org/10.1029/2019GL082471) showed the electron crescent generates high-frequency waves. We investigate harmonics of upper-hybrid (UH) waves using both observation and particle-in-cell (PIC) simulation, and the generation of electromagnetic radiation from PIC simulation. Harmonics of UH are linearly polarized and propagate along the perpendicular direction to the ambient magnetic field. Compared with two-dimensional PIC simulation and nonlinear kinetic theory, we show that the nonlinear beam-plasma interaction between the agyrotropic electrons and the core electrons generates harmonics of UH. Moreover, PIC simulation shows that agyrotropic electron beam can lead to electromagnetic (EM) radiation at the plasma frequency and harmonics.

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  • 204.
    Dokgo, Kyunghwan
    et al.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Hwang, Kyoung-Joo
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Burch, James L.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Yoon, Peter H.
    Univ Maryland, Inst Phys Sci & Technol, College Pk, MD 20742 USA.;Korea Astron & Space Sci Inst, Daejeon, South Korea.;Kyung Hee Univ, Sch Space Res, Yongin, South Korea..
    Graham, Daniel B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Li, Wenya
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    High-Frequency Waves Driven by Agyrotropic Electrons Near the Electron Diffusion Region2020In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 47, no 5, article id e2020GL087111Article in journal (Refereed)
    Abstract [en]

    National Aeronautics and Space Administration's Magnetosphere Multiscale mission reveals that agyrotropic electrons and intense waves are prevalently present in the electron diffusion region. Prompted by two distinct Magnetosphere Multiscale observations, this letter investigates by theoretical means and the properties of agyrotropic electron beam-plasma instability and explains the origin of different structures in the wave spectra. The difference is owing to the fact that in one instance, a continuous beam mode is excited, while in the other, discrete Bernstein modes are excited, and the excitation of one mode versus the other depends on physical input parameters, which are consistent with observations. Analyses of dispersion relations show that the growing mode becomes discrete when the maximum growth rate is lower than the electron cyclotron frequency. Making use of particle-in-cell simulations, we found that the broadening angle Delta in the gyroangle space is also an important factor controlling the growth rate. Ramifications of the present finding are also discussed. Plain Language Summary Magnetospheric Multiscale mission has observed magnetic reconnection process, which converts magnetic energy to kinetic energy of charged particles. Extremely rapid time scale data reveal that electron scale high-frequency waves exist near the electron diffusion region of magnetic reconnection. Recently, two different types of waves observed; one is discrete electron-Bernstein waves, and the other is continuous beam modes. In this study, we formulated a unified theory for both types of waves. Comparing Magnetosphere Multiscale observations, the theory, and particle-in-cell simulations, this study shows that the same cause (agyrotropic electrons) can make two different wave structures depending on plasma parameters. The condition that the maximum growth rate of instabilities equals the electron cyclotron frequency can be considered as a threshold of the transition from discrete electron Bernstein waves to continuous beam modes.

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  • 205.
    Dokgo, Kyunghwan
    et al.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Hwang, Kyoung-Joo
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Burch, James L.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Yoon, Peter H.
    Univ Maryland, Inst Phys Sci & Technol, College Pk, MD 20742 USA..
    Graham, Daniel B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Li, Wenya
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Chinese Acad Sci, State Key Lab Space Weather, Natl Space Sci Ctr, Beijing, Peoples R China..
    The Effects of Upper-Hybrid Waves on Energy Dissipation in the Electron Diffusion Region2020In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 47, no 19, article id e2020GL089778Article in journal (Refereed)
    Abstract [en]

    Using a two-dimensional particle-in-cell simulation, we investigate the effects and roles of upper-hybrid waves (UHW) near the electron diffusion region (EDR). The energy dissipation via the wave-particle interaction in our simulation agrees withJ center dot E(')measured by magnetospheric multiscale (MMS) spacecraft. It means that UHW contributes to the local energy dissipation. As a result of wave-particle interactions, plasma parameters which determine the larger-scale energy dissipation in the EDR are changed. They-directional current decreases while the pressure tensorP(yz)increases/decreases when the agyrotropic beam density is low/high, where(x, y, z)-coordinates correspond the(L, M, N)-boundary coordinates. Because the reconnection electric field comes from- partial differential P-yz/ partial differential z, our result implies that UHW plays an additional role in affecting larger-scale energy dissipation in the EDR by changing plasma parameters. We provide a simple diagram that shows how the UHW activities change the profiles of plasma parameters near the EDR comparing cases with and without UHW.

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  • 206.
    Dreyer, Joshua
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. The University Centre in Svalbard.
    A detailed study of auroral fragments2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Aurora occurs in various shapes, one of which is the hitherto unreported phenomenon of auroral fragments. For three periods of occurrence of these fragments their properties were studied in detail during this master’s thesis, using mainly ground-based instrumentation located near Longyearbyen on Svalbard, Norway. A base dataset was constructed from 103 all-sky camera images, manually marking 305 fragments for further analysis. This thesis reports and describes the fragment observations during the observed events, including the auroral and geomagnetic context. Fragments generally seem to fall into two categories, the first being singular, apparently randomly distributed fragments, and the second being periodic fragments that occur in groups with a regular spacing close to auroral arcs. A typical fragment has a small horizontal size below 20 km, a short lifetime of less than a minute and shows no field-aligned extent in the emission. The fragments appear mainly west of zenith (73%) during the three observation nights, whereas their north-south distribution is symmetric around the zenith. Almost all of them exhibit westward drift, the estimated speed for one of the fragments passing the field of view of ASK is ∼1 km/s. A spectral signature can be seen in the green auroral wavelength of O at 557.7 nm and red emission line of N2 at 673.0 nm, but no emission enhancement was observed in the blue wavelengths. One fragment passing the EISCAT Svalbard radar’s field of view shows a local ion temperature increase in a small altitude range of ∼15 km, whereas there is no visible increase in electron density. This could be explained by fragment generation due to locally strong horizontal electric fields. A potential mechanism for this might be electric fields of atmospheric waves superposing with the converging electric fields of auroral arcs created by particle precipitation and the corresponding field-aligned currents. The resulting field would be perpendicular to the magnetic field and the auroral arcs, leading to wave-like density variations of excited plasma close to the arcs. Further study is required to verify this hypothesis and improve the understanding of fragment properties determined from the limited dataset used for this thesis.

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  • 207.
    Dreyer, Joshua
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Diving Deep into Saturn's Equatorial Ionosphere with Cassini: Insights from the Grand Finale2023Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the summer of 2017, the Cassini mission concluded its nearly 13 years orbiting Saturn with a series of daring dives between the rings and the upper reaches of Saturn's atmosphere. This last phase of the mission, called the Grand Finale, revealed a highly variable equatorial ionosphere dominated by a large influx of ring material from Saturn's D ring. The papers included in this thesis utilize data gathered during these proximal orbits to gain insights into the nature and effects of the infalling ring material.

    Initially, we derive upper limits for the effective recombination coefficient in Saturn's equatorial ionosphere at altitudes below 2500 km, where photochemical equilibrium can be assumed, to constrain the composition of the positive ion species. Our inceptive results indicate that ion species with low recombination coefficients are dominant.

    We follow up on this by developing a photochemical model, incorporating grain charging, to investigate the effects of the ring influx on the plasma composition. The model results at an altitude of 1700 km yield vastly different abundances of two types of neutral species when compared to those derived from measurements, ultimately representing the difficulty of reconciling the observed H+ and H3+ densities with our and other model results.

    Exploring the nature of narrow decreases in the ionospheric H2+ densities reveals a time shift in the ion data. After correcting for this, the decreases line up very well with calculated shadows for substructures in Saturn's C ring. We can further estimate the optical depths of these substructures and investigate at which altitudes photochemical equilibrium for H2+ is applicable.

    The direct measurement of heavier neutral species during the proximal orbits is complicated by the high spacecraft speed. We devise a method to utilize helium ion chemistry to independently derive the mixing ratios of these heavier neutrals in Saturn's ionosphere. Our results show considerable variability, which may suggest temporal and/or spatial changes in the ring influx. A comparison with other studies indicates that potentially only the most volatile ring-sourced species significantly ablate to enter the gas phase in this region of Saturn's ionosphere.

    Finally, we compare the fixed-bias Langmuir probe electron densities and the light ion densities. They exhibit a strong positive correlation for most parts of the proximal orbits even on short timescales. We find three distinct regions in the proximal orbits, which can provide further insight into the ionospheric composition, connection to the rings, and measurement uncertainties.

    List of papers
    1. Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient
    Open this publication in new window or tab >>Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient
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    2021 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 2, no 1, article id 39Article in journal (Refereed) Published
    Abstract [en]

    The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini's last four orbits into Saturn's lower ionosphere to constrain the effective recombination coefficient α300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10−7 cm3 s−1 at an electron temperature of 300 K.

    Place, publisher, year, edition, pages
    American Astronomical SocietyAmerican Astronomical Society, 2021
    National Category
    Fusion, Plasma and Space Physics
    Identifiers
    urn:nbn:se:uu:diva-458301 (URN)10.3847/psj/abd6e9 (DOI)000912867900001 ()
    Funder
    Swedish National Space Board, 143/18Swedish National Space Board, 168/15
    Available from: 2021-11-08 Created: 2021-11-08 Last updated: 2024-01-15Bibliographically approved
    2. Empirical Photochemical Modeling of Saturn's Ionization Balance Including Grain Charging
    Open this publication in new window or tab >>Empirical Photochemical Modeling of Saturn's Ionization Balance Including Grain Charging
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    2022 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 3, no 2, article id 49Article in journal (Refereed) Published
    Abstract [en]

    We present a semianalytical photochemical model of Saturn's near-equatorial ionosphere and adapt it to two regions (similar to 2200 and similar to 1700 km above the 1 bar level) probed during the inbound portion of Cassini's orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near similar to 1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 angstrom.

    Place, publisher, year, edition, pages
    Institute of Physics (IOP), 2022
    National Category
    Fusion, Plasma and Space Physics
    Identifiers
    urn:nbn:se:uu:diva-495353 (URN)10.3847/PSJ/ac4eee (DOI)000911845800001 ()
    Funder
    Swedish National Space Board
    Available from: 2023-01-30 Created: 2023-01-30 Last updated: 2023-10-03Bibliographically approved
    3. Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn's Ionosphere
    Open this publication in new window or tab >>Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn's Ionosphere
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    2022 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 3, no 7, article id 168Article in journal (Refereed) Published
    Abstract [en]

    For orbits 288 and 292 of Cassini's Grand Finale, clear dips (sharp and narrow decreases) are visible in the H-2(+) densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini's position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn's C ring. The calculated shadowed altitudes along Cassini's trajectory line up well with the dips in the H-2(+) data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H-2(+) densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H2+ at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini's trajectory. The resulting estimate of similar to 0.3 km s(-1) at an altitude of similar to 3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

    Place, publisher, year, edition, pages
    Institute of Physics (IOP), 2022
    National Category
    Fusion, Plasma and Space Physics
    Identifiers
    urn:nbn:se:uu:diva-497057 (URN)10.3847/PSJ/ac7790 (DOI)000914478200001 ()
    Funder
    Swedish National Space Board, 143/18E. och K.G. Lennanders StipendiestiftelseSwedish National Space Board, Dnr 195/20
    Available from: 2023-02-23 Created: 2023-02-23 Last updated: 2023-10-03Bibliographically approved
    4. Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere
    Open this publication in new window or tab >>Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere
    2023 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 6, article id e2023JA031488Article in journal (Refereed) Published
    Abstract [en]

    A surprisingly strong influx of organic-rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady-state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H-2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of similar to 2 x 10(-4) at closest approaches and order-of-magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions. Plain Language Summary During the final orbits of the Cassini mission, the spacecraft flew between Saturn's rings and the planets upper atmosphere. The onboard plasma instruments detected a large amount of ring particles falling toward the planet, but direct measurements of the composition of these grains are complicated due to the high spacecraft speed and instrumental effects. In this study, we present an independent method to estimate the abundance of heavier neutral species entering the atmosphere from infalling ring material. This method relies on helium ion chemistry and the measured light ion and neutral densities. Our results generally fall below those inferred from direct measurements. Together with comparisons to other studies, this potentially suggests that a large fraction of the infalling neutral species do not significantly ablate in the considered part of Saturn's atmosphere (and remain bound to the dust grains instead) and are thus unavailable for reactions.

    Place, publisher, year, edition, pages
    American Geophysical Union (AGU), 2023
    Keywords
    Saturn, Cassini, ionosphere, space plasma, ion chemistry, planetary science
    National Category
    Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
    Identifiers
    urn:nbn:se:uu:diva-508434 (URN)10.1029/2023JA031488 (DOI)001011943200001 ()
    Funder
    Swedish National Space Board, 143/18
    Available from: 2023-08-02 Created: 2023-08-02 Last updated: 2023-10-06Bibliographically approved
    5. Electron to Light Ion Density Ratios during Cassini's Grand Finale: Addressing Open Questions About Saturn's Low-Latitude Ionosphere
    Open this publication in new window or tab >>Electron to Light Ion Density Ratios during Cassini's Grand Finale: Addressing Open Questions About Saturn's Low-Latitude Ionosphere
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    (English)Manuscript (preprint) (Other academic)
    Keywords
    Saturn, Cassini, Grand Finale, Ionosphere, Photochemistry, Planetary Rings, Planetary Science, Space Plasma, Space Physics
    National Category
    Fusion, Plasma and Space Physics
    Research subject
    Physics with specialization in Space and Plasma Physics
    Identifiers
    urn:nbn:se:uu:diva-512832 (URN)
    Funder
    Swedish National Space Board, 143/18
    Available from: 2023-10-03 Created: 2023-10-03 Last updated: 2023-10-10
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  • 208.
    Dreyer, Joshua
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Saturn's dusty equatorial ionosphere from Cassini's Grand Finale observations2021Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Cassini mission provided unprecedented insights into the Kronian system and revealed many unexpected phenomena relating to dusty plasma environments around Saturn and its rings. Almost 13 years after its arrival, Cassini began a series of 22 orbits which brought it between Saturn's upper atmosphere and the innermost D ring, culminating in a final plunge into Saturn's ionosphere. This concluding period of Cassini's journey is referred to as the Grand Finale and data collected during these final orbits serves as the foundation for the two papers included in this thesis.

    We constrain the composition of positive ions in Saturn's equatorial ionosphere by deriving upper limits for the effective recombination coefficient. Our results suggest an ionosphere dominated by species with low recombination coefficients, such as HCO+. The analysis of Grand Finale data further revealed small-scale variations in the H2+ density for two of the orbits. We link these to narrow substructures in Saturn's C ring with increased optical depths, which attenuate the solar extreme ultraviolet flux and thus reduce photoionisation rates within their respective shadowed regions.

  • 209.
    Dreyer, Joshua
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Swedish Inst Space Phys IRF, Space Plasma Phys Grp, Uppsala, Sweden.;Uppsala Univ, Dept Phys & Astron, Uppsala, Sweden..
    Partamies, Noora
    Univ Ctr Svalbard, Dept Arctic Geophys, Longyearbyen, Norway.;Univ Bergen, Birkeland Ctr Space Sci, Bergen, Norway..
    Whiter, Daniel
    Univ Southampton, Dept Phys & Astron, Southampton, Hants, England..
    Ellingsen, Pal G.
    UiT Arctic Univ Norway, Dept Elect Engn, Narvik, Norway..
    Baddeley, Lisa
    Univ Ctr Svalbard, Dept Arctic Geophys, Longyearbyen, Norway.;Univ Bergen, Birkeland Ctr Space Sci, Bergen, Norway..
    Buchert, Stephan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Characteristics of fragmented aurora-like emissions (FAEs) observed on Svalbard2021In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 39, no 2, p. 277-288Article in journal (Refereed)
    Abstract [en]

    This study analyses the observations of a new type of small-scale aurora-like feature, which is further referred to as fragmented aurora-like emission(s) (FAEs). An all-sky camera captured these FAEs on three separate occasions in 2015 and 2017 at the Kjell Henriksen Observatory near the arctic town of Longyearbyen, Svalbard, Norway. A total of 305 FAE candidates were identified. They seem to appear in two categories - randomly occurring individual FAEs and wave-like structures with regular spacing between FAEs alongside auroral arcs. FAEs show horizontal sizes typically below 20 km, a lack of field-aligned emission extent, and short lifetimes of less than a minute Emissions were observed at the 557.7 nm line of atomic oxygen and at 673.0 nm (N-2; first positive band system) but not at the 427.8 nm emission of N-2(+) or the 777.4 nm line of atomic oxygen. This suggests an upper limit to the energy that can be produced by the generating mechanism. Their lack of field-aligned extent indicates a different generation mechanism than for aurorae, which are caused by particle precipitation. Instead, these FAEs could be the result of excitation by thermal ionospheric electrons. FAE observations are seemingly accompanied by elevated electron temperatures between 110-120 km and increased ion temperatures at F-region altitudes. One possible explanation for this is Farley-Buneman instabilities of strong local currents. In the present study, we provide an overview of the observations and discuss their characteristics and potential generation mechanisms.

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  • 210.
    Dreyer, Joshua
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik
    ESTEC, European Space Agency, Noordwijk, Netherlands.
    Hadid, Lina
    Laboratoire de Physique des Plasmas, Palaiseau, France.
    Morooka, Michiko
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Waite, J. Hunter
    Waite Science LLC, Pensacola, FL, USA.
    Electron to Light Ion Density Ratios during Cassini's Grand Finale: Addressing Open Questions About Saturn's Low-Latitude IonosphereManuscript (preprint) (Other academic)
  • 211.
    Dreyer, Joshua
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik L.
    European Space Agcy, European Space Res & Technol Ctr, Noordwijk, Netherlands..
    Waite, J. Hunter
    Waite Sci LLC, Pensacola, FL USA..
    Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere2023In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 6, article id e2023JA031488Article in journal (Refereed)
    Abstract [en]

    A surprisingly strong influx of organic-rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady-state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H-2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of similar to 2 x 10(-4) at closest approaches and order-of-magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions. Plain Language Summary During the final orbits of the Cassini mission, the spacecraft flew between Saturn's rings and the planets upper atmosphere. The onboard plasma instruments detected a large amount of ring particles falling toward the planet, but direct measurements of the composition of these grains are complicated due to the high spacecraft speed and instrumental effects. In this study, we present an independent method to estimate the abundance of heavier neutral species entering the atmosphere from infalling ring material. This method relies on helium ion chemistry and the measured light ion and neutral densities. Our results generally fall below those inferred from direct measurements. Together with comparisons to other studies, this potentially suggests that a large fraction of the infalling neutral species do not significantly ablate in the considered part of Saturn's atmosphere (and remain bound to the dust grains instead) and are thus unavailable for reactions.

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  • 212.
    Dreyer, Joshua
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Shebanits, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morooka, Michiko
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Perryman, Rebecca S.
    Southwest Res Inst, Space Sci & Engn Div, San Antonio, TX USA..
    Waite, Jack Hunter
    Southwest Res Inst, Space Sci & Engn Div, San Antonio, TX USA..
    Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn's Ionosphere2022In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 3, no 7, article id 168Article in journal (Refereed)
    Abstract [en]

    For orbits 288 and 292 of Cassini's Grand Finale, clear dips (sharp and narrow decreases) are visible in the H-2(+) densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini's position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn's C ring. The calculated shadowed altitudes along Cassini's trajectory line up well with the dips in the H-2(+) data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H-2(+) densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H2+ at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini's trajectory. The resulting estimate of similar to 0.3 km s(-1) at an altitude of similar to 3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

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  • 213.
    Dreyer, Joshua
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morooka, Michiko
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Buchert, Stephan C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik L.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Waite, Jack Hunter
    Space Science and Engineering Division, Southwest Research Institute, San Antonio, USA .
    Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient2021In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 2, no 1, article id 39Article in journal (Refereed)
    Abstract [en]

    The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini's last four orbits into Saturn's lower ionosphere to constrain the effective recombination coefficient α300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10−7 cm3 s−1 at an electron temperature of 300 K.

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    Dreyer_2021_Planet._Sci._J._2_39
  • 214.
    Duan, Suping
    et al.
    Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    Dai, Lei
    Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    Wang, Chi
    Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    He, Zhaohai
    Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    Cai, Chunlin
    Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    Zhang, Y. C.
    Chinese Acad Sci, Natl Space Sci Ctr, State Key Lab Space Weather, Beijing, Peoples R China..
    Dandouras, I.
    Univ Toulouse, UPS OMP, IRAP, Toulouse, France.;CNRS, IRAP, Toulouse, France..
    Reme, H.
    Univ Toulouse, UPS OMP, IRAP, Toulouse, France.;CNRS, IRAP, Toulouse, France..
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Khotyaintsev, Yuri V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Oxygen Ions O+ Energized by Kinetic Alfven Eigenmode During Dipolarizations of Intense Substorms2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 11, p. 11256-11273Article in journal (Refereed)
    Abstract [en]

    Singly charged oxygen ions, O+, energized by kinetic Alfven wave eigenmode (KAWE) in the plasma sheet boundary layer during dipolarizations of two intense substorms, 10: 07 UT on 31 August 2004 and 18: 24 UT on 14 September 2004, are investigated by Cluster spacecraft in the magnetotail. It is found that after the beginning of the expansion phase of substorms, O+ ions are clearly energized in the direction perpendicular to the magnetic field with energy larger than 1 keV in the near-Earth plasma sheet during magnetic dipolarizations. The pitch angle distribution of these energetic O+ ions is significantly different from that of O+ ions with energy less than 1 keV before substorm onset that is in the quasi-parallel direction along the magnetic field. The KAWE with the large perpendicular unipolar electric field, E-z similar to -20 mV/m, significantly accelerates O+ ions in the direction perpendicular to the background magnetic field. We present good evidences that O+ ion origin from the ionosphere along the magnetic field line in the northward lobe can be accelerated in the perpendicular direction during substorm dipolarizations. The change of the move direction of O+ ions is useful for O+ transferring from the lobe into the central plasma sheet in the magnetotail. Thus, KAWE can play an important role in O+ ion transfer process from the lobe into the plasma sheet during intense substorms.

  • 215.
    Dubart, M.
    et al.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Battarbee, M.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Ganse, U.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Osmane, A.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Spanier, F.
    Heidelberg Univ, Inst Theoret Astrophys, Ctr Astron, D-69120 Heidelberg, Germany..
    Suni, J.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Johlander, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Alho, M.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Bussov, M.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Cozzani, G.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    George, H.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Grandin, M.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Horaites, K.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Papadakis, K.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Pfau-Kempf, Y.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Tarvus, V.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Turc, L.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Zaitsev, I.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Zhou, H.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland..
    Palmroth, M.
    Univ Helsinki, Dept Phys, POB 68, Uusimaa 00014, Finland.;Finnish Meteorol Inst, Space & Earth Observat Ctr, POB 503, FI-00101 Helsinki, Finland..
    Sub-grid modeling of pitch-angle diffusion for ion-scale waves in hybrid-Vlasov simulations with Cartesian velocity space2022In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 29, no 10, article id 103902Article in journal (Refereed)
    Abstract [en]

    Numerical simulations have grown to play a central role in modern sciences over the years. The ever-improving technology of supercomputers has made large and precise models available. However, this accuracy is often limited by the cost of computational resources. Lowering the simulation's spatial resolution in order to conserve resources can lead to key processes being unresolved. We have shown in a previous study how insufficient spatial resolution of the proton cyclotron instability leads to a misrepresentation of ion dynamics in hybrid-Vlasov simulations. This leads to larger than expected temperature anisotropy and loss-cone shaped velocity distribution functions. In this study, we present a sub-grid numerical model to introduce pitch-angle diffusion in a 3D Cartesian velocity space, at a spatial resolution where the relevant wave-particle interactions were previously not correctly resolved. We show that the method is successfully able to isotropize loss-cone shaped velocity distribution functions, and that this method could be applied to simulations in order to save computational resources and still correctly model wave-particle interactions.

  • 216.
    Dubinin, E.
    et al.
    Max Planck Inst Solar Syst Res, D-37077 Gottingen, Germany..
    Fraenz, M.
    Max Planck Inst Solar Syst Res, D-37077 Gottingen, Germany..
    Andrews, David
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morgan, D.
    Univ Iowa, Iowa City, IA USA..
    Martian ionosphere observed by Mars Express. 1. Influence of the crustal magnetic fields2016In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 124, p. 62-75Article in journal (Refereed)
    Abstract [en]

    We present multi-instrument observations of the effects of the crustal magnetic field on the Martian ionosphere at different altitudes and solar zenith angles by Mars Express. Total electron content (TEC) at solar zenith angles 55 degrees >= SZA >= 105 degrees over the ionosphere with crustal sources increases with the strength of the magnetic field. A similar trend is observed in a dependence of the local electron density in the upper ionosphere on the crustal magnetic field. On the nightside, at SZA >= 110 degrees, the opposite trend of TEC increase with decrease in the magnetic field value is observed. A dependence on the magnetic field inclination also varies between the day and night sides. TEC decreases for vertical field inclination at 90 degrees >= SZA >= 70 degrees and increases at SZA >= 110 degrees. This effect becomes stronger for larger magnetic field values. A different dependence of the local electron densities in the upper ionosphere at small and high SZA is observed too. An ionospheric exhaust for vertical field inclination in the regions with strong crustal sources is probably caused by escape to space along open field lines which arise due to reconnection that is confirmed by the case studies. The existence of such localized ionospheric depressions is also observed by the in-situ plasma observations. In contrast, on the nightside downward plasma transport and electron precipitation along the field lines produce patches of enhanced ionization.

  • 217. Dubinin, E.
    et al.
    Fraenz, M.
    Fedorov, A.
    Lundin, R.
    Edberg, Niklas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Duru, F.
    Vaisberg, O.
    Ion Energization and Escape on Mars and Venus2011In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 162, no 1-4, p. 173-211Article, review/survey (Refereed)
    Abstract [en]

    Mars and Venus do not have a global magnetic field and as a result solar wind interacts directly with their ionospheres and upper atmospheres. Neutral atoms ionized by solar UV, charge exchange and electron impact, are extracted and scavenged by solar wind providing a significant loss of planetary volatiles. There are different channels and routes through which the ionized planetary matter escapes from the planets. Processes of ion energization driven by direct solar wind forcing and their escape are intimately related. Forces responsible for ion energization in different channels are different and, correspondingly, the effectiveness of escape is also different. Classification of the energization processes and escape channels on Mars and Venus and also their variability with solar wind parameters is the main topic of our review. We will distinguish between classical pickup and 'mass-loaded' pickup processes, energization in boundary layer and plasma sheet, polar winds on unmagnetized planets with magnetized ionospheres and enhanced escape flows from localized auroral regions in the regions filled by strong crustal magnetic fields.

  • 218.
    Dubinin, E.
    et al.
    Max Planck Inst Solar Syst Res, D-37077 Gottingen, Germany..
    Fraenz, M.
    Max Planck Inst Solar Syst Res, D-37077 Gottingen, Germany..
    Paetzold, M.
    Rheinische Inst Umweltforsch, Abt Planetforsch, Cologne, Germany..
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vaisberg, O.
    Inst Space Res, Moscow, Russia..
    Zelenyi, L.
    Inst Space Res, Moscow, Russia..
    Barabash, S.
    Swedish Inst Space Phys, Kiruna, Sweden..
    Martian ionosphere observed by Mars Express. 2. Influence of solar irradiance on upper ionosphere and escape fluxes2017In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 145, p. 1-8Article in journal (Refereed)
    Abstract [en]

    We present multi-instrument observations of the effects of solar irradiance on the upper Martian ionosphere and escape fluxes based on Mars Express measurements obtained over almost 12 years. It is shown that the variations in the upper ionosphere caused by solar irradiance lead to significant changes in the trans-terminator fluxes of low-energy ions and total ion losses through the tail. The observed dependence of the electron number density in the upper ionosphere at altitudes above 300 km on solar irradiance implies that the ionosphere at such altitudes was denser by a factor of ten during the periods of solar maxima in solar cycles 22-23. Correspondingly, the trans terminator fluxes of cold ions and escape fluxes through the tail were also significantly higher. We estimate an increase of total ion losses through the tail during these solar maxima by a factor of 5-6.

  • 219.
    Dumbovic, M.
    et al.
    Univ Zagreb, Hvar Observ, Fac Geodesy, Zagreb, Croatia..
    Kramaric, L.
    Univ Zagreb, Hvar Observ, Fac Geodesy, Zagreb, Croatia.;Univ Zagreb, Fac Transport & Traff Sci, Zagreb, Croatia..
    Benko, Ilona
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Univ Zagreb, Hvar Observ, Fac Geodesy, Zagreb, Croatia..
    Heber, B.
    Christian Albrechts Univ Kiel, Dept Extraterr Phys, Kiel, Germany..
    Vrsnak, B.
    Univ Zagreb, Hvar Observ, Fac Geodesy, Zagreb, Croatia..
    A new method of measuring Forbush decreases2024In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 683, article id A168Article in journal (Refereed)
    Abstract [en]

    Context: Forbush decreases (FDs) are short-term depressions in the galactic cosmic ray (GCR) flux and one of the common signatures of coronal mass ejections (CMEs) in the heliosphere. They often show a two-step profile, the second one associated with the CME's magnetic structure. This second step can be described by the recently developed analytical FD model for flux ropes (FRs) - ForbMod.

    Aims: The aim of this study is to utilise ForbMod to develop a best-fit procedure to be applied on FR-related FDs as a convenient measurement tool. Our motivation is to develop a best-fit procedure that can be applied to a data series from an arbitrary detector. Thus, the basic procedure would facilitate measurement estimation of the magnitude of the FR-related FD, with the possibility of being adapted for the energy response of a specific detector for a more advanced analysis.

    Methods: The non-linear fitting was performed by calculating all possible ForbMod curves constrained within the FR borders to the designated dataset and minimising the mean square error (MSE). In order to evaluate the performance of the ForbMod best-fit procedure, we used synthetic measurements produced by calculating the theoretical ForbMod curve for a specific example CME and then applying various effects to the data to mimic the imperfection of the real measurements. We also tested the ForbMod best-fit function on the real data, measured by detector F of the SOHO/EPHIN instrument on a sample containing 30 events, all of which have a distinct FD corresponding to the magnetic obstacle. The extraction of FD profiles (from the onset to the end) was performed manually by an observer, whereby we applied two different versions of border selection and assigned a quality index to each event.

    Results: We do not find notable differences between events marked by a different quality index. For events with a selection of two different borders, we find that the best fit applied on extended interplanetary coronal mass ejection (ICME) structure borders results in a slightly larger MSE and differences compared to the traditional method due to a larger scatter of the data points. We find that the best-fit results can visually be categorised into six different FD profile types. Although some profiles do not show a visually pleasing FD, the ForbMod best-fit function still manages to find a solution with a relatively small MSE.

    Conclusions: Overall, we find that the ForbMod best-fit procedure performs similar to the traditional algorithm-based observational method, but with slightly smaller values for the FD amplitude, as it's taking into account the noise in the data. Furthermore, we find that the best-fit procedure has an advantage compared to the traditional method as it can estimate the FD amplitude even when there is a data gap at the onset of the FD.

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  • 220.
    Dwivedi, N. K.
    et al.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria.
    Kumar, S.
    Kyung Hee Univ, Sch Space Res, Yongin 446701, Gyeonggi Do, South Korea;Shandong Univ, Inst Space Sci, Shandong Prov Key Lab Opt Astron & Solar Terr Env, Weihai, Peoples R China.
    Kovacs, P.
    Min & Geol Survey Hungary, Budapest, Hungary.
    Yordanova, Emiliya
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Echim, M.
    Inst Royale Aeron Spatiale Belgique, B-1180 Brussels, Belgium.
    Sharma, R. P.
    Indian Inst Technol Delhi, Ctr Energy Studies, Delhi, India.
    Khodachenko, M. L.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria;Moscow MV Lomonosov State Univ, Skobeltsyn Inst Nucl Phys, Moscow, Russia;Russian Acad Sci, Inst Astron, Moscow 119017, Russia.
    Sasunov, Y.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria;Moscow MV Lomonosov State Univ, Skobeltsyn Inst Nucl Phys, Moscow, Russia.
    Implication of kinetic Alfvén waves to magnetic field turbulence spectra: Earth's magnetosheath2019In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 364, no 6, article id 101Article in journal (Refereed)
    Abstract [en]

    In the present paper, we investigate the power-law behaviour of the magnetic field spectra in the Earth's magnetosheath region using Cluster spacecraft data under solar minimum condition. The power spectral density of the magnetic field data and spectral slopes at various frequencies are analysed. Propagation angle, kB, and compressibility, R vertical bar, are used to test the nature of turbulent fluctuations. The magnetic field spectra have the spectral slopes, , between -1.5 to 0 down to spatial scales of 20i (where i is ion gyroradius), and show clear evidence of a transition to steeper spectra for small scales with a second power-law, having between -2.6 to -1.8. At low frequencies, fsc<0.3fci (where fci is ion gyro-frequency), kB approximate to 90 degrees to the mean magnetic field, B0, and R vertical bar shows a broad distribution, 0.1R vertical bar 0.9. On the other hand at fsc>10fci, kB exhibits a broad range, 30 degrees kB90 degrees, while R vertical bar has a small variation: 0.2R vertical bar 0.5. We conjecture that at high frequencies, the perpendicularly propagating Alfven waves could partly explain the statistical analysis of spectra. To support our prediction of kinetic Alfven wave dominated spectral slope behaviour at high frequency, we also present a theoretical model and simulate the magnetic field turbulence spectra due to nonlinear evolution of kinetic Alfven waves. The present study also shows the analogy between the observational and simulated spectra.

  • 221.
    Eastwood, J. P.
    et al.
    Univ London Imperial Coll Sci Technol & Med, Blackett Lab, London, England..
    Phan, T. D.
    Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA..
    Cassak, P. A.
    W Virginia Univ, Dept Phys & Astron, Morgantown, WV 26506 USA..
    Gershman, D. J.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.;Univ Maryland, Dept Astron, College Pk, MD 20742 USA..
    Haggerty, C.
    Univ Delaware, Dept Phys & Astron, Newark, DE 19716 USA..
    Malakit, K.
    Mahidol Univ, Dept Phys, Bangkok 10700, Thailand..
    Shay, M. A.
    Univ Delaware, Dept Phys & Astron, Newark, DE 19716 USA..
    Mistry, R.
    Univ London Imperial Coll Sci Technol & Med, Blackett Lab, London, England..
    Oieroset, M.
    Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA..
    Russell, C. T.
    Univ Calif Los Angeles, Dept Earth Planetary & Space Sci, Los Angeles, CA USA..
    Slavin, J. A.
    Univ Michigan, Dept Climate & Space Sci & Engn, Ann Arbor, MI 48109 USA..
    Argall, M. R.
    Univ New Hampshire, Inst Study Earth Oceans & Space, Durham, NH 03824 USA..
    Avanov, L. A.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.;Univ Maryland, Dept Astron, College Pk, MD 20742 USA..
    Burch, J. L.
    SW Res Inst, San Antonio, TX USA..
    Chen, L. J.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.;Univ Maryland, Dept Astron, College Pk, MD 20742 USA..
    Dorelli, J. C.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Ergun, R. E.
    Univ Colorado, Atmospher & Space Phys Lab, Campus Box 392, Boulder, CO 80309 USA..
    Giles, B. L.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Khotyaintsev, Yuri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Lavraud, B.
    Univ Toulouse, Inst Rech Astrophys & Planetol, Toulouse, France.;CNRS, UMR 5277, Toulouse, France..
    Lindqvist, P. A.
    Royal Inst Technol, Sch Elect Engn, Stockholm, Sweden..
    Moore, T. E.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Nakamura, R.
    Austrian Acad Sci, Space Res Inst, A-8010 Graz, Austria..
    Paterson, W.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Pollock, C.
    Denali Sci, Healy, AK USA..
    Strangeway, R. J.
    Univ Calif Los Angeles, Dept Earth Planetary & Space Sci, Los Angeles, CA USA..
    Torbert, R. B.
    Univ New Hampshire, Inst Study Earth Oceans & Space, Durham, NH 03824 USA.;SW Res Inst, San Antonio, TX USA..
    Wang, S.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.;Univ Maryland, Dept Astron, College Pk, MD 20742 USA..
    Ion-scale secondary flux ropes generated by magnetopause reconnection as resolved by MMS2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 10, p. 4716-4724Article in journal (Refereed)
    Abstract [en]

    New Magnetospheric Multiscale (MMS) observations of small-scale (similar to 7 ion inertial length radius) flux transfer events (FTEs) at the dayside magnetopause are reported. The 10 km MMS tetrahedron size enables their structure and properties to be calculated using a variety of multispacecraft techniques, allowing them to be identified as flux ropes, whose flux content is small (similar to 22 kWb). The current density, calculated using plasma and magnetic field measurements independently, is found to be filamentary. Intercomparison of the plasma moments with electric and magnetic field measurements reveals structured non-frozen-in ion behavior. The data are further compared with a particle-in-cell simulation. It is concluded that these small-scale flux ropes, which are not seen to be growing, represent a distinct class of FTE which is generated on the magnetopause by secondary reconnection.

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  • 222. Eastwood, J. P.
    et al.
    Sibeck, D. G.
    Angelopoulos, V.
    Phan, T. D.
    Bale, S. D.
    McFadden, J. P.
    Cully, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Mende, S. B.
    Larson, D.
    Frey, S.
    Carlson, C. W.
    Glassmeier, K. -H
    Auster, H. U.
    Roux, A.
    Le Contel, O.
    THEMIS observations of a hot flow anomaly: Solar wind, magnetosheath, and ground-based measurements2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no 17, p. L17S03-Article in journal (Refereed)
    Abstract [en]

    The THEMIS spacecraft encountered a Hot Flow Anomaly ( HFA) on the dusk flank of the Earth's bow shock on 4 July 2007, observing it on both sides of the shock. Meanwhile, the THEMIS ground magnetometers traced the progress of the associated Magnetic Impulse Event along the dawn flank of the magnetosphere, providing a unique opportunity to study the transmission of the HFA through the shock and the subsequent downstream response. THEMIS-A, in the solar wind, observed classic HFA signatures. Isotropic electron distributions inside the upstream HFA are attributed to the action of the electron firehose instability. THEMIS-E, just downstream, observed a much more complex disturbance with the pressure perturbation decoupled from the underlying discontinuity. Simple calculations show that the pressure perturbation would be capable of significantly changing the magnetopause location, which is confirmed by the ground-based observations.

  • 223. Eastwood, J.P.
    et al.
    Phan, T.-D.
    Moser, F.S.
    Shay, M.A.
    Fujimoto, M.
    Retino, Alessandro
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Dandouras, I.
    Multi-point observations of the Hall electromagnetic field and secondary island formation during magnetic reconnection2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no A6, p. A06235-Article in journal (Refereed)
    Abstract [en]

    A key feature of collisionless magnetic reconnection is the formation of Hall magnetic and electric field structure in the vicinity of the diffusion region. Here we present multi‐point Cluster observations of a reconnection event in the near‐Earth magnetotail where the diffusion region was nested by the Cluster spacecraft; we compare observations made simultaneously by different spacecraft on opposite sides of the magnetotail current sheet. This allows the spatial structure of both the electric and magnetic field to be probed. It is found that, close to the diffusion region, the magnetic field displays a symmetric quadrupole structure. The Hall electric field is symmetric, observed to be inwardly directed on both sides of the current sheet. It is large (∼40 mV m−1) on the earthward side of the diffusion region, but substantially weaker on the tailward side, suggesting a reduced reconnection rate reflected by a similar reduction in Ey. A small‐scale magnetic flux rope was observed in conjunction with these observations. This flux rope, observed very close to the reconnection site and entrained in the plasma flow, may correspond to what have been termed secondary islands in computer simulations. The core magnetic field inside the flux rope is enhanced by a factor of 3, even though the lobe guide field is negligible. Observations of the electric field inside the magnetic island show extremely strong (∼100 mV m−1) fields which may play a significant role in the particle dynamics during reconnection.

  • 224.
    Edberg, Niklas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Auster, U.
    Barabash, S.
    Boesswetter, A.
    Brain, D. A.
    Burch, J. L.
    Carr, C. M.
    Cowley, S. W. H.
    Cupido, E.
    Duru, F.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Fraenz, M.
    Glassmeier, K. -H
    Goldstein, R.
    Lester, M.
    Lundin, R.
    Modolo, R.
    Nilsson, H.
    Richter, I.
    Samara, M.
    Trotignon, J. G.
    Rosetta and Mars Express observations of the influence of high solar wind pressure on the Martian plasma environment2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 12, p. 4533-4545Article in journal (Refereed)
    Abstract [en]

    We report on new simultaneous in-situ observations at Mars from Rosetta and Mars Express (MEX) on how the Martian plasma environment is affected by high pressure solar wind. A significant sharp increase in solar wind density, magnetic field strength and turbulence followed by a gradual increase in solar wind velocity is observed during similar to 24 h in the combined data set from both spacecraft after Rosetta's closest approach to Mars on 25 February 2007. The bow shock and magnetic pileup boundary are coincidently observed by MEX to become asymmetric in their shapes. The fortunate orbit of MEX at this time allows a study of the inbound boundary crossings on one side of the planet and the outbound crossings on almost the opposite side, both very close to the terminator plane. The solar wind and interplanetary magnetic field (IMF) downstream of Mars are monitored through simultaneous measurements provided by Rosetta. Possible explanations for the asymmetries are discussed, such as crustal magnetic fields and IMF direction. In the same interval, during the high solar wind pressure pulse, MEX observations show an increased amount of escaping planetary ions from the polar region of Mars. We link the high pressure solar wind with the observed simultaneous ion outflow and discuss how the pressure pulse could also be associated with the observed boundary shape asymmetry.

  • 225.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Alho, M.
    Aalto Univ, Sch Elect Engn, Dept Radio Sci & Engn, POB 13000, FI-00076 Aalto, Finland..
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Behar, E.
    Swedish Inst Space Phys, Box 812, SE-98128 Kiruna, Sweden..
    Burch, J. L.
    Southwest Res Inst, 6220 Culebra Rd, San Antonio, TX 78238 USA..
    Carr, C. M.
    Imperial Coll London, Exhibit Rd, London SW7 2AZ, England..
    Cupido, E.
    Imperial Coll London, Exhibit Rd, London SW7 2AZ, England..
    Engelhardt, Ilka. A. D.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Glassmeier, K. -H
    Goetz, C.
    TU Braunschweig, Inst Geophys & Extraterr Phys, Mendelssohnstr 3, D-38106 Braunschweig, Germany..
    Goldstein, R.
    Southwest Res Inst, 6220 Culebra Rd, San Antonio, TX 78238 USA..
    Henri, P.
    Lab Phys & Chim Environm & Espace, F-45071 Orleans 2, France..
    Johansson, Fredrik L.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Koenders, C.
    TU Braunschweig, Inst Geophys & Extraterr Phys, Mendelssohnstr 3, D-38106 Braunschweig, Germany..
    Mandt, K.
    Southwest Res Inst, 6220 Culebra Rd, San Antonio, TX 78238 USA.;Univ Texas San Antonio, Dept Phys & Astron, San Antonio, TX 78249 USA..
    Moestl, C.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria..
    Nilsson, H.
    Swedish Inst Space Phys, Box 812, SE-98128 Kiruna, Sweden..
    Odelstad, Elias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Richter, I.
    TU Braunschweig, Inst Geophys & Extraterr Phys, Mendelssohnstr 3, D-38106 Braunschweig, Germany..
    Wedlund, C. Simon
    Univ Oslo, Dept Phys, Box 1048 Blindern, N-0316 Oslo, Norway..
    Wieser, G. Stenberg
    Swedish Inst Space Phys, Box 812, SE-98128 Kiruna, Sweden..
    Szego, K.
    Wigner Res Ctr Phys, Konkoly Thege Miklos Ut 29-33, H-1121 Budapest, Hungary..
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Volwerk, M.
    Austrian Acad Sci, Space Res Inst, Schmiedlstr 6, A-8042 Graz, Austria..
    CME impact on comet 67P/Churyumov-Gerasimenko2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 462, p. S45-S56Article in journal (Refereed)
    Abstract [en]

    We present Rosetta observations from comet 67P/Churyumov-Gerasimenko during the impact of a coronal mass ejection (CME). The CME impacted on 2015 Oct 5-6, when Rosetta was about 800 km from the comet nucleus, and 1.4 au from the Sun. Upon impact, the plasma environment is compressed to the level that solar wind ions, not seen a few days earlier when at 1500 km, now reach Rosetta. In response to the compression, the flux of suprathermal electrons increases by a factor of 5-10 and the background magnetic field strength increases by a factor of similar to 2.5. The plasma density increases by a factor of 10 and reaches 600 cm(-3), due to increased particle impact ionization, charge exchange and the adiabatic compression of the plasma environment. We also observe unprecedentedly large magnetic field spikes at 800 km, reaching above 200 nT, which are interpreted as magnetic flux ropes. We suggest that these could possibly be formed by magnetic reconnection processes in the coma as the magnetic field across the CME changes polarity, or as a consequence of strong shears causing Kelvin-Helmholtz instabilities in the plasma flow. Due to the limited orbit of Rosetta, we are not able to observe if a tail disconnection occurs during the CME impact, which could be expected based on previous remote observations of other CME-comet interactions.

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  • 226.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA..
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Bertucci, C.
    IAFE, Buenos Aires, DF, Argentina..
    Gurnett, D. A.
    Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA..
    Holmberg, Mika K. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Jackman, C. M.
    Univ Southampton, Southampton, Hants, England..
    Kurth, W. S.
    Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA..
    Menietti, J. D.
    Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA..
    Opgenoorth, Hermann J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Shebanits, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Effects of Saturn's magnetospheric dynamics on Titan's ionosphere2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 10, p. 8884-8898Article in journal (Refereed)
    Abstract [en]

    We use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn's magnetosphere influences Titan's ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan's ionosphere, in the altitude range 1600-2400km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21-03h) compared to when on the dayside (SLT 09-15 h). For lower altitudes (1100-1600km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan's ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from approximate to 200eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn's nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan's ionosphere against the dynamic pressure and energetic particle pressure in Saturn's magnetosphere.

  • 227.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Shebanits, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Ågren, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Opgenoorth, Hermann J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Cravens, T. E.
    Girazian, Z.
    Solar cycle modulation of Titan's ionosphere2013In: Journal of Geophysical Research-Space Physics, ISSN 2169-9380, Vol. 118, no 8, p. 5255-5264Article in journal (Refereed)
    Abstract [en]

    During the six Cassini Titan flybys T83-T88 (May 2012 to November 2012) the electron density in the ionospheric peak region, as measured by the radio and plasma wave science instrument/Langmuir probe, has increased significantly, by 15-30%, compared to previous average. These measurements suggest that a longterm change has occurred in the ionosphere of Titan, likely caused by the rise to the new solar maximum with increased EUV fluxes. We compare measurements from TA, TB, and T5, from the declining phase of solar cycle 23 to the recent T83-T88 measurements during cycle 24, since the solar irradiances from those two intervals are comparable. The peak electron densities normalized to a common solar zenith angle N-norm from those two groups of flybys are comparable but increased compared to the solar minimum flybys (T16-T71). The integrated solar irradiance over the wavelengths 1-80nm, i.e., the solar energy flux, F-e, correlates well with the observed ionospheric peak density values. Chapman layer theory predicts that Nnorm<mml:msubsup>Fek</mml:msubsup>, with k=0.5. We find observationally that the exponent k=0.540.18. Hence, the observations are in good agreement with theory despite the fact that many assumptions in Chapman theory are violated. This is also in good agreement with a similar study by Girazian and Withers (2013) on the ionosphere of Mars. We use this power law to estimate the peak electron density at the subsolar point of Titan during solar maximum conditions and find it to be about 6500cm(-3), i.e., 85-160% more than has been measured during the entire Cassini mission.

  • 228.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Shebanits, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Opgenoorth, Hermann J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Roussos, E.
    Garnier, P.
    Cravens, T. E.
    Badman, S. V.
    Modolo, R.
    Bertucci, C.
    Dougherty, M. K.
    Extreme densities in Titan's ionosphere during the T85 magnetosheath encounter2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 12, p. 2879-2883Article in journal (Refereed)
    Abstract [en]

    We present Cassini Langmuir probe measurements of the highest electron number densities ever reported from the ionosphere of Titan. The measured density reached 4310cm(-3) during the T85 Titan flyby. This is at least 500cm(-3) higher than ever observed before and at least 50% above the average density for similar solar zenith angles. The peak of the ionospheric density is not reached on this flyby, making the maximum measured density a lower limit. During this flyby, we also report that an impacting coronal mass ejection (CME) leaves Titan in the magnetosheath of Saturn, where it is exposed to shocked solar wind plasma for at least 2 h 45 min. We suggest that the solar wind plasma in the magnetosheath during the CME conditions significantly modifies Titan's ionosphere by an addition of particle impact ionization by precipitating protons.

  • 229.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Auster, U.
    Barabash, S.
    Boesswetter, A.
    Carr, C. M.
    Cowley, S. W. H.
    Cupido, E.
    Fraenz, M.
    Glassmeier, K. -H
    Goldstein, R.
    Lester, M.
    Lundin, R.
    Modolo, Ronan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nilsson, H.
    Richter, I.
    Samara, M.
    Trotignon, J. G.
    Simultaneous measurements of Martian plasma boundaries by Rosetta and Mars Express2009In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 57, no 8-9, p. 1085-1096Article in journal (Refereed)
    Abstract [en]

    We present the first two-spacecraft near-simultaneous observations of the Martian bow shock (BS), magnetic pileup boundary (MPB) and photo-electron boundary (PEB) obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars fly by on February 25, 2007. Our observations are compared with shape models for the BS and MPB derived from previous statistical studies. The MPB is found at its expected position but the BS for this event is found significantly closer to the planet than expected for the rather slow and moderately dense solar wind. Cross-calibration of the density measurements on the two spacecraft gives a density profile through the magnetosheath, indicating an increasing solar wind flux during the Rosetta passage which is consistent with the multiple BS crossings at the Rosetta exit.

  • 230.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Odelstad, Elias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Henri, P.
    Lebreton, J. -P
    Gasc, S.
    Rubin, M.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Gill, Reine
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Erik P. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Carr, C. M.
    Cupido, E.
    Glassmeier, K. -H
    Goldstein, R.
    Koenders, C.
    Mandt, K.
    Nemeth, Z.
    Nilsson, H.
    Richter, I.
    Wieser, G. Stenberg
    Szego, K.
    Volwerk, M.
    Spatial distribution of low-energy plasma around comet 67P/CG from Rosetta measurements2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 11, p. 4263-4269Article in journal (Refereed)
    Abstract [en]

    We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be approximate to 1-210(-6), at a cometocentric distance of 10km and at 3.1AU from the Sun. A clear 6.2h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260km from the nucleus falls off with radial distance as approximate to 1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet.

  • 231.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Odelstad, Elias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, D. J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Burch, J. L.
    SW Res Inst, San Antonio, TX USA..
    Carr, C. M.
    Univ London Imperial Coll Sci Technol & Med, Space & Atmospher Phys Grp, London, England..
    Cupido, E.
    Univ London Imperial Coll Sci Technol & Med, Space & Atmospher Phys Grp, London, England..
    Glassmeier, K. -H
    Goldstein, R.
    SW Res Inst, San Antonio, TX USA..
    Halekas, J. S.
    Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA..
    Henri, P.
    Lab Phys & Chim Environm & Espace, Orleans, France..
    Koenders, C.
    TU Braunschweig, Inst Geophys & Extraterr Phys, Braunschweig, Germany..
    Mandt, K.
    SW Res Inst, San Antonio, TX USA..
    Mokashi, P.
    SW Res Inst, San Antonio, TX USA..
    Nemeth, Z.
    Wigner Res Ctr Phys, Budapest, Hungary..
    Nilsson, H.
    Swedish Inst Space Phys, S-98128 Kiruna, Sweden..
    Ramstad, R.
    Swedish Inst Space Phys, S-98128 Kiruna, Sweden..
    Richter, I.
    TU Braunschweig, Inst Geophys & Extraterr Phys, Braunschweig, Germany..
    Wieser, G. Stenberg
    Swedish Inst Space Phys, S-98128 Kiruna, Sweden..
    Solar wind interaction with comet 67P: Impacts of corotating interaction regions2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 2, p. 949-965Article in journal (Refereed)
    Abstract [en]

    We present observations from the Rosetta Plasma Consortium of the effects of stormy solar wind on comet 67P/Churyumov-Gerasimenko. Four corotating interaction regions (CIRs), where the first event has possibly merged with a coronal mass ejection, are traced from Earth via Mars (using Mars Express and Mars Atmosphere and Volatile EvolutioN mission) to comet 67P from October to December 2014. When the comet is 3.1-2.7AU from the Sun and the neutral outgassing rate approximate to 10(25)-10(26)s(-1), the CIRs significantly influence the cometary plasma environment at altitudes down to 10-30km. The ionospheric low-energy (approximate to 5eV) plasma density increases significantly in all events, by a factor of >2 in events 1 and 2 but less in events 3 and 4. The spacecraft potential drops below -20V upon impact when the flux of electrons increases. The increased density is likely caused by compression of the plasma environment, increased particle impact ionization, and possibly charge exchange processes and acceleration of mass-loaded plasma back to the comet ionosphere. During all events, the fluxes of suprathermal (approximate to 10-100eV) electrons increase significantly, suggesting that the heating mechanism of these electrons is coupled to the solar wind energy input. At impact the magnetic field strength in the coma increases by a factor of 2-5 as more interplanetary magnetic field piles up around the comet. During two CIR impact events, we observe possible plasma boundaries forming, or moving past Rosetta, as the strong solar wind compresses the cometary plasma environment. We also discuss the possibility of seeing some signatures of the ionospheric response to tail disconnection events.

  • 232.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nilsson, H.
    Swedish Inst Space Phys IRF, Kiruna, Sweden..
    Gunell, H.
    Umeå Univ, Dept Phys, Umeå, Sweden..
    Götz, C.
    Northumbria Univ, Dept Math Phys & Elect Engn, Newcastle Upon Tyne, England..
    Richter, I.
    Tech Univ Carolo Wilhelmina Braunschweig, Inst Geophys & Extraterr Phys, Braunschweig, Germany..
    Henri, P.
    CNRS, Lab Phys & Chim Environm & Espace, F-45071 Orleans, France.;Lab Lagrange, OCA, CNRS, UCA, F-06304 Nice, France..
    De Keyser, J.
    Royal Belgian Inst Space Aeron, BIRA IASB, Brussels, Belgium..
    Scale size of cometary bow shocks2024In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 682, article id A51Article in journal (Refereed)
    Abstract [en]

    Context. In past decades, several spacecraft have visited comets to investigate their plasma environments. In the coming years, Comet Interceptor will make yet another attempt. This time, the target comet and its outgassing activity are unknown and may not be known before the spacecraft has been launched into its parking orbit, where it will await a possible interception. If the approximate outgassing rate can be estimated remotely when a target has been identified, it is desirable to also be able to estimate the scale size of the plasma environment, defined here as the region bound by the bow shock.

    Aims. This study aims to combine previous measurements and simulations of cometary bow shock locations to gain a better understanding of how the scale size of cometary plasma environments varies. We compare these data with models of the bow shock size, and we furthermore provide an outgassing rate-dependent shape model of the bow shock. We then use this to predict a range of times and cometocentric distances for the crossing of the bow shock by Comet Interceptor, together with expected plasma density measurements along the spacecraft track.

    Methods. We used data of the location of cometary bow shocks from previous spacecraft missions, together with simulation results from previously published studies. We compared these results with an existing model of the bow shock stand-off distance and expand on this to provide a shape model of cometary bow shocks. The model in particular includes the cometary outgassing rate, but also upstream solar wind conditions, ionisation rates, and the neutral flow velocity.

    Results. The agreement between the gas-dynamic model and the data and simulation results is good in terms of the stand-off distance of the bow shock as a function of the outgassing rate. For outgassing rates in the range of 1027–1031–s-1, the scale size of cometary bow shocks can vary by four orders of magnitude, from about 102 km to 106 km, for an ionisation rate, flow velocity, and upstream solar wind conditions typical of those at 1 AU. The proposed bow shock shape model shows that a comet plasma environment can range in scale size from the plasma environment of Mars to about half of that of Saturn.

    Conclusions. The model-data agreement allows for the planning of upcoming spacecraft comet encounters, such as that of Comet Interceptor, when a target has been identified and its outgassing rate is determined. We conclude that the time a spacecraft can spend within the plasma environment during a flyby can range from minutes to days, depending on the comet that is visited and on the flyby speed. However, to capture most of the comet plasma environment, including pick-up ions and upstream plasma waves, and to ensure the highest possible scientific return, measurements should still start well upstream of the expected bow shock location. From the plasma perspective, the selected target should preferably be an active comet with the lowest possible flyby velocity.

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  • 233.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Goetz, Charlotte
    Tech Univ Carolo Wilhelmina Braunschweig, Inst Geophys & Extraterr Phys, Braunschweig, Germany.
    Nilsson, Hans
    Swedish Inst Space Phys, POB 812, SE-98128 Kiruna, Sweden;Lulea Univ Technol, Dept Comp Sci Elect & Space Engn, Rymdcampus 1, SE-98128 Kiruna, Sweden.
    Gilet, Nicolas
    CNRS, LPC2E, Orleans, France.
    Henri, Pierre
    CNRS, LPC2E, Orleans, France.
    The Convective Electric Field Influence on the Cold Plasma and Diamagnetic Cavity of Comet 67P2019In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 158, no 2, article id 71Article in journal (Refereed)
    Abstract [en]

    We studied the distribution of cold electrons (<1 eV) around comet 67P/Churyumov-Gerasimenko with respect to the solar wind convective electric field direction. The cold plasma was measured by the Langmuir Probe instrument and the direction of the convective electric field E-conv = -nu x B was determined from magnetic field (B) measurements inside the coma combined with an assumption of a purely radial solar wind velocity nu. We found that the cold plasma is twice as likely to be observed when the convective electric field at Rosetta's position is directed toward the nucleus (in the -E(conv )hemisphere) compared to when it is away from the nucleus (in the +E-conv hemisphere). Similarly, the diamagnetic cavity, in which previous studies have shown that cold plasma is always present, was also found to be observed twice as often when in the -E-conv hemisphere, linking its existence circumstantially to the presence of cold electrons. The results are consistent with hybrid and Hall magnetohydrodynamic simulations as well as measurements of the ion distribution around the diamagnetic cavity.

  • 234.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Hajra, R.
    CNRS, LPC2E, Orleans, France;Natl Atmospher Res Lab, Gadanki 517112, India.
    Henri, P.
    CNRS, LPC2E, Orleans, France.
    Wedlund, C. S.
    Univ Oslo, Dept Phys, Box 1048 Blindern, N-0316 Oslo, Norway.
    Alho, M.
    Aalto Univ, Sch Elect Engn, Dept Radio Sci & Engn, Aalto, Finland.
    Thiemann, E.
    Univ Colorado, Lab Atmospher & Space Phys, 3665 Discovery Dr, Boulder, CO 80303 USA.
    Solar flares observed by Rosetta at comet 67P/Churyumov-Gerasimenko2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, article id A49Article in journal (Refereed)
    Abstract [en]

    Context. The Rosetta spacecraft made continuous measurements of the coma of comet 67P/Churyumov-Gerasimenko (67P) for more than two years. The plasma in the coma appeared very dynamic, and many factors control its variability. Aims. We wish to identify the effects of solar flares on the comet plasma and also their effect on the measurements by the Langmuir Probe Instrument (LAP). Methods. To identify the effects of flares, we proceeded from an existing flare catalog of Earth-directed solar flares, from which a new list was created that only included Rosetta-directed flares. We also used measurements of flares at Mars when at similar longitudes as Rosetta. The flare irradiance spectral model (FISM v.1) and its Mars equivalent (FISM-M) produce an extreme-ultraviolet (EUV) irradiance (10-120 nm) of the flares at 1 min resolution. LAP data and density measurements obtained with the Mutual Impedence Probe (MIP) from the time of arrival of the flares at Rosetta were examined to determine the flare effects. Results. From the vantage point of Earth, 1504 flares directed toward Rosetta occurred during the mission. In only 24 of these, that is, 1.6%, was the increase in EUV irradiance large enough to cause an observable effect in LAP data. Twenty-four Mars-directed flares were also observed in Rosetta data. The effect of the flares was to increase the photoelectron current by typically 1-5 nA. We find little evidence that the solar flares increase the plasma density, at least not above the background variability. Conclusions. Solar flares have a small effect on the photoelectron current of the LAP instrument, and they are not significant in comparison to other factors that control the plasma density in the coma. The photoelectron current can only be used for flare detection during periods of calm plasma conditions.

  • 235.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Johansson, Fredrik L.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Eriksson, Anders I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Henri, P.
    CNRS, Lab Phys & Chim Environm & Espace, Orleans, France.;UCA, Lab Lagrange, OCA, CNRS, Nice, France..
    De Keyser, J.
    BIRA IASB, Royal Belgian Inst Space Aeron, Brussels, Belgium..
    Radial distribution of plasma at comet 67P: Implications for cometary flyby missions2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 663, article id A42Article in journal (Refereed)
    Abstract [en]

    Context. The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years at a slow walking pace (similar to 1 m s(-1)) within 1500 km from the nucleus. During one of the radial movements of the spacecraft in the early phase of the mission, the radial distribution of the plasma density could be estimated, and the ionospheric density was found to be inversely proportional to the cometocentric distance r from the nucleus (a 1/r distribution). Aims. This study aims to further characterise the radial distribution of plasma around 67P throughout the mission and to expand on the initial results. We also aim to investigate how a 1/r distribution would be observed during a flyby with a fast (similar to 10's km s(-1)) spacecraft, such as the upcoming Comet Interceptor mission, when there is also an asymmetry introduced to the outgassing over the comet surface. Methods. To determine the radial distribution of the plasma, we used data from the Langmuir probe and Mutual Impedance instruments from the Rosetta Plasma Consortium during six intervals throughout the mission, for which the motion of Rosetta was approximately radial with respect to the comet. We then simulated what distribution a fast flyby mission would actually observe during its passage through a coma when there is a 1/r plasma density distribution as well as a sinusoidal variation with a phase angle (and then a sawtooth variation) multiplied to the outgassing rate. Results. The plasma density around comet 67P is found to roughly follow a 1/r dependence, although significant deviations occur in some intervals. If we normalise all data to a common outgassing rate (or heliocentric distance) and combine the intervals to a radial range of 10-1500 km, we find a 1/r(1.19) average distribution. The simulated observed density from a fast spacecraft flying through a coma with a 1/r distribution and an asymmetric outgassing can, in fact, appear anywhere in the range from a 1/r distribution to a 1/r(2) distribution, or even slightly outside of this range. Conclusions. The plasma density is distributed in such a way that it approximately decreases in a manner that is inversely proportional to the cometocentric distance. This is to be expected from the photoionisation of a collision-less, expanding neutral gas at a constant ionisation rate and expansion speed. The deviation from a pure 1/r distribution is in many cases caused by asymmetric outgassing over the surface, additional ionisation sources being present, electric fields accelerating plasma, and changing upstream solar wind conditions. A fast flyby mission can observe a radial distribution that deviates significantly from a 1/r trend if the outgassing is not symmetric over the surface. The altitude profile that will be observed depends very much on the level of outgassing asymmetry, the flyby velocity, the comet rotation rate, and the rotation phase. It is therefore essential to include data from both the inbound and outbound legs, as well as to compare plasma density to neutral density to get a more complete understanding of the radial distribution of the plasma.

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  • 236.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Lester, M.
    Cowley, S. W. H.
    Brain, D. A.
    Fraenz, M.
    Barabash, S.
    Magnetosonic Mach number effect of the position of the bow shock at Mars in comparison to Venus2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, p. A07203-Article in journal (Refereed)
    Abstract [en]

    We study the effect of the magnetosonic Mach number on the position of the bow shock (BS) at Mars. The magnetosonic Mach number is calculated from solar wind data obtained by the ACE satellite upstream of Earth and extrapolated to Mars during two intervals, starting in 2005 and 2007, when Mars and Earth were close to opposition. An increased Mach number is observed to cause the Martian BS to move to lower altitudes and the variation in the terminator altitude is proportional to the Mach number change. When the Mach number is lowered, the BS flares more. We also compare our results to previous studies at Venus. The variation in BS altitude with magnetosonic Mach number is found to be very similar to the variation of the Venusian BS, which has previously been shown to decrease linearly in altitude with increasing Mach number.

  • 237.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nilsson, H.
    Futaana, Y.
    Stenberg, G.
    Lester, M.
    Cowley, S. W. H.
    Luhmann, J. G.
    McEnulty, T. R.
    Opgenoorth, Herman J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Fedorov, A.
    Barabash, S.
    Zhang, T. L.
    Atmospheric erosion of Venus during stormy space weather2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, p. A09308-Article in journal (Refereed)
    Abstract [en]

    We study atmospheric escape from Venus during solar minimum conditions when 147 corotating interaction regions (CIRs) and interplanetary coronal mass ejections (ICMEs) combined impact on the planet. This is the largest study to date of the effects of stormy space weather on Venus and we show for the first time statistically that the atmosphere of Venus is significantly affected by CIRs and ICMEs. When such events impact on Venus, as observed by the ACE and Venus Express satellites, the escape rate of Venus's ionosphere is measured to increase by a factor of 1.9, on average, compared to quiet solar wind times. However, the increase in escape flux during impacts can occasionally be significantly larger by orders of magnitude. Taking into account the occurrence rate of such events we find that roughly half (51%) of the outflow occurs during stormy space weather. Furthermore, we particularly discuss the importance of the increased solar wind dynamic pressure as well as the polarity change of the interplanetary magnetic field (IMF) in terms of causing the increase escape rate. The IMF polarity change across a CIR/ICME could cause dayside magnetic reconnection processes to occur in the induced magnetosphere of Venus, which would add to the erosion through associated particle acceleration.

  • 238.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vigren, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Snowden, D.
    Cent Washington Univ, Dept Phys, Ellensburg, WA USA.
    Regoli, L. H.
    Univ Michigan, Climate & Space Sci & Engn, Ann Arbor, MI 48109 USA.
    Shebanits, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Imperial Coll London, Dept Phys, London, England.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, David J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Bertucci, C.
    IAFE, Ciudad Univ, Buenos Aires, DF, Argentina.
    Cui, J.
    Sun Yat Sen Univ, Sch Atmospher Sci, Zhuhai, Peoples R China;Chinese Acad Sci, Key Lab Lunar & Deep Space Explorat, Beijing, Peoples R China.
    Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys2018In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 17, p. 8721-8728Article in journal (Refereed)
    Abstract [en]

    We report on unusual dynamics in Titan's ionosphere as a significant difference in ionospheric electron density is observed between the T118 and T119 Cassini nightside flybys. Two distinct nightside electron density peaks were present during T118, at 1,150 and 1,200km, and the lowest density ever observed in Titan's ionosphere at altitudes 1,000-1,350km was during T118. These flybys were quite similar in geometry, Saturn local time, neutral density, extreme ultraviolet flux, and ambient magnetic field conditions. Despite this, the Radio and Plasma Waves/Langmuir Probe measured a density difference up to a factor of 6 between the passes. The overall difference was present and similar during both inbound and outbound legs. By ruling out other factors, we suggest that an exceptionally low rate of particle impact ionization in combination with dynamics in the ionosphere is the explanation for the observations. Plain Language Summary Using the Cassini satellite in orbit around Saturn, we make measurements during two close passes of the moon Titan. We observe how the electron density in the uppermost part of the moon's atmosphere-the ionosphere-changes drastically from one pass to the next. We also observe unexpectedly high peaks of electron density in a specific altitude range during the first pass. The findings are attributed to low influx of charged particles from Saturn's magnetosphere as well as to increased dynamics of the plasma in the ionosphere. The study emphasizes the complexity of the physical process at play at the moon and aims at gaining further understanding of this environment.

  • 239.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morooka, Michiko W.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andrews, D. J.
    Cowley, S. W. H.
    Wellbrock, A.
    Coates, A. J.
    Bertucci, C.
    Dougherty, M. K.
    Structured ionospheric outflow during the Cassini T55-T59 Titan flybys2011In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 59, no 8, p. 788-797Article in journal (Refereed)
    Abstract [en]

    During the final three of the five consecutive and similar Cassini Titan flybys T55-T59 we observe a region characterized by high plasma densities (electron densities of 1-8 cm(-3)) in the tail/nightside of Titan. This region is observed progressively farther downtail from pass to pass and is interpreted as a plume of ionospheric plasma escaping Titan, which appears steady in both location and time. The ions in this plasma plume are moving in the direction away from Titan and are a mixture of both light and heavy ions with composition revealing that their origin are in Titan's ionosphere, while the electrons are more isotropically distributed. Magnetic field measurements indicate the presence of a current sheet at the inner edge of this region. We discuss the mechanisms behind this outflow, and suggest that it could be caused by ambipolar diffusion, magnetic moment pumping or dispersive Alfven waves.

  • 240.
    Edberg, Niklas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Lester, M.
    Cowley, S. W. H.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Statistical analysis of the location of the Martian magnetic pileup boundary and bow shock and the influence of crustal magnetic fields2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A8, p. A08206-Article in journal (Refereed)
    Abstract [en]

    We use the data set from the magnetometer and electron reflectometer instruments on board the Mars Global Surveyor spacecraft to show that the crustal magnetic fields of Mars affect the location of the magnetic pileup boundary (MPB) and bow shock (BS) globally. We search for crossings of the MPB and BS in the data that were observed over the first 16 months of the mission. To identify the influence of the crustal magnetic fields, all crossings are extrapolated to the terminator plane in order to remove the solar zenith angle (SZA) dependence, and to make it possible to compare crossings independently of location. The MPB crossings that were observed over regions on Mars, which contain strong crustal magnetic fields, are on average located further out than crossings observed over regions with weak crustal fields. This is shown in three separate longitude intervals. We also find that the dayside BS crossings observed over the southern hemisphere of Mars are on average located further out than the BS crossings observed over the northern hemisphere, possibly because of the influence of the crustal fields. We also study the magnetic field strength and its variation at the inside of the MPB and their dependence on the SZA and altitude. We find that the magnitude of the magnetic field in the MPB is closely linked to the altitude of the MPB, with the magnitude increasing as the MPB is observed closer to the planet.

  • 241.
    Edberg, Niklas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nilsson, H.
    Williams, A. O.
    Lester, M.
    Milan, S. E.
    Cowley, S. W. H.
    Fränz, M.
    Barabash, S.
    Futaana, Y.
    Pumping out the atmosphere of Mars through solar wind pressure pulses2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, p. L03107-Article in journal (Refereed)
    Abstract [en]

    We study atmospheric escape from Mars during solar wind pressure pulses. During the solar minimum of 2007 08 we have observed 41 high pressure events, which are predominantly identified as corotating interaction regions (CIR) while a few are coronal mass ejections (CME), in data from the Advanced Composition Explorer (ACE) upstream of the Earth. 36 of these events are also identified using Mars Express (MEX) data at Mars. We use MEX measurements at Mars to compare the antisunward fluxes of heavy planetary ions during the passage of these pulses to the fluxes during quiet solar wind conditions. The ion fluxes are observed to increase by a factor of similar to 2.5, on average. Hence, a third of the total outflow from Mars takes place during similar to 15% of the time, when a solar wind pressure pulse impacts on the planet. This can have important consequences for the total time-integrated outflow of plasma from Mars.

  • 242.
    Edberg, Niklas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morooka, Michiko W.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Modolo, R.
    Bertucci, C.
    Dougherty, M. K.
    Electron density and temperature measurements in the cold plasma environment of Titan: Implications for atmospheric escape2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, no 20, p. L20105-Article in journal (Refereed)
    Abstract [en]

    We present electron temperature and density measurements of Titan's cold ionospheric plasma from the Langmuir probe instrument on Cassini from 52 flybys. An expression of the density as a function of temperature is presented for altitudes below two Titan radii. The density falls off exponentially with increased temperature as log(n(e)) = -2.0log(T-e) + 0.6 on average around Titan. We show that this relation varies with location around Titan as well as with the solar illumination direction. Significant heating of the electrons appears to take place on the night/wake side of Titan as the density-temperature relation is less steep there. Furthermore, we show that the magnetospheric ram pressure is not balanced by the thermal and magnetic pressure in the topside ionosphere and discuss its implications for plasma escape. The cold ionospheric plasma of Titan extends to higher altitudes in the wake region, indicating the loss of atmosphere down the induced magnetospheric tail.

  • 243. Ekeberg, J.
    et al.
    Stasiewicz, Kristof
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wannberg, G.
    Sergienko, T.
    Eliasson, L.
    Incoherent scatter ion line enhancements and auroral arc-induced Kelvin-Helmholtz turbulence2015In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 122, p. 119-128Article in journal (Refereed)
    Abstract [en]

    We present two cases of incoherent-scatter ion line enhancements in conjunction with auroral arcs drifting through the radar beam. The up- and downshifted ion line shoulders as well as the spectral region between them are enhanced equally and simultaneously. The power enhancements are one order of magnitude above the thermal level and are concentrated in less than 15 km wide altitude ranges at the ionospheric F region peak. The auroral arc passages are preceded by significantly enhanced ion temperatures in the E region, assumed to be caused by transient electric fields associated with velocity shears. We use a Hall MHD model of velocity shears perpendicular to the geomagnetic field and show that a Kelvin-Helmholtz instability will grow for the two presented cases.

  • 244. Ekeberg, J.
    et al.
    Wannberg, G.
    Eliasson, L.
    Stasiewicz, Kristof
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ion-acoustic solitary waves and spectrally uniform scattering cross section enhancements2010In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, no 6, p. 1299-1306Article in journal (Refereed)
    Abstract [en]

    Spectra measured by incoherent scatter radars are formed predominantly by scattering of the incident signal off ion-acoustic and Langmuir waves in the ionosphere. Occasionally, the upshifted and/or downshifted lines produced by the ion-acoustic waves are enhanced well above thermal levels and referred to as naturally enhanced ion-acoustic lines. In this paper, we study another kind of enhancement, which is spectrally uniform over the whole ion-line, i.e. the up- and downshifted shoulder and the spectral valley in between. Based on observations made with the EISCAT Svalbard radar (ESR) facility, we investigate the transient and spectrally uniform power enhancements, which can be explained by ion-acoustic solitary waves. We use a theory of nonlinear waves in a magnetized plasma to determine the properties of such waves and evaluate their effects on scattered signals measured by ESR. We suggest a new mechanism that can explain backscattered power enhancements by one order of magnitude above the thermal level and show that it is consistent with observations.

  • 245.
    Ekvall, Cornelia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Investigation of sub-surface ocean induction on Jupiter's icy moon Europa2022Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Following previous studies, a theoretical model for the induced magnetic field by Europa, one of Jupiter's icy moons, is presented. The aim of the model is to find evidence for the existence of a sub-surface ocean on the moon. Moreover, the accuracy of the theoretical model is evaluated using data from the Galileo space probe and a discussion of improvements, with the upcoming mission JUICE in mind, is given.

    The magnetic field from Jupiter is modeled using a dipole field and the moon is assumed to have the properties of a perfect homogeneous conductive layer (i.e a superconductor with no resistance). Europa is assumed to possess an electrically conductive subsurface ocean with conductivity $\sigma$. As the moon orbits Jupiter, the moon will experience a time-varying magnetic field since the magnetic dipole axis is tilted with an angle with respect to the rotation axis of the planet. The fact that the moon experiences a time-varying magnetic field will cause an inductive response inside the moon if a conductive material is present. 

    However, since this set-up reflects the ideal case, a discussion of constraints and improvements is submitted as a compliment. This thesis shows that the ocean model for Europa is supported, but further evidence is needed to fully understand the structure of the moon. The model shows a clear induction in almost all Galileo-flybys investigated, especially flyby E4 and E14. Thereby, it can be argued that the model gives a representative picture of the true induced magnetic field, with room for improvement. 

    In conclusion, further data is needed to fully reveal the structure of the moon, a fact that lays the foundation for the coming JUICE mission. JUICE will study both the magnetic and the electric field of Jupiter, and analyze the inner structures of the Galilean moons with higher precision than ever

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  • 246.
    Eliasson, B.
    et al.
    Univ Strathclyde, Dept Phys, SUPA, Glasgow G4 0NG, Lanark, Scotland..
    Leyser, Thomas B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Swedish Inst Space Phys, S-75121 Uppsala, Sweden..
    Numerical study of upper hybrid to Z-mode leakage during electromagnetic pumping of groups of striations in the ionosphere2015In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 33, no 8, p. 1019-1030Article in journal (Refereed)
    Abstract [en]

    We investigate numerically the interaction between ionospheric magnetic field-aligned density striations and a left-hand circularly polarized (L)-mode wave. The L-mode wave is scattered into upper hybrid (UH) waves which are partially trapped in the striations, but leak energy to electromagnetic waves in the Z-mode branch. For small-amplitude (1 %) striations, this loss mechanism leads to a significant reduction in amplitude of the UH waves. For several striations organized in a lattice, the leaking of Z-mode waves is compensated by influx of Z-mode radiation from neighboring striations, leading to an increased amplitude of the weakly trapped UH waves. For large-amplitude (10 %) striations the trapped UH waves rapidly increase in amplitude far beyond the threshold for parametric instabilities, and the Z-mode leakage is less important. The results have relevance for the growth of striations and the onset of UH and lower hybrid turbulence during electromagnetic high-frequency pumping of ionospheric plasma, which require large-amplitude UH waves.

  • 247. Eliasson, B.
    et al.
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Simulation study of the interaction between large-amplitude HF radio waves and the ionosphere2007In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, no 6, p. L06106-Article in journal (Refereed)
    Abstract [en]

    The time evolution of a large-amplitude electromagnetic (EM) wave injected vertically into the overhead ionosphere is studied numerically. The EM wave has a carrier frequency of 5 MHz and is modulated as a Gaussian pulse with a width of approximately 0.1 milliseconds and a vacuum amplitude of 1.5 V/m at 50 km. This is a fair representation of a modulated radio wave transmitted from a typical high-power HF broadcast station on the ground. The pulse is propagated through the neutral atmosphere to the critical points of the ionosphere, where the L-O and R-X modes are reflected, and back to the neutral atmosphere. We observe mode conversion of the L-O mode to electrostatic waves, as well as harmonic generation at the turning points of both the R-X and L-O modes, where their amplitudes rise to several times the original ones. The study has relevance for ionospheric interaction experiments in combination with ground-based and satellite or rocket observations.

  • 248. Eliasson, B.
    et al.
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A2, p. A02313-Article in journal (Refereed)
    Abstract [en]

    [1] We have performed a simulation study of Langmuir turbulence in the Earth's ionosphere by means of a Zakharov model with parameters relevant for the F layer. The model includes dissipative terms to model collisions and Landau damping of the electrons and ions, and a linear density profile, which models the ionospheric plasma inhomogeneity whose length scale is of the order 10-100 km. The injection of energy into the system is modeled by a constant source term in the Zakharov equation. Langmuir turbulence is excited "on-demand" in controlled ionospheric modification experiments where the energy is provided by an HF radio beam injected into the overhead ionospheric plasma. The ensuing turbulence can be studied with radars and in the form of secondary radiation recorded by ground-based receivers. We have analyzed spectral signatures of the turbulence for different sets of parameters and different altitudes relative to the turning point of the linear Langmuir mode where the Langmuir frequency equals the local plasma frequency. By a parametric analysis, we have derived a simple scaling law, which links the spectral width of the turbulent frequency spectrum to the physical parameters in the ionosphere. The scaling law provides a quantitative relation between the physical parameters ( temperatures, electron number density, ionospheric length scale, etc.) and the observed frequency spectrum. This law may be useful for interpreting experimental results.

  • 249. Emile, Olivier
    et al.
    Brousseau, Christian
    Emile, Janine
    Niemiec, Ronan
    Madhjoubi, Kouroch
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Electromagnetically Induced Torque on a Large Ring in the Microwave Range2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 5, p. 053902-Article in journal (Refereed)
    Abstract [en]

    We report on the exchange of orbital angular momentum between an electromagnetic wave and a 30 cm diameter ring. Using a turnstile antenna in the GHz range, we induce a torque on a suspended copper strip of the order of 10(-8) N m. Rotations of a few degrees and accelerations up to 4 x 10(-4) degrees/s(2) are observed. A linear dependence of the acceleration as a function of the applied power is found. There are many applications in the detection of angular momentum in electromagnetics, in acoustics, and also in the magnetization of nanostructures.

  • 250.
    Emile, Olivier
    et al.
    Univ Rennes 1, F-35042 Rennes, France..
    Niemiec, Ronan
    Univ Rennes 1, F-35042 Rennes, France.;Univ Rennes 1, CNRS, UMR 6164, IETR, F-35042 Rennes, France..
    Brousseau, Christian
    Univ Rennes 1, CNRS, UMR 6164, IETR, F-35042 Rennes, France..
    Emile, Janine
    Univ Rennes 1, CNRS, UMR 6251, IPR, F-35042 Rennes, France..
    Mahdjoubi, Kouroch
    Univ Rennes 1, CNRS, UMR 6164, IETR, F-35042 Rennes, France..
    Wei, Wenlong
    Univ Rennes 1, CNRS, UMR 6164, IETR, F-35042 Rennes, France..
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Mechanism of angular momentum transfer from microwaves to a copper ring2016In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 70, no 8, article id 172Article in journal (Refereed)
    Abstract [en]

    In the exchange of orbital angular momentum between an electromagnetic wave and a copper ring we examine the origin of the Angular Momentum. We then investigate the transfer mechanism between the microwave and the object, and compare it with other mechanisms. We evidence a transfer mechanism based on the reflection of the electromagnetic field on the copper ring. In particular, at a microscopic scale, we show that the electromagnetic field induces alternative electric currents in the ring, with a small drift. Although little, the resistivity of copper leads to a force that rotates the ring. The estimation of the torque, which is of the order of 10(-8) Nm, is in good agreement with the experimental measurements. We also show that the transfer of electromagnetic orbital angular momentum to objects could be a way to measure the orbital angular momentum carried by electromagnetic fields, and we discuss possible applications.

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