Survey of Magnetic Field Parameters Associated With Large Solar Flares

Until now, how the magnetic fields in M/X-class flaring active regions (ARs) differ from C-class flaring ARs remains unclear. Here, we calculate the key magnetic field parameters within the area of high photospheric free energy density (HED region) for 323 ARs (217 C- and 106 M$/$X-flaring ARs), including total photospheric free magnetic energy density Efree, total unsigned magnetic flux ΦHED$, mean unsigned current helicity h$c$, length of the polarity inversion lines $LPIL with a steep horizontal magnetic gradient, etc., and compare these with flare/coronal mass ejection (CME) properties. We first show the quantitative relations among the flare intensity, the eruptive character and ΦHED$. We reveal that $ is a measure for the GOES flux upper limit of the flares in a given region. For a given ΦHED$, there exists the lower limit of F$_SXR$ for eruptive flares. This means that only the relatively strong flares with the large fraction of energy release compared to the total free energy are likely to generate a CME. We also find that the combinations of Efree-LPIL$ and E$free$-h$c$ present a good ability to distinguish between C-class and M$/$X-class flaring ARs. Using determined critical values of E$free$ and $LPIL, one predicts correctly 93 out of 106 M/X-class flaring ARs and 159/217 C-class flaring ARs. The large LPIL$ or h$c$ for M$/$X-class flaring ARs probably implies the presence of a compact current with twisted magnetic fields winding about it.