The Electron Temperature and Anisotropy in the Solar Wind. I. Comparison of the Core and Halo Populations

TL;DR

This study analyzes the temperatures and anisotropies of core and halo electron populations in the solar wind across various distances, revealing anti-correlations and shared deviations from isotropy that inform understanding of solar wind dynamics.

Contribution

It provides the first detailed observational comparison of core and halo electron temperatures and anisotropies over a wide range of heliocentric distances, clarifying their interdependence.

Findings

Anti-correlation of core and halo temperatures consistent with Kappa model evolution

Both components tend to deviate from isotropy in the same direction

Presence of anti-correlated anisotropies indicating dynamic interplay

Abstract

Estimating the temperature of the solar wind particles and their anisotropies is particularly important for understanding the origin of these deviations from thermal equilibrium as well as their effects. In the absence of energetic events the velocity distribution of electrons reveal a dual structure with a thermal (Maxwellian) core and a suprathermal (Kappa) halo. This paper presents a detailed observational analysis of these two components, providing estimations of their temperatures and temperature anisotropies and decoding any potential interdependence that their properties may indicate. The data set used in this study includes more than 120$\,$000 the events detected by three missions in the ecliptic within an extended range of heliocentric distances from 0.3 to over 4~AU. The anti-correlation found for the core and halo temperatures is consistent with the radial evolution of the Kappa model, clarifying an apparent contradiction in previous observational analysis and providing valuable clues about the temperature of the Kappa-distributed populations. However, these two components manifest a clear tendency to deviate from isotropy in the same direction, that seems to confirm the existence of mechanisms with similar effects on both components, e.g., the solar wind expansion, or the particle heating by the fluctuations. On the other hand, the existence of plasma states with anti-correlated anisotropies of the core and halo populations suggests a dynamic interplay of these components, mediated, most probably, by the anisotropy-driven instabilities.