| 1. |
- Cao, D., et al.
(författare)
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MMS observations of whistler waves in electron diffusion region
- 2017
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 44:9, s. 3954-3962
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Tidskriftsartikel (refereegranskat)abstract
- Whistler waves that can produce anomalous resistivity by affecting electrons' motion have been suggested as one of the mechanisms responsible for magnetic reconnection in the electron diffusion region (EDR). Such type of waves, however, has rarely been observed inside the EDR so far. In this study, we report such an observation by Magnetospheric Multiscale (MMS) mission. We find large-amplitude whistler waves propagating away from the X line with a very small wave-normal angle. These waves are probably generated by the perpendicular temperature anisotropy of the -300eV electrons inside the EDR, according to our analysis of dispersion relation and cyclotron resonance condition; they significantly affect the electron-scale dynamics of magnetic reconnection and thus support previous simulations.
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| 2. |
- Chen, Z. Z., et al.
(författare)
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Electron-Driven Dissipation in a Tailward Flow Burst
- 2019
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 46:11, s. 5698-5706
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Tidskriftsartikel (refereegranskat)abstract
- Traditionally, the magnetotail flow burst outside the diffusion region is known to carry ions and electrons together (V-i = V-e), with the frozen-in condition well satisfied (E + V-e x B = 0). Such picture, however, may not be true, based on our analyses of the high-resolution MMS (Magnetospheric Multiscale mission) data. We find that inside the flow burst the electrons and ions can be decoupled (V-e not equal V-i), with the electron speed 5 times larger than the ion speed. Such super-Alfvenic electron jet, having scale of 10 d(i) (ion inertial length) in X-GSM direction, is associated with electron demagnetization (E + V-e x B not equal 0), electron agyrotropy (crescent distribution), and O-line magnetic topology but not associated with the flow reversal and X-line topology; it can cause strong energy dissipation and electron heating. We quantitatively analyze the dissipation and find that it is primarily attributed to lower hybrid drift waves. These results emphasize the non-MHD (magnetohydrodynamics) behaviors of magnetotail flow bursts and the role of lower hybrid drift waves in dissipating energies.
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| 3. |
- Fu, Huishan, et al.
(författare)
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Occurrence rate of earthward-propagating dipolarization fronts
- 2012
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Ingår i: Geophysical Research Letters. - 2000 Florida Ave NW Washington, DC 20009, USA : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 39, s. L10101-
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Tidskriftsartikel (refereegranskat)abstract
- The occurrence rate of earthward-propagating dipolarization fronts (DFs) is investigated in this paper based on the 9 years (2001-2009) of Cluster 1 data. For the first time, we select the DF events by fitting the characteristic increase in B-z using a hyperbolic tangent function. 303 earthward-propagating DFs are found; they have on average a duration of 4 s and a B-z increase of 8 nT. DFs have the maximum occurrence at Z(GSM) approximate to 0 and r approximate to 15 R-E with one event occurring every 3.9 hours, where r is the distance to the center of the Earth in the XYGSM plane. The maximum occurrence rate at Z(GSM) approximate to 0 can be explained by the steep and large increase of B-z near the central current sheet, which is consistent with previous simulations. Along the r direction, the occurrence rate increases gradually from r approximate to 20 to r approximate to 15 R-E but decreases rapidly from r approximate to 15 to r approximate to 10 R-E. This may be due to the increasing pileup of the magnetic flux from r approximate to 20 to r approximate to 15 R-E and the strong background magnetic field at r
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| 4. |
- Fu, Huishan S., et al.
(författare)
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Electric structure of dipolarization front at sub-proton scale
- 2012
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Ingår i: Geophysical Research Letters. - 2000 Florida Ave NW Washington, DC 20009, USA : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 39, s. L06105-
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Tidskriftsartikel (refereegranskat)abstract
- Using Cluster data, we investigate the electric structure of a dipolarization front (DF) - the ion inertial length (c/omega(pi)) scale boundary in the Earth's magnetotail formed at the front edge of an earthward propagating flow with reconnected magnetic flux. We estimate the current density and the electron pressure gradient throughout the DF by both single-spacecraft and multi-spacecraft methods. Comparison of the results from the two methods shows that the single-spacecraft analysis, which is capable of resolving the detailed structure of the boundary, can be applied for the DF we study. Based on this, we use the current density and the electron pressure gradient from the single-spacecraft method to investigate which terms in the generalized Ohm's law balance the electric field throughout the DF. We find that there is an electric field at ion inertia scale directed normal to the DF; it has a duskward component at the dusk flank of DF but a dawnward component at the dawn flank of DF. This electric field is balanced by the Hall (j x B/ne) and electron pressure gradient (del P-e/ne) terms at the DF, with the Hall term being dominant. Outside the narrow DF region, however, the electric field is balanced by the convection (V-i x B) term, meaning the frozen-in condition for ions is broken only at the DF itself. In the reference frame moving with the DF the tangential electric field is almost zero, indicating there is no flow of plasma across the DF and that the DF is a tangential discontinuity. The normal electric field at the DF constitutes a potential drop of similar to 1 keV, which may reflect and accelerate the surrounding ions.
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| 5. |
- Fu, H. S., et al.
(författare)
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Evidence of Magnetic Nulls in Electron Diffusion Region
- 2019
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 46:1, s. 48-54
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Fu, H. S., et al.
(författare)
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First observation of rising-tone magnetosonic waves
- 2014
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:21, s. 7419-7426
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Tidskriftsartikel (refereegranskat)abstract
- Magnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90 degrees and frequency below the lower hybrid frequency. Such waves, also termed equatorial noise, were traditionally known to be "temporally continuous" in their time-frequency spectrogram. Here we show for the first time that MS waves actually have discrete wave elements with rising-tone features in their spectrogram. The frequency sweep rate of MS waves, similar to 1 Hz/s, is between that of chorus and electromagnetic ion cyclotron (EMIC) waves. For the two events we analyzed, MS waves occur outside the plasmapause and cannot penetrate into the plasmasphere; their power is smaller than that of chorus. We suggest that the rising-tone feature of MS waves is a consequence of nonlinear wave-particle interaction, as is the case with chorus and EMIC waves.
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| 7. |
- Huang, S. Y., et al.
(författare)
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Electron acceleration in the reconnection diffusion region : Cluster observations
- 2012
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 39:L11103
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Tidskriftsartikel (refereegranskat)abstract
- We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration.
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| 8. |
- Huang, S. Y., et al.
(författare)
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MMS observations of ion-scale magnetic island in the magnetosheath turbulent plasma
- 2016
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:15, s. 7850-7858
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Tidskriftsartikel (refereegranskat)abstract
- In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 d(i), where d(i) is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma.
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| 9. |
- Huang, S. Y., et al.
(författare)
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Observations of Flux Ropes With Strong Energy Dissipation in the Magnetotail
- 2019
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:2, s. 580-589
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Tidskriftsartikel (refereegranskat)abstract
- An ion-scale flux rope (FR), embedded in a high-speed electron flow (possibly an electron vortex), is investigated in the magnetotail using observations from the Magnetospheric Multiscale (MMS) spacecraft. Intense electric field and current and abundant waves are observed in the exterior and interior regions of the FR. Comparable parallel and perpendicular currents in the interior region imply that the FR has a non-force-free configuration. Electron demagnetization occurs in some subregions of the FR. It is surprising that strong dissipation (I x E' up to 2,000 pW/m(3)) occurs in the center of the FR without signatures of secondary reconnection or coalescence of two FRs, implying that FR may provide another important channel for energy dissipation in space plasmas. These features indicate that the observed FR is still highly dynamical, and hosts multiscale coupling processes, even though the FR has a very large scale and is far away from the reconnection site. Plain Language Summary: Flux ropes, 3-D helical magnetic structures, in which magnetic field lines twist with each other, play an important role in the macroscopic and microscopic physical process during magnetic reconnection. Most of previous studies focused on the flux ropes in the reconnection region. However, some physical process inside macroscopic flux ropes far away from the reconnection site in the magnetotail is still unclear due to the lack of high time resolution data. In this letter, thanks to the unprecedented high time resolution data of the Magnetospheric Multiscale (MMS) mission, we report an ion-scale flux rope and study its dynamics. Our observations demonstrate that the observed flux rope is still highly dynamical, and hosting multiscale coupling processes and strong energy dissipation, even though the flux rope has very large scale and is far away from the reconnection site.
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| 10. |
- Huang, S. Y., et al.
(författare)
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Observations of turbulence within reconnection jet in the presence of guide field
- 2012
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 39:L11104
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Tidskriftsartikel (refereegranskat)abstract
- We present the first comprehensive observations of turbulence properties within high speed reconnection jet in the plasma sheet with moderate guide field. The power spectral density index is about -1.73 in the inertial range, and follows the value of -2.86 in the ion dissipation range. The turbulence is strongly anisotropic in the wave-vector space with the major power having its wave-vector highly oblique to the ambient magnetic field, suggesting that the turbulence is quasi-2D. The measured "dispersion relations" obtained using the k-filtering technique are compared with theory and are found to be consistent with the Alfven-Whistler mode. In addition, both Probability Distribution Functions and flatness results show that the turbulence in the reconnection jet is intermittent (multifractal) at scales less than the proton gyroradius/inertial lengths. The estimated electric field provided by anomalous resistivity caused by turbulence is about 3 mV/m, which is close to the typical reconnection electric field in the magnetotail.
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