Ye Pang

Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble

Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasma-depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near-Earth tail around X = -18 R-E. The bubble is bounded by both sharp leading (partial derivative b(z)/partial derivative x 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field-aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super-Alfvenic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E x B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of B-z, or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field-aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed. Citation: Zhou, M., X. Deng, M. Ashour-Abdalla, R. Walker, Y. Pang, C. Tang, S. Huang, M. El-Alaoui, Z. Yuan and H. Li (2013), Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble, J. Geophys. Res. Space Physics, 118, 674-684, doi:10.1029/2012JA018323.

Cold electron heating by EMIC waves in the plasmaspheric plume with observations of the Cluster satellite

We report in situ observations by the Cluster spacecraft of plasmaspheric electron heating in the plasmaspheric plume. Electron heating events were accompanied by enhancements of electromagnetic ion cyclotron (EMIC) waves in the increased density ducts on the negative density gradient side for two substructures of the plasmaspheric plume. Electron heating is much stronger for the pitch angle of 0° and 180° than for the pitch angle of 90°. Theoretical calculations of the Landau resonant interaction between electrons and observed EMIC waves demonstrate that Landau damping of oblique EMIC waves is a reasonable candidate to heat cold electrons in the presence of O + ions in the outer boundary of the plasmaspheric plume. Therefore, this observation is considered in situ evidence of plasmaspheric electron heating through Landau damping of EMIC waves in plasmaspheric plumes.

Observation of large‐amplitude magnetosonic waves at dipolarization fronts

Various plasma waves have been observed in the vicinity of dipolarization fronts (DFs) and the rarefaction regions behind them. It was suggested that these waves not only play crucial roles in regulating particle kinetics at the DFs but also may potentially affect the large-scale dynamics of the magnetotail. In this paper, we present the observations of large-amplitude electromagnetic waves at DFs that occurred during magnetospheric substorms. The DFs were embedded in either the tailward or earthward flows in the near-Earth magnetotail. The wave frequencies were near the local proton cyclotron frequency. The waves propagated at highly oblique angles with respect to the ambient magnetic field (~80°–100°). Their corresponding wavelengths were on the order of the local ion gyroradii. The major magnetic field fluctuations were along the background magnetic field, while the electric field fluctuations were predominantly perpendicular to the background magnetic field. The waves were compressional waves as there was an anticorrelation between the plasma density and the wave magnetic field strength. The electric potential associated with the waves reached to more than half of the electron temperature, indicating the waves are nonlinear. We suggest that the waves were magnetosonic or ion Bernstein mode waves driven by the ion ring distribution. The waves were able to provide significant anomalous resistivity at the front, with major contributions from the electric field fluctuations. The effects of these waves on the electron pitch angle scattering and energy diffusion are also discussed.

Characteristic distribution and possible roles of waves around the lower hybrid frequency in the magnetotail reconnection region

It has long been suggested that waves around the lower hybrid frequency play a significant role in magnetic reconnection. In this paper we statistically study the distribution and possible roles of waves around lower hybrid frequency based on the wave data recorded by the Cluster spacecraft during 21 magnetotail reconnection events. We find that, as the plasma β increments, magnetic field fluctuations associated with waves increase while electric field fluctuations decrease. As β exceeds 10, both magnetic and electric field fluctuations decrease. Furthermore, a two-dimensional wave distribution is constructed based on the two-dimensional reconnection model. The most intense magnetic field fluctuations occur in the outflow region, while they are weaker in the inflow region and separatrix region. The most intense electric field fluctuations occur around the separatrix region, while they are weaker in the inflow and outflow regions. There are positive correlations between wave strength and energetic electron acceleration, as well as between wave strength and reconnection rate. Our results may be important for fully understanding the role of waves around lower hybrid frequency in the dissipation process of magnetic reconnection.

MMS Observations of Ion-scale Magnetic Island in the Magnetosheath Turbulent Plasma

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 ...

Plasma physics of magnetic island coalescence during magnetic reconnection

A large-scale two-dimensional electromagnetic particle-in-cell simulation was employed to study the magnetic island coalescence/merging process during magnetic reconnection with guide field. The merged island after coalescence is characterized by strong core field and plasma density dip at the island center. We found that the core field enhancement is caused by the out-of-plane magnetic field pileup, as well as the field line twisting due to Hall effect. There is an in-plane electric current loop, which is mainly carried by electrons, circumventing the enhanced core field region. Total force points away or tangentially to the surface of density dip within the merged island, which prevents electrons from higher-density region entering the lower density region between two merging islands. This is contrary to the force in the secondary island, inside which the total force points toward the island center and constrains plasma there. The coalescence process involves a reconnection at the merging sheet between two islands. Electron frozen-in condition is violated locally along the merging sheet. It is contributed by both the divergence of electron pressure tensor and electron inertial. Energy dissipation is concentrated on the merging line during coalescence, while a train of dynamo (j′ · E′   0) regions are distributed along the merging sheet after coalescence. Electrons and ions within the density dip at the island center are accelerated antiparallel to the ambient magnetic field, i.e., along the out-of-plane direction. The resultant distribution can excite the Buneman instability and two-streaming instability, which probably account for the electrostatic solitary waves observed by satellite.

Direct evidence for kinetic effects associated with solar wind reconnection

Kinetic effects resulting from the two-fluid physics play a crucial role in the fast collisionless reconnection, which is a process to explosively release massive energy stored in magnetic fields in space and astrophysical plasmas. In-situ observations in the Earths magnetosphere provide solid consistence with theoretical models on the point that kinetic effects are required in the collisionless reconnection. However, all the observations associated with solar wind reconnection have been analyzed in the context of magnetohydrodynamics (MHD) although a lot of solar wind reconnection exhausts have been reported. Because of the absence of kinetic effects and substantial heating, whether the reconnections are still ongoing when they are detected in the solar wind remains unknown. Here, by dual-spacecraft observations, we report a solar wind reconnection with clear Hall magnetic fields. Its corresponding Alfvenic electron outflow jet, derived from the decouple between ions and electrons, is identified, showing direct evidence for kinetic effects that dominate the collisionless reconnection. The turbulence associated with the exhaust is a kind of background solar wind turbulence, implying that the reconnection generated turbulence has not much developed.

A statistical study on the whistler waves behind dipolarization fronts

We present a statistical study of whistler waves behind dipolarization fronts (DFs) based on the Cluster satellites measurements during the years 2001–2007. We find 732 DFs during the 7 year tail periods (XGSM ≤ −8 RE and |YGSM| ≤ 10 RE) in the plasma sheet. By constraining the whistler waves in a 1 min interval behind the DFs (the maximum Bz), we find that 381 DFs (about 50%) are followed by whistler waves. We study the occurrence rate of whistler waves, the wave characteristic parameters, and the corresponding electron distribution, not only in a global view but also in the local DF coordinate. In a global view, behind the DFs, the whistler waves mostly occur in the radial distance between 17 and 18 RE. They have a higher occurrence rate on the dawnside than the duskside. On the other hand, in the local DF coordinate, whistler waves have a higher occurrence rate around the meridian of DF. In addition, the average wave amplitudes increase toward the dawnside of DF. Associated with the whistler waves, electron distributions have a dominant perpendicular anisotropy for electrons with energy higher than 5 keV. Lower energy electron distributions do not have such perpendicular anisotropy dominance. Moreover, the perpendicular anisotropy for electrons >5 keV increases toward the dawnside of DF, which may be caused by the drift-betatron acceleration. We suggest that the free energy source for whistler waves behind the DFs is probably the perpendicular anisotropy of >5 keV electrons caused by the betatron acceleration.

Evidence of deflected super-Alfvenic electron jet in a reconnection region with weak guide field

Recent numerical simulations demonstrated that electron diffusion region develops into two-scale structure, i.e., the inner electron diffusion region and the outer electron diffusion region. The outer diffusion region is manifested as super-Alfvenic electron jet embedded in central current sheet. However, the electron jets are deflected from neutral sheet with a weak guide field. In this paper we present the in situ evidence of deflected super-Alfvenic electron jet in a reconnection region with a weak guide field in the Earths magnetotail. The electron-scale jet was detected at about 37 ion inertial lengths from the X line. There was a strong electric field at the jet. The strong electric field at the jet was primarily balanced by Hall electric field, as the intense current was mainly carried by magnetized electrons. Another event in the magnetosheath also supports our conclusion that guide field deflects the electron jet away the neutral sheet.

Statistics of energetic electrons in the magnetotail reconnection

Magnetic reconnection has long been regarded as an important site for producing energetic electrons in solar terrestrial and astrophysical plasmas. The motivation of this paper is to provide the average properties of energetic electrons in reconnection region, which are crucial for understanding electron energization mechanism but are rarely known. We statistically analyzed the energetic electrons through 21 magnetotail reconnection events observed by Cluster spacecraft during the years of 2001–2005. Approximately 1200 data points with time resolution of 8 s have been collected for each spacecraft. Two parameters are examined: energetic electron rate (EER) and power law index. EER, which is defined as the ratio of the integrated energetic electron flux to the lower energy electron flux, is used to quantify the electron acceleration efficiency. We find that EER and energetic electron flux (EEF) are positively correlated with the power law index, i.e., the higher rate and flux generally corresponds to softer spectrum. This unexpected correlation is probably caused by some nonadiabatic heating/acceleration mechanisms that tend to soft the spectrum with high temperature. EER is much larger within the earthward flow than the tailward flow. It is positively correlated with the outflow speed Vx, while the correlation between EER and Bz is less clear. With the increment of earthward outflow speed, the occurrence rate of high EER also monotonically increases. We find that EER generally does not increase with the increment of perpendicular electric field |E⊥|, suggesting that adiabatic betatron and Fermi acceleration probably play minor roles in electron energization during magnetotail reconnection.