The Latitudinal Structure of the Solar Wind in the Vicinity of the Solar Equator near Solar Minimum: 1986 and Predictions for 1997

Abstract

In situ observations by different spacecraft have shown that the heliospheric current sheet (HCS) is associated with a narrow band of low speed high density solar wind. The inclination of the HCS and the three‐dimensional structure of solar wind streams is related to the inclination of the coronal neutral line. Observations from the Ulysses spacecraft during the descending phase of solar cycle 22, when Wilcox Solar Observatory coronal field models suggest that the inclination of the coronal neutral line was between 50° and 70°, showed that the inclination of the HCS was ≃20° while solar wind streams were confined to heliographic latitudes less than 35°. At higher latitudes, low and medium speed solar wind disappeared and the solar wind speed was uniformly high. Observations from the Pioneer 11 ‐ Voyager 2 spacecraft pair near the time of the 1986 solar minimum revealed a different structure. At that time the inclination of the coronal neutral line was unusually low (<10°–15°). Solar wind streams vanished near the solar equator and may have disappeared at most heliographic latitudes in the outer heliosphere. This has implications for the forthcoming Ulysses equatorial pass at the end of 1997, when the inclination of the coronal neutral line and the HCS are again expected to be low. If the HCS is sufficiently flat, conditions may resemble those observed near the time of the 1986 solar minimum and the solar wind stream structure may disappear at Ulysses near the solar equator.