M.K. Georgoulis, D.M. Rust
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
20906, USA
Based on a statistical sample of nearly 300 solar active regions, observed over much of solar cycle 23, we show that regions hosting major (X- and M-class) flares and coronal mass ejections (CMEs) tend to have twist and writhe of the same sense. Virtually all nonflaring regions exhibit opposite twist and writhe. Same-sign twist and writhe is a necessary condition for the helical kink instability. Moreover, many eruptive regions with like-sign twist and writhe form both persistent and transient sigmoids in soft X-rays, a clear sign of magnetic helicity with a dominant sense. The photospheric boundary of these regions is often a delta-type sunspot. Reconciling existing interpretations of sigmoids and delta-sunspots with our analysis of twist and writhe, we conclude that the dominant instability mechanism of major solar eruptions is the helical kink instability. Further observational studies of the active-region photosphere and corona, as well as quantitative analyses of energy and helicity budgets in active regions are under way. The project receives partial support by NASA LWS TR&T Grant NNG05-GM47G.