The Highest Solar Wind Velocity in a Polar Region Estimated from IPS Tomography Analysis

Abstract

The Ulysses spacecraft has revealed that the velocity of high-speed solar wind increases with latitude for latitudes > 30° (Woch et al. 1997; Goldstein et al. 1996). We have investigated this velocity increase up to the polar regions using a tomography method from interplanetary scintillation observations made in 1996. For this analysis the tomography method was modified from its previous version (Kojima et al. 1998) so that it has enough sensitivity for the observations at the polar regions, and the reliability and uniqueness of its solution was tested. The modified tomography method analyzes the latitudinal structure in two steps: first the structure within ±80° was derived, and then the latitudes of 80°-90° are analyzed. Synoptic maps of velocity in Carrington longitude and heliographic latitude were derived with the modified tomography method, and latitudinal structures were obtained by averaging velocities along a longitude (Figure 1a). All analysis results show that the velocity increases with latitude in both hemispheres. In Carrington rotations 1911 to 1915, the velocities in the southern polar regions are 100–150 km/s larger than the velocity expected from the extrapolation of the observations at mid-latitudes, while in the northern polar region they were 35–75 km/s larger than the velocities similarly expected from mid-latitude observations. In rotations 1908 to 1911, the velocities in both polar regions were within ±60 km/s of the expected velocity. A large velocity decrease was not seen in the polar regions. At mid-latitudes, the velocity in the northern hemisphere is higher than that in the southern hemisphere.