Non-thermal Electrons at the Earth's Bow Shock: A `Gradual' Event

TL;DR

This study analyzes non-thermal electron behavior and whistler wave activity at Earth's bow shock, revealing exponential growth of waves and electron flux prior to shock encounter, and suggesting whistlers influence gradual electron acceleration.

Contribution

It provides new insights into the role of upstream whistler waves in electron acceleration at quasi-perpendicular shocks below the whistler critical Mach number.

Findings

Whistler wave amplitudes grow exponentially before shock impact.

Suprathermal electron flux increases with different e-folding times.

Pitch angle scattering indicates upstream electron interactions.

Abstract

Earth's bow shock is known to produce non-thermal electrons which are generally observed as a `spike' in their flux profile. Here, in this paper, we present an analysis of electron and whistler wave properties for a quasi-perpendicular shock crossing that is supercritical, but subcritical to the so-called whistler critical Mach number, M$^w_{\rm crit}$, above which whistler waves cannot propagate upstream. We have found that the amplitudes of whistler waves increased exponentially as a function of time prior to the shock encounter, while the suprathermal ($>$ 2 keV) electron flux similarly increased with time, although with differing $e$-folding time scales. Comparison of the electron energy spectrum measured within the ramp with predictions from diffusive shock acceleration theory was poor, but the variation of pitch angle distribution showed scattering of non-thermal electrons in the upstream region. While not finding a specific mechanism to account for the electron diffusion, we suggest that the whistlers seen probably account for the differences observed between this `gradual' event and the `spike' events seen at shocks with no upstream whistlers.