Ulysses observations of pressure-balance structures in the polar solar wind

Throughout its high-latitude excursion Ulysses has observed several types of solar wind structures. In addition to structures such as Alfven waves, compressional features, and microstreams, the solar wind also contains low contrast pressure-balance structures (PBSs). The PBSs are marked by an anti-correlation of the plasma and field pressures; their signatures are often hidden by the comparatively larger effects of compressions. Previous studies of high-latitude solar wind structures have only shown a few examples of PBSs. In this study we examine Ulysses polar pass solar wind plasma and interplanetary magnetic field data sets to provide the first statistical analyses of high-latitude PBSs. Two independent lines of analysis are pursued: first, we identify 78 PBSs in the data and analyze their average properties compared to the rest of the solar wind; second, we use high pass filtering of the data to separate the pressure balance aspects of the solar wind from the longer period compressional features. While our findings indicate that PBSs occur more frequently at small than at large heliocentric distances, indicating a possible solar origin for these structures, they do not have characteristically different plasma or held properties from that of the rest of the polar solar wind. Such differences would indicate that PBSs are possibly the remnants of differing populations of solar wind source plasma back in the polar corona. Our null result does not support the suggestion that PBSs are the interplanetary signature of polar plumes.