Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

SIMULATION OF THE TRANSPORT OF SO L A R PROTONS THROUGH THE ATMO SPHERE IN THE 1 3 D EC EM B ER 2006 G L E E.A. Maurchev1, Yu.V. Balabin1, E.V. Vashenyuk1, B.B. Gvozdevsky1, G.A. Bazilevskaya2, V.S. Makhmutov2 1Polar Geophysical Institute, Russian Academy o f Sci., Apatity, Russia 2Lebedev Physical Institute, Russian Academy o fSci., Moscow, Russia Abstract. Using the PLANETOCOSMICS simulation framework we simulated solar proton transport through the Earth’s atmosphere and estimated angular and energy distributions of secondaries (protons, electrons, positrons, muons, photons and neutrons) at various atmospheric levels. As the source spectrum of solar protons at the boundary of atmosphere the spectrum obtained with the GLE modeling from the data of neutron monitor network in the event of 13 December, 2006 has been used. These Monte Carlo simulation results were compared with the solar cosmic ray neutron monitor and balloon measurements during 13 December, 2006 SPE. The calculated solar proton spectra are in good agreement with the balloon and neutron monitor observational data. ‘‘Physics o fAurora! Phenomena ", Proc. XXXIVAnnual Seminar, Apatity, pp. 110 -113 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute 1 Introduction The ground level enhancement (GLE) 13 December, 2006 was related to the parent flare X3.4/2B, heliocoordinates S06 W24. The flare was accompanied by halo type CME and radio emission of II and IV types. The type II onset was reported at 02.26 UT. By feature of the event was that it has occurred in conditions on the Sun and in interplanetary medium appropriate to a minimum of a solar cycle. At the same time the event of 13.12.2006 concerns to be large ones. It was registered by more than 30 neutron monitors of a worldwide network. The modeling study of this GLE was carried out in [1, 2]. With usage of a modeling procedure [3] characteristics of a flux of primary solar protons outside the magnetosphere on the data of neutron monitors have been obtained and their dynamics during the event studied. The characteristics of primary solar protons obtained in the GLE model study have been used then as input parameters on the atmosphere boundary at simulation of a transport through the atmosphere of these particles. Our recent modeling technique takes into account the contribution into the neutron monitor response not only vertical, but also oblique incident particles. This kind of analysis requires the data of no less than 25-30 ground- based cosmic ray stations, and consists of a few steps: 1. Definition of asymptotic viewing cones (taking into account not only vertical but also oblique incident on detector particles) of the NM stations under study by the particle trajectory computations in a model magnetosphere. 2. Calculation of the NM responses at variable primary solar proton flux parameters. 3. Application of a least square procedure for determining primary solar proton parameters (namely, energy spectrum, anisotropy axis direction, and pitch- angle distribution) outside the magnetosphere by comparison of computed ground based detector responses with observations. For calculation of cascades of particles in the atmosphere in this paper the PLANETOCOSMICS 2.0 package [4] based on GEANT4 [5] has been used. For the initial geometry the "Plane" geometry has been taken (Flat Geometry), with gradient separation of layers, each o f which contained 5 % from a whole column of air. For the representation o f physical properties of the atmosphere the NRLMSISE-00 model is used. On boundary of the atmosphere (80 km) the source of primary protons with the given energy distribution (0,5 - 10 GeV, Fig.2) has been used. Lesser energies have not been used owing to restricted resources of a computer. For an evaluation of secondary particle fluxes by means of PLANETOCOSMICS have been given detecting layers at definite altitudes. Resulting fluxes for a model absorption profiles are obtained, as integral. 2 Experimental data Figure 1 shows time profiles o f NM count rate increase at several stations on 13 December, 2005. The marked point on a profile of the Apatity station shows results of calculation of the ground based neutron monitor response to the neutron flux at the ground level. It is obtained as a result o f simulation of transport of solar particles through the atmosphere. This event gave an opportunity to estimate energy spectrum of solar protons in the wide energy range (from 10 MeV to 10 GeV) using satellite, balloon measurements and ground-based neutron monitors observations [1, 2]. A pair of modeled relativistic solar proton spectra are presented in Figure 2. They agree well with the direct solar proton data obtained at the GOES-11 spacecraft. 110