Author:

Keywords:

Science & Technology, Physical Sciences, Astronomy & Astrophysics, solar wind, plasmas, methods: data analysis, methods: statistical, STRAHL ELECTRONS, 1 AU, TEMPERATURE, CORE, 0201 Astronomical and Space Sciences, 5101 Astronomical sciences, 5107 Particle and high energy physics, 5109 Space sciences

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). However, the origin, properties and implications of this population are poorly known. Here 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 anti-correlation 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 to the high-speed winds, where the halo electrons and their properties (e.g., number densities, temperature, plasma beta) exhibit a progressive enhancement already distinguishable at about 0.5 AU, when magnetic focusing of the electron strahls becomes weaker and may be counterbalanced by the interactions of electrons with wave fluctuations. This evolution of halo electrons between 0.5 and 3.0 AU in the fast winds complements well with previous results 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.