| 1. |
- Cao, D., et al.
(författare)
-
MMS observations of whistler waves in electron diffusion region
- 2017
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 44:9, s. 3954-3962
-
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.
|
|
| 2. |
- Fu, H. S., et al.
(författare)
-
Discrete magnetosonic waves as an evidence of nonlinear wave-particle interaction
- 2014
-
Ingår i: 2014 XXXITH URSI General Assembly And Scientific Symposium (URSI GASS). - 9781467352253
-
Konferensbidrag (refereegranskat)abstract
- Two events, showing strong emissions near the lower hybrid frequency, are studied in detail in this paper. By analyzing the polarization degree, wave normal angle, and ellipticity, we conclude that the emissions are magnetosonic (MS) waves. These MS waves have opposite poynting fluxes in the radial and azimuthal direction, indicating that they were detected near the source region. In the wave spectrogram, discrete and "rising-tone" elements are found, suggesting that MS waves probably are a consequence of nonlinear wave-particle interaction.
|
|
| 3. |
- Fu, H. S., et al.
(författare)
-
Evidence of Magnetic Nulls in Electron Diffusion Region
- 2019
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 46:1, s. 48-54
-
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.
|
|
| 4. |
- Fu, H. S., et al.
(författare)
-
First observation of rising-tone magnetosonic waves
- 2014
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:21, s. 7419-7426
-
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.
|
|
| 5. |
- Fu, H. S., et al.
(författare)
-
How to find magnetic nulls and reconstruct field topology with MMS data?
- 2015
-
Ingår i: Journal of Geophysical Research - Space Physics. - 2000 Florida Ave NW Washington, DC 20009, USA : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 120:5, s. 3758-3782
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, we apply a new method-the first-order Taylor expansion (FOTE)-to find magnetic nulls and reconstruct magnetic field topology, in order to use it with the data from the forthcoming MMS mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the FOTE method can find a null both inside and outside the tetrahedron and also deduce its drift velocity. In addition, the FOTE method can (1) avoid limitations of the PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3-D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2-D (X and O); (3) reconstruct the magnetic field topology. We quantitatively test the accuracy of FOTE in positioning magnetic nulls and reconstructing field topology by using the data from 3-D kinetic simulations. The influences of SC separation (0.05 similar to 1 d(i)) and null-SC distance (0 similar to 1 d(i)) on the accuracy are both considered. We find that (1) for an isolated null, the method is accurate when the SC separation is smaller than 1 d(i), and the null-SC distance is smaller than 0.25 similar to 0.5 d(i); (2) for a null pair, the accuracy is same as in the isolated-null situation, except at the separator line, where the field is nonlinear. We define a parameter xi vertical bar(lambda(1) +lambda(2) +lambda(3))vertical bar/vertical bar lambda vertical bar(max) in terms of the eigenvalues (lambda(i)) of the null to quantify the quality of our method-the smaller this parameter the better the results. Comparing to the previously used parameter (eta vertical bar del center dot B vertical bar/vertical bar del x B vertical bar), xi is more relevant for null identification. Using the new method, we reconstruct the magnetic field topology around a radial-type null and a spiral-type null, and find that the topologies are well consistent with those predicted in theory. We therefore suggest using this method to find magnetic nulls and reconstruct field topology with four-point measurements, particularly from Cluster and the forthcoming MMS mission. For the MMS mission, this null-finding algorithm can be used to trigger its burst-mode measurements.
|
|
| 6. |
- Fu, H. S., et al.
(författare)
-
Identifying magnetic reconnection events using the FOTE method
- 2016
-
Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 121:2, s. 1263-1272
-
Tidskriftsartikel (refereegranskat)abstract
- A magnetic reconnection event detected by Cluster is analyzed using three methods: Single-spacecraft Inference based on Flow-reversal Sequence (SIFS), Multispacecraft Inference based on Timing a Structure (MITS), and the First-Order Taylor Expansion (FOTE). Using the SIFS method, we find that the reconnection structure is an X line; while using the MITS and FOTE methods, we find it is a magnetic island (O line). We compare the efficiency and accuracy of these three methods and find that the most efficient and accurate approach to identify a reconnection event is FOTE. In both the guide and nonguide field reconnection regimes, the FOTE method is equally applicable. This study for the first time demonstrates the capability of FOTE in identifying magnetic reconnection events; it would be useful to the forthcoming Magnetospheric Multiscale (MMS) mission. ion
|
|
| 7. |
- Fu, H. S., et al.
(författare)
-
Intermittent energy dissipation by turbulent reconnection
- 2017
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 44:1, s. 37-43
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 8. |
- Fu, H. S., et al.
(författare)
-
Whistler-mode waves inside flux pileup region : Structured or unstructured?
- 2014
-
Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 119:11, s. 9089-9100
-
Tidskriftsartikel (refereegranskat)abstract
- During reconnection, a flux pileup region (FPR) is formed behind a dipolarization front in an outflow jet. Inside the FPR, the magnetic field magnitude and Bz component increase and the whistler-mode waves are observed frequently. As the FPR convects toward the Earth during substorms, it is obstructed by the dipolar geomagnetic field to form a near-Earth FPR. Unlike the structureless emissions inside the tail FPR, we find that the whistler-mode waves inside the near-Earth FPR can exhibit a discrete structure similar to chorus. Both upper band and lower band chorus are observed, with the upper band having a larger propagation angle (and smaller wave amplitude) than the lower band. Most chorus elements we observed are rising-tone type, but some are falling-tone type. We notice that the rising-tone chorus can evolve into falling-tone chorus within <3s. One of the factors that may explain why the waves are unstructured inside the tail FPR but become discrete inside the near-Earth FPR is the spatial inhomogeneity of magnetic field: we find that such inhomogeneity is small inside the near-Earth FPR but large inside the tail FPR.
|
|
| 9. |
- Lu, H. Y., et al.
(författare)
-
Electric structure of dipolarization fronts associated with interchange instability in the magnetotail
- 2013
-
Ingår i: Journal of Geophysical Research: Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:10, s. 6019-6025
-
Tidskriftsartikel (refereegranskat)abstract
- The electric structure of dipolarization fronts (DFs) is very important to both DF dynamics and particle acceleration. We performed two-dimensional Hall MHD simulation to study the electric structure of DF produced by interchange instability on the scale of ion inertial length in the flow braking region of near-Earth tail. The results indicate that the Hall effect makes the structures of plasma density and magnetic field deformed in the dawn-dusk direction. This deformation is caused by the induced Lorentz force along the tangent plane of DF, which is associated with the outward moving of demagnetized ions driven by the ion-scale Earthward electric field on DF. In addition, the x component of electric field contributed jointly by Hall and electron pressure gradient terms along with Bz can produce a dawnward ExB drift to the whole mushroom structure. Inside the DF, the electric field is mainly produced by Hall term, and the contributions from the convectional and electron pressure gradient electric fields are very small. This indicates that the ion frozen-in condition of magnetic field is violated inside the DF. Therefore, it is the electric field contributed by Hall term inside the DF that changes the overall MHD mushroom pattern. The comparison between the simulation results and the observations of THEMIS satellites demonstrates that the model of Hall MHD simulation can reproduce the plasma and electric field observed at DF.
|
|
| 10. |
- Peng, F. Z., et al.
(författare)
-
Quadrupolar pattern of the asymmetric guide-field reconnection
- 2017
-
Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 122:6, s. 6349-6356
-
Tidskriftsartikel (refereegranskat)abstract
- With high-resolution data of the recently launched Magnetospheric Multiscale mission, we report a magnetic reconnection event at the dayside magnetopause. This reconnection event, having a density asymmetry N-high/N-low approximate to 2 on the two sides of the reconnecting current sheet and a guide field B-g approximate to 0.4B(0) in the out-of-plane direction, exhibit all the two-fluid features: Alfvenic plasma jets in the outflow region, bipolar Hall electric fields toward the current sheet center, quadrupolar Hall magnetic fields in the out-of-plane direction, and the corresponding Hall currents. Obviously, the density asymmetry N-high/N-low approximate to 2 and the guide field B-g approximate to 0.4B(0) are not sufficient to dismiss the quadrupolar pattern of Hall reconnection. This is different from previous simulations, where the bipolar pattern of Hall reconnection was suggested.
|
|
