Characteristics of solar wind suprathermal halo electrons

¹ Institute for Theoretical Physics IV, Ruhr-Universität Bochum, 44780 Bochum, Germany
e-mail: mlazar@tp4.rub.de
² Centre for Mathematical Plasma Astrophysics, Celestijnenlaan 200B, 3001 Leuven, Belgium
³ Royal Belgian Institute for Space Aeronomy, Space Physics and STCE, 3 av. Circulaire, 1180 Brussels, Belgium
⁴ Université Catholique de Louvain (UCL), Center for Space Radiations (CSR) and Georges Lemaître Centre for Earth and Climate Research (TECLIM), Earth and Life Institute (ELI), Place Louis Pasteur 3 bte L4.03.08, 1348 Louvain-La-Neuve, Belgium
⁵ Institute of Physics, University of Maria Curie-Skłodowska, ul. Radziszewskiego 10, 20-031 Lublin, Poland
⁶ Research Dept. Plasmas with Complex Interactions, Ruhr-University Bochum, 44780 Bochum, Germany

Received: 3 July 2020
Accepted: 15 August 2020

Abstract

A suprathermal halo population of electrons is ubiquitous in space plasmas, as evidence of their departure from thermal equilibrium even in the absence of anisotropies. The origin, properties, and implications of this population, however, are poorly known. We provide a comprehensive description of solar wind halo electrons in the ecliptic, contrasting their evolutions with heliospheric distance in the slow and fast wind streams. At relatively low distances less than 1 AU, the halo parameters show an anticorrelation with the solar wind speed, but this contrast decreases with increasing distance and may switch to a positive correlation beyond 1 AU. A less monotonic evolution is characteristic of the high-speed winds, in which halo electrons and their properties (e.g., number densities, temperature, plasma beta) exhibit a progressive enhancement already distinguishable at about 0.5 AU. At this point, magnetic focusing of electron strahls becomes weaker and may be counterbalanced by the interactions of electrons with wave fluctuations. This evolution of halo electrons between 0.5 AU and 3.0 AU in the fast winds complements previous results well, indicating a substantial reduction of the strahl and suggesting that significant fractions of strahl electrons and energy may be redistributed to the halo population. On the other hand, properties of halo electrons at low distances in the outer corona suggest a subcoronal origin and a direct implication in the overheating of coronal plasma via velocity filtration.