Electron energetics in the expanding solar wind via Helios observations

We present an observational analysis of electron cooling/heating rates in the fast and slow solar wind between0.3 and1AU. We fit electron velocity distribution functions acquired in situ by Helios1 and2 spacecraft by a three-component (core-halo-strahl) analytical model. The resulting radial profiles of macroscopic characteristics (density, temperatures, and heat fluxes) are employed to examine properties of theoretical energy balance equations and to estimate external cooling/heating terms. Our analysis indicates that in contrast to solar wind protons the electrons do not require important heating mechanisms to explain the observed temperature gradients. The electron heating rates are actually found to be negative for both the slow and fast solar wind, namely, due to the significant degradation of the electron heat flux with increasing radial distance from the Sun. Cooling mechanisms acting on electrons are found to be significantly stronger in the slow wind than in the fast wind streams.