Ulysses observations of microstreams in the solar wind from coronal holes

Abstract

During its south polar passage in 1994, the Ulysses spacecraft continuously sampled the properties of the solar wind emanating from the south polar coronal hole. At latitudes poleward of ~-60°, the solar wind speed had an average value of 764 km/s and a range of 700-833 km/s. The principal variations in the vector velocity were associated with either outward propagating Alfvén waves with periods up to about half a day or with longer-period high- or low-speed ``microstreams.'' The microstreams had an amplitude of ~40 km/s and a mean half width of 0.4 days, and they recurred on timescales of 2-3 days (power spectral peaks at 1.9 and 3.3 days). The density and temperature profiles showed the expected evidence of pileup and compression on the leading edges of high-speed microstreams, although no forward or reverse shocks were observed. The particle fluxes were nearly the same for both the fast and slow microstreams. The higher-speed microstreams had higher proton temperatures and higher alpha-particle abundances than did the slower microstreams. The absence of latitude variations in the thickness or the recurrence rate suggests that the microstreams are caused by temporal rather than long lived (> a few days) spatial variations in the source region at the Sun. Some speculations are made about the possible cause of the microstreams.