JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. A4, PAGES 6651-6665, APRIL 1, 1994 Propagation of solar neutrons through the atmosphere of the Earth S. Shibata Department of Physics, Nagoya University, Nagoya, Japan Abstract. The propa•ga•tion of sola•r neutrons throughthe a•tmosphere of the E•rth h•s been cMcul•ted by • Monte C•rlo method using• nuclear interaction modelwhichc•n reproduce the existing da•t•obtained by •ccelera•tor experiments. With this calcula•tion, it h•s been found tha•t the ela•stic scattering process plays •n importamt role in the propagation of solarneutrons in the •tmosphere •t •n incident energy below,,•200MeV. By a•pplying this c•lcula•tion to the amalysis of a• solar neutron event observed•t Mount Norikur• Cosmic Racy Observa•tory on June 4, 1991,it is possible to interpret the event a•s 7 ra•ys amd neutrons halving been produced impulsively a•tthe time of the sola•r fl•re. The eventobserved Jungfra•ujoch on June 3, 1982 c•n be expladned simila•rly. 1. Introduction The observation of solar neutrons at Mount Norikura Cosmic Ray Observatory ( 2770 m above sea level) has been carried out since October 1990 with a new solar neutron telescope proposed by Muraki et al. [1991a; Shibata et al., 1991; Miyazaki, 1992]. Solar neutrons were observed,in association with a large solar flare in the region of NOAA 6659 [Sakurai el al., 1992], on June 4 and6 of 1991 [Murakiel al., 1991b, 1992]. This was 9 yearsafter the first reliable detectionof solarneu- trons made on June 3, 1982 at Jungfraujoch [Debrunner el al., 1983; Chupp el al., 1983, 1987]and other high mountains [Efimov el al., 1983] using neutron moni- tors. Other groups also reported observations of solar neutrons recently [Kocharov, 1991;Pyle and Simpson, 1991; Shea el al., 1991]. A solar flare is an impulsive astronomical phe- nomenon which occurs at the solar surface. In a so- lar flare, particles are accelerated to high energy and emitted. This is in addition to the emission of electro- magneticwaves over a wide frequency range including X rays and ? rays. Particles are accelerated to tens of GeV energy. An event which occurredon September 29, 1989, called ground level enhancement (GLE), was a clear exampleof acceleration of particlesto suchhigh energies [Shea, 1990;Lee, 1991]. A GLE occurs when protonswhich have been accelerated in a solar flare fly directly to the Earth. High-energy protonsejectedin a flare are expected to produce solarneutronsthrough collisions with the solar atmosphere [Lingenfeller et al., 1965a, b]. Among the Copyright 1994 by the American GeophysicalUnion. Paper number 93JA03175. 0148- 0227/94/93JA-03175 $05.00 variousparticlesejectedfrom a solarflare, neutronsare the most suitablefor probing the mechanism of parti- cle acceleration. Since neutrons haveno electriccharge, they travel directly to the Earth without beingdeflected by the intense magnetic field at the flare site or by the coronal or interplanetary magnetic fields. We can ob- tain information on the time of neutron production at a solar flare from the arrival times of solar neutrons at the Earth after correctingfor times of flight between the Sun and the Earth. The latter are determined by their energies. The relationships betweenthe produc- tion time of neutrons and the observation times of radio, Ha, X ray, and ? ray radiation determine the time pro- file of particle acceleration in a solarflare. Also, the en- ergy spectrum of solarneutrons provides key knowledge for understanding the particle acceleration mechanism in a solar flare. After entering the atmosphere of the Earth, solarneu- trons are scattered or absorbedby air nuclei. One must fully understand these effectsin order to be able to de- duceinformation on the intensity of neutronsat the top of the atmosphere from data obtained at observational sites at mountain altitudes. Until recently,two calculations of the propagation of solar neutronsin the atmosphere have been known. One is the calculation of neutron intensities down to an at- mospheric depth of 300 gcm -2 presented by Alsmillev andBoughnet [1968], andthe other is the calculation of Debrunner el al. [1983,1989] of solar neutron"sensi- tivity" usingneutron monitors. Neither of these calcu- lations can be applied to our observation [Muraki el al., 1991b, 1992], because ourobservational site(at a depth of 776 gcm-2 along the direction of the Sun at a zenith angle of 18.9 o ) is much deeper than the former and the detection efficiencies of ourdetectors (neutron telescope andmuon telescope) aredifferent fromthose of neutron monitors of the latter. Therefore a new calculation of 6651