Statistical characteristics of EMIC waves: Van Allen Probe observations

Abstract

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 significant in the dawn sector. This phenomenon may result from the distribution characteristics of the plasmasphere. The location of the plasmapause is usually lower in the dawn sector than that in other sectors, and the plasmapause is considered to be the favored region for the generation of EMIC waves. In higher 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.

Geomagnetic storms and EMIC waves: Van Allen Probe observations

“…On the other hand, this study concentrates on the distribution of EMIC waves in the inner magnetosphere in different geomagnetic condition indicated by SYM‐H index. By advantage of 64 Hz magnetic field data from EMFISIS instrument [ Kletzing et al , ] on board Van Allen Probe A, the upper frequency limit of EMIC waves in this study extends to 10 Hz [ Wang et al , ]. Peak occurrences are also found in noon to dusk sector during storm main phase but in the region with L between 5 and 6.5.…”

“…Furthermore, they identify the location of EMIC waves relative to plasmapause according to empirical models, while plasma density data are shown in this manuscript. Actually, in our previous research [ Wang et al , ], utilizing the plasma density data, majority of EMIC waves observed by Van Allen Probe A were identified inside the plasmasphere. On the other hand, most of oxygen band EMIC waves reported in Yu et al [] were identified in the plasmasphere.…”

“…Thus, that oxygen band EMIC wave event is not included in this paper. Additionally, different band EMIC waves on the same time interval are treated as one event, as in Wang et al []. The oxygen band EMIC wave event which was observed on 8 October 2012 took place at the same time with a helium band EMIC wave event [see Yu et al , , Figure 1].…”

“…All those trajectories on which these eight oxygen band EMIC waves were observed are depicted in green color in Figure . In addition, 11 hydrogen or helium band (or both bands at the same time) EMIC wave events which were observed beyond the L shell range (3 ~ 6) investigated in [ Wang et al , ] are involved in this study, which are shown in red color in Figure . Among these events, two of them were observed where L < 3, and the rest nine of them were observed where L > 6.…”

Ground-based instruments of the PWING project to investigate dynamics of the inner magnetosphere at subauroral latitudes as a part of the ERG-ground coordinated observation network

“…On the other hand, the electron-cyclotron (ELF/VLF) waves and EMIC waves are localized mainly in the dusk and dawn sectors, respectively (e.g., Li et al 2011;Wang et al 2015). Thus, observations of longitudinal distribution of these waves and drifting plasmas are essentially important to understand particle energization and loss in the inner magnetosphere quantitatively.…”