What Makes Active Regions Grow?

Abstract

We have studied the growth, or failure to grow, of well over 100 active regions. Most growth is connected with the emergence of a large batch of flux in the shape of a new Arch Filament System (AFS). During the recent sunspot maximum, new AFSs appeared at a rate of nearly one per day over the entire Sun. We see them popping up at random points, not only in quiet parts of the Sun, but also within old active regions; and more often than chance would predict, they come up right in the middle, on the site of preexisting AFSs. These AFS appearances and rejuvenations account for most of the growth of active regions. We present evidence for the hypotheses that (1) a twist in the flux tubes of new AFSs is a key factor in determining which new AFSs will grow, and (2) this twist is related to the well-known asymmetry of sunspot groups.