The area asymmetry in bipolar magnetic fields

Abstract

Context. The area asymmetry between the preceding and following regions of opposite magnetic polarity in a bipolar sunspot group has been known since the studies of Hale and his colleagues in the early 20th century. This area asymmetry, however, has not yet been investigated quantitatively using magnetograms. Aims. We quantitatively define the area asymmetry of bipolar magnetic fields in the photosphere of active regions on the Sun, and investigate correlations between the area asymmetry and other parameters. Methods. We selected 138 bipolar regions including eleven recurrent regions from magnetograms observed by the Michelson Doppler Imager (MDI) from 23 April 1996 to 2 September 2001. These regions are on the southern hemisphere and around the solar meridian. The area asymmetry (A), tilt angle (θ), and magnetic orientation (M) are investigated separately in the preceding and following polarities of the respective active regions. Results. It is found that in 37% (51/138) of our events the preceding polarity regions have larger areas than the following polarity regions. The ratio of the area of the preceding and following polarity regions become close to be unity in four recurrent active regions. In the other four regions, the area ratios do not change, and in three regions, the area ratios become far from unity. Conclusions. Our results quantitatively confirmed our impression that areas of the preceding polarities are lower than those of the following polarities in many bipolar magnetic regions. © 2012 ESO.