Summary
Atmospheric ionization by solar protons has traditionally been modeled using top-of-atmosphere (TOA) particle fluxes measured by near-Earth spacecraft, and as a consequence have been restricted to particle energies below ~500 MeV. However, as inferred from measurements by Earth-based muon and neutron monitor telescopes during Ground Level Enhancement (GLE) events, protons accompanying the most energetic SPEs can have significant short term (<1 day duration) fluxes at energies up to ~20 GeV. Here we the model atmospheric ionization in the polar stratosphere and mesosphere during the large GLEs of September-October 1989 and January 2005 . We compare the ionization profiles for these events using TOA proton spectra derived from the standard (satellite-based) approach, with spectra that have been augmented with 5-minute resolution neutron monitor data. We show that inclusion of the highest available energies can have an small short timescale influence on the ionization profile at the greatest penetration depths (i.e., the lower stratosphere). However the overall effect of the highest energy particles on atmospheric chemistry at these altitudes is likely to be of low significance.