Paper Title

A large number of solar protons are accelerated into high energies by solar flares. They are observed as Ground Level Enhancements (GLEs). In previous cosmic ray conferences the percentage increase of cosmic rays induced by solar flares was reported. This was sufficient to describe briefly the effect of solar flares at Earth. However, a problem is encountered if we wish to describe the energy spectra of protons at the Earth. Particle detectors are commonly located at different places on the world, and at different altitudes. The attenuation of neutrons and protons in the atmosphere is therefore different from one detector to another. Corrections need to be made to account for these differences in order to deter mine energy spectra. Monte Carlo calculations performed by Shibata provided atmospheric attenuation curves for neutrons with energies from 50 MeV to 1 GeV. In this paper, we employ the GEANT 4 and CORSIKA Monte Carlo simulation codes to obtain correction data for the analysis of Solar Energetic Particles (SEPs) with energies in the range 100MeV to 1000GeV. Purpose of Monte Carlo simulation Large numbers of particles are accelerated to high energies in solar flares, sometimes beyond 10 GeV. These particles reach Earth and produce Ground Level Enhancements (GLEs). They used to be monitored by the GOES satellite. When we try to deduce the energy spectra of SEPs at the top of the atmosphere, it is necessary to combine data obtained by the GOES satellite with data obtained by neutron monitors at ground levels. A conversion factor of the neutron monitor data is then needed to deduce fluxes at the top of the atmosphere. Neutron monitors are sited at various altitudes observations are often made at different altitudes. Atmospheric attenuation factors of SEPs at various altitudes are therefore required. A good code of Monte Carlo calculations was not made available until recently. This is because the interaction processes of very low energy neutrons in nuclear cascades are complicated. However, the new GEANT 4 code (version 4.6.2.p02) together with the interaction model, QGSP_BERT, is now available with improved simulations of neutron cascade processes at low energies down to a few MeV. It is now possible to deduce initial proton fluxes at the top of the atmosphere with use of these codes. In this paper, we present new results on atmospheric attenuation and compare these with results calculated by CORSIKA. The new results should be useful in future cosmic ray research, especially for understanding SEPs. A valuable catalogue of SEPs was published by Moscow University Press in 1998 [1]. In addition, they were summarized in two books on cosmic rays [2, 3]. IC R C 2007 P rocedings P re-C onrence E dtion PARTICLE COMPOSITION IN THE ATMOSPHERE FOR THE SEP Conditions of calculations and Results We calculated integral spectra of electrons, photons, muons, protons and neutrons for primary incident protons and neutrons with energies 10, 50, 100, 500, and 1000 GeV respectively. The power indices of proton and neutron primaries were both assumed to be -2.5 above 1 GeV. The calculations were made using CORSIKA and GEANT 4 independently, and examined for consistency. Typical results for electrons are shown in Fig. 1, for photons in Fig. 2, for negative muons in Fig. 3, for neutrons in Fig. 4, and for protons in Fig. 5, at an atmospheric depth of 600g/cm2. Fig. 1 The integral electron energy spectrum: top panel by CORSIKA (in unit of GeV) ; bottom panel by GEANT 4. Fig. 2 The gamma-ray energy spectrum: top panel by CORSIKA and bottom panel by