The Radial Evolution of the Solar Wind as Organized by Electron Distribution Parameters

TL;DR

This study uses Parker Solar Probe data to analyze how electron distribution parameters influence the radial evolution of solar wind speed, temperature, and electric potential, revealing different acceleration mechanisms for slow and fast solar wind streams.

Contribution

It provides new insights into the electron-driven acceleration processes and compares observational data with theoretical models for the first time across different wind speeds.

Findings

Electron temperature and electric potential are anti-correlated with wind speed.

Electron thermal pressure and electric fields contribute more to slow wind acceleration.

Models explain slow wind speeds but not the faster streams, indicating additional acceleration mechanisms.

Abstract

We utilize observations from the Parker Solar Probe (PSP) to study the radial evolution of the solar wind in the inner heliosphere. We analyze electron velocity distribution functions observed by the Solar Wind Electrons, Alphas, and Protons suite to estimate the coronal electron temperature and the local electric potential in the solar wind. From the latter value and the local flow speed, we compute the asymptotic solar wind speed. We group the PSP observations by asymptotic speed, and characterize the radial evolution of the wind speed, electron temperature, and electric potential within each group. In agreement with previous work, we find that the electron temperature (both local and coronal) and the electric potential are anti-correlated with wind speed. This implies that the electron thermal pressure and the associated electric field can provide more net acceleration in the slow wind than in the fast wind. We then utilize the inferred coronal temperature and the extrapolated electric + gravitational potential to show that both electric field driven exospheric models and the equivalent thermally driven hydrodynamic models can explain the entire observed speed of the slowest solar wind streams. On the other hand, neither class of model can explain the observed speed of the faster solar wind streams, which thus require additional acceleration mechanisms.