Statistical Study of Approaching Strong Diffusion of Low‐Energy Electrons by Chorus and ECH Waves Based on In Situ Observations

Abstract

Inner magnetospheric electrons are precipitated in the ionosphere via pitch‐angle (PA) scattering by lower band chorus (LBC), upper band chorus (UBC), and electrostatic electron cyclotron harmonic (ECH) waves. However, the PA scattering efficiency of low‐energy electrons (0.1–10 keV) has not been investigated via in situ observations because of difficulties in flux measurements within loss cones at the magnetosphere. In this study, we demonstrate that LBC, UBC, and ECH waves contribute to PA scattering of electrons at different energy ranges using the Arase (ERG) satellite observation data and successively detected the moderate loss cone filling, that is, approaching strong diffusion. Approaching strong diffusion by LBC, UBC, and ECH waves occurred at ∼2–20 keV, ∼1–10 keV, and ∼0.1–2 keV, respectively. The occurrence rate of the approaching strong diffusion by high‐amplitude LBC (>50 pT), UBC (>20 pT), and ECH (>10 mV/m) waves, respectively, reached ∼70%, ∼40%, and ∼30% higher than that without simultaneous wave activity. The energy range in which the occurrence rate was high agreed with the range where the PA diffusion rate of each wave exceeded the strong diffusion level based on the quasilinear theory.

Variation of Electron Lifetime Due To Scattering by Electrostatic Electron Cyclotron Harmonic Waves in the Inner Magnetosphere With Electron Distribution Parameters

“…This is consistent with the results from Fukizawa et al. (2022), who showed that the pitch angle diffusion coefficient for ECH waves can exceed the strong diffusion level up to electron energies of around 2 keV if E w = 10 mV/m. However, Ni et al.…”

A missing dusk-side loss process in the terrestrial electron ring current

“…Other types of waves may be also responsible for the missing loss process. ECH waves are currently not included in VERB-4D, although they are capable of very efficiently scattering electrons of a few eV up to 38 – 40 . Statistical studies of satellite measurements show that ECH waves occur mostly at radial distances of 5 and above on the nightside, including the pre-midnight sector 39 , 41 , 42 , where the missing loss process operates.…”

Global Occurrence of Higher‐Bands ECH Waves in Radiation Belts Based on a Novel Noise Reduction Algorithm (NORA)

“…They have focused on the average wave amplitude in three sectors (12–18, 18–24, and 00–06, respectively), but without the prenoon sector due to the orbit limitation of the CRRES mission. Numerous previous studies on the global distribution of ECH waves (e.g., Fukizawa et al., 2022; Liu et al., 2020; Ni, Thorne, Liang, et al., 2011; Ni et al., 2017; Zhang et al., 2015) have focused on the first harmonic band without differentiation of different bands. Considering that ECH waves occur mainly at low geomagnetic latitudes (Meredith et al., 2009; Ni, Thorne, Liang, et al., 2011), we shall investigate their global distribution based on 7 years of observations from Van Allen Probes orbiting near the magnetic equator.…”

“…Fukizawa et al. (2022) has investigated the occurrence rate of ECH waves in the first band as a function of the Roederer's L ‐parameter and Mlat, and has revealed energy ranges where ECH waves contribute to scattering electrons in loss cones. Ni, Thorne, Horne, et al.…”