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Fu, H. S., Cao, J. B., Cao, D., Wang, Z., Vaivads, A., Khotyaintsev, Y. V., . . . Huang, S. Y. (2019). Evidence of Magnetic Nulls in Electron Diffusion Region. Geophysical Research Letters, 46(1), 48-54
Open this publication in new window or tab >>Evidence of Magnetic Nulls in Electron Diffusion Region
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2019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 1, p. 48-54Article in journal (Refereed) Published
Abstract [en]

Theoretically, magnetic reconnection—the process responsible for solar flares and magnetospheric substorms—occurs at the X‐line or radial null in the electron diffusion region (EDR). However, whether this theory is correct is unknown, because the radial null (X‐line) has never been observed inside the EDR due to the lack of efficient techniques and the scarcity of EDR measurements. Here we report such evidence, using data from the recent MMS mission and the newly developed First‐Order Taylor Expansion (FOTE) Expansion technique. We investigate 12 EDR candidates at the Earth's magnetopause and find radial nulls (X‐lines) in all of them. In some events, spacecraft are only 3 km (one electron inertial length) away from the null. We reconstruct the magnetic topology of these nulls and find it agrees well with theoretical models. These nulls, as reconstructed for the first time inside the EDR by the FOTE technique, indicate that the EDR is active and the reconnection process is ongoing.

Plain Language Summary: Magnetic reconnection is a key process responsible for many explosive phenomena in nature such as solar flares and magnetospheric substorms. Theoretically, such process occurs at the X‐line or radial null in the electron diffusion region (EDR). However, whether this theory is correct is still unknown, because the radial null (X‐line) has never been observed inside the EDR due to the lack of efficient technique and the scarcity of EDR measurements. Here we report such evidence, using data from the recent MMS mission and the newly developed FOTE technique.

Keywords
Electron diffusion region, Magnetic null, Magnetic reconnection, FOTE method, Magnetic topology, Reconstruction
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-377360 (URN)10.1029/2018GL080449 (DOI)000456938600006 ()
Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-02-19Bibliographically approved
Cozzani, G., Retino, A., Califano, F., Alexandrova, A., Contel, O. L., Khotyaintsev, Y. V., . . . Burch, J. L. (2019). In situ spacecraft observations of a structured electron diffusion region during magnetopause reconnection. Physical review. E, 99(4), Article ID 043204.
Open this publication in new window or tab >>In situ spacecraft observations of a structured electron diffusion region during magnetopause reconnection
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2019 (English)In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 4, article id 043204Article in journal (Refereed) Published
Abstract [en]

The electron diffusion region (EDR) is the region where magnetic reconnection is initiated and electrons are energized. Because of experimental difficulties, the structure of the EDR is still poorly understood. A key question is whether the EDR has a homogeneous or patchy structure. Here we report Magnetospheric Multiscale (MMS) spacecraft observations providing evidence of inhomogeneous current densities and energy conversion over a few electron inertial lengths within an EDR at the terrestrial magnetopause, suggesting that the EDR can be rather structured. These inhomogenenities are revealed through multipoint measurements because the spacecraft separation is comparable to a few electron inertial lengths, allowing the entire MMS tetrahedron to be within the EDR most of the time. These observations are consistent with recent high-resolution and low-noise kinetic simulations.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-382381 (URN)10.1103/PhysRevE.99.043204 (DOI)000463898200002 ()
Available from: 2019-04-25 Created: 2019-04-25 Last updated: 2019-04-25Bibliographically approved
Steinvall, K., Khotyaintsev, Y. V., Graham, D. B., Vaivads, A., Lindqvist, P.-A. -., Russell, C. T. & Burch, J. L. (2019). Multispacecraft Analysis of Electron Holes. Geophysical Research Letters, 46(1), 55-63
Open this publication in new window or tab >>Multispacecraft Analysis of Electron Holes
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2019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 1, p. 55-63Article in journal (Refereed) Published
Abstract [en]

Electron holes (EHs) are nonlinear electrostatic structures in plasmas. Most previous in situ studies of EHs have been limited to single‐ and two‐spacecraft methods. We present statistics of EHs observed by Magnetospheric Multiscale on the magnetospheric side of the magnetopause during October 2016 when the spacecraft separation was around 6 km. Each EH is observed by all four spacecraft, allowing EH properties to be determined with unprecedented accuracy. We find that the parallel length scale, l, scales with the Debye length. The EHs can be separated into three groups of speed and potential based on their coupling to ions. We present a method for calculating the perpendicular length scale, l. The method can be applied to a small subset of the observed EHs for which we find shapes ranging from almost spherical to more oblate. For the remaining EHs we use statistical arguments to find l/l≈5, implying dominance of oblate EHs.

Plain Language Summary: Electron holes are positively charged structures moving along the magnetic field and are frequently observed in space plasmas in relation to strong currents and electron beams. Electron holes interact with the plasma, leading to electron heating and scattering. In order to understand the effect of these electron holes, we need to accurately determine their properties, such as velocity, length scale, and potential. Most earlier studies have relied on single‐ or two‐spacecraft methods to analyze electron holes. In this study we use the four satellites of the Magnetospheric Multiscale mission to analyze 236 electron holes with unprecedented accuracy. We find that the holes can be divided into three distinct groups with different properties. Additionally, we calculate the width of individual electron holes, finding that they are typically much wider than long, resembling

Keywords
Electron holes
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-377361 (URN)10.1029/2018GL080757 (DOI)000456938600007 ()
Funder
Swedish Research Council, 2016-05507Swedish National Space Board, 2016-05507
Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-02-19Bibliographically approved
Fu, H. S., Xu, Y., Vaivads, A. & Khotyaintsev, Y. V. (2019). Super-efficient Electron Acceleration by an Isolated Magnetic Reconnection. Astrophysical Journal Letters, 870(2), Article ID L22.
Open this publication in new window or tab >>Super-efficient Electron Acceleration by an Isolated Magnetic Reconnection
2019 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 870, no 2, article id L22Article in journal (Refereed) Published
Abstract [en]

Magnetic reconnection-the process typically lasting for a few seconds in space-is able to accelerate electrons. However, the efficiency of the acceleration during such a short period is still a puzzle. Previous analyses, based on spacecraft measurements in the Earth's magnetotail, indicate that magnetic reconnection can enhance electron fluxes up to 100 times. This efficiency is very low, creating an impression that magnetic reconnection is not good at particle acceleration. By analyzing Cluster data, we report here a remarkable magnetic reconnection event during which electron fluxes are enhanced by 10,000 times. Such acceleration, 100 times more efficient than those in previous studies, is caused by the betatron mechanism. Both reconnection fronts and magnetic islands contribute to the acceleration, with the former being more prominent.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
acceleration of particles, magnetic reconnection
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-375876 (URN)10.3847/2041-8213/aafa75 (DOI)000455938700002 ()
Available from: 2019-02-04 Created: 2019-02-04 Last updated: 2019-02-04Bibliographically approved
Eriksson, E., Vaivads, A., Graham, D. B., Divin, A., Khotyaintsev, Y. V., Yordanova, E. & André, M. (2018). Electron Energization at a Reconnecting Magnetosheath Current Sheet [Letter to the editor]. Geophysical Research Letters, 45(16)
Open this publication in new window or tab >>Electron Energization at a Reconnecting Magnetosheath Current Sheet
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 16Article in journal, Letter (Refereed) Published
Abstract [en]

We present observations of electron energization within a sub-ion-scale magnetosheath current sheet (CS). A number of signatures indicate ongoing reconnection, including the thickness of the CS (∼0.7 ion inertial length), nonzero normal magnetic field, Hall magnetic fields with electrons carrying the Hall currents, and electron heating. We observe localized electron acceleration and heating parallel to the magnetic field at the edges of the CS. Electrostatic waves observed in these regions have low phase velocity and small wave potentials and thus cannot provide the observed acceleration and heating. Instead, we find that the electrons are accelerated by a parallel potential within the separatrix regions. Similar acceleration has been reported based on magnetopause and magnetotail observations.Thus, despite the different plasma conditions in magnetosheath, magnetopause, and magnetotail,the acceleration mechanism and corresponding heating of electrons is similar.

Keywords
magnetic reconnection, magnetosheath, electron heating, electron acceleration, Magnetospheric Multiscale
National Category
Other Physics Topics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-359592 (URN)10.1029/2018GL078660 (DOI)000445612500023 ()
Funder
Swedish Research Council, 2013-4309Swedish National Infrastructure for Computing (SNIC), m.2017-1-422Swedish National Infrastructure for Computing (SNIC), m.2016-457
Available from: 2018-09-04 Created: 2018-09-04 Last updated: 2018-10-25Bibliographically approved
Liu, C. M., Fu, H. S., Vaivads, A., Khotyaintsev, Y. V., Gershman, D. J., Hwang, K.-J., . . . Le Contel, O. (2018). Electron Jet Detected by MMS at Dipolarization Front. Geophysical Research Letters, 45(2), 556-564
Open this publication in new window or tab >>Electron Jet Detected by MMS at Dipolarization Front
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 2, p. 556-564Article in journal (Refereed) Published
Abstract [en]

Using MMS high-resolution measurements, we present the first observation of fast electron jet (V-e similar to 2,000 km/s) at a dipolarization front (DF) in the magnetotail plasma sheet. This jet, with scale comparable to the DF thickness (similar to 0.9 d(i)), is primarily in the tangential plane to the DF current sheet and mainly undergoes the E x B drift motion; it contributes significantly to the current system at the DF, including a localized ring-current that can modify the DF topology. Associated with this fast jet, we observed a persistent normal electric field, strong lower hybrid drift waves, and strong energy conversion at the DF. Such strong energy conversion is primarily attributed to the electron-jet-driven current (E.j(e) approximate to 2 E.j(i)), rather than the ion current suggested in previous studies.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2018
Keywords
electron jet, dipolarization front, electric field, LHD waves, electron current, energy conversion
National Category
Fusion, Plasma and Space Physics Geophysics
Identifiers
urn:nbn:se:uu:diva-347650 (URN)10.1002/2017GL076509 (DOI)000425514300009 ()
Available from: 2018-04-06 Created: 2018-04-06 Last updated: 2018-04-06Bibliographically approved
Norgren, C., Graham, D. B., Khotyaintsev, Y. V., André, M., Vaivads, A., Hesse, M., . . . Russell, C. T. (2018). Electron Reconnection in the Magnetopause Current Layer. Journal of Geophysical Research - Space Physics, 123(11), 9222-9238
Open this publication in new window or tab >>Electron Reconnection in the Magnetopause Current Layer
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 11, p. 9222-9238Article in journal (Refereed) Published
Abstract [en]

The electron dynamics within thin current sheets plays a key role both for the process of magnetic reconnection and other energy transfer mechanisms but, from an observational point of view, is not well understood. In this paper we report observations of a reconnecting current sheet with intermediate guide field B-G = 0.5B(in), where B-in is the magnetic field amplitude in the inflow regions. The current sheet width is comparable to electron spatial scales. It shows a bifurcated structure and is embedded within the magnetopause current layer with thickness of several ion scales. The electron scale current sheet has strong out-of-plane and in-plane currents, Hall electric and magnetic fields, a finite magnetic field component normal to the current sheet, and nongyrotropic electron distributions formed due to finite gyroradius effects at the boundary of the current sheet. Comparison between test particle simulations and electron data shows that electrons approaching from the edge of the largest magnetic curvature are scattered to perpendicular pitch angles in the center of the current sheet while electrons entering from the opposite side remain close to field aligned. The comparison also shows that an observed depletion in phase space at antiparallel pitch angles can be explained if an out-of-plane electric field, which due to the guide field is close to antiparallel to the magnetic field, is present in the center of the current sheet. This electric field would be consistent with the reconnection electric field, and we therefore interpret the depletion of electron phase space density as a manifestation of ongoing reconnection.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
National Category
Geophysics Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-372832 (URN)10.1029/2018JA025676 (DOI)000453227400022 ()
Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-01-09Bibliographically approved
Graham, D. B., Vaivads, A., Khotyaintsev, Y. V., Eriksson, A., André, M., Malaspina, D. M., . . . Plaschke, F. (2018). Enhanced Escape of Spacecraft Photoelectrons Caused by Langmuir and Upper Hybrid Waves. Journal of Geophysical Research - Space Physics, 123(9), 7534-7553
Open this publication in new window or tab >>Enhanced Escape of Spacecraft Photoelectrons Caused by Langmuir and Upper Hybrid Waves
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 9, p. 7534-7553Article in journal (Refereed) Published
Abstract [en]

The spacecraft potential is often used to infer rapid changes in the thermal plasma density. The variations in spacecraft potential associated with large-amplitude Langmuir and upper hybrid waves are investigated with the Magnetospheric Multiscale (MMS) mission. When large-amplitude Langmuir and upper hybrid waves are observed, the spacecraft potential increases. The changes in spacecraft potential are shown to be due to enhanced photoelectron escape from the spacecraft when the wave electric fields reach large amplitude. The fluctuations in spacecraft potential follow the envelope function of the Langmuir and upper hybrid waves. Comparison with the high-resolution electron moments shows that the changes in spacecraft potential associated with the waves are not due to density perturbations. Indeed, using the spacecraft potential as a density probe leads to unphysically large density fluctuations. In addition, the changes in spacecraft potential are shown to increase as density decreases: larger spacecraft potential changes are observed in the magnetosphere, than in the magnetosheath and solar wind. These results show that external electric fields can lead to unphysical results when the spacecraft potential is used as a density probe. The results suggest that fluctuations in the spacecraft potential alone cannot be used to determine whether nonlinear processes associated with Langmuir and upper hybrid waves, such as the ponderomotive force and three-wave decay, are occurring.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
Keywords
Langmuir waves, photoelectron current, spacecraft potential, upper hybrid waves
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-369520 (URN)10.1029/2018JA025874 (DOI)000448376600029 ()
Funder
Swedish National Space Board, 175/15Swedish National Space Board, 128/17
Available from: 2018-12-17 Created: 2018-12-17 Last updated: 2018-12-17Bibliographically approved
Schwartz, S. J., Avanov, L., Turner, D., Zhang, H., Gingell, I., Eastwood, J. P., . . . Wilder, F. (2018). Ion Kinetics in a Hot Flow Anomaly: MMS Observations. Geophysical Research Letters, 45(21), 11520-11529
Open this publication in new window or tab >>Ion Kinetics in a Hot Flow Anomaly: MMS Observations
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 21, p. 11520-11529Article in journal (Refereed) Published
Abstract [en]

Hot Flow Anomalies (HFAs) are transients observed at planetary bow shocks, formed by the shock interaction with a convected interplanetary current sheet. The primary interpretation relies on reflected ions channeled upstream along the current sheet. The short duration of HFAs has made direct observations of this process difficult. We employ high resolution measurements by NASA's Magnetospheric Multiscale Mission to probe the ion microphysics within a HFA. Magnetospheric Multiscale Mission data reveal a smoothly varying internal density and pressure, which increase toward the trailing edge of the HFA, sweeping up particles trapped within the current sheet. We find remnants of reflected or other backstreaming ions traveling along the current sheet, but most of these are not fast enough to out-run the incident current sheet convection. Despite the high level of internal turbulence, incident and backstreaming ions appear to couple gyro-kinetically in a coherent manner. Plain Language Summary Shock waves in space are responsible for energizing particles and diverting supersonic flows around planets and other obstacles. Explosive events known as Hot Flow Anomalies (HFAs) arise when a rapid change in the interplanetary magnetic field arrives at the bow shock formed by, for example, the supersonic solar wind plasma flow from the Sun impinging on the Earth's magnetic environment. HFAs are known to produce impacts all the way to ground level, but the physics responsible for their formation occur too rapidly to be resolved by previous satellite missions. This paper employs NASA's fleet of four Magnetospheric Multiscale satellites to reveal for the first time clear, discreet populations of ions that interact coherently to produce the extreme heating and deflection.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-372713 (URN)10.1029/2018GL080189 (DOI)000451832600002 ()
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-01-08Bibliographically approved
Graham, D. B., Vaivads, A., Khotyaintsev, Y. V., André, M., Le Contel, O., Malaspina, D. M., . . . Torbert, R. B. (2018). Large-Amplitude High-Frequency Waves at Earth's Magnetopause. Journal of Geophysical Research - Space Physics, 123(4), 2630-2657
Open this publication in new window or tab >>Large-Amplitude High-Frequency Waves at Earth's Magnetopause
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 4, p. 2630-2657Article in journal (Refereed) Published
Abstract [en]

Large-amplitude waves near the electron plasma frequency are found by the Magnetospheric Multiscale (MMS) mission near Earth's magnetopause. The waves are identified as Langmuir and upper hybrid (UH) waves, with wave vectors either close to parallel or close to perpendicular to the background magnetic field. The waves are found all along the magnetopause equatorial plane, including both flanks and close to the subsolar point. The waves reach very large amplitudes, up to 1Vm(-1), and are thus among the most intense electric fields observed at Earth's magnetopause. In the magnetosphere and on the magnetospheric side of the magnetopause the waves are predominantly UH waves although Langmuir waves are also found. When the plasma is very weakly magnetized only Langmuir waves are likely to be found. Both Langmuir and UH waves are shown to have electromagnetic components, which are consistent with predictions from kinetic wave theory. These results show that the magnetopause and magnetosphere are often unstable to intense wave activity near the electron plasma frequency. These waves provide a possible source of radio emission at the magnetopause.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
Keywords
plasma waves, magnetopause
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-357762 (URN)10.1002/2017JA025034 (DOI)000433498400012 ()
Funder
Swedish National Space Board, 175/15
Available from: 2018-08-23 Created: 2018-08-23 Last updated: 2018-08-23Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-1654-841x

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