Structures in the polar solar wind: Plasma and field observations from Ulysses

Abstract

Ulysses has frequently observed solar wind structures lasting from several hours to several days since it passed into the region of solar wind emanating from the southern polar coronal hole at ~36°S. In addition to previously reported Alfven waves and coronal mass ejections, two other sorts of structures are routinely evident. This paper provides the first report of these structures in the high-latitude solar wind: (1) small-scale compressional structures and (2) pressure balance structures. The compressional structures are driven by faster solar wind overtaking slower solar wind ahead of it and exhibit the plasma and field properties expected for compressions.

Unlike large-scale stream interaction regions observed in and near the ecliptic plane, these structures are much smaller scale and are nonrecurring from one rotation to the next. These smaller-scale compressional structures appear to arise preferentially from a limited (~120° of heliolongitude) region in the southern polar coronal hole. The pressure balance structures, on the other hand, are indicated by roughly equal increases in the plasma pressure and decreases in the magnetic field pressure. These structures, which are several degrees across, are more dense and have higher plasma pressures and betas than the surrounding solar wind. These pressure balance structures may be the interplanetary manifestations of ``polar plumes.''