Non-locality of Earth's quasi-parallel bow shock: injection of thermal protons in a hybrid-Vlasov simulation

Markus Battarbee,Tuomas Koskela,Urs Ganse,Lucile Turc,Minna Palmroth,Maxime Grandin,Thiago Brito,Yann Pfau-Kempf

doi:10.5194/angeo-38-625-2020

Abstract

Abstract. We study the interaction of solar wind protons with Earth's quasi-parallel bow shock using a hybrid-Vlasov simulation. We employ the global hybrid model Vlasiator to include effects due to bow shock curvature, tenuous upstream populations, and foreshock waves. We investigate the uncertainty of the position of the quasi-parallel bow shock as a function of several plasma properties and find that regions of non-locality or uncertainty of the shock position form and propagate away from the shock nose. Our results support the notion of upstream structures causing the patchwork reconstruction of the quasi-parallel shock front in a non-uniform manner. We propose a novel method for spacecraft data to be used to analyse this quasi-parallel reformation. We combine our hybrid-Vlasov results with test-particle studies and show that proton energization, which is required for injection, takes place throughout a larger shock transition zone. The energization of particles is found regardless of the instantaneous non-locality of the shock front, in agreement with it taking place over a larger region. Distortion of magnetic fields in front of and at the shock is shown to have a significant effect on proton injection. We additionally show that the density of suprathermal reflected particles upstream of the shock may not be a useful metric for the probability of injection at the shock, as foreshock dynamics and particle trapping appear to have a significant effect on energetic-particle accumulation at a given position in space. Our results have implications for statistical and spacecraft studies of the shock injection problem.

Highlights

Collisionless plasma shocks are a ubiquitous source of plasma acceleration, common within stellar, planetary, and interplanetary environments
We show that the density of suprathermal reflected particles upstream of the shock may not be a useful metric for the probability of injection at the shock, as foreshock dynamics and particle trapping appear to have a significant effect on energetic-particle accumulation at a given position in space
In regions of shock geometry where the angle θBn between the shocknormal direction nand the upstream interplanetary magnetic field (IMF) direction B is small ( 45◦), the shock is considered quasi-parallel

Summary

Introduction

Collisionless plasma shocks are a ubiquitous source of plasma acceleration, common within stellar, planetary, and interplanetary environments. The streaming energized particles excite instabilities such as a right-hand ion–ion beam instability, building a wave field of ultra-low-frequency (ULF) waves (Hoppe et al, 1981) with periods around ∼ 30 s, which further interact with the particles themselves and are convected toward the bow shock. The incident ULF waves can experience nonlinear steepening, possibly forming shocklets (Hada et al, 1987; Wilson, 2016) or short large-amplitude magnetic structures (SLAMS; Schwartz et al, 1992; Burgess, 1995; Lucek et al, 2008), eventually causing the patchwork reformation of the bow shock (Scholer and Terasawa, 1990; Thomas and Winske, 1990; Schwartz and Burgess, 1991; Burgess, 1995) as incoming structures proceed to build a new shock front periodically (Burgess, 1989).

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