Low-latitude coronal holes during solar maximum

Abstract

The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO has been used to observe large low-latitude coronal holes during solar maximum that produced fast solar wind streams. UVCS observations show that large low-latitude coronal holes at solar maximum, coronal holes of at least 10° in longitude, have plasma properties that seem to bridge the gap between solar minimum polar coronal holes and streamers. The ion kinetic perpendicular temperatures in equatorial coronal holes are about 2 times larger than those in a solar minimum equatorial streamer, and about a factor of 2 smaller than those in polar coronal holes above 2 R ⊙. The outflow speeds for the large equatorial coronal holes observed by UVCS are 3–4 times lower than those in polar coronal holes between 2 and 3 R ⊙. The values for high- and mid-latitude coronal holes are in between those. In all these cases, the in situ data corresponding to these coronal holes showed high-speed wind streams with asymptotic speeds of 600–750 km s −1. These wind speeds approach those observed over polar coronal holes at solar minimum, but the outflow speeds in these coronal holes between 2 and 3 R ⊙ are different. In contrast to the polar coronal holes, the bulk of the solar wind acceleration must occur above 3 R ⊙ for large low-latitude coronal holes at solar maximum. These observations provide detailed empirical constraints for theoretical models and may be key to understanding how the various types of solar wind plasma are heated and accelerated.