Large‐scale variations of thermal electron parameters in the solar wind between 0.3 and 1 AU

Abstract

Variations of thermal electron parameters in the solar wind are investigated using data obtained from the Helios probes in the years from 1974 to 1976, shortly before solar minimum, at heliocentric distances ranging from 0.3 to 1 AU. The main part of the present analysis is based on Helios 2 data obtained in 1976. Variations across the sector structure of the interplanetary magnetic field (IMF) and across the plasma stream structures are studied. These studies support the hypothesis that the thermal electron properties, i.e., the electron temperatures, the core temperatures, the heat flux, and the normalized heat flux, are strongly correlated with the distance from the neutral sheet in the IMF (implying also a correlation with the plasma stream structures). The results do not indicate significant electron heating in compression regions. Also, power laws for the variations of the electron parameters with distance from the Sun as well as polytrope laws are derived for different solar wind structures and distance ranges. The core temperatures, determined by bi‐Maxwellian fits to the electron distributions below 20 or 30 eV, mostly decrease faster with distance from the Sun than the electron temperatures Te∥ and Te⊥, parallel and perpendicular to the magnetic field, respectively, as determined from the entire electron distributions. Most of the Helios 2 observations indicate that the radial profiles of electron temperatures and of electron heat flux tend to be flatter in high‐speed streams embedded within the interior of magnetic sectors than in the slow‐ and intermediate‐speed solar wind and in compression regions mostly at or near sector boundaries. For high‐speed streams, the Helios data indicate a flattening of the radial profiles for Te⊥ with increasing distance from the Sun. For Te∥ this effect is not as evident in the case of Helios 2 data but is indicated by Helios 1 data. Analogous trends are observed for most of the derived polytrope indices for the core and electron temperatures. The data provide evidence that in high‐speed streams the electrons are heated not only by the electron heat flux but also by external energy sources. Finally, estimated decreases with heliocentric distance from 0.3 to 1 AU of the interplanetary electrostatic potential could provide an increase of the solar wind bulk velocity of about 5 to 20 km s−1.