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

The August 27, 1996, boot-shaped coronal hole is shown to rotate nearly rig idly at a rate of 13.25 degrees/day, greater than the equatorial rotation r ate 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 in fluence of the changing photospheric field on the location of the coronal h ole boundary, a better approximation of the instantaneous global magnetic f ield distribution is developed and used as input to a potential-field sourc e-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 th e magnetic field and plasma properties in the corona, as well as by the cha nging photospheric field.