Radial evolution of thermal and suprathermal electron populations in the slow solar wind from 0.13 to 0.5 au : Parker Solar Probe Observations

TL;DR

This study analyzes Parker Solar Probe data to understand how the electron populations in the slow solar wind evolve from 0.13 to 0.5 au, revealing growth patterns of halo and strahl populations close to the Sun.

Contribution

It introduces a new fitting routine to characterize electron populations and provides novel insights into their radial evolution near the Sun.

Findings

Halo population is negligible at small distances.

Halo and strahl populations grow from 0.13 to 0.17 au.

Strahl density remains constant below 0.2 au.

Abstract

We develop and apply a bespoke fitting routine to a large volume of solar wind electron distribution data measured by Parker Solar Probe (PSP) over its first five orbits, covering radial distances from 0.13 to 0.5 au. We characterise the radial evolution of the electron core, halo and strahl populations in the slow solar wind during these orbits. The fractional densities of these three electron populations provide evidence for the growth of the combined suprathermal halo and strahl populations from 0.13 to 0.17 au. Moreover, the growth in the halo population is not matched by a decrease of the strahl population at these distances, as has been reported for previous observations at distances greater than 0.3 au. We also find that the halo is negligible at small heliocentric distances. The fractional strahl density remains relatively constant ~1 % below 0.2 au, suggesting that the rise in the relative halo density is not solely due to the transfer of strahl electrons into the halo.