Coronal radio sounding with Ulysses: solar wind electron density near 0.1 AU during the 1995 conjunction

Dual-frequency ranging measurements were recorded for a three-week interval during the 1995 solar conjunction of the Ulysses spacecraft. The unusual occultation geometry resulting from the high inclinationUlysses orbit enabled coronal radio sounding from the solar South Pole to the heliographic equator at solar ray path offsets near 0.1 AU. The electron column density along the line-of-sight between Ulysses and Earth, derived from the differential time delays of ranging signals transmitted simultaneously on S-band and X-band carriers, clearly show the signatures of the polar coronal hole and a streamer belt at low latitudes. The observations are utilized in this work to test models for the heliographic latitude dependence of the electron density in the inner heliosphere. Traditional models for solar activity minimum, whereby the density decreases rather slowly from equator to pole, were found to fare rather poorly in representing the measurements. In contrast, a model based onUlysses in situ solar windmeasurements, when combined with ancillary observations to derive the location of the heliospheric current sheet, was found to provide satisfactory agreement with the radio sounding data.