Statistical properties of the Bipolar Magnetic Regions

Using the observations from Michelson Doppler Imager (MDI) onboard Solar and Heliospheric Observatory (SOHO), we develop a computational algorithm to automatically identify the bipolar magnetic regions (BMRs) in the active regions, and then study their statistical properties. The individual magnetic (positive or negative) pole of the BMR is determined from the region with an absolute strength above 55 G and with an area above 250 pixel (∼495 Mm), while a BMR is identified as a pair of positive and negative poles with a shortest area-weight distance between them. Based on this method, 2234 BMRs are identified from the MDI synoptic magnetograms between the Carrington Rotation 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 paraments (e.g., latitudes, separations, fragments, and strength) are similar to those of active regions (ARs). Moreover, based on the maximum likelihood estimation (MLE) method, the frequency distributions of these BMRs occurrence as functions of area size and magnetic flux exhibit a power-law behavior, i.e., dN/dx ∝ xx , with an index of αA = 1.98 ± 0.06 and αF = 1.93 ± 0.05. We also find that their orientation angles (θ) follow the “Hale’s Polarity Laws” and deviate slightly to the solar equator direction. Consistent with the previous findings, we obtain the orientation angles dependence on the latitudes for the normal BMRs during the 23rd solar cycle. The north-south asymmetry of these BMRs is also detected here.