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 The data presented in Fig. 3 and Fig. 4 completely rule out the possibility of flare chromospheric mechanism and indicate dissipation of the magnetic field o f the coronal current system, which appeared in the pre-flare state. Current sheet in corona The simplest possibility of current sheet creation above AR appears during emerging of two magnetic fluxes in the highly conductive plasma of the corona. The diagrams of two emerging magnetic fluxes that appeared in the same plane are shown in Fig. 5. If increase of the magnetic fluxes occurs in the vacuum, the magnetic vector addition takes place, and a zero X- type magnetic field line perpendicular to the figure plane is formed. In highly conducting plasma the magnetic field lines are frozen into the plasma, and therefore the magnetic fluxes with opposite directed magnetic lines cannot merge immediately. The plasma motion with frozen-in magnetic lines builds a current sheet that separates two different magnetic fluxes. In the initial stage of magnetic fluxes floating-up the currents are generated in the moving boundary between magnetic flux and the unperturbed plasma. These currents are shown by circles. Magnetic fluxes expanding take place. After some time the boundaries of two independent fluxes meet each other. The independent magnetic fluxes are separated by a current sheet. During a local current sheet formation, the distortion of the magnetic field takes place only in a local area of the corona. Reverse current is also localized in the corona. So, the current is completely closed in the corona. No new magnetic lines appear in the active region due current sheet creation. The new magnetic field lines are closed inside the corona. The plasma pressure in the current sheet is balanced by the magnetic pressure on both of its sides. The decay of such a current sheet is accompanied by dissipation of energy stored in the current sheet magnetic field. The solar flare appears. The current sheet decay should not cause disturbance of the magnetic field on the photosphere. Conclusion 1. The necessary condition for appearing of a big flare is an increase of AR magnetic flux up to 1022Mx. 2. Big flares occur in the corona over active regions with complex (|3у5) magnetic field distribution. 3. The bipolar region cannot produce a flare. The magnetic field in the corona above the bipolar AR does not contain a singular line in the vicinity of which a current sheet can be formed. 4. No change of the magnetic flux and magnetic field distribution in AR are generated due to solar flare appearance. A c kn ow led gm en ts. This work was supported by RFBR grant № 13-02-00064 and by 22 Program of RAS Prezidium. References 1. Lin R. P ., K ru ck er S., H urford, G. J. et al. A strophys. J. 595, L 69, 2003. 2. M asuda S., K osugi Т., Нага H ., T suneta S., O gaw ara Y. N ature. 371. № 6497, 495, 1994. 3. P o d g orn y A. I. and P odgom y I. M . G eom agn. and A eronom y. 52, 150, 2012. 4. P o d g om y A. I. and P odgom y I. M . G eom agn. and A eronom y. 52, 162, 2012. 5. P o d g om y A. I. Solar Phys. 123, 285, 1989. 6. http://soi.stanford.edu/m agnetic/index5.htm l 7. h ttp://jsoc.stanford.edu/ajax/lookdata.htm l 8. F alcon er D. A ., M oor R. L., and G ary G. A. A strophys. J. 644, 1258, 2006. 9. L eka K. D. and B am es G. A strophys. J. 656, 1173, 2007. 10. P etrie G. J. D ., Sudol J. J. A strophys. J. 724, 1218, 2010. 11. W ang J., Z hao М ., Z hou G. M agnetic changes in the course o f th e X7.1 solar flare on 2005 January 20. A strophys. J. V. 690, 862, 2009. 12. Jiang Y ., Z h eng R., Y ang J. et al. A strophys. J., 744. P. 500-505, 2012. 13. K usano K. A dv. Space Res. 32, 1917, 2003. 14. P o d g om y A. I. and P o d g om y I. M . A stronom y R eports. 55, 629, 2011. 15. P o d g om y I. М ., and P od g om y A. I. Phys. A uroral Phen. Proc. 33 A nnual Sem inar. A patity, 2010. P. 87. 16. P o d g om y I. М ., and P od g om y A. I. Phys. A uroral Phen. Proc. 35 A nnual Sem inar. A patity, 2012. P. 88. 17. P o dgom y 1. M . and P o d g om y A. I. Journal o f A tm ospheric and Solar-T errestrial Physics. Journal o f A tm ospheric and Solar- T errestrial P hysics 92, 59, 2013. 18. P o dgom y I. M . and P o dg om y A . I. A stronom icheskii T sirkular. № 1586. January 24, 2013. 19. Ishkov V. N. A stronom ical and A strophysical Trans. 20, 563, 2001. 20. P o dgom y I. М ., P o d g om y A. 1. P ulkovo. 2010. P. 315. 125