Upper atmospheric geoeffectiveness of energetic proton precipitation with beam spreading.

Abstract

A 3-D Monte Carlo proton transport model has been developed to investigate the dynamic effects of high-energy magnetospheric ion precipitation on the Earth's Mesosphere and Lower Thermosphere/Ionosphere (MLTI) region from 60 to 300 km altitude. Interaction of fast particles with a three-species atmosphere (O, N₂ and O₂) is considered in the model through charge exchange, electron stripping, ionization, excitation, and elastic scattering collisions. Unlike electron injection, major difficulties in modeling ion precipitation arise from the coupling of ion and neutral transport through repeated electron capture from and loss to the ambient atmospheric constituents. The spreading effect is a unique feature for an incident energetic ion beam, and thus an important focus of our study. The beam spreading effect is extensively simulated using our 3-D Monte Carlo model for both a fine proton beam and a proton arc of longitudinal and latitudinal extent. Our model is validated through a variety of comparisons with observations and other model simulation results. In the comparison with 1-D calculations, it is found that a single correction factor, often introduced by 1-D models for incident ions with finite arc dimensions, cannot completely account for the spatial spreading effect. In general, ionization rates are overestimated at high altitudes and underestimated at low altitudes by 1-D models. The availability of the NOAA/POES satellite data helps to yield a global pattern of energetic ion precipitation. We have carried out 3-D Monte Carlo simulations for the energetic proton precipitation during the April 2002 magnetic storm events, and examined for the first time how the structure of the global proton energy input to the Earth's upper atmosphere varies with time in response to different types of solar and magnetospheric activities. In addition, our simulation results are fed into the Global Ionosphere and Thermosphere model (GITM) by providing ionization rates and atmospheric heating. Through the coupling with GITM, the impact of high-energy precipitating protons on the ionosphere-thermosphere system has been for the first time evaluated by assuming realistic global particle precipitation. It is shown that, in certain regions, electron and nitric oxide densities can be enhanced by an order of magnitude, and ion and neutral winds can be affected by tens of percent.