FORMATION AND EVOLUTION OF CORONAL HOLES FOLLOWING THE EMERGENCE OF ACTIVE REGIONS

Abstract

The low level of solar activity over the past four years has provided unusually favorable conditions for tracking the formation and evolution of individual coronal holes and their wind streams. Employing extreme-ultraviolet images recorded with the Solar Terrestrial Relations Observatory during 2007-2009, we analyze three cases in which small coronal holes first appear at the edges of newly emerged active regions and then expand via flux transport processes, eventually becoming attached to the polar holes. The holes form gradually over timescales comparable to or greater than that for the active regions to emerge, without any obvious association with coronal mass ejections. The evolving hole areas coincide approximately with the footpoints of open field lines derived from potential-field source-surface extrapolations of the photospheric field. One of these coronal-hole systems, centered at the equator and maintained by a succession of old-cycle active regions emerging in the same longitude range, persists in one form or another for up to two years. The other two holes, located at midlatitudes and originating from new-cycle active regions, become strongly sheared and decay away after a few rotations. The hole boundaries and the small active-region holes, both of which are sources of slow wind, are observed to undergo continual short-term (1 day) fluctuations on spatial scales comparable to that of the supergranulation. From in situ measurements, we identify a number of plasma sheets associated with pseudostreamers separating holes of the same polarity.