Solar wind Halo electrons from 1–4 AU

Abstract

In this study we use observations from the Ulysses solar wind electron spectrometer to make a first examination of the evolution of the solar wind suprathermal or halo electron population as a function of heliocentric distance beyond 1 AU. Solar wind electron analyzers previously flown to distances >1 AU did not resolve the halo electron distribution so that the 3‐D suprathermal solar wind electron observations from Ulysses represent a unique data set. As the core population cools with increasing heliocentric distance, no gap is formed between the core and halo populations. Rather, the halo electrons extend to increasingly lower energies. As predicted previously on theoretical grounds, the ratio of the core electron temperature to the low energy cutoff of the halo population appears to be roughly constant with a value of ∼7.5. The total integrated heat flux drops rapidly with increasing heliocentric distance; we find a best fit power law of R−2.36. In addition, we find that the ratio of the halo to core densities is roughly constant over heliocentric distance with the halo representing 4% of the total electron distribution. These results suggest that the halo population may not consist of truly non‐interactive test particles over the heliocentric range of 1–4 AU.