Xiongdong Yu

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.

Electromagnetic energy conversion at dipolarization fronts: Multispacecraft results

Dipolarization fronts (DFs) are believed to play important roles in transferring plasmas, magnetic fluxes, and energies in the magnetotail. Using the Cluster observations in 2003, electromagnetic energy conversion at the DFs is investigated by case and statistical studies. The case study indicates strongest energy conversion at the DF. The statistical study shows the similar features that the energy of the fields can be significantly transferred to the plasmas (load, J · E > 0) at the DFs. These results are consistent with some recent simulations. Examining the electromagnetic fluctuations at the DFs, we suggest that the wave activities around the lower hybrid frequency may play an important role in the energy dissipation.

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.

In situ observations of magnetosonic waves modulated by background plasma density

We report in situ observations by the Van Allen Probe mission that magnetosonic (MS) waves are clearly relevant to the background plasma number density. As the satellite moved across dense and tenuous plasma alternatively, MS waves occurred only in lower density region. As the observed protons with “ring” distributions provide free energy, local linear growth rates are calculated and show that magnetosonic waves can be locally excited in tenuous plasma. With variations of the background plasma density, the temporal variations of local wave growth rates calculated with the observed proton ring distributions show a remarkable agreement with those of the observed wave amplitude. Therefore, the paper provides a direct proof that background plasma densities can modulate the amplitudes of magnetosonic waves through controlling the wave growth rates.

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.

The enhancement of cosmic radio noise absorption due to hiss-driven energetic electron precipitation during substorms

The Van Allen probes, low-altitude NOAA satellite, MetOp satellite, and riometer are used to analyze variations of precipitating energetic electron fluxes and cosmic radio noise absorption (CNA) driven by plasmaspheric hiss with respect to geomagnetic activities. The hiss-driven energetic electron precipitations (at L ~ 4.7–5.3, magnetic local time (MLT) ~ 8–9) are observed during geomagnetic quiet condition and substorms, respectively. We find that the CNA detected by riometers increased very little in the hiss-driven event during quiet condition on 6 September 2012. The hiss-driven enhancement of riometer was still little during the first substorm on 30 September 2012. However, the absorption detected by the riometer largely increased, while the energies of the injected electrons became higher during the second substorm on 30 September 2012. The enhancement of CNA (ΔCNA) observed by the riometer and calculated with precipitating energetic electrons is in agreement during the second substorm, implying that the precipitating energetic electrons increase CNA to an obviously detectable level of the riometer during the second substorm on 30 September 2012. The conclusion is consistent with Rodger et al. (2012), which suggest that the higher level of ΔCNA prefers to occur in the substorms, because substorms may produce more intense energetic electron precipitation associated with electron injection. Furthermore, the combination of the observations and theory calculations also suggests that higher-energy electron (>55 keV) precipitation contributes more to the ΔCNA than the lower energy electron precipitation. In this paper, the higher-energy electron precipitation is related to lower frequency hiss.

Geomagnetic storms and EMIC waves: Van Allen Probe observations

Utilizing the data from magnetometer instrument of EMFISIS suite on board Van Allen Probe A, the occurrences of Electromagnetic Ion Cyclotron (EMIC) waves during geomagnetic storms and non storm periods are investigated. 270 EMIC wave events and 76 geomagnetic storms were identified during the period under research, from 8 September 2012 to 30 April 2014, when the apogee of Van Allen Probe A covered all the MLT sectors. 50 of the 76 storms observed 124 EMIC wave events, of which 80 are found in the recovery phase, more than those observed in the main phase. Majority EMIC wave events (~54%) were observed during the non-storm periods. Occurrence rates of EMIC waves as a function of L and MLT during different geomagnetic conditions are also examined, whose peaks in main phase are higher than those in recovery phase. However, occurrences of EMIC waves in recovery phase distribute more uniformly than those do in main phase. Evolution of the distribution characteristics of EMIC waves respect to L and MLT in different geomagnetic phases is investigated, consistent with that of the plasmasphere during geomagnetic storms, implying that the cold and dense plasma in the plasmasphere or plasmaspheric plume play a significant role in the generation of EMIC waves in the inner magnetosphere. Few EMIC waves in the dayside sector during the pre-onset periods are observed, suggesting that the effect of solar wind dynamic pressure on the generation of EMIC waves in the inner magnetosphere in those periods is not so significant as expected.

A statistical study of kinetic‐size magnetic holes in turbulent magnetosheath: MMS observations

Kinetic-size magnetic holes (KSMHs) in the turbulent magnetosheath are statistically investigated using high time resolution data from the MMS mission. The KSMHs with short duration (i.e., < 0.5 s) have their cross section smaller than the ion gyro-radius. Superposed epoch analysis of all events reveals that an increase in the electron density and total temperature, significantly increase (resp. decrease) the electron perpendicular (resp. parallel) temperature, and an electron vortex inside KSMHs. Electron fluxes at ~ 90° pitch angles with selective energies increase in the KSMHs, are trapped inside KSMHs and form the electron vortex due to their collective motion. All these features are consistent with the electron vortex magnetic holes obtained in 2D and 3D particle-in-cell simulations, indicating that the observed KSMHs seem to be best explained as electron vortex magnetic holes. It is furthermore shown that KSMHs are likely to heat and accelerate the electrons.

EMIC waves covering wide L shells: MMS and Van Allen Probes observations

During 04:45:00 – 08:15:00 UT on September 13 in 2015, a case of Electromagnetic ion cyclotron (EMIC) waves covering wide L shells (L =3.6 ~ 9.4), observed by the Magnotospheric Multiscale 1 (MMS1) are reported. During the same time interval, EMIC waves observed by Van Allen Probes–A (VAP-A) only occurred just outside the plasmapause. As the Van Allen Probes move outside into a more tenuous plasma region, no intense waves were observed. Combined observations of MMS1 and VAP-A suggest that in the terrestrial magnetosphere, an appropriately dense background plasma would make contributions to the growth of EMIC waves in lower L shells while the ion anisotropy, driven by magnetospheric compression, might play an important role in the excitation of EMIC waves in higher L shells. These EMIC waves are observed over wide L shells after three continuous magnetic storms, which suggests that these waves might obtain their free energy from those energetic ions injected during storm times. These EMIC waves should be included in radiation belt modeling, especially during continuous magnetic storms. Moreover, two-band structures separated in frequencies by local He2+ gyro-frequencies were observed in large L shells (L >~6), implying sufficiently rich solar wind origin He2+ likely in the outer ring current. It is suggested that multi-band structured EMIC waves can be used to trace the coupling between solar wind and the magnetosphere.