The Relation Between Electron Cyclotron Harmonic Waves and Plasmapause: Case and Statistical Studies

Observationally, electron cyclotron harmonic (ECH) waves are often terminated at the outer boundary of the plasmasphere boundary layer (PBL, i.e., plasmapause). Physics of this empirical relation is not well established. In this study, two categories of ECH waves are shown by their different behaviors near PBL. For Category I, all bands of ECH waves terminate at PBL because the density ratio (nh/nc) between hot and cold electrons decreases dramatically across PBL. For Category II, ECH waves, especially the lower harmonic bands, can be excited deeper inside the plasmasphere because nh/nc gradually decreases across PBL. A statistical study using Van Allen Probes observation is performed for these two categories. We find that the two categories of ECH waves are preferred to occur at nightside and dawnside. The two categories of ECH waves may be separated by the wave intensity outside the PBL or nh/nc with the threshold on the order of 10−10–10−9 (V/m)2 and 10−2, respectively. Plain Language Summary Electron cyclotron harmonic (ECH) waves, an electrostatic emission with wave frequency lying between electron cyclotron harmonic frequencies, are considered to be responsible for the formation of nightside diffuse and pulsating auroras in the outer magnetosphere. Observationally, ECH waves are likely to terminate at the plasmapause (Category I ECH wave). However, (1) physics of this empirical relation is not well established. (2) There exists another category of ECH waves (Category II ECH wave), which are extended inside the plasmapause. The excitation mechanism of these two‐category ECH waves are analyzed by a linear growth rate solver of ECH waves. Then, we perform a statistical study for the characteristics of these two category ECH waves by Van Allen Probe B from 2013 to 2016. Our study successfully explains the excitation mechanisms and different behaviors of the two categories of ECH waves. Our results reveal how the ECH waves are excited and attenuated in the vicinity of the plasmapause, and significantly advance our understanding of the excitation characteristics of ECH waves in the inner magnetosphere. Key Points Often terminated outside the plasmapause, ECH waves are occasionally extended inside the plasmapause The instability analysis explains why ECH waves often terminate near the plasmapause Statistically, Category II ECH waves are only confined at nightside and dawnside, with much stronger intensity