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.

“Physics o f Auroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 103 -105, 2013 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute THE FIRST GLE OF THE NEW 24thSOLAR CYCLE Yu.V . Balabin, E.V. Vashenyuk, B.B. Gvozdevsky, A .V . Germanenko Polar Geophysical institute, Apatity, Russia Introduction The first ground level enhancement (GLE) in the 24th solar cycle was registered by the worldwide network of neutron monitors (NM) at 17.05.2012 after 5-year pause. Besides solar cosmic rays, this event was also registered in soft X-rays as a weak eruption flare 1F/M5.1. The event was associated with a solar flare in region 1476 located at N11W76. The region was beta type and quite small (the square was 230) [1]. GLE started at 01:54 UT (Oulu), the maximum was 18% on Oulu and Apatity stations at 02:08 UT. The South Pole station, which was the third on magnitude, registered around 7% increase, as reduced to the sea level. Total duration of the GLE was about 1.5 hour. On the whole GLE71 is weak and odd. Calculation methods Based on our advanced method (which was used in modeling more than four tens of GLEs) we derived main parameters of the GLE: spectrum of relativistic solar proton (RSP) flux, pitch-angle distribution and direction of anisotropy. The method is based on an inverse problem solution. The data of the worldwide network of NMs (27 stations) [2, 3] were used. To calculate asymptotic cones of the stations we used the Tsyganenko magnetosphere model T01 [4]. Spectral function I(R) was taken in the common form: I (R) = J 0 -Rr~ ^ R-l) (1) where J0 is a flux intensity at 1 GV rigidity, у is a spectral index, Ду is a spectral correction. Ду parameter provides variable slope o f the spectral function. Note especially that the spectral function is not pure power law or exponential. However, for given pair of values (y and Ду) it is possible to obtain a near-power or near-exponential form within the rigidity range 1-20 GV. Basic pitch-angle distribution of the RSP flux is given in the form of Gaussian function normalized to 1 G(6) = exp(-6>2/с ) (2) where с is a parameter characterizing the flux width, 0 is a pitch angle of a particle with rigidity R moving in the interplanetary magnetic field (IMF). To determine 0 it is necessary to know the direction of the anisotropy axis. It can be defined by two angles X and <p, latitude and longitude. So there are additionally three spatial parameters (с, X and <p) describing RSP flux. The final expression is 20 AN ( J 0, у, Ay, с, Л, p) = £ / ( Д) ■ S(R) ■A(R ) • G(0(R, y, q>)) • AR (3) R =1 where AN is a derived increase at a station, S(R) is a specific yield function [5], A(R) is a quantized function of asymptotic cones, which is ” 1” for allowed trajectory and ”0” for forbidden one, AR is a rigidity step. Discrepancy function is <P(J0, y , A y , c , A,<p) = '^ j [AN l ■(J0, y ,Ay , c ,A,<p) -AMLf (4) i and the least square method is used to find the minimum o f Ф. More information about our method one can find in the papers, for example [6, 7]. ^ Results and discussion GLE71 was processed by the described method from 02:05 UT till 03:10 UT with 5 minute step. All 6 parameters of RSP were obtained each time. Furthermore, responses in same time points for each station of NM were calculated (see Figures 1 and 2). It should be noted that GLE71 is really odd. In this study it was found that in a given form of the pitch-angle distribution (expression 2) a suitable solution can’t be found. The analysis of NM increase profiles and directions o f asymptotic cones (Figure 3) has shown that at the pitch angles close to 90° an intensity gap is present. This gap can’t be described by expression (2). Therefore a function has been created to add a gap into Gaussian distribution at -90°. Due to it two parameters were added. After such fitting solutions with a satisfactory accuracy (-10-15%) were obtained. Physically, this gap is easy to explain. GLE71 occurred on extremely weak and quiet IMF. Under such conditions RSP propagation was along the magnetic field lines almost without scattering. It 103