The Correlation Between Whistler Mode Waves and Electron Beam‐Like Distribution: Test Particle Simulations and THEMIS Observations

Abstract

The beam‐like electron population is often observed along with whistler mode waves in the Earth's magnetosphere, but its generation mechanism is still under debate. In this study, we use a test particle simulation model to study the Landau resonance between whistler mode waves and electrons, and quantify the proportion nb/nh of beam‐like electrons. There is a trend that the nb/nh first increases and then decreases with the increasing temperature of hot electrons or ratio of plasma frequency to electron gyrofrequency. While the nb/nh is positively correlated with the parallel electric field, magnetic amplitude, wave normal angle, or frequency of whistler mode waves if other parameters are fixed. Essentially the parallel electric field is the key parameter in determining nb/nh. Furthermore, we have also compared simulation results with THEMIS observations for one year. The results suggest the observed beam‐like population could be generated by simultaneously observed whistler mode waves for 44.5% quasi‐parallel cases. But the beam‐like population is unlikely to be generated by simultaneously observed waves for the remaining 55.5% of quasi‐parallel cases and nearly all oblique cases. Our study provides a better understanding of the formation of beam‐like electron distribution in the Earth's magnetosphere.

On the Nature of Intense Sub‐Relativistic Electron Precipitation

“…Accordingly, such very oblique waves are mainly associated with electron distributions having a plateau or weak field-aligned beams around the Landau resonant energy at the equator (Li, Mourenas, et al, 2016;R. Chen et al, 2019;Kong et al, 2021). Figure 4c shows that THEMIS E measured such a plateau both in the field-aligned and transverse phase space densities during the entire interval of whistler-mode wave observations (note also a weak field-aligned anisotropy of ∼1-5 keV electrons in Figure 4b).…”

“…For typical electron distributions with a smoothly decreasing density toward higher parallel electron energy, Landau damping by these dense suprathermal electrons would quickly reduce the wave intensity to the noise level (Kennel, 1966; Bortnik et al., 2007; L. Chen et al., 2013). Accordingly, such very oblique waves are mainly associated with electron distributions having a plateau or weak field‐aligned beams around the Landau resonant energy at the equator (Li, Mourenas, et al., 2016; R. Chen et al., 2019; Artemyev & Mourenas, 2020; Kong et al., 2021). Figure 4c shows that THEMIS E measured such a plateau both in the field‐aligned and transverse phase space densities during the entire interval of whistler‐mode wave observations (note also a weak field‐aligned anisotropy of ∼1–5 keV electrons in Figure 4b).…”

Unraveling the Role of Electron Plateau Distributions in the Power Gap Formation of Chorus Waves: Van Allen Probes Observations

“…The cold electron component is not considered, because its thermal velocity is much lower than 0.5 V Ae . Since electron plateau components along with chorus waves are typically observed at ∼0.5 V Ae (Chen et al., 2019; Kong et al., 2021; Li et al., 2019; Min et al., 2014), then the v b is determined by the positive gradient of PSD in the range of 0.4–0.6 V Ae (Min et al., 2014). In Event I, v b = 0.431 V Ae in both directions.…”

“…Their number densities are estimated as n p /n 0 ∼ 10 −3 , consistent with those in banded chorus events from observations (Figures 2d and 2f). In the magnetosphere, the plateau components are commonly observed along with chorus waves (Chen et al, 2019;Kong et al, 2021;Li et al, 2019;Li, Mourenas, et al, 2016;Min et al, 2014). With THEMIS data, Min et al (2014) have suggested that the bulk velocities of plateau components generally match the Landau resonant velocities of chorus waves, indicating these components are formed due to Landau resonance.…”

“…Their number densities are estimated as n p / n 0 ∼ 10 −3 , consistent with those in banded chorus events from observations (Figures 2d and 2f). In the magnetosphere, the plateau components are commonly observed along with chorus waves (Chen et al., 2019; Kong et al., 2021; Li et al., 2019; Li, Mourenas, et al., 2016; Min et al., 2014). With THEMIS data, Min et al.…”