Similarities and Differences of SARs in Solar Cycle 24

Abstract

Solar Cycle 24 is a particularly weak cycle and there are only six super active regions (SARs). Except for NOAA active region (AR) 12192, all other SARs are coronal mass ejection rich (CME-rich) and produced CMEs faster than 1000 kms−¹. By studying the similarities and differences between CME-poor SARs and CME-rich SARs in Solar Cycle 24, we aim to improve our understanding of the cause of major explosive solar activity events. Using the magnetograms of the Helioseismic and Magnetic Imager (HMI) and the images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), the magnetic-field characteristics of the six SARs and their established overall or even global magnetic connectivity are studied. It is found that the area of strong magnetic shear (A_Ψ) of all the six SARs is larger than 100 Mm², and the composite magnetic-field index (Ico_m: Chen and Wang in Astron. Astrophys.543, A49, 2012) of them is higher than 1. However, the total free magnetic-energy density (Efre_e) and the Ico_m for the majority of them are relatively low compared to the SARs in Solar Cycles 22 and 23. Compared with their very high total unsigned magnetic flux, the CME-poor AR 12192 has a lower Efre_e, a shorter magnetic-neutral line with steep horizontal gradient, a smaller A_Ψ, and a lower Ico_m. It is also revealed that the current helicity proxy, in terms of either the twist parameter or the best force-free parameter, has a good correlation with the fastest CME velocities of the SARs and serves as a good criterion to distinguish the CME-poor SARs from the CME-rich SARs. Except for NOAA AR 12192, all other SARs are found to connect closely with other ARs, and the AR cluster is a common feature of CME-rich SARs.