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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.

Place, publisher, year, edition, pages
2000 Florida Ave NW Washington, DC 20009, USA: American Geophysical Union (AGU), 2019
Keywords
Electron diffusion region, Magnetic null, Magnetic reconnection, FOTE method, Magnetic topology, Reconstruction
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-253246 (URN)10.1029/2018GL080449 (DOI)000456938600006 ()2-s2.0-85059894810 (Scopus ID)
Available from: 2019-06-14 Created: 2019-06-14 Last updated: 2019-06-18Bibliographically approved
Cozzani, G., Retino, A., Califano, F., Alexandrova, A., Contel, O. L., Khotyaintsev, Y., . . . 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
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-249783 (URN)10.1103/PhysRevE.99.043204 (DOI)000463898200002 ()2-s2.0-85064403029 (Scopus ID)
Note

QC 20190429

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-06-18Bibliographically approved
Steinvall, K., Khotyaintsev, Y. V. 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 6km. 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(vertical bar), 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(vertical bar)approximate to 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 pancakes.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2019
Keywords
Electron holes
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-244119 (URN)10.1029/2018GL080757 (DOI)000456938600007 ()2-s2.0-85059915386 (Scopus ID)
Funder
Swedish National Space Board, 128/17Swedish Research Council, 2016-05507
Note

QC 20190219

Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-06-18Bibliographically 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
Institute of Physics Publishing (IOPP), 2019
Keywords
acceleration of particles, magnetic reconnection
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-253253 (URN)10.3847/2041-8213/aafa75 (DOI)000455938700002 ()2-s2.0-85060198327 (Scopus ID)
Note

QC 20190625

Available from: 2019-06-14 Created: 2019-06-14 Last updated: 2019-06-25Bibliographically 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.

Place, publisher, year, edition, pages
Blackwell Publishing Ltd, 2018
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-253640 (URN)10.1029/2018GL080189 (DOI)000451832600002 ()2-s2.0-85056153852 (Scopus ID)
Note

QC 20190618

Available from: 2019-06-15 Created: 2019-06-15 Last updated: 2019-06-18Bibliographically approved
Graham, D. B., Vaivads, A., Khotyaintsev, Y. V. V., Andre, 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
Identifiers
urn:nbn:se:kth:diva-231239 (URN)10.1002/2017JA025034 (DOI)000433498400012 ()2-s2.0-85045218412 (Scopus ID)
Note

QC 20180627

Available from: 2018-06-27 Created: 2018-06-27 Last updated: 2019-06-18Bibliographically approved
Huang, S. Y., Yuan, Z. G., Fu, H. S., Vaivads, A., Sahraoui, F., Khotyaintsev, Y. V., . . . Wang, D. D. (2018). Observations of Whistler Waves in the Magnetic Reconnection Diffusion Region. In: 2ND URSI ATLANTIC RADIO SCIENCE MEETING (AT-RASC): . Paper presented at 2nd URSI Atlantic Radio Science Meeting (AT-RASC), MAY 28-JUN 01, 2018, Meloneras, SPAIN. IEEE
Open this publication in new window or tab >>Observations of Whistler Waves in the Magnetic Reconnection Diffusion Region
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2018 (English)In: 2ND URSI ATLANTIC RADIO SCIENCE MEETING (AT-RASC), IEEE , 2018Conference paper, Published paper (Refereed)
Abstract [en]

Whistler waves are believed to play an important role during magnetic reconnection. In this paper, we report the simultaneous occurrence of two types of the whistler waves in the magnetotail reconnection diffusion region. The first type is observed in the pileup region of downstream and propagates away along the field lines to downstream, and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type is found around the separatrix region and propagates towards the X-line, and is possibly aenerated by the electron beam-driven whistler instability or Cerenkov emission from electron phase-space holes. Our observations of two different types of whistler waves are well consistent with recent kinetic simulations, and suggest that the observed whistler waves are the consequences of magnetic reconnection.Moreover, we statistically investigate the whistler waves in the magnetotail reconnection region, and construct the global distribution and occurrence rate of the whistler waves based on the two-dimensional reconnection model. It is found that the occurrence rate of the whistler waves is large in the separatrix region (113,1B0j>0.4) and pileup region ([B,./Bol<0.2, 161>45'), but very small in the X-line region. The statistical results are well consistent with the case study.

Place, publisher, year, edition, pages
IEEE, 2018
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-253491 (URN)10.23919/URSI-AT-RASC.2018.8471382 (DOI)000462069500089 ()978-90-82598-73-5 (ISBN)
Conference
2nd URSI Atlantic Radio Science Meeting (AT-RASC), MAY 28-JUN 01, 2018, Meloneras, SPAIN
Note

QC 20190826

Available from: 2019-06-15 Created: 2019-06-15 Last updated: 2019-08-26Bibliographically approved
Johlander, A., Vaivads, A., Khotyaintsev, Y. V., Gingell, I., Schwartz, S. J., Giles, B. L., . . . Russell, C. T. (2018). Shock ripples observed by the MMS spacecraft: ion reflection and dispersive properties. Plasma Physics and Controlled Fusion, 60, Article ID 125006.
Open this publication in new window or tab >>Shock ripples observed by the MMS spacecraft: ion reflection and dispersive properties
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2018 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 60, article id 125006Article in journal (Refereed) Published
Abstract [en]

Shock ripples are ion-inertial-scale waves propagating within the front region of magnetized quasi-perpendicular collisionless shocks. The ripples are thought to influence particle dynamics and acceleration at shocks. With the four magnetospheric multiscale (MMS) spacecraft, it is for the first time possible to fully resolve the small scale ripples in space. We use observations of one slow crossing of the Earth’s non-stationary bow shock by MMS. From multi-spacecraft measurements we show that the non-stationarity is due to ripples propagating along the shock surface. We find that the ripples are near linearly polarized waves propagating in the coplanarity plane with a phase speed equal to the local Alfvén speed and have a wavelength close to 5 times the upstream ion inertial length. The dispersive properties of the ripples resemble those of Alfvén ion cyclotron waves in linear theory. Taking advantage of the slow crossing by the four MMS spacecraft, we map the shock-reflected ions as a function of ripple phase and distance from the shock. We find that ions are preferentially reflected in regions of the wave with magnetic field stronger than the average overshoot field, while in the regions of lower magnetic field, ions penetrate the shock to the downstream region.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-253498 (URN)10.1088/1361-6587/aae920 (DOI)000449418100001 ()2-s2.0-85056350872 (Scopus ID)
Note

QC 20190710

Available from: 2019-06-15 Created: 2019-06-15 Last updated: 2019-07-10Bibliographically approved
Fu, H. S., Vaivads, A., Khotyaintsev, Y. V., André, M., Cao, J. B., Olshevsky, V., . . . Retino, A. (2017). Intermittent energy dissipation by turbulent reconnection. Geophysical Research Letters, 44(1), 37-43
Open this publication in new window or tab >>Intermittent energy dissipation by turbulent reconnection
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2017 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 1, p. 37-43Article in journal (Refereed) Published
Abstract [en]

Magnetic reconnectionthe process responsible for many explosive phenomena in both nature and laboratoryis efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the diffusion region at the sub-ion scale. Here we report such a measurement by Clusterfour spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, Ej, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2017
Keywords
turbulent reconnection, energy dissipation, turbulence, magnetic nulls, current filaments, intermittence
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-253251 (URN)10.1002/2016GL071787 (DOI)000393954900005 ()2-s2.0-85010638265 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20190625

Available from: 2019-06-14 Created: 2019-06-14 Last updated: 2019-06-25Bibliographically approved
Graham, D. B., Khotyaintsev, Y. V. V., Norgren, C., Vaivads, A., Andre, M., Toledo-Redondo, S., . . . Burch, J. L. (2017). Lower hybrid waves in the ion diffusion and magnetospheric inflow regions. Journal of Geophysical Research - Space Physics, 122(1), 517-533
Open this publication in new window or tab >>Lower hybrid waves in the ion diffusion and magnetospheric inflow regions
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2017 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 1, p. 517-533Article in journal (Refereed) Published
Abstract [en]

The role and properties of lower hybrid waves in the ion diffusion region and magnetospheric inflow region of asymmetric reconnection are investigated using the Magnetospheric Multiscale (MMS) mission. Two distinct groups of lower hybrid waves are observed in the ion diffusion region and magnetospheric inflow region, which have distinct properties and propagate in opposite directions along the magnetopause. One group develops near the ion edge in the magnetospheric inflow, where magnetosheath ions enter the magnetosphere through the finite gyroradius effect and are driven by the ion-ion cross-field instability due to the interaction between the magnetosheath ions and cold magnetospheric ions. This leads to heating of the cold magnetospheric ions. The second group develops at the sharpest density gradient, where the Hall electric field is observed and is driven by the lower hybrid drift instability. These drift waves produce cross-field particle diffusion, enabling magnetosheath electrons to enter the magnetospheric inflow region thereby broadening the density gradient in the ion diffusion region.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2017
Keywords
Magnetic reconnection, Ion diffusion region, Lower hybrid waves
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-205170 (URN)10.1002/2016JA023572 (DOI)000395655800038 ()2-s2.0-85010693276 (Scopus ID)
Note

QC 20170411

Available from: 2017-04-11 Created: 2017-04-11 Last updated: 2019-06-18Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-1654-841x

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