On Solar-Wind Electron Heating at Large Solar Distances

We study the temperature of electrons advected with the solar wind to large solar distances far beyond 1 AU. Almost nothing is known about the thermodynamics of these electrons from in-situ plasma observations at these distances, and usually it is tacitly assumed that electrons, due to adiabatic behaviour and vanishing heat conduction, rapidly cool off to very low temperatures at larger distances. In this article we show, however, that electrons on their way to large distances undergo non-adiabatic interactions with travelling shocks and solar-wind bulk-velocity jumps and thereby are appreciably heated. Examining this heating process on an average statistical basis, we find that solar-wind electrons first cool down to a temperature minimum, which depending on the occurrence frequency of bulk velocity jumps is located between 3 and 6 AU, but beyond this the lowest electron temperature again starts to increase with increasing solar distance, finally achieving temperatures of about 7×104 K to 7×105 K at the location of the termination shock. Hence these electrons are unexpectedly shown to play an important dynamical role in structuring this shock and in determining the downstream plasma properties.