Radial Evolution of Non-Maxwellian Solar Wind Electrons Derived From Quasi-Thermal Noise Spectroscopy: Parker Solar Probe Observation

Abstract

Precise measurement of the electron temperature and their non-thermal characteristics in the solar wind is essential for understanding the energy transport in the solar wind plasma. The present study applies the quasi-thermal noise (QTN) spectroscopy fitting method to analyze the QTN measurements from the Parker Solar Probe FIELDS radio instrument during the antenna-unbiased intervals of Encounters 2 to 13. The solar wind electrons are assumed to follow kappa velocity distributions in our QTN analysis and the fitting yields the relevant electron parameters across the heliocentric distance (r) from 12 to 76 solar radii. The results obtained indicate that the electron density scales as n_e  ∝ r^(-2.09±0.05), the electron temperature varies as T_kappa   ∝ r^(-0.65~-0.67), and the kappa index reveals three distinct segments of variations with r. Additionally, a statistical analysis of the solar wind energy flux demonstrates a power-law relation with r, w_total  ∝ r^(-1.92~-1.96). These findings contribute valuable information to the understanding of the energy transport dynamics in the solar wind.