Constraining Suprathermal Electron Evolution in a Parker Spiral Field With Cassini Observations

G A Graham,M R Bakrania,M J Owens,C J Owen,I J Rae,A P Walsh

doi:10.1029/2020ja028669

Abstract

AbstractSuprathermal electrons in the solar wind consist of the “halo,” present at all pitch angles, and the “strahl” which is a field‐aligned, beam‐like population. Examining the heliospheric evolution of strahl beams is key to understanding the in‐transit processing of solar wind suprathermal electrons, in particular, to identify electron scattering mechanisms and to establish the origin of the halo population. Not only does this have significant implications with regard to the kinetic processes occurring within the solar wind but also its thermodynamic evolution, as the suprathermal electrons carry the majority of the solar wind heat flux. In this investigation, an established model for suprathermal electron evolution in a Parker spiral interplanetary magnetic field is adapted from its original use. The model is constrained using solar wind strahl observed by the Cassini mission on its interplanetary journey to Saturn. The effects of large scale IMF geometry due to different solar wind velocities and application of different electron scattering factors are examined. It is found that slow solar wind speeds provide the closest match to the strahl width observations, both in terms of radial distance and electron energy trends, and that predominantly slower solar wind speeds were therefore likely observed by the Cassini mission en‐route to Saturn. It is necessary to include a strahl scattering factor which increases with electron energy in order to match observations, indicating that the strahl scattering mechanism must have an inherent energy dependence.

Highlights

Solar wind electrons consist of a thermal component population known as the core and suprathermal electrons, which generally comprise of a relatively isotropic population known as the halo, and a field-aligned, beam-like population known as strahl (e.g., Feldman et al, 1975)
When we model the evolution of higher energy electrons, it can be seen that the pitch angle change per AU does not continue to decrease linearly with energy
This can be seen in Panel (a), in which, beyond ∼ 250 eV, the simulated energy relation for all electron energies flattens out and departs from the linear realtion given in Equation 2

Summary

Introduction

Solar wind electrons consist of a thermal component population known as the core and suprathermal electrons, which generally comprise of a relatively isotropic population known as the halo, and a field-aligned, beam-like population known as strahl (e.g., Feldman et al, 1975). Strahl electrons typically travel away from the Sun along the interplanetary magnetic field (IMF) direction, certain IMF typologies, such as local inversion in the field or closed loops associated with ICMEs, can result in observation of a sunward or bi-directional strahl (e.g., Feldman et al, 1975; Pilipp, Miggenrieder, Mühlhäuser, et al, 1987; Gosling et al, 1994). As IMF field strength decreases with distance from the Sun as it expands outwards with the solar wind plasma, strahl electrons are subject to adiabatic focusing. This should result in the formation of a strongly collimated beam (e.g., Owens et al, 2008).

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