Electrons in the Young Solar Wind: First Results from the Parker Solar Probe

TL;DR

This paper presents initial measurements of electron velocity distributions in the solar wind near the Sun using the Parker Solar Probe, revealing how electron populations evolve with proximity to the Sun and their relation to solar wind properties.

Contribution

First detailed analysis of electron velocity distributions close to the Sun, combining modeling and numerical integration, revealing new insights into electron populations and their dynamics in the young solar wind.

Findings

Electron core density and temperature increase closer to the Sun.

Strahl density increases, halo density plateaus or decreases at perihelion.

Electron distribution characteristics correlate with solar wind speed, beta, and collisional age.

Abstract

The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. We derive the parameters of the core, halo, and strahl populations utilizing a combination of fitting to model distributions and numerical integration for $\sim 100,000$ electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as $\sim 0.17$ AU. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure ($\beta_e$) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, $\beta_e$, and/or collisional age. Near the Sun, some trends not clearly seen at 1 AU become apparent, including anti-correlations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.