Abstract

<p>The foreshock is a region of space in front of the Earth's bow shock, extending along the interplanetary magnetic field. It is permeated by ions and electrons reflected at the shock, low-frequency waves, and various plasma transients. The ion foreshock is dominated by a number of proton populations such as field-aligned beams, gyrating distributions and diffuse ions, as well as proton-excited waves. As the solar wind can contain a significant fraction of helium, it is of great interest to investigate how alpha-particles (He<sup>2+</sup>) are reflected into forming their own foreshock. We investigate the extent of the helium foreshock in relation to foreshock ultra-low frequency waves and protons using Vlasiator, a global hybrid-Vlasov simulation. We confirm a number of historical spacecraft observations at the foreshock regions associated with field-aligned beams, gyrating ion distributions, and specularly reflected particles, performing the first numerical global survey of the helium foreshock. We present wavelet analysis at multiple positions within the foreshock and evaluate the dynamics of gyrating ion populations in response to the transverse and compressive wave components. We also present Magnetosphere Multiscale (MMS) spacecraft crossings of the foreshock edge and compare Hot Plasma Composition Analyzer (HPCA) measurements of energetic ions with our simulation data, showing the variability of the foreshock edge suprathermal ion profiles.</p>

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

  • We examine the properties of ion velocity distribution functions (VDFs) at the edge of and within the foreshock

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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 interaction of suprathermal ions with the solar wind results in instabilities able to generate ULF waves (Gary, 1991)

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