Flare-related changes of an active region magnetic field

It was shown recently that current helicity, calculated using the photospheric magnetic field vector measurements, possesses a well-pronounced scaling behavior. The sign singularity of two-dimensional structures of current helicity, h_c = B_z · (∇ x B)_z, can be studied by introducing a signed measure and by calculating the power-law exponent, the cancellation exponent K. The time variations of the cancellation exponent seem to be related to flare activity of an active region (AR). Periods of enhanced flaring are accompanied by a drop and subsequent rise of the cancellation exponent. Here we focus on the changes in the vortex structure of the photospheric magnetic field during the transition of an active region from low flaring to enhanced flaring state. We analyzed variations of the cancellation exponent, helicity imbalance, and total electric current in four flaring active regions and one quiet one. We show that the transition of an active region from a low flaring state to an enhanced one is always accompanied (in this study, the corresponding time interval is less than 2 hr) by the 30%-45% decrease of the cancellation exponent. In two active regions, a reliable 13%-22% decrease of the total electric current took place, and in three active regions the helicity imbalance changed. This, possibly, implies a reinforcement of the anticoriolis twist of the whole magnetic configuration. For comparison, the decrease of K in the quiet active region does not exceed 10%. This can be interpreted as a real preflare reorganization of the vortex structure of the photospheric magnetic field: a combination of the exhausting of small-scale eddies along with the reinforcement of the total anticoriolis twist of the magnetic structure.