Summary
We evaluate atmospheric ionization during the Ground Level Enhancement (GLE) of 2005 Jan 20 using two methods: a Monte-Carlo (MC) air-shower simulation [1] and a continuous slowing down (CSD) energy loss approximation [2,3]. For both methods, top-of-atmosphere particle fluxes are derived using satellite measurements (for particle energies below 500 MeV) and proton spectra inferred from the global neutron monitor network (for energies from 500 MeV to ~20 GeV). We consider three 5-minute intervals during the rise, peak and decline of the GLE and evaluate the three-dimensional ionization distribution in the middle atmosphere (stratosphere and mesosphere) taking into account the evolution of the particle pitch angle distribution and geomagnetic cut-off. We show that during the peak of this event there were significant intra- and inter-hemispheric asymmetries in ionization, particularly in the stratosphere. We compare the MC and CDS ionization evaluations in terms of the implications for NOx and HOx chemistry in the middle atmosphere.