TANGENTIAL DISCONTINUITIES AT HIGH HELIOGRAPHIC LATITUDES (SIMILAR-TO-80-DEGREES)

In this study, tangential discontinuities (TDs) near -80 degrees helio graphic latitude have been studied. Based on our quantitative definiti ons, we find that the TDs may generally be divided into two clear type s. There is a distinct population of TDs small (< 40 degrees) directio nal changes. About 19% of all high-latitude TDs are in this category. These types of TDs are found at the edges of mirror mode structures. T he latter are presumed to be locally generated by ion anisotropies, ba sed on examination of the mirror mode instability criteria. Another 50 % of high latitude TDs are associated with large field directional cha nges (> 60 degrees). These TDs typically are found at the boundary bet ween two streams. The discontinuities have large normals in the plane perpendicular to the background field, as might be expected for featur es generated by currents flowing along the field. These discontinuitie s usually have larger current intensities than those of the mirror-mod e related TDs. The remainder of the discontinuities have angular chang es which lie in between the mirror-mode-associated and the current-she et-associated TDs.