Solar wind at solar maximum
Abstract
During the last solar minimum in 1996 Ulysses, on its first polar orbit, found a heliosphere that was relatively simply structured. Large fast streams emanating from the polar coronal holes dominated the heliosphere poleward of30 , separated by a zone of generally slow and very variable solar wind within20 from the solar equator. Composition data were essential in defining these two quasi-stationary stream types and in particular the boundary between them, which appears to be remarkably sharp and well-defined from down in the chromosphere out to Ulysses at 5 AU. Now, around the time of solar maximum activity of cycle 23, Ulysses is on its second polar orbit, and the picture of the heliosphere that we obtain this time around is radically different. Superficially it looks as if the fast streams had almost completely vanished and the slow, variable solar wind subtended the full solid angle. Yet composition data of both Ulysses- and ACE-SWICS reveal that the fast streams are, and have been, present all along. As the solar magnetic field reverses around maximum activity, the large polar coronal holes fragment and are replaced by a number of smaller coronal holes at low latitudes (possibly of about equal integrated area). The fast streams emanating from these equatorial coronal holes are affected by stream-stream interaction due to the solar rotation and their kinetic signature may be altered thereby. But compositional signatures such as a low O7+ /O6+ charge state ratio are unaffected by such interactions and clearly indicate their coronal hole origin. Moreover, the solar wind around solar maximum is interspersed with interplanetary coronal mass ejections of any speed, and again compositional signatures such as a high average Fe charge state can be used to identify many of these.
- Publication:
-
34th COSPAR Scientific Assembly
- Pub Date:
- 2002
- Bibcode:
- 2002cosp...34E1201V