Physics of auroral phenomena : proceedings of the 38th annual seminar, Apatity, 2-6 march, 2015 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Издательство Кольского научного центра РАН, 2015. - 189 с. : ил., табл.

about the mechanism of flare is introduced. The numerical MHD calculations shows formation of the current sheet in the vicinity of the X-type magnetic field line. This means that the configuration of the field in the AR for the flare appearance must be quite complicated. Magnetic energy acumulation in the coronal current sheetfor a solarflare Figure 1. Number o f sunspots according to the Royal Observatory o f Belgium I960 1970 1980 1990 2000 2010 years: Conditions of flare appearance It was shown in [6-9] that the main condition for the large flare generation is the AR magnetic flux exceeding 10 22 Mx. Neither the imbalance of the northern and southern fluxes, nor rate of the magnetic flux change are signs of the flare appearance. In [10] it is noted that a big flare occurs over the photospheric field configuration resembling the fractal development. It is shown in [6-9] that a flare occurs over AR with a complicated distribution o f the magnetic field, when a singular magnetic field line of type X appears in the corona. In the vicinity o f this singular line magnetic perturbations are concentrated arriving in the pre-flare state from the photosphere, and a current sheet appears. The simple bipolar AR, which lines in the corona have the form of arches, does not produce flares. This fundamental fact is confirmed here by the new data. One of the main results of the numerical MHD simulation is demonstration of the current sheet formation before a flare and the accumulation of energy in the magnetic field of this current sheet. The energy amount is sufficient for the observed flare and coronal mass ejection. This energy is released due to the current sheet transition in an unstable state and its decay. In the numerical solution o f the complete system of 3D MHD equations with dissipative terms the initial and boundary conditions on the photosphere are set from the magnetic field measurements by SOHO or SDO spacecraft before a real flare [4, 5, 11]. No assumptions about the mechanism of a solar flare are used in these numerical MHD calculations. The results of calculations performed at the real situation demonstrate the current sheet creation and the energy accumulation in the magnetic field for a flare. Formation o f the current sheet takes place due to freezing o f the magnetic field in the plasma when the MHD perturbations propagating from the photosphere in the conducting plasma create currents in the corona. These currents are concentrated in the singular magnetic field line vicinity o f the X- type magnetic line, accumulating the free magnetic energy in the corona. The rapid dissipation of this magnetic energy due to the transition of the current sheet in an unstable state takes place. Current sheet decay should produce the complex phenomena observed at the flare, including a coronal mass ejection and generation o f relativistic protons [ 1 2 ]. Current dissipation in the corona during a flare should not disturb the magnetic flux in the AR, since the flare occurs high in the corona over of AR, and cannot cause a strong perturbation o f the magnetic field frozen into the dense matter of the photosphere. Therefore, the flare should not produce of any significant impact on the evolution of the AR magnetic field. Given the short duration of the flare (10 - 30 min) compared with the duration of the evolution of the magnetic field prior to the flare (3 - 5 days), the probability o f random disturbances in AR during a flare is small. Perturbations AR during a flare can only wear casual character, as it is sometimes the case in the evolution o f AR before and after the flare. Attempts to detect magnetic field AR perturbations that correlated with the flare occurrence, carried out repeatedly, but without any success. A number o f studies examined the possibility o f energy supply from AR during a flare due to the magnetic helicity injection from the photosphere, i.e. to observe the possible current increasing along the magnetic field lines of AR during a flare. However, the correlation between the appearance o f the helicity injection and flare occurrence was not detected. Any significant changes in the magnetic field AR was not registered during the flare, which could be attributed to the fast energy influx from the photosphere. The lack of flare correlation with appearance of strong photospheric disturbances excludes the possibility of mechanisms for development of photospheric flares. A detailed study of this phenomenon is presented in this paper. Analysis of the dynamics of a ARs before class X flares made in [6-9] showed that large flares occur over ARs with the magnetic flux greater than Ф= 10 22 Mx. Typical behavior of the AR magnetic flux that appeared on the visible disk and gave a series of large flares is shown in Fig. 2. Vertical arrows indicate the time occurrence o f large (Class X) flares. However, the condition Ф> 10 22 Mx is the necessary but not a sufficient. Flares occur only over the 80