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.

COMMON FEATURES OF GLEs IN 19-24 SOLAR CYCLES Yu.V . Balabin, E.Y. Vashenyuk, B.B. Gvozdevsky, A .V . G erm anenko (P o la r G eophysica l institute, Apatity, Russia) Introduction During strong solar flares the energetic particles are emitted into interplanetary space. Sometimes these particles reach the Earth and cause the events of ground level enhancement (GLE) o f cosmic rays recorded by neutron monitors and sometimes even by muon telescopes. The worldwide NM network during its existence registered totally 71 ground level enhancements (GLEs). The first one was in 1942 and the last one was on 17 May 2012. Based on the data of the worldwide neutron monitor network it is possible to restore the primary spectrum of RSP by solving an inverse problem. It was done by many groups since the 70-ies o f the XX century [1]. This paper presents our results of unified methodology processing o f most o f all GLEs. We have selected 51 events: from GLE05 (23.02.1956, 19th solar cycle) to GLE71 (17.05.2012, 24th solar cycle) suitable for modeling. The other events was either too weak or there was data from too few stations. Selected GLEs have a large range of increases from -3% up to -5000%. This amplitude threshold is necessary because the standard NM has accuracy of about 1.5% (as for five-minute data). For the selection and processing o f these GLEs we used a unified method. Technique of solving the inverse problem The basis of the technique is a searching the RSP spectrum parameters which give the minimum difference between the calculated and measured response of NM. To calculate a NM response one have firstly to determine the asymptotic cone (AC) of acceptance for given station. AC refers to which direction the radiation arrives at the NM. In our work, AC is calculated for particles of 1 to 20 GV with a step 0.001 GV for all stations regardless o f their cut­ off rigidity. When calculating the AC for each station generates additional array o f rigidities R, which indicating is this forbidden or allowed rigidity. This eliminates the problem of calculating the penumbra. When data about the state of the interplanetary medium are available, the magnetosphere model T01 is used. In other rare cases we used the T89 model. The calculation of AC is performed by the test particle technique. From the edge o f the atmosphere (conditionally accepted height of 20 km) at the location of the station straight up a particle with rigidity R, mass of a proton and a negative charge (antiproton) is launched. Its path to escape from the Earth’s magnetosphere is traced. Particle velocity vector outside the magnetosphere indicates the direction from which the proton with rigidity R must entry the magnetosphere to reach the NM station. If antiproton whith rigidity R can not get out o f the magnetosphere, then the proton with rigidity R from outside the magnetosphere can not reach the station. Such rigidity is called forbidden for this station. Otherwise it is called allowed. NM response to an isotropic flux of RSP is defined as: 20 A N = ^ I ( R ) - S ( R ) - A ( R ) - A R (1) «=i where DN is a relative increase of the NM count rate; I(R) is a rigidity spectrum; S(R) is a Specific Yield Function (SYF) [2]; A(R)=1 for allowed rigidities and 0 for forbidden ones; DR is the rigidity step. SYF is a tabular value. Array A(R) is generated for each station in the calculation of the asymptotic cone. The general form o f the specrum is I ( R ) = J a - R ' - w -'> (2) where J0 is an intensity of the flux at R=1 GV, g is a spectral index, Dg is a correction factor. Dg option provides variable slope of the spectral function. The three parameters of the spectral function (JO; g;Dg) are the values that uniquely determine it. They are energy parameters of the flux. Note especially that the spectral function takes no any particular form (power or exponential). However, for certain values of g and Dg it is possible to obtain a power law or an exponential form. The real RSP flux is not isotropic. To take into account the anisotropy we take a basic pitch-angle distribution in the form o f Gaussian: G( 6) = e x p ( -# 2 / c) (3) where с is a parameter characterizing the angle width of the flux, 0 is the pitch angle o f the 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 cp, latitude and longitude. So there are three parameters (c, /, and <p) describing RSP flux. They are spatial characteristics of RSP. As a result the expression (1) takes the form "Physics o f Auroral Phenomena”, Proc. XXXVI Annual Seminar, Apatity, pp. 106 - 109, 2 0 1 3 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute 106