Summary
Atmospheric ionization during Solar Proton Events (SPE) has traditionally been modeled using top-of-atmosphere (TOA) particle fluxes measured by near- Earth spacecraft, and as a consequence the modeling has been restricted to particle energies below ¡ 500 MeV. However, as inferred from measurements by Earth-based muon telescopes and neutron monitors 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 at least 20GeV. Here, we examine atmospheric chemistry changes in the polar atmosphere during the large GLE of 20 January 2005. We use TOA particle spectra derived from satellite measurements augmented with proton spectra inferred from ground-based neutron monitor data to drive NOy and OH production in the SOCRATES two-dimensional atmospheric chemistry model. We show that the energetic particles of the January 20 event and associated particle enhancements during the preceding 5 days produced sufficient short timescale ionization in the lower stratosphere for measurable effects on HNO3 and O3 concentrations in the polar regions of both hemispheres. We also show that the detailed consideration of the most energetic particles that contributed to this GLE is of low importance for modeling ionization in the lower stratosphere, at least on a daily average timescale.