Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

Magneticfield dynamics o f large active regions in the pre-flare state during solarflares August 13, and then a series of flares of class M appear. Until August 13, the class of solar flares produced by the region NOAA 10656, is not exceed the class C. The flare o f class XI appears only on August 13 at magnetic flux ~ 4 x l022Mx. This big flare is accompanied by a series of flares of class M occurred on August 14 and 15. From comparison of magnetic flux time dependence and the magnetogramms shown in Fig. 2 one can conclude that flare activity takes place not only due to slow increasing of AR magnetic flux, but also because of the complicity of magnetic field distribution. NOAA 10656 GOES Xray Flux Fig. 2 X-ray dynamic produced by AR NOAA 10656 and magnetograms In contrast to AR NOAA 10656, which have a big magnetic flux for 5 days, but not complicated distribution of the magnetic field, AR NOAA 10365, 10720, 11158, discussed in [13-17], have a complex distribution of the magnetic fields o f the type Py5 and caused a series of very big flares at lower magnetic flux. All the time of NOAA 10656 traveling on the visible disk, AR field distribution is not as complex as the field of NOAA 10365, 10720, 11158 that produced many flares. AR NOAA 10656 have produced only one flare of class X (XI) after distribution complicity increasing. The data presented here suggest two factors that determine the onset of pre-flare situation for big flares: the increase of the magnetic flux above the critical value of ~1022 Mx and the complexity of the distribution of the magnetic sources on the photosphere, in particular the complexity of the shape of the field inversion line. The constancy of active region magnetic field distribution during a flare It is has been shown in [14- 20] that the magnetic flux o f AR remains unchangeable even during large flares. This conclusion seems contradicts the fact that the source of the flare energy is the Sun. However, by visual analysis of photographs of the magnetogramms and magnetic field distributions in the active regions during the flares any even slight changes that could be associated with flare appearance has not found. Only at the giant flare X17 28 October 2003 in NOAA 10486 a narrow maximum of ~ 1000 G has been registered, which is not cause a noticeable change in the magnetic flux o f the active region. The magnetic flux during this flare also has remained constant to within 1 %. Comparison o f the magnetic field distributions measured on February 15, 2011 near the flare X2.2 maximum has been carried out in details in the present work. The time interval between measurements is 90 s. The half-width of X-ray flare pulse does not exceed ~ 20 min. Fig. 3 shows the distribution of the magnetic fields at two time moments around the flare maximum (1:44:15 and 1:45:45) and the difference of these distributions. It is clearly seen the preservation o f magnetic field distribution at the flare maximum. The north and south magnetic fluxes of AR remain constant to within 1%. During this flare the AR position is S21 W21. The angle between the line-of-sight at the measurement and the normal to the Sun surface is about 30°. Highly constancy of the distribution of the line-of- sight component shows that at the maximum energy release of the X2.2 flare on February 15, 2011 not only the normal magnetic field component is preserved, but the tangential component remains also constant. This means that the energy of the magnetic field, which quickly released during the flare, has been transported to the corona in the pre-flare state and accumulated in the current system (current sheet) above the active region. 123