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Publications (10 of 19) Show all publications
Cravens, T. E., Morooka, M., Renzaglia, A., Moore, L., Waite, J. H., Perryman, R., . . . Hadid, L. Z. (2019). Plasma Transport in Saturn's Low-Latitude Ionosphere: Cassini Data. Journal of Geophysical Research - Space Physics, 124(6), 4881-4888
Open this publication in new window or tab >>Plasma Transport in Saturn's Low-Latitude Ionosphere: Cassini Data
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 6, p. 4881-4888Article in journal (Refereed) Published
Abstract [en]

In 2017 the Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn. The Ion and Neutral Mass Spectrometer in its ion mode measured densities of light ion species (H+, H-2(+), H-3(+), and He+), and the Radio and Plasma Wave Science instrument measured electron densities. During proximal orbit 287 (denoted P287), Cassini reached down to an altitude of about 3,000 km above the 1 bar atmospheric pressure level. The topside ionosphere plasma densities measured for P287 were consistent with ionospheric measurements during other proximal orbits. Spacecraft potentials were measured by the Radio and Plasma Wave Science Langmuir probe and are typically about negative 0.3 V. Also, for this one orbit, Ion and Neutral Mass Spectrometer was operated in an instrument mode allowing the energies of incident H+ ions to be measured. H+ is the major ion species in the topside ionosphere. Ion flow speeds relative to Saturn's atmosphere were determined. In the southern hemisphere, including near closest approach, the measured ion speeds were close to zero relative to Saturn's corotating atmosphere, but for northern latitudes, southward ion flow of about 3 km/s was observed. One possible interpretation is that the ring shadowing of the southern hemisphere sets up an interhemispheric plasma pressure gradient driving this flow.

National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-392052 (URN)10.1029/2018JA026344 (DOI)000477723100067 ()
Available from: 2019-09-10 Created: 2019-09-10 Last updated: 2019-09-10Bibliographically approved
Morooka, M., Wahlund, J.-E., Hadid, L. Z., Eriksson, A. I., Edberg, N. J. T., Vigren, E., . . . Perry, M. (2019). Saturn's Dusty Ionosphere. Journal of Geophysical Research - Space Physics, 124(3), 1679-1697
Open this publication in new window or tab >>Saturn's Dusty Ionosphere
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 3, p. 1679-1697Article in journal (Refereed) Published
Abstract [en]

Measurements of electrons and ions in Saturn's ionosphere down to 1,500-km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (N-e and N-i) near the closest approach. The small N-e/N-i ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the Langmuir probe obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when N-e/N-i << 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (T-e) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2,500-km altitude in a region where electron-neutral collisions should be prominent. A well-defined relationship between T-e and N-e/N-i ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2019
Keywords
Saturn's ionosphere, dusty plasma, Langmuir probe, plasma density, electron temperature
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-387566 (URN)10.1029/2018JA026154 (DOI)000466087900016 ()
Funder
Swedish Research Council, 621-2013-4191Swedish Research Council, 2.2.1-312/16Swedish National Space Board, Dnr 174/15 135/13 162/14
Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-06-24Bibliographically approved
Hadid, L. Z., Morooka, M. W., Wahlund, J.-E., Persoon, A. M., Andrews, D. J., Shebanits, O., . . . Eriksson, A. I. (2019). Saturn's Ionosphere: Electron Density Altitude Profiles and D-Ring Interaction From The Cassini Grand Finale. Geophysical Research Letters, 46(16), 9362-9369
Open this publication in new window or tab >>Saturn's Ionosphere: Electron Density Altitude Profiles and D-Ring Interaction From The Cassini Grand Finale
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2019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 16, p. 9362-9369Article in journal (Refereed) Published
Abstract [en]

We present the electron density (n(e)) altitude profiles of Saturn's ionosphere at near-equatorial latitudes from all 23 orbits of Cassini's Grand Finale. The data are collected by the Langmuir probe part of the Radio and Plasma Wave Science investigation. A high degree of variability in the electron density profiles is observed. However, organizing them by consecutive altitude ranges revealed clear differences between the southern and northern hemispheres. The n(e) profiles are shown to be more variable and connected to the D-ring below 5,000 km in the southern hemisphere compared to the northern hemisphere. This observed variability is explained to be a consequence of an electrodynamic interaction with the D-ring. Moreover, a density altitude profile is constructed for the northern hemisphere indicating the presence of three different ionospheric layers. Similar properties were observed during Cassini's final plunge, where the main ionospheric peak is crossed at similar to 1,550-km altitude. Plain Language Summary The Cassini Langmuir probe measured directly the uppermost layer of Saturn's atmosphere, the ionosphere, during its Grand Finale. The observations revealed a layered electron density altitude profile with evidence in the southern hemisphere of an electrodynamic type of interaction with the planet innermost D-ring. Moreover, the main peak of the ionosphere is observed for the first time in the final plunge around 1,550 km.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-396547 (URN)10.1029/2018GL078004 (DOI)000490966700007 ()
Funder
Swedish National Space BoardSwedish Research Council, 2016-05364Swedish National Space Board, Dnr 174/15Swedish National Space Board, Dnr 135/13Swedish Research Council, 621-2013-4191
Available from: 2019-11-07 Created: 2019-11-07 Last updated: 2019-11-07Bibliographically approved
Menietti, J. D., Averkamp, T. F., Ye, S.-Y. -., Sulaiman, A. H., Morooka, M., Persoon, A. M., . . . Wahlund, J.-E. (2018). Analysis of Intense Z-Mode Emission Observed During the Cassini Proximal Orbits. Geophysical Research Letters, 45(14), 6766-6772
Open this publication in new window or tab >>Analysis of Intense Z-Mode Emission Observed During the Cassini Proximal Orbits
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 14, p. 6766-6772Article in journal (Refereed) Published
Abstract [en]

The role of Z-mode emission in the diffusive scattering and resonant acceleration of electrons is believed to be important at Saturn. A survey of the 5kHz component of this emission at Saturn earlier reported strong intensity in the lower density regions where the ratio of plasma frequency to cyclotron frequency, f(p)/f(c)<1. At Saturn this occurs along the inner edge of the Enceladus torus near the equator and at higher latitudes. Using the Cassini Radio and Plasma Wave Science instrument observations during the Cassini proximal orbits, we have now identified these emissions extending down to and within the ionosphere. Wave polarization measurements and unique frequency cutoffs are used to positively identify the wave mode. Analogous to the role of whistler mode chorus at Earth, Saturn Z-mode emissions may interact with electrons contributing to the filling or depleting of Saturn's inner radiation belts.

Keywords
Z-mode emission, Saturn proximal orbits, wave source region
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-364054 (URN)10.1002/2018GL077354 (DOI)000442582100002 ()
Available from: 2018-12-07 Created: 2018-12-07 Last updated: 2018-12-07Bibliographically approved
Waite, J. H., Perryman, R. S., Perry, M. E., Miller, K. E., Bell, J., Cravens, T. E., . . . Ip, W.-H. -. (2018). Chemical interactions between Saturn's atmosphere and its rings. Science, 362(6410), Article ID eaat2382.
Open this publication in new window or tab >>Chemical interactions between Saturn's atmosphere and its rings
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2018 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 362, no 6410, article id eaat2382Article in journal (Refereed) Published
Abstract [en]

The Pioneer and Voyager spacecraft made close-up measurements of Saturn's ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn's atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft's Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H-2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

Place, publisher, year, edition, pages
AMER ASSOC ADVANCEMENT SCIENCE, 2018
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-367034 (URN)10.1126/science.aat2382 (DOI)000446547100037 ()30287634 (PubMedID)
Funder
Swedish National Space Board
Available from: 2018-11-29 Created: 2018-11-29 Last updated: 2018-11-29Bibliographically approved
Ye, S.-Y. -., Kurth, W. S., Hospodarsky, G. B., Persoon, A. M., Sulaiman, A. H., Gurnett, D. A., . . . Srama, R. (2018). Dust Observations by the Radio and Plasma Wave Science Instrument During Cassini's Grand Finale. Geophysical Research Letters, 45(19), 10101-10109
Open this publication in new window or tab >>Dust Observations by the Radio and Plasma Wave Science Instrument During Cassini's Grand Finale
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 19, p. 10101-10109Article in journal (Refereed) Published
Abstract [en]

Dust particles in the Saturn system can be detected by the Radio and Plasma Wave Science (RPWS) instrument on board Cassini via antenna voltage signals induced by dust impacts. These impact signals have been simulated in the laboratory by accelerating dust particles onto a Cassini model with electric field antennas. RPWS dust measurements have been shown to be consistent with the Cosmic Dust Analyzer. During the Grand Finale orbits, Cassini flew through the gap between the D ring and Saturn's atmosphere 22 times. In situ measurements by RPWS helped quantify the hazards posed to the spacecraft and instruments on board, which showed a micron-sized dust density orders of magnitude lower than that observed during the Ring Grazing orbits. Close inspection of the waveforms indicated a possible dependence of the impact signal decay time on ambient plasma density. Plain Language Summary Cassini flew through the gap between Saturn and its rings for 22 times before plunging into the atmosphere of Saturn, ending its 20-year mission. The radio and plasma waves instrument on board Cassini helped quantify the dust hazard in this previously unexplored region. The measured density of large dust particles was much lower than expected, allowing high-value science observations during the subsequent Grand Finale orbits.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-387258 (URN)10.1029/2018GL078059 (DOI)000448656800009 ()
Available from: 2019-06-27 Created: 2019-06-27 Last updated: 2019-06-27Bibliographically approved
Menietti, J. D., Averkamp, T. F., Ye, S.-Y. -., Persoon, A. M., Morooka, M., Groene, J. B. & Kurth, W. S. (2018). Extended Survey of Saturn Z-Mode Wave Intensity Through Cassini's Final Orbits. Geophysical Research Letters, 45(15), 7330-7336
Open this publication in new window or tab >>Extended Survey of Saturn Z-Mode Wave Intensity Through Cassini's Final Orbits
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 15, p. 7330-7336Article in journal (Refereed) Published
Abstract [en]

Similar to whistler mode chorus, Z-mode emission is an efficient diffusive scatterer of electrons possibly resulting in resonant acceleration. We present results of a survey of both the low-band (5 kHz) and for the first time the high-band (20 kHz) intensity of these emissions, based on over 11 years of Cassini Radio and Plasma Wave Science instrument data including nine ring-grazing orbits and two proximal orbits, which occurred at the end of the mission. We distinguish these emissions using density and polarization measurements and calculate the mean intensity as a function of frequency and spatial coordinates. We find that the average low-band Z-mode intensity peak is P-0 similar to 7 x 10(-8) nT(2), while the high-band peak is much lower at P-0 similar to 10(-9) nT(2). The spatial distribution of intensity differs for each emission band implying different source regions and perhaps different source mechanisms.

Plain Language Summary

Intense narrow band waves (Z-mode) are observed at Saturn when the spacecraft is located in regions of relatively low density and high magnetic field. These waves are of special importance because they are not seen at such high intensity or over as large a spatial range at Earth. In addition, these waves are known to be very efficient at accelerating electrons under certain conditions and could be responsible for a portion of the observed radiation belts at Saturn. We present an extensive survey of the observations of Z-mode extending over more than 11 years. The survey includes for the first time both the low and high-frequency emissions and orbits from the Cassini final mission, where these waves were seen at a high rate of occurrence. Contour plots and graphs of wave intensity as a function of radius, latitude, and longitude are shown, which will be of value to scientists who model the dynamic processes controlling the electron population at Saturn.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
Keywords
all frequency Z-mode survey, Cassini final orbits, wave source region
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-364397 (URN)10.1029/2018GL079287 (DOI)000443129500011 ()
Available from: 2018-11-01 Created: 2018-11-01 Last updated: 2018-11-01Bibliographically approved
Wahlund, J.-E., Morooka, M. W., Hadid, L. Z., Persoon, A. M., Farrell, W. M., Gurnett, D. A., . . . Vigren, E. (2018). In situ measurements of Saturn's ionosphere show that it is dynamic and interacts with the rings. Science, 359(6371), 66-68
Open this publication in new window or tab >>In situ measurements of Saturn's ionosphere show that it is dynamic and interacts with the rings
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2018 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 359, no 6371, p. 66-68Article in journal (Refereed) Published
Abstract [en]

The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (for example, rings) and hosts ion-molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science instrument detected a cold, dense, and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cubic centimeters, and electron temperatures were below 1160 kelvin near closest approach. The density varied between orbits by up to two orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a north-south asymmetry.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-341494 (URN)10.1126/science.aao4134 (DOI)000419324700067 ()29229651 (PubMedID)
Available from: 2018-02-21 Created: 2018-02-21 Last updated: 2018-02-21Bibliographically approved
Taylor, S. A., Coates, A. J., Jones, G. H., Wellbrock, A., Fazakerley, A. N., Desai, R. T., . . . Waite, J. H. (2018). Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini. Journal of Geophysical Research - Space Physics, 123(1), 287-296
Open this publication in new window or tab >>Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 1, p. 287-296Article in journal (Refereed) Published
Abstract [en]

The Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer has observed photoelectrons produced in the plume of Enceladus. These photoelectrons are observed during Enceladus encounters in the energetic particle shadow where the spacecraft is largely shielded from penetrating radiation by the moon. We present a complex electron spectrum at Enceladus including evidence of two previously unidentified electron populations at 6–10 eV and 10–16 eV. We estimate that the proportion of “hot” (>15 eV) to “cold” (<15 eV) electrons during the Enceladus flybys is ≈ 0.1–0.5%. We have constructed a model of photoelectron production in the plume and compared it with ELS Enceladus flyby data by scaling and energy shifting according to spacecraft potential. We suggest that the complex structure of the electron spectrum observed can be explained entirely by photoelectron production in the plume ionosphere.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-348789 (URN)10.1002/2017JA024536 (DOI)000425637600021 ()
Available from: 2018-04-24 Created: 2018-04-24 Last updated: 2018-04-24Bibliographically approved
Moore, L., Cravens, T. E., Mueller-Wodarg, I., Perry, M. E., Waite, J. H., Perryman, R., . . . Morooka, M. (2018). Models of Saturn's Equatorial Ionosphere Based on In Situ Data From Cassini's Grand Finale. Geophysical Research Letters, 45(18), 9398-9407
Open this publication in new window or tab >>Models of Saturn's Equatorial Ionosphere Based on In Situ Data From Cassini's Grand Finale
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 18, p. 9398-9407Article in journal (Refereed) Published
Abstract [en]

We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low-altitude ionosphere, with a mean ion mass of 11Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H-3(+) variability and an overabundance of modeled H-3(+) potentially hint at missing physical processes in the model, including a loss process that affects H-3(+) but not H+. Plain Language Summary Cassini's Grand Finale enabled the first-ever direct measurements of Saturn's upper atmosphere. Here we use Cassini's unique measurements to construct new models of the plasma in this important boundary region that separates the dense lower atmosphere from space. Based on the complex array of observed gases, we find that heavy molecular ions are dominant near Saturn's equator. This surprising result demonstrates that the chemistry in Saturn's equatorial upper atmosphere is substantially more complex than anticipated. The presence of these unexpected ions potentially represents a new method of monitoring Saturn's ionosphere remotely. Furthermore, as other Cassini measurements indicate that the complex chemistry is likely driven by an influx of ring-derived material, such observations may even help to track the evolution of Saturn's rings as they lose mass to its atmosphere.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
Keywords
Saturn, Cassini, ionosphere
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-387262 (URN)10.1029/2018GL078162 (DOI)000447761300008 ()
Available from: 2019-06-20 Created: 2019-06-20 Last updated: 2019-06-20Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-9958-0241

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