Electron temperature anisotropy constraints in the solar wind

TLDR

Coulomb collisions in the solar wind are quantified by the electron collisional age, the number of collisions an electron experiences during solar wind expansion. The study statistically analyzes solar wind electron data to determine how plasma instabilities and Coulomb collisions constrain electron temperature anisotropy. The authors analyzed electron velocity distribution functions from Helios I, Cluster II, and Ulysses (0.3–4 AU) using a core–halo two‑population model to estimate temperature anisotropy, compare it with parallel beta and instability growth rates, and study its radial evolution. The results indicate that Coulomb collisions dominate isotropisation of the bulk electron population, whereas large anisotropies are limited by plasma instabilities.

Abstract

We have performed a statistical study of a substantial amount of electron data acquired in the solar wind to understand the constraints on electron temperature anisotropy by plasma instabilities and Coulomb collisions. We use a large data set of electron measurements from three different spacecraft (Helios I, Cluster II, and Ulysses) collected in the low ecliptic latitudes covering the radial distance from the Sun from 0.3 up to 4 AU. We estimate the electron temperature anisotropy using fits of the measured electron velocity distribution functions acquired in situ. We use a two population (core and halo) analytical model and properties of both populations are studied separately. We examine all the acquired data in terms of temperature anisotropy versus parallel electron plasma beta, and we relate the measurements to the growth rates of unstable modes. The effect of Coulomb collisions is expressed by the electron collisional age A e defined as the number of collisions suffered by an electron during the expansion of the solar wind. We show that both instabilities and collisions are strongly related to the isotropisation process of the electron core population. In addition we examine the radial evolution of these effects during the expansion of the solar wind. We show that the bulk of the solar wind electrons are constrained by Coulomb collisions, while the large departures from isotropy are constrained by instabilities.