Characteristics of flare-related photospheric magnetic fields in asymmetric hard X-ray footpoints

It is commonly observed that the conjugate footpoints of newly reconnected magnetic fields have different hard X-ray (HXR) emissions. Besides the acceleration process in the corona, such asymmetry of chromospheric HXR sources are thought to be associated with the different plasma or magnetic field conditions during the transportation in a flaring loop. Due to the effect of asymmetric magnetic mirroring at the conjugate footpoints, strong HXR emission is supposed to be observed in the region of weak magnetic field because of the high precipitation rate of energetic electrons there. In this study, the RHESSI flares larger than M-class during 2002-2009 with two well-identified HXR footpoints near the solar disk center and with a series of one-minute averaged MDI magnetograms are analyzed to examine the correlation between HXR fluxes and photospheric magnetic fields in asymmetric HXR footpoints statistically. Moreover, all the studied time intervals are classified into impulsive, main, and gradual phases to figure out the significance of magnetic mirroring effect in different flare periods.