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 o fthe transport o fsolarprotons through the atmosphere in the 13 December 2006 GLE Universal Time, hour Fig. 1. Increase profiles during GLE 13.12.2006 at a number of neutron monitor stations: Ou-Oulu, Ap- Apatity, Mo-Moscow, Bar- Barentsburg, F.S- Fort Smith. The marked point on a profile of the Apatity station shows the calculated NM response using the results of simulations of solar particle transport through the atmosphere. During the SPE 13.12.2006 balloon measurements of solar protons were carried out too. Determination of solar proton spectra on the measured by a balloon an absorption curve in the air. was carried out using a standard method [6]. The long-term balloon observations of ionizing particles in the atmosphere from the ground level up to 30 - 35 km are carried out by Lebedev Physical Institute (LPI) using light radio sounds since 1957 [7]. If the high energy proton fluxes are large enough to essentially enhance the nucleonic component of the atmospheric cascade at the Earth’s surface the effect is recorded by the ground based neutron monitors. Recently we developed a new approach to the determination of solar proton spectra at the top of Earth’s atmosphere based on Monte Carlo simulations of solar proton transport through the Earth’s atmosphere [6]. These simulations allow us to estimate angular and energy distributions of secondaries (protons, electrons, positrons, muons, photons and neutrons) produced by the primary solar proton flux in the atmosphere. We present here results of simulations obtained for the SPE of 13 December, 2006. 3 Simulation results By using the Monte Carlo PLANETOCOSMICS code based on GEANT4 [4, 5] we have computed interaction of different solar proton populations with the Earth’s atmosphere. The code takes into account the following processes: bremsstrahlung, ionization, multiple scattering, pair production, Compton scattering, photoelectric effect, elastic and inelastic nuclear interaction, and the decay of particles. The solar proton populations are considered as isotropic at the top of the atmosphere. The proton energy spectrum in the energy range of Emin -Emax was defined by a power law (J(>E) = A£Ej°) using a set of °, Emin and Emax. For each proton population (characterised by °, Emin and Emax), we calculated the total flux of secondary particles (e, photons, protons, and neutrons) at different atmospheric depth. We compared the calculated depth dependence of the total secondary particle flux with the data recorded in balloon experiment in the Earth’s atmosphere during several SEP events [8]. In present analysis we used the solar proton energy spectrum (fig. 2) determined on 3.05 UT on December 13, 2006 as an input in the Monte Carlo simulations. The results of simulations are presented in Figures 3-6. Energy spectra of protons and neutrons at selected levels of the atmosphere are shown in Figure 3 and 4. Resulting absorption profiles of secondary particle flux in the atmosphere (protons, electrons, positrons and muons) are presented in Figure 5. We note that the recorded absorption profile of particles in the atmosphere (circles) and the calculated one as total flux J(x) = Jprotons + Jmuons + J(ej+e+) + 0.01*Jphotons are in good agreement. This result is a validity check of the solar proton spectrum (Fig. 2) derived from the set of the satellite, balloon and ground-based NM measurements. e io' Energy, GeV Fig. 2. The derived energetic spectra of RSP for 2 moments of time. The direct solar proton data are shown by filled squares (GOES-11). Fig. 3. The energy spectra of protons at various altitudes, obtained as a result of simulation of transport of primary solar protons through the atmosphere layers. Input data have been smoothed by means of approximating. I l l