Evidence of Microbursts Observed Near the Equatorial Plane in the Outer Van Allen Radiation Belt

Abstract

We present the first evidence of electron microbursts observed near the equatorial plane in Earth's outer radiation belt. We observed the microbursts on 31 March 2017 with the Magnetic Electron Ion Spectrometer and Radiation Belt Storm Probes Ion Composition Experiment on the Van Allen Probes. Microburst electrons with kinetic energies of 29–92 keV were scattered over a substantial range of pitch angles, and over time intervals of 150–500 ms. Furthermore, the microbursts arrived without dispersion in energy, indicating that they were recently scattered near the spacecraft. We have applied the relativistic theory of wave‐particle resonant diffusion to the calculated phase space density, revealing that the observed transport of microburst electrons is not consistent with the hypothesized quasi‐linear approximation.

Electron Diffusion and Advection During Nonlinear Interactions With Whistler‐Mode Waves

“…Here, we show that significant diffusion and advection occur during short-timescale (≈100 ms) dynamics due to interactions with high-amplitude hisslike whistler-mode waves (≈100 nT at saturation), which are generated by strong (thermal-anisotropy driven) instabilities. Of particular note for comparison is the work by Shumko et al (2018), in which Van Allen Probe data of near-parallel-propagating and near-equatorial hiss-like whistler-mode waves at L ⋆ ≈ 6 (with background electron number density ≈ 10 7 m −3 ) indicated electron microbursts with a duration 150−500 ms. These parameters and characteristics are very similar to those presented in this study.…”

“…A background magnetic field inhomogeneity is considered to be fundamentally important to promote rising‐tone and falling‐tone chorus waves to high amplitudes (Omura et al., 2008) (however note recent experiments that demonstrate rising‐tone and falling‐tone chorus in a uniform magnetic field; Wu et al., 2020). However, less structured whistler‐mode waves have been observed by a number of studies to be prevalent in the equatorial/near‐equatorial regions outside of the plasmasphere (Gao et al., 2014; W. Li et al., 2012; Shumko et al., 2018; Tsurutani & Smith, 1974; Tsurutani et al., 2013). These wave modes have been termed “hiss‐like” or even “hiss‐like chorus” (hereafter we use the term “hiss‐like”).…”

Earth's Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era

“…These nonlinear effects include phase trapping that leads to a rapid, dramatic increase in pitch angle and energy, and phase bunching effects, accounting for decreased pitch angle and energy (e.g., Albert, 2002; Bortnik et al, 2008; Inan et al, 1978). Nonlinear interactions between the rising tone chorus and energetic electrons are suggested to lead to the electron microbursts observed by various LEO satellites (e.g., Breneman et al, 2017; Hikishima et al, 2010; Lorentzen et al, 2001; O'Brien et al, 2004; Shumko et al, 2018).…”

Modeling of Bouncing Electron Microbursts Induced by Ducted Chorus Waves

“…Previous simulation with self-consistent chorus waves (Hikishima et al, 2010) has demonstrated a one-to-one correspondence between microbursts of precipitating electrons and chorus elements, which attributes the formation of an individual microburst to an interaction with one chorus element. The first observations of microburst counterpart near the equatorial plane within the outer radiation belt itself were reported (Shumko, Turner, et al, 2018). Direct testing of the process(es) of microburst creation is exceedingly difficult due to the large separation over which the chorus waves and resultant microbursts are observed.…”