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.

MAGNETIC FIELD DYNAMICS OF LARGE ACTIVE R IN THE PRE-FLARE STATE DURING SOLAR FLARES I.M. P odgom y1, A.I. Podgomy2 1Institute f o r Astronomy RAS, Moscow, Russia, podgorny@ inasan.ru 2Lebedev Physical Institute RAS, Moscow , Russia Abstract. Publications of the active region (AR) behavior before flares and during flares are controversial. In our report in Apatity Seminar 2012 the preliminary data are demonstrated the growth of the magnetic flux of the AR before flares. The X class flares appear, when the magnetic flux of AR becomes bigger than 1022 Mx. Apparently the new magnetic flux flowing up from the Sun surface is responsible for energy accumulation in the corona. Another important condition for strong flare appearances is the high magnetic distribution complexity - py5. The simple p - type AR does not produce a flare. During a solar flare, when the accumulated energy is fast released, there are no any specific magnetic flux changes in AR. This surprising fact follows from the analysis of the array data obtained in recent years for big flares with the SOHO and SDO spacecrafts. It is shown the conservation of the magnetic field distribution in AR during the majority of flares. Small magnetic field changes in the distribution of the field sometimes appear that are typical for the time interval at the flare absence. These results are consistent with the flare theory based on the slow accumulation of the magnetic energy in a current sheet and its explosive realize due to current sheet instability. Introduction Long-term observations show that solar flares occur at the AR area, but only the X-ray measurements on spacecrafts Yohkoh and RHESSI [1,2] have revealed that the primary flare energy release takes place not on the surface of the Sun, but in the corona above AR at the altitude about 20 000 km. The possibility o f energy accumulation for the flare in the coronal current sheet has been previously shown in numerical 3D MHD experiments [3, 4]. It has been found that the current sheet is formed in the corona before the flare, and magnetic energy that accumulated in such current sheet is sufficient to produce a flare and coronal mass ejection. This energy is released due to current sheet transition in an unstable state and sheet decay [5]. At the numerical solution of the full system of three-dimensional MHD equations all dissipative terms are taken into account. The initial and boundary conditions on the photosphere are set using the magnetic maps obtained by SDO and SOHO space crafts measurements [6, 7]. No assumptions about the mechanism o f the flare are introduced. The current sheet creation occurs because of magnetic field frozen in the plasma o f the solar corona. Disturbances propagating from photosphere in the highly conducting plasma create currents in the corona. These currents are concentrated in the vicinity of a singular magnetic line above AR. Dissipation of currents in the corona during the flare should not change the magnetic flux of the active region. Therefore, the flare may not have an impact on the o f the magnetic evolution field of the active region. Due to short duration of the flare (~ 10 min) compared to the duration of magnetic field evolution before the flare (about 5 days), the probability of disturbance appearing during the flare is low. So, the field perturbation of AR during the flare can be only random, as in the case of the field evolution before and after a flare. Magnetic field distribution in AR should not be changed during the appearance of most o f the observed flares. In this paper we present new data showing that big (class X) flares occur, when the magnetic fluxes exceed 1022 Max, and the magnetic field distribution is very complex. At high magnetic field complexity the conditions for singular lines appear and a possibility arises for current sheet creation in the singular line vicinity [2, 3]. Attempts to detect disturbances of the magnetic field responsible for a flare are carried out many times, but the strong photospheric disturbances during the flare, which could explain the energy released during an flare with the energy of ~ 1032 erg, are not detected [8 - 11]. A number of studies examined the possibility of the energy input in the corona during the flare by helicity injection [12]. However, the correlation between the occurrence of helicity injection and flare occurrence is not detected [13]. Analysis o f the magnetic field dynamics in AR before the X-class flares have shown an increase of the AR magnetic flux for several days before the flare with the energy above 1022 Mx [14 - 18]. In this paper we present new data showing that big (X-class) flares occur when magnetic flux of AR becomes greater than 1022Mx and this AR has a complex field distribution (type Py5). p means existence of the magnetic inversion line in AR, у means a complex form a field inversion line (or lines), and 5 means that magnetic sources o f one polarity are intruded in the field of another polarity. The numerical MHD simulation demonstrate that a current sheet can be formed only at the complex field distribution when a singular magnetic line exists in the corona above AR [1, 2] The simple AR, consisting of two sunspots (leading and following), cannot form a singular line in the corona, and therefore and “Physics o f Auroral Phenomena ”, Proc. XXXVI Annual Seminar, Apatity, pp. 121 -125, 2013 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute EGIONS 121