Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

“Physics ofAuroral Phenomena", Proc. XXXVAnnual Seminar, Apatity, pp. 88 - 91, 2012 © Kola Science Centre, Russian Academy of Science, 2012 THE ACTIVE REGION MAGNETIC FIELD ASSOCIATION WITH SOLAR FLARES I. M. Podgomy1, and A. I. Podgomy 2 1Institutefor Astronomy RAS, Moscow, Russia, podgorny@inasan.ru 2Lebedev Physical Institute RAS, Moscow, Russia Abstract. The powerful active region with a dipole type magnetic field distribution does not produce flares. For flare appearance the magnetic field distribution in the active region should not have a regular character - magnetic field sources must be intruded in the area of the opposite magnetic field component. The weak active region (AR) 11158 appeared on the eastern solar limb. The region becomes stronger and stronger traveling across the solar disk. The flare X2.2 appears, when magnetic flux is increased up to Ф > 10 22 Mx. Six X-class flares are produced in the NOAA 10720. All of them appeared after northern and southern magnetic fluxes reached the critical value o f - 10 22 Mx. The comparison of flare productive active AR with regions that produce no flare shows that the magnetic flux increasing is necessary, but not a sufficient condition for the flare occurrence. The strong AR (even at Ф > 10 22 Mx) with a simple magnetic polarity-reversed line does not produce a powerful flare. No considerable change of AR 11158 active region is observed during the X-class flare. The observed weak localized change of the line-of- sight magnetic component does not produce appreciable influence on the magnetic field distribution in the active region during a flare. The SDO data with cadence of 45s have been used. Introduction The space craft measurements [1,2] have shown that the solar flare energy is released in the corona above an active region (AR). The only source of energy in the corona, where p = 8 тткТ/В 2 - 10'6, is the magnetic field, but the potential magnetic field of photospheric sources cannot be transferred in plasma kinetic energy. 3D MHD numerical simulation shows that before the flare the current sheet is formed above AR. The magnetic energy stored in such a sheet is sufficient for a flare [3, 4]. In [5 - 8 ] it is concluded that flares can occur only over AR where the magnetic flux of AR becomes big enough. Analysis of magnetic flux [7, 8 ] in magnetic fields of NOAA 10486 and 10365 that produced a series of strong flares (class X) shows that such flares occur when the magnetic flux exceeds 10 22 Mx. The flare appears, if field distribution on the photosphere is complex and has a complex polarity-inversion-lines. The work [ 8 ] also draws attention to the increased probability of large flare production at the strong magnetic field gradient across the polarity-inversion line. Here, we consider the magnetic field evolution of AR that produced of X- class flares, and to study AR magnetic field behavior before and during flares. Such information is needed to elucidate the mechanism of flares - whether the dissipation of magnetic energy during the flare occurs in the corona or on the solar surface. The results are also important for improving the flare prognoses. The attempts to detect magnetic field evolution during a flare [9, 10] have demonstrated only a small (-100 G) field fluctuations at different points of AR, which the magnetic field reaches 3000 G. The work [10] draws attention to the movement of penumbra elements of of AR 11158, which are treated as the spot rotation with a speed of 90 deg/day. This movement began for the day before the X2.2 flare and continued after the flare, without showing any features during the flare. Magnetograms o f active regions during and before the flare The AR 11158 began to form 02.11.2011 near the eastern limb (Fig. 1), and for four days became a strong region with two main complex polarity-inversion-lines and the strong fields of both polarities. This type of AR is highly likely to produce a powerful flare. Indeed, flares of classes C 8 and then X2.2 appear (Fig. 1). The magnetograms show that NOAA 11158 magnetic fluxes of both polarities are increased within four days before the flare, and the distribution of the field was of complex nature with the intrusion of the field of one polarity in a spot of other polarity. It is clearly seen the sharp boundary between the magnetic fluxes of opposite polarity, indicating a large magnetic gradient across the polarity -reversion line. This is a typical preflare PyS-configuration. Another important feature of the preflare evolution of this field is the lack of movement of magnetic fields that could lead to an accumulation of energy at the expense of helicity. The SDO HMI measurements are also demonstrate the growth of magnetic activity before the flares. In Fig. 1 below the SDO measurements demonstrate the growth of X-rays activity before the flares with increasing the NOAA 11158 magnetic flux. Another important feature of the preflare evolution of this AR is the lack of movement in the magnetogramms, which could lead to an accumulation o f energy at the expense of helicity. Polar Geophysical Institute 88