Investigation of magnetic free energy dynamics in the M1.2-class complex solar flare of March 15, 2015 using the 135-second HMI vector magnetograms

We investigate magnetic free energy (MFE) dynamics in the M1.2 solar flare occurred on March 15, 2015, 22:45 UT. We use the nonlinear force-free field (NLFFF) model of the coronal magnetic field on the basis of 135-second HMI vector magnetograms (standard magnetograms have a temporal resolution of 720 s) from the Heliosesmic and Magnetic Imager (HMI). Since we consider a rather long event (about 100 minutes) with several episodes of energy release of different temporal dynamics (pulsed or more gradual), we can find features of the MFE dynamics relative to the flare energy release stages using 135-second vector magnetograms. It is shown that the main MFE dissipation (approximately 30 % of the initial level) occurred during the two first subflare bursts seen in microwave hard X-ray ranges. These two bursts developed at low (less than 5 Mm) magnetic structures extremely elongated along the magnetic field polarity inversion line (PIL), while the other long-lasting bursts without appreciable MFE dissipation occurred in a growing flare arcade of magnetic loops. The obtained results are in favor of the fact that most of the MFE is localized in magnetic structures with a strong electric current at the PIL. It is possible that local monitoring of the preflare state around the PIL based on magnetic extrapolations with high temporal and spatial resolution will allow further improvement of methods for predicting solar flares. The dynamics of relative helicity are also studied in relation to the dynamics of the eruptions, and we found that the first impulsive subflares were accompanied by the most significant drop in helicity.