Summary
We evaluate atmospheric ionization during the Ground Level Enhancement (GLE) of 2005 Jan 20 using the continuous slowing down (CSD) energy loss approximation [1,2]. The top-of-atmosphere particle flux is derived using satellite measurements (for particle energies below 500MeV) and proton spectra inferred from the global neutron monitor network (for energies from 500MeV 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. During the peak of this event there were significant intra- and inter-hemispheric asymmetrics in ionization, particularly in the stratosphere. Using our ionization model, we infer the atmospheric production rate of hydroxyl (OH) and show that the inferred spatial enhancement of OH in the lower polar stratosphere at the peak of the GLE is consistent with measurements by the Microwave Limb Sounder on the Aura spacecraft. We discuss the implication of the OH enhancement for ozone chemistry in the lower stratosphere. References [1] F.M. Vitt and C.H. Jackman, J. Geophys. Res. 101, 6729 (1996). [2] A.R. Klekociuk, D.J. Bombardieri, M.L. Duldig and K.J. Michael, Advances in Geosciences 14:Solar Terrestrial, 305-319