COLLISIONAL DEGRADATION OF THE PROTON-H ATOM FLUXES IN THE ATMOSPHERE- A COMPARISON OF THEORETICAL TECHNIQUES

Three methods for calculating the transport of energetic protons and h ydrogen atoms within the Earth's atmosphere are compared. The methods are (1) a Monte Carlo (MC) simulation, (2) a discrete energy loss solu tion to the linear transport equations, and (3) a continuous slowing-d own approximation (CSDA). Ln the calculations performed, all three mod els use the same cross sections, three-component (N-2, O-2, O) neutral atmosphere, and incident isotropic Maxwellian proton fluxes of variou s characteristic energies (1-20 keV). To ensure that all three methods include the same physical processess, the effects of magnetic mirrori ng and the lateral spreading of particles are ''turned off'' in the MC simulations as these processes are not included in the present linear transport or CSDA models. A variety of quantities are calculated and compared including energy deposition rates, eV/ion pair, hemispherical ly averaged differential fluxes of protons and H atoms, energy integra ted differential fluxes, and total proton and H atom fluxes. The agree ment between all three models is excellent except at the lowest altitu des. Apart from these altitudes, the differences that do exist are sma ll compared to the. errors that generally result from poorly known inp uts and compared to the typical errors quoted for geophysical observat ions. The altitudes where the results do differ significantly are wher e the proton and H atom fluxes are severely attenuated and are below t he altitudes where the bulk of the energy deposition and ionization ta kes place. The success of these comparisons suggests that our ability to model actual observations is presently limited by uncertainties in cross sections and the lack of suitable observations rather than our a bility to solve the equations that describe the known physics of proto n-H atom transport.