Changes of the boot-shaped coronal hole boundary during Whole Sun Month near sunspot minimum

Abstract

The August 27, 1996, boot-shaped coronal hole is shown to rotate nearly rigidly at a rate of 13.25°/day, greater than the equatorial rotation rate of bipolar magnetic regions such as active regions and plages. The day-to-day variation of the coronal hole border is determined by comparing the rigid rotation projection of the disk-center hole boundary to coronal hole boundaries observed in successive daily coronal images. To determine the influence of the changing photospheric field on the location of the coronal hole boundary, a better approximation of the instantaneous global magnetic field distribution is developed and used as input to a potential-field source-surface model to compute the foot-point areas of open field lines. Day-to-day variations of the coronal hole boundary may be caused by changes of the magnetic field and plasma properties in the corona, as well as by the changing photospheric field. Copyright 1999 by the American Geophysical Union.