Ulysses Observations of the Properties of Multiple Ion Beams in the Solar Wind

Properties of multiple ion beams in the solar wind beyond 1 AU as observed by the SWOOPS experiment on Ulysses are discussed. The solar wind proton distributions are approximated by a two beam bi-Maxwellian model. The slower outward traveling beam typically comprises the majority of the solar wind density. Differential streaming between the slower and faster proton beams decreases with distance from the Sun. The greatest difference between the beams in their evolution with distance from the Sun is that the parallel temperature component of the faster beam decreases more rapidly, r(-1.22), than that of the slower beam, r(-0.39). The difference in behavior for the perpendicular components (r(-0.46) for the faster beam and r(-0.73) for the slower beam) is real but less marked. The indication that relative perpendicular cooling is less for the faster beam while relative parallel cooling is greater and differential beam speed decreases is generally consistent with expectations from a streaming instability between the two proton beams. We have observed a dependence of the temperature anisotropy of the faster proton beam on the drift speed of the faster beam with respect to the slower beam; for large drifts (about 1.6 V-A) the anisotropy, T-vertical bar/T-perpendicular to, is smaller (about 0.8), whereas for slower drifts the parallel temperature is relatively hotter (anisotropy ratio of about 1.5).