Shiyong Huang

Dipolarization fronts as a consequence of transient reconnection : In situ evidence

Dipolarization fronts (DFs) are frequently detected in the Earths magnetotail from X-GSM=-30 R-E to X-GSM=-7 R-E. How these DFs are formed is still poorly understood. Three possible mechanisms have been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near-Earth region (X-GSM>-10 R-E) are primarily attributed to jet braking, while the DFs detected in the mid- or far-tail region (X-GSM<-15 R-E) are primarily attributed to transient reconnection or spontaneous formation. In the jet-braking mechanism, the high-speed flow pushes the preexisting plasmas to produce the DF so that there is causality between high-speed flow and DF. In the transient-reconnection mechanism, there is no causality between high-speed flow and DF, because the frozen-in condition is violated.

First observation of rising‐tone magnetosonic waves

Magnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90° 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, ~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.

Statistical characteristics of EMIC wave‐driven relativistic electron precipitation with observations of POES satellites: Revisit

Electromagnetic ion cyclotron (EMIC) waves play an important role in the magnetospheric dynamics and can scatter relativistic electrons in the outer radiation belt into the loss cone leading to the rapid loss of relativistic electrons. In this paper, we present characteristics of EMIC wave-driven relativistic electron precipitation (REP) with observations of six Polar Orbiting Environmental Satellites (POES). Based on the simultaneity between spikes in the P1 0° (Ep = 30 keV–80 keV) and P6 0° (Ee > 1 MeV) channels, in comparison with the criterion of Carson et al. (2013), we improve the algorithm and make it stricter. A total of 436,286 individual half orbits between 1998 and 2010 are inspected by this algorithm. The majority of selected events are observed at L values within the outer radiation belt (3 < L < 7) and more common in 1800–2200 magnetic local time. The distribution of normalized events follows the location of plasmapause contracting toward lower L value with the decrease of the Dst index, implying a strong link between detected events and the plasmapause. The cluster of normalized events moves to later afternoon sector where the peak occurrence of plasmaspheric plumes is located during geomagnetic storms. It is suggested that there is a connection between plasmaspheric plumes and detected events. Corresponding to the peak of event occurrence in 2003, solar wind dynamic pressure has a same peak. In addition, the minimum values of them are coincident. These results indicate that the increase of the solar wind dynamic pressure enhances the likelihood of EMIC wave-driven relativistic electron precipitation.

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.

In situ observations of EMIC waves in O+ band by the Van Allen Probe A

Through polarization and spectra analysis of the magnetic field observed by the Van Allen Probe A, we present two typical cases of O+ band electromagnetic ion cyclotron (EMIC) waves in the outer plasmasphere or plasma trough. Although such O+ band EMIC waves are rarely observed, 18 different events of O+ band EMIC waves (16 events in the outer plasmasphere and two events in the plasma trough) are found from September 2012 to August 2014 with observations of the Van Allen Probe A. We find that the preferred region for the occurrence of O+ band EMIC waves is in L = 2–5 and magnetic local time  = 03–13, 19–20, which is in accordance with the occurrence region of O+ ion torus. Therefore, our result suggests that the O+ ion torus in the outer plasmasphere during geomagnetic activities should play an important role in the generation of EMIC waves in O+ band.

Statistical characteristics of EMIC waves: Van Allen Probe observations

Utilizing the data from the magnetometer instrument which is a part of the Electric and Magnetic Field Instrument Suite and Integrated Science instrument suite on board the Van Allen Probe A from September 2012 to April 2014, when the apogee of the satellite has passed all the magnetic local time (MLT) sectors, we obtain the statistical distribution characteristics of electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere over all magnetic local times from L = 3 to L = 6. Compared with the previous statistical results about EMIC waves, the occurrence rates of EMIC waves distribute relatively uniform in the MLT sectors in lower L shells. On the other hand, in higher L shells, there are indeed some peaks of the occurrence rate for the EMIC waves, especially in the noon, dusk, and night sectors. EMIC waves appear at lower L shells in the dawn sector than in other sectors. In the lower L shells (L   4) the occurrence rates of EMIC waves are most significant in the dusk sector, implying the important role of the plasmapause or plasmaspheric plume in generating EMIC waves. We have also investigated the distribution characteristics of the hydrogen band and the helium band EMIC waves. Surprisingly, in the inner magnetosphere, the hydrogen band EMIC waves occur more frequently than the helium band EMIC waves. Both of them have peaks of occurrence rate in noon, dusk, and night sectors, and the hydrogen band EMIC waves have more obvious peaks than the helium band EMIC waves in the night sector, while the helium band EMIC waves are more concentrated than the hydrogen band EMIC waves in the dusk sector. Both of them occur significantly in the noon sector, which implies the important role of the solar wind dynamic pressure.

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.

In situ evidence of the modification of the parallel propagation of EMIC waves by heated He+ ions

With observations of the Van Allen Probe B, we report in situ evidence of the modification of the parallel propagating electromagnetic ion cyclotron (EMIC) waves by heated He+ ions. In the outer boundary of the plasmasphere, accompanied with the He+ ion heating, the frequency bands of H+ and He+ for EMIC waves merged into each other, leading to the disappearance of a usual stop band between the gyrofrequency of He+ ions (ΩHe+) and the H+ cut-off frequency (ωH+co) in the cold plasma. Moreover, the dispersion relation for EMIC waves theoretically calculated with the observed plasma parameters also demonstrates that EMIC waves can indeed parallel propagate across ΩHe+. Therefore, the paper provides an in situ evidence of the modification of the parallel propagation of EMIC waves by heated He+ ions.