Solar wind velocity jumps across tangential discontinuities: Ulysses observations and kinetic interpretation

Some tangential discontinuities (TDs) observed by theUlysses spacecraft interface solar wind regions that differ only in their bulk velocity and magnetic field orientation; composition, density, temperature and magnetic field intensity are essentially the same in both regions. The relation between the plasma velocity jump across the TD and the magnetic field is investigated through the analysis and simulation of equilibrium plane TD configurations. These theoretical results are compared with Ulysses observations. It is concluded that (a) the theoretically predicted magnetic field profile agrees with the morphology of the observed profile, (b) solar wind transitions are essentially of mixed type, i.e. both ion and electron velocity distribution functions are non-Maxwellian inside the transition layer, (c) there are constraints on the orientation and magnitude of the velocity jump that can be supported across a single transition, (d) large magnetic field rotations correspond to wide transition layers, and (e) in addition to density and temperature inhomogeneities, variations in the bulk velocity are a major reason for the solar wind plasma to set up current-carrying boundary layers.