Abstract :
Measurements of the vector magnetic field in an active region NOAA 6757 were used to compare current helicity and turbulent regime of the magnetic field 2.5 – 0.5 hours prior and 2 minutes after the maximum of a strong 2B/X1.5 solar flare. First, we calculated the imbalance, ϱh, of current helicity over the area of the flare. During a forty minute time interval, which includes the impulsive phase of the flare, total positive (negative) helicity decreased by 20% (27%), while the imbalance ϱh changed from −5% to +1%. This implies that before the flare the necessary conditions for the α-effect in the solar photosphere were fulfilled, whereas after the flare maximum the generation of the electromotive force due to small-scale fluctuations of the magnetic field seems to be exhausted. Second, we calculated a cancellation exponent, kh, of current helicity and an exponent, Σ(q), of structure functions of the longitudinal magnetic field. The curvature of Σ(q) indicates multifractality (intermittent turbulence) of the magnetic field, while kh describes the intensity of oscillations of the sign of current helicity and, therefore, the strength of tangential discontinuities in the magnetic field. We found that the value of kh was large until the last field measurement before the flare. After the flare maximum, the cancellation exponent was found to be significantly smaller. Meanwhile, observed behavior of Σ(q) implies that multifractality of the Bz component became more complicated before the flare, whereas, immediately after the flare maximum monofractality (non-intermittent turbulence) of the Bz component was set. Such changes in both kh and Σ(q) complement each other as evidence that a flare can be treated as an avalanche of small-scale reconnections at tangential discontinuities of the magnetic field.
