Nighttime mesospheric hydroxyl enhancements during SEP events and accompanying geomagnetic storms: Ionization rate modeling and Aura satellite observations

Abstract

We quantify the effects of combined precipitating solar protons and magnetospheric electrons on nighttime odd hydrogen density enhancements during two solar energetic particle (SEP) events accompanied by strong geomagnetic storms. We perform detailed modeling of ionization rates for 7-17 November 2004 and 20-30 August 2005 intervals with improved version 1.6 of the Atmospheric Ionization Module Osnabruck model. Particle measurements from Geostationary Operational Environmental Satellites and Polar Orbiting Environmental Satellites are sorted and combined in 2h intervals to create realistic particle precipitation maps that are used as the modeling input. We show that modeled atmospheric ionization rates and estimated peak odd hydrogen (primarily hydroxyl) production from 0.001hPa to 0.1hPa atmospheric pressure levels during these intervals are consistent with enhancements in nighttime averaged zonal odd hydrogen densities derived from newly reprocessed and improved data set of Microwave Limb Sounder instrument on board Aura satellite. We show that both precipitating SEPs and magnetospheric electrons contribute to mesospheric ionization and their relative contributions change throughout the intervals. Our event-based modeling results underline the importance of the combined ionization sources for odd hydrogen chemistry in the middle atmosphere.