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.

1. M. Podgomy and A. I. Podgomy therefore cannot accumulate energy for a flare. Another important feature o f AR is the constancy o f the field magnetic distribution during the most of flares, which eliminates the possibility o f all the mechanisms o f the primary flare energy release on the chromosphere. The flare occurs in the corona above AR. Flares should not cause disturbances of the magnetic field frozen into the dense matter of the photosphere. It should be emphasized that a current sheet is the only known object in the space that is able to accumulate magnetic energy and then to release it explosively. The current sheet in the geomagnetic tail is causing geomagnetic storms. The role of the current sheet in the Earth's magnetosphere became known only after the flight o f spacecrafts in the geomagnetic tail. Up to now we have no possibility of magnetic field measurements in the solar corona. The only possibility to get information about magnetic field distribution above AR is MHD numerical simulation using photosphere magnetic field as boundary conditions. Active region 10656 The work [19] has concludes that a big flare occurs after increasing the magnetic flux o f AR up to 10 Mx. In [14 - 18] it is shown that magnetic flux of AR greater than 1022 Mx is the necessary condition for X-class flares appearance. The typical rise time of the magnetic flux is a few days. This condition is necessary but not sufficient. Flares occur only above AR of with the complex field distribution. Bipolar AR with one leading and one following spots, separated by field inversion line of simple form (P-type), does not produce a flare. The typical bipolar region (P-type), and the typical Py5 region with a line inversion of complex shape (y) and embedded sources of one magnetic polarity in the magnetic field of another polarity (6), which caused a flare o f X-class, are shown in Fig. 1. Feb-201303:58:12 HMI Magnetogram frfff- И н т И В ■- ......... ■ MDI Magnetogram 18 - May - 2003 12:47:59 Fig. l NOAA 1 1673 magnetogram with bipolar field distribution (P), which produces no flares, and NOAA 10365 magnetogram with a complex field distribution (Py5), causing the X3.6 flare. Fig. 2 shows the X-ray emission according to GOES and NOAA 10656 region magnetogramms from 7 to 17 August 2004. NOAA 10656 is formed on the back side of the Sun. It appeared on the eastern limb on the August 7, 2004 with the north and south magnetic fluxes greater than 1022Mx. NOAA 10656 demonstrates the absence of big flares (class X) over several days passage across the solar disk with the magnetic flux o f up to 4 Ч 1022 Mx. Until 11.08.2004 the magnetic field distribution o f NOAA 10656 shows two compact groups o f field sources o f north and south polarity. The magnetic sources of the same polarity are clearly separated by a single inversion line from sources other polarity - the typical type P distribution is demonstrated. Such a bipolar distribution o f field sources in the AR forms magnetic loops in the corona. A singular magnetic field line cannot be formed above AR. North and south magnetic fluxes smoothly increase during AR moving across the disk. But photos o f magnetogramms shown in Fig. 2 are the results of SOHO MDI measurements of the magnetic field component directed along the line-of- sight, and therefore provide only qualitative estimation of the magnetic flux dynamics. The magnetic field measured by SOHO MDI depends not only on its true value, but also on the angle between the line-of-sight and the Sun surface. To eliminate this effect the magnetic flux is calculated using the field component normal to the solar surface [14]. The normal component is determined by solution of the Laplace equation with boundary conditions in the form of the oblique magnetic potential derivative taken from SOHO MDI measurement [7]. This approximation is valid, if the currents that are responsible for the energy storage for the flare are located high in the corona, as it should be in the case of the flare model based on current sheet creation above AR. Such approach eliminates the dependence of the measured magnetic components on the position of the AR on the Sun. The magnetic flux - time dependence in Fig. 2 is obtained by calculating the normal component o f the magnetic field [4]. The magnetic flux of NOAA 10656 increases during a week from 1022 Mx to ~ 4 x l0 22 Mx. Despite the great importance for northern and southern magnetic fluxes for flare appearance, AR NOAA 10656 does not produce any big flares until August 13. All this time AR NOAA 10656 magnetic configuration does not reveal high magnetic field complexity. Later, the field configuration o f AR becomes a more complex. By this time the configuration of the field has changed: the sources of the one polarity are embedded in the field o f other polarity, and the field inversion line has a rather complicated form. Field distribution becomes Руб type. Magnetic flux in AR reaches (3- 4 )x l022 Mx, and the field distribution complexity is also gradually increased. Correlation of flare activity with the increasing complexity of the field distribution in the area is clearly seen. Along with the magnetic flux increasing and the complexity of the field distribution in NOAA 10656 the flare activity also increases. XI flare appears on 122