Polar low‐speed solar wind at the solar activity maximum

Abstract

The tomographic analysis of interplanetary scintillation (IPS) showed that low‐speed winds (≤ 370 km s−1) emanated out from the polar region at the last solar activity maximum. In order to investigate the origin of those low‐speed winds, we compared the velocity distribution derived from the IPS tomographic analysis to the magnetic field structure derived from the potential field analysis. We found that the polar low‐speed winds appeared for a short period just before and after the disappearance of polar open fields. When the polar coronal hole shrank very small before its disappearance, the coronal polar open field was encircled by large‐scale closed loops and became super radially diverging field into the interplanetary space. A low‐speed region appeared in this diverging polar magnetic field region. This situation is a condition very similar to the compact low‐speed streams associated with equatorial active regions, which were found by Kojima et al. [1999]. After the open field regions had disappeared from the pole, the polar regions were occupied with closed loops. These closed loops were overlapped by the magnetic field which fanned out from the midlatitudes. A low‐speed streamer located above these closed loops even after the polar open field had disappeared. The velocities of polar low‐speed streams before polar hole disappearance were much lower than those after disappearance. This result suggests that the physical conditions to generate much lower speed streams are closely associated with large expansion from small open field regions encircled by large‐scale closed loops. Finally, a reliability of the IPS measurement of polar low‐speed wind was examined by simulating synthetic IPS observations in hypothetical model polar streams.