Statistical properties of bipolar magnetic regions

Abstract

Using observations from the Michelson Doppler Imager (MDI) onboard Solar and Heliospheric Observatory (SOHO), we develop a computational algorithm to automatically identify bipolar magnetic regions (BMRs) in active regions (ARs), and then study their statistical properties. The individual magnetic (positive or negative) pole of a BMR is determined from the region with an absolute strength above 55 G and with an area larger than 250 pixel2 (∼495 Mm2), while a BMR is identified as a pair of positive and negative poles with the shortest area-weight distance between them. Based on this method, 2234 BMRs are identified from MDI synoptic magnetograms between Carrington Rotations 1909 (1996 May 06) and 2104 (2010 December 10). 1005 of them are located in the northern hemisphere, while the other 1229 are in the southern hemisphere. We find that the BMR parameters (e.g., latitude, separation, fragment number and strength) are similar to those of ARs. Moreover, based on the maximum likelihood estimation (MLE) method, the frequency distributions representing the occurrence of these BMRs as functions of area and magnetic flux exhibit a power-law behavior, i.e., , with indices of and respectively. We also find that their orientation angles (θ) follow “Hale’s Polarity Law” and deviate slightly toward the direction of the solar equator. Consistent with previous findings, we obtain the dependence of orientation angles on latitudes for normal BMRs during the 23rd solar cycle. The north-south asymmetry of these BMRs is also detected here.