To understand the relationship between generation of electromagnetic ion cyclotron (EMIC) waves and energetic particle injections, we performed a statistical study of EMIC waves associated with and without injections based on the Van Allen Probes (Radiation Belt Storm Probes) and Geostationary Operational Environmental Satellite (GOES; GOES‐13 and GOES‐15) observations. Using 47 months of observations, we identified wave events seen by the Van Allen Probes relative to the plasmapause and to energetic particle injections seen by GOES‐13 and GOES‐15 on the nightside. We separated the events into four categories: EMIC waves with (without) injections inside (outside) the plasmasphere. We found that He+ EMIC waves have higher occurrence rate inside the plasmasphere, while H+ EMIC waves predominantly occur outside the plasmasphere. Meanwhile, the time duration and peak occurrence rate of EMIC waves associated with injections are shorter and limited to a narrower magnetic local time region than those without injections, indicating that these waves have localized source regions. He+ EMIC waves inside the plasmasphere associated with injection are usually accompanied by an increase in H+ flux within energies of 1–50 keV through all magnetic local time regions, while most wave events outside the plasmasphere show less relationship with H+ flux increase. From these observations, we suggest that injected hot ions are the major driver of He+ EMIC waves inside the plasmasphere during active time. Expanding plasmasphere during quiet times can provide broad wave source regions for He+ EMIC waves on the dayside. However, H+ EMIC waves outside the plasmasphere show different characteristics, suggesting that these waves are generated by other processes.
To understand the generation and propagation processes of electromagnetic ion cyclotron (EMIC) waves under different geomagnetic conditions in the inner magnetosphere, we performed a statistical study of EMIC wave properties observed by the Van Allen Probes from February 2013 to December 2016. We divided EMIC waves into two groups: those associated with and those occurring without injections observed by the Geostationary Operational Environmental Satellites (GOES-13 and GOES-15). We found that the EMIC wave polarization sense increased and the normalized frequency X decreased with increasing |MLAT|. Inside the plasmasphere, He + EMIC waves were predominantly observed with left-hand polarization ( < −0.3) and higher wave normal angles ( k = 30-40 • ). Those associated with injections showed the most intense wave power at 14-16 MLT, compared to periods without injections when these waves exhibit a similar wave power but on the dayside. H + EMIC waves were predominantly observed outside the plasmasphere on the dayside and showed a mixture of left-hand and linear polarizations ( = −0.3-0.0) with lower wave normal angles ( k = 20-30 • ) regardless of injections. Moreover, H + EMIC waves were accompanied by a solar wind dynamic pressure enhancement ( Psw = 0.5 nPa). From these observations, we suggest that hot injected plasma contributes to the generation of intense He + EMIC waves in the afternoon sector. A mixture of expanding cold plasmaspheric ions and coexisting hot ring current ions acts as the free energy source for He + EMIC waves on the dayside during quiet times. Solar wind dynamic pressure enhancements are likely the major driver of H + EMIC waves outside the plasmasphere.
Electromagnetic ion cyclotron (EMIC) waves constitute a significant loss process of energetic protons and sub-relativistic electrons through pitch-angle scattering by wave-particle interactions (e.g., Anderson et al., 1992aAnderson et al., , 1992bThorne, 2010). These interactions contribute to the generation of the isolated proton auroras at subauroral latitudes (e.g.,
Plasma kinetic theory predicts that sufficiently anisotropic proton distribution will excite electromagnetic ion cyclotron (EMIC) waves, which in turn relax the proton distribution to a marginally stable state creating an upper bound on the relaxed proton anisotropy. Here, using EMIC wave observations and coincident plasma measurements made by Van Allen Probes in the inner magnetosphere, we show that the proton distributions are well constrained by this instability to a marginally stable state. Near the threshold, the probability of EMIC wave occurrence is highest, having left‐handed polarization and observed near the magnetic equator with relatively small wave normal angles, indicating that these waves are locally generated. In addition, EMIC waves are distributed in two magnetic local time regions with different intensity. Compared with helium band waves, hydrogen band waves behave similarly except that they are often observed in low‐density regions. These results reveal several important features regarding EMIC waves excitation and propagation.
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