Electron cooling between 0.3 and 1 AU from Helios observations

Based on Helios observations we analyze the cooling process of solar wind electrons between 0.3 and 1 AU. In the present study we assume a steady radial expansion in a Parker spiral interplanetary magnetic field and anisotropic electron velocity distribution functions. The empirical energization rates are consequently derived for both the slow and fast solar wind streams using fits of power law radial profiles to observed macroscopic electron characteristics (i.e. density, temperature and heat flux). Our results indicate that compared to protons no external heating mechanisms are required in case of solar wind electrons and the observed temperature radial gradients are simply explained by the adiabatic cooling, internal electron heat flux and energy exchange provided by electron-electron Coulomb collisions. While the energization rate in the fast solar wind is found more or less equal to zero, it is even negative in the slow wind representing thus a source of a free energy.