Xiaohua Deng

Link between EMIC waves in a plasmaspheric plume and a detached sub‐auroral proton arc with observations of Cluster and IMAGE satellites

[1]xa0In this paper, we report observations from a Cluster satellite showing that ULF wave occurred in the outer boundary of a plasmaspheric plume on September 4, 2005. The band of observed ULF waves is between the He+ ion gyrofrequency and O+ ion gyrofrequency at the equatorial plane, implying that those ULF waves can be identified as EMIC waves generated by ring current ions in the equatorial plane and strongly affected by rich cold He+ ions in plasmaspheric plumes. During the interval of observed EMIC waves, the footprint of Cluster SC3 lies in a subauroral proton arc observed by the IMAGE FUV instrument, demonstrating that the subauroral proton arc was caused by energetic ring current protons scattered into the loss cone under the Ring Current (RC)-EMIC interaction in the plasmaspheric plume. Therefore, the paper provides a direct proof that EMIC waves can be generated in the plasmaspheric plume and scatter RC ions to cause subauroral proton arcs.

Wave‐particle interaction in a plasmaspheric plume observed by a Cluster satellite

[1]xa0The wave-particle interaction is a possible candidate for the energy coupling between the ring current and plasmaspheric plumes. In this paper, we present wave and particle observations made by the Cluster C1 satellite in a plasmaspheric plume in the recovery phase of the geomagnetic storm on 18 July 2005. Cluster C1 simultaneously observed Pc1-2 waves and extremely low frequency (ELF) hiss in the plasmaspheric plume. Through an analysis of power spectral density and polarization of the perturbed magnetic field, we identify that the observed Pc1-2 waves are linearly polarized electromagnetic ion cyclotron (EMIC) waves and show that the ELF hiss propagates in the direction of the ambient magnetic field in whistler mode. In the region where the EMIC waves were observed, the pitch angle distribution of ions becomes more isotropic, likely because of the pitch angle scattering by the EMIC waves. It is shown that the ELF hiss and EMIC waves are spatially separated: The ELF hiss is located in the vicinity of the electron density peak within the plume while the EMIC waves are detected in the outer boundary of the plume because of the different propagation characteristics of the ELF hiss and EMIC waves.

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 Lu2009=u20093 to Lu2009=u20096. 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 (Lu2009 u20094) 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.

Characteristics of precipitating energetic ions/electrons associated with the wave-particle interaction in the plasmaspheric plume

In this paper, we present characteristics of precipitating energetic ions/electrons associated with the wave-particle interaction in the plasmaspheric plume during the geomagnetic storm on July 18, 2005 with observations of the NOAA15 NOAA16, IMAGE satellites and Finnish network of search coil magnetometers. Conjugate observations of the NOAA15 satellite and the Finnish network of search coil magnetometers have demonstrated that a sharp enhancement of the precipitating ion flux is a result of ring current (RC) ions scattered into the loss cone by EMIC waves. Those precipitating RC ions lead to a detached subauroral proton arc observed by the IMAGE FUV. In addition, with observations of NOAA15 and NOAA16, the peak of precipitating electron flux was equatorward to that of precipitating proton flux, which is in agreement with the region separation of ELF hiss and EMIC waves observed by the Cluster C1 in the Yuan et al. (2012) companion paper. In combination with the result of the companion paper, we demonstrate the link between the wave activities (ELF hiss, EMIC waves) in plasmaspheric plumes and energetic ion/electron precipitation at ionospheric altitudes. Therefore, it is an important characteristic of the plasmaspheric plumes-RC-ionosphere interaction during a geomagnetic storm that the precipitation of energetic protons is latitudinally separated from that of energetic electrons.

DMSP/GPS observations of intense ion upflow in the midnight polar ionosphere associated with the SED plume during a super geomagnetic storm

[1]xa0We report observations from the GPS TEC and DMSP F-15 satellite showing that a very strong upward field-aligned (FA) plasma velocity and flux at F-region heights in the auroral zone/polar cap boundary during a passage of the polar tongue of ionization (TOI) full of storm-enhanced density (SED) materials occurred in the event of the super geomagnetic storm on Nov. 20, 2003. The upward FA ion velocities in excess of 460 m/s are obtained from observations of the DMSP F-15 satellite. With enhancements of the plasma density caused by the TOI, FA ion fluxes are estimated to about 1.2 × 1014 ions m−2s−1, which are comparable to those observed by the ground-based radar in the polar cusp. Therefore, during the super geomagnetic storm a s SED can cause a TOI plume at F region altitudes, which leads to a strong upward ion flux reaching 1–2 orders of magnitude larger than that of the typical flux at midnight auroral zone/polar cap boundary. Through estimations of the influence of those upflow ions associated with SED plumes on the development of the super storm, our result suggests that the midnight auroral zone/polar cap boundary becomes an important source to directly provide rich O+ to the ring current during super geomagnetic storms.

Energetic particle precipitation and the influence on the sub‐ionosphere in the SED plume during a super geomagnetic storm

[1]xa0In this paper, we present precipitation characteristic of energetic particles and the influence on the sub-ionosphere in the storm enhanced density (SED) plume in the event of the geomagnetic storm during March 31, 2001. With observations of the NOAA16 satellite, the peak of precipitating electron flux was located near the TEC peak in region of a SED plume recognized on two-dimensional GPS TEC maps. On the other hand, the peak of precipitating energetic proton flux was observed in the outer boundary of the SED plume. Those precipitations were associated with energetic ions/electrons injected into ring currents (RC) caused by a sequence of substorms. When the Digisonde at Millstone Hill was located in the SED plume, the minimum frequency of ionogram echoes (fmin) was observed to keep a high level. Considering the SED plume as a signature of a plasmaspheric drainage plume, the precipitation of energetic electrons is attributed to the RC-ELF hiss interaction in the plasmaspheric plume. Calculations of a simple sub-ionospheric model demonstrate that those energetic electrons can precipitate into the sub-ionosphere and cause sub-ionospheric ionization enhancements in the SED plume. As a result, the fmin observed by the Digisonde in the SED plume was kept at a high level. Therefore, the paper provides direct evidences that precipitating energetic RC electrons play a significant role in the coupling between magnetosphere and sub-ionosphere in SED plumes.

The turbulence evolution in the high β region of the Earth's foreshock

[1]xa0In this paper, we study the foreshock turbulence evolution in high β region via both Cluster observation on 29 March 2002 and 1-D hybrid simulation. The quasi-sinusoidal and the irregular wave trains are both detected in this event. The former one is believed to be generated by ion-ion right-hand nonresonant instability due to the right-hand polarization and antisunward propagation in the plasma frame. Since the ion distribution associated with the wave train is more “intermediate” rather than “diffused”, we suggest that the wave train reported in this paper can be viewed as a “midstep” of “isolated” and “irregular”. From the quasi-sinusoidal to the irregular waveform, the corresponding polarizations appear to transit: right-hand wave of higher frequency (wave number) is substituted by a left-hand wave with lower frequency (wave number) in spacecraft frame. Then the 1-D hybrid simulation is applied for two cases with various velocities to study such polarization transition. By comparing observation results with the simulation, such polarization transition and “inversed cascade” (wave energy transferring from large wave number to small wave number) can be understood as the consequence of decay instability. Although decay instability cannot be initiated in high beta (βu2009>u20091) plasma in magnetohydrodynamic theory, such β dependence can be modified by ion kinetic effect. Moreover, it is found that in simulation no matter which right-hand instability is dominant in early stage, left-hand wave will be the prime component of magnetic field disturbance in the final stage.