Radial Evolution of Thermal and Suprathermal Electron Populations in the Slow Solar Wind from 0.13 to 0.5 au: Parker Solar Probe Observations

We develop and apply a bespoke fitting routine to a large volume of solar wind electron distribution data measured by Parker Solar Probe over its first five orbits, covering radial distances from 0.13 to 0.5 au. We characterize 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 in 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 at similar to 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.

Automated summary

We develop and apply a fitting routine to analyze solar wind electron distribution data measured by Parker Solar Probe during its first five orbits, revealing the radial evolution of electron core, halo, and strahl populations in the slow solar wind. The results show that the combined suprathermal halo and strahl populations grow from 0.13 to 0.17 au, contradicting previous observations at distances greater than 0.3 au where an increase in halo is accompanied by a decrease in strahl. Additionally, the halo is negligible at small heliocentric distances, and the fractional strahl density remains relatively constant below 0.2 au.