Pioneer Venus and IMP 8 observations of the latitude dependence of the solar wind

Abstract

Solar wind speeds and magnetic field data from the Pioneer Venus orbiter (0.7 AU) and IMP 8 spacecraft (1 AU) are compared to infer the latitudinal structure of the solar wind in the inner heliosphere between 1984 and 1987. It is found that solar wind streams and the interplanetary magnetic field (IMF) exhibited an unusual configuration during the last solar minimum (1986‐1987). This configuration only appeared during a limited period around the time of solar minimum, and was similar to the configuration that appeared during the previous solar minimum in 1975. Prior to solar minimum, the solar wind speed did not appear to vary with heliographic latitude, and the same high‐speed streams appeared at both spacecraft even when they were widely separated. The IMF had a two‐sector structure at all latitudes. During solar minimum, this situation changed, and high‐speed streams were excluded from low (<3°) latitudes, while the vicinity of the solar equator was characterized by slow solar wind and irregular variations in speed. The sector structure of the IMF also changed during solar minimum, and a single polarity predominated at latitudes greater than 4°. The onset of these changes appears to be related to a decrease in the inclination of the coronal neutral line; from approximately 40° at the end of 1984 to approximately 10° in the beginning of 1986. This is consistent with suggestions by previous observers that the coronal neutral line is associated with a sheet of low‐speed solar wind without high‐speed streams.