The magnetic fields of active regions

Abstract

Daily magnetogram data are analyzed to study the orientation angles of the magnetic axes of active regions defined by magnetic fields measured with the Mount Wilson magnetograph. The period covered by this daily data set is 1967 through April 9, 1990. The well-known tilt of the magnetic axes of active regions is seen clearly in these data. The dependence of tilt angle on latitude is different from that found by previous investigators, and it is suggested that this is due to the fact that this study uses all active regions, not spot groups or young active regions, and that there may be systematic variations in time of the tilt angles of regions. The variation of tilt angle with latitude is shown not to be a variation with cycle phase. Regions with smaller absolute tilt angles are larger than those with larger absolute tilt angles. Regions tilted in the normal orientation, with leading fields equatorward of following fields (positive tilt angles), are larger on average than those regions with negative tilt angles. Although there is no obvious relationship between tilt angle and daily region area change, it is found that regions with large tilt angles show on average rapid separation of the magnetic poles of the regions. This is not an effect of differential rotation shear. Normally oriented regions with small positive tilt angles rotate slower on average than those with small negative tilt angles. Some, but not all, of these results suggest that regions rise from subsurface flux ropes.