Helium in the Earth's foreshock: a global Vlasiator survey

Markus Battarbee,Vertti Tarvus,Maxime Grandin,Yann Pfau-Kempf,Maxime Dubart,Jonas Suni,Thiago Brito,Urs Ganse,Primož Kajdič,Lucile Turc,Andreas Johlander,Tomas Karlsson,Karlheinz Trattner,Markku Alho,Mojtaba Akhavan-Tafti,Minna Palmroth,Savvas Raptis,Xóchitl Blanco-Cano,Stephen Fuselier

doi:10.5194/angeo-38-1081-2020

Abstract

Abstract. The foreshock is a region of space upstream of the Earth's bow shock extending along the interplanetary magnetic field (IMF). It is permeated by shock-reflected ions and electrons, low-frequency waves, and various plasma transients. We investigate the extent of the He2+ foreshock using Vlasiator, a global hybrid-Vlasov simulation. We perform the first numerical global survey of the helium foreshock and interpret some historical foreshock observations in a global context. The foreshock edge is populated by both proton and helium field-aligned beams, with the proton foreshock extending slightly further into the solar wind than the helium foreshock and both extending well beyond the ultra-low frequency (ULF) wave foreshock. We compare our simulation results with Magnetosphere Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA) measurements, showing how the gradient of suprathermal ion densities at the foreshock crossing can vary between events. Our analysis suggests that the IMF cone angle and the associated shock obliquity gradient can play a role in explaining this differing behaviour. We also investigate wave–ion interactions with wavelet analysis and show that the dynamics and heating of He2+ must result from proton-driven ULF waves. Enhancements in ion agyrotropy are found in relation to, for example, the ion foreshock boundary, the ULF foreshock boundary, and specular reflection of ions at the bow shock. We show that specular reflection can describe many of the foreshock ion velocity distribution function (VDF) enhancements. Wave–wave interactions deep in the foreshock cause de-coherence of wavefronts, allowing He2+ to be scattered less than protons.

Highlights

The Earth’s bow shock forms due to the interaction of the supermagnetosonic solar wind with our planet’s magnetic field
We show that specular reflection can describe many of the foreshock ion velocity distribution function (VDF) enhancements
The solar wind and/or foreshock thermal ion distribution was accumulated from the velocity space constrained within a sphere of 500 km s−1, centred at the solar wind speed usw,x = −750 km s−1, and all ions outside this sphere were considered part of the suprathermal distribution

Summary

Introduction

The Earth’s bow shock forms due to the interaction of the supermagnetosonic solar wind with our planet’s magnetic field. As in other heliospheric shocks, solar wind particles interacting with the shock undergo a variety of processes, including reflection and acceleration. Upstream of the bow shock, in regions where plasma is magnetically connected to the shock, the reflected particles form a region called the foreshock It is a very complex environment, populated by a variety of suprathermal ion distributions (Thomsen, 1985; Fuselier, 1995; Wilson, 2016), waves (Hoppe et al, 1981; Blanco-Cano et al, 2009; Wilson, 2016), and non-. The edges of the foreshock are magnetically connected to quasi-perpendicular regions of the Earth’s bow shock (where the angle between the shock normal and the magnetic field θBn 45◦), whereas the central region of the foreshock is magnetically connected to the quasi-parallel bow shock (where θBn 45◦)

Methods

Results

Conclusion