Narrow lanes of transverse magnetic field in sunspots

SOLAR flares are closely associated with magnetic activity on the surface of the Sun. They typically occur¹ in complex sunspot groups, where the vertical magnetic fields reverse abruptly, and the horizontal (transverse) fields connecting the vertical poles are both sheared and strong. A single field inversion line may be the site of many flares¹. Here we report observations of a large, active sunspot group which reveal a series of oppositely directed vertical-field inversions separated by extremely narrow elongated channels of intense horizontal fields. In a minimum-energy configuration, lines of force connecting oppositely directed vertical fields simply arch across the inversion line; but when newly emerged sunspots move through older magnetic-field configurations, the poles are pushed together and the field lines turn sharply along the inversion line to reconnect with the vertical field some distance away. These multiple channels of horizontal field imply a large curl term (▽×B), and hence a substantial electric current. Our observations show that almost all of the larger flares in this region occur in these highly convoluted fields.