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V. B. Yurchyshyn (Big Bear Solar Obs., NJIT \& Crimean Astrophysical Obs., Ukraine), V. I. Abramenko (Crimean Astrophysical Obs., Ukraine), V. Carbone (Dipartimento di Fisica, U. Cosenza, Italy)
Recently it has been shown (Abramenko et al., Sol. Phys. 178) the current helicity calculated by the photospheric magnetic field vector possesses a well pronounced scaling behavior. The sign-singularity of the 2D structures of current helicity can be studied by introducing the signed measure and by calculating some power-law exponent: cancellation exponent \kappa. The time variations of the cancellation exponent seem to be related to flare activity of an active region: the periods of enhanced flaring are accompanied by a drop and subsequent rise of the cancellation exponent (Abramenko et al., A&A. 334). Here we continue to study the flare-related changes of cancellation exponent paying special attention to the preflare situation. We have found that the significant decrease of \kappa begins 1-3 hours prior to a strong flare. The reduce of the \kappa means the changes in the transverse magnetic field structure. The preflare energy release produced by small-scale currents dissipation seems to supply observed preflare energetic phenomena such as preflare flocculi brightening and enhanced X-ray and microwave emission. So we are able to conclude that small-scale energy release in the transverse photospheric magnetic field take place prior to a strong flare. As soon as the evaluation of \kappa needs only the calibrated vector magnetogram and some minutes for calculations the preflare drop of the cancellation exponent could be used for the short-time prediction of strong flares.
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