Density Depletions Associated With Enhancements of Electron Cyclotron Harmonic Emissions: An ERG Observation

Abstract

In this study, we describe small-scale density depletions of cold electrons associated with electrostatic electron cyclotron harmonic (ECH) waves observed by the ERG (Exploration of energization and Radiation in Geospace) spacecraft during a plasmapause crossing near the magnetic equator in the postmidnight sector. The total electron density was roughly 2 orders of magnitude larger than that of the hot electrons, indicating existence of a cold and dense electron population below ∼20 eV. The cold electron density profile showed small-scale density depletion regions, in which the ECH emissions were intensified. Moreover, a flux enhancement of hot electrons in the perpendicular direction was also associated with the ECH emissions intensity. This relation suggests perpendicular energization of electrons by electric field oscillation of the ECH waves.Plain Language Summary This study clarifies a relation of cold-electron density irregularities and electron cyclotron harmonic (ECH) waves, both of which are frequently observed in the inner magnetosphere but their relation had not been clearly indicated by observations. We analyzed a plasmapause crossing event on 12 April 2017 observed by the ERG spacecraft and found that the plasmapause had small-scale density depletions of cold electrons and ECH emissions were intensified inside the depletion regions. This suggests that an ECH emission intensity can be controlled by a background cold-electron population. In addition, enhancements of hot-electron fluxes in the direction perpendicular to the local magnetic field were also observed in response to the ECH emissions. This is probably due to perpendicular energization of cold electrons by the oscillating electric field of the ECH waves. The close relation between cold electrons and ECH emission intensification, shown in this analysis, gives indication of leakage of ionospheric cold electrons and its control of equatorial ECH waves just outside the plasmapause.

The Modulation of Plasma and Waves by Background Electron Density Irregularities in the Inner Magnetosphere

“…However, clear and sharp decreases in the density profiles as a function of radial distance are detected during less than 20% of plasmapause crossings based on CRRES spacecraft observation (Moldwin et al, 2002). In contrast, density irregularities are frequently observed near the plasmapause (Carpenter et al, 1993, 2000; Darrouzet et al, 2004; Kazama et al, 2018; Yuan et al, 2019). These density structures are believed to play an important role in the generation and propagation of various plasma waves (e.g., Chen et al, 2009; de Soria‐Santacruz et al, 2013; Lemaire & Gringauz, 1998; Ma et al, 2014).…”

“…While previous studies have analyzed different types of waves affected by the density irregularities near the plasmapause (e.g., Kazama et al, 2018; Yuan et al, 2019), no comprehensive investigation has been performed to study the simultaneous modulation of particle fluxes, EMIC, MS, and electrostatic waves by the background plasma density structure as presented in this letter. Here, we demonstrate that the variation of suprathermal protons and helium ions with high specific anisotropies is well correlated with the background electron density variation, and the left‐hand‐polarized quasi‐parallel EMIC and quasi‐perpendicular MS waves are simultaneously observed in high plasma density regions but vanish in low‐density regions, when Van Allen Probe B crossed the plasmapause near the postdusk magnetic equator on 9 October, 2013.…”

Response of the Ionosphere‐Plasmasphere Coupling to the September 2017 Storm: What Erodes the Plasmasphere so Severely?

“…In L ≥ 3 of #3 profile, the density showed perturbations, in other words, increase and decrease of density. These perturbations are often observed by Arase (Kazama et al, 2018). On the other hand, in #4 profile, it had a smooth density structure except for an increase in the density in L = 3.4-4.3 (probably, it is the plume).…”

Pitch‐Angle Scattering of Inner Magnetospheric Electrons Caused by ECH Waves Obtained With the Arase Satellite

“…ECH waves are electrostatic emissions excited in frequency bands between a multiple of the electron cyclotron frequency f ce , and they are sometimes called (n + 1/2) f ce waves (Kazama et al, 2018; Kennel et al, 1970) while the wave frequency is not centered exactly at half‐harmonics of f ce (Zhou et al, 2017). LBC waves are electromagnetic and right‐handed polarized waves excited in the lower frequency band of 0.5 f ce .…”