Squashing Factor Q in Relation to Extreme Ultraviolet Late

Abstract:

In this study, we utilized magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) to investigate the magnetic topology within two active regions, in which two M-class flares occurred on 2010 October 16 and 2011 February 18 with typical EUV late phases. Based on extrapolated potential fields, we mapped the magnetic connectivities, calculated the squashing factor Q of elemental magnetic flux tubes, and found a close match between the distribution of high-Q lines on the photosphere and the circular–shaped flare ribbons observed in UV 160 nm with SDO's Atmospheric Imaging Assembly (AIA). Further locating the coronal nulls embedded in the quasi-separtrix layers, we recognized that the large-scale loops in AIA 33.5 nm that have been identified by K. Liu et al (2013, ApJ, 768, 150) to be responsible for the EUV late phases are associated with the spines of the coronal nulls.