PIONEER VENUS AND IMP-8 OBSERVATIONS OF THE LATITUDE DEPENDENCE OF THE SOLAR-WIND

Solar wind speeds and magnetic field data from the Pioneer Venus orbit er (0.7 AU) and IMP 8 spacecraft (1 AU) are compared to infer the lati tudinal structure of the solar wind in the inner heliosphere between 1 984 and 1987. It is found that solar wind streams and the interplaneta ry magnetic field (IMF) exhibited an unusual configuration during the last solar minimum (1986-1987). This configuration only appeared durin g a limited period around the time of solar minimum, and was similar t o 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 appea red at both spacecraft even when they were widely separated. The IMF h ad a two-sector structure at all latitudes. During solar minimum, this situation changed, and high-speed streams were excluded from low (<3- degrees) latitudes, while the vicinity of the solar equator was charac terized by slow solar wind and irregular variations in speed. The sect or structure of the IMF also changed during solar minimum, and a singl e polarity predominated at latitudes greater than 4-degrees. The onset of these changes appears to be related to a decrease in the inclinati on of the coronal neutral line; from approximately 40-degrees at the e nd of 1984 to approximately 10-degrees 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 withou t high-speed streams.