Analysis of the MiddleAtmosphere's Response toEnergetic Particle Events

Abstract

The chemical composition of the middle atmosphere can be strongly influenced by Solar Proton Events (SPEs) and Energetic Electron Precipitation Events (EEPs). Theseevents are well known sources of NOx (N, NO, NO2) and HOx (H, OH, HO2), which both contribute to ozone loss in the middle atmosphere. Due to its long lifetime, significantamounts of NOx produced by large particle events in the mesosphere and the upper stratosphere can be transported down into the middle and lower stratosphere duringpolar winter, where NOx is a key species in ozone loss. Thus large particle events can potentially contribute significantly to stratospheric ozone loss. This study usesmeasurements of the Halogen Occultation Experiment (HALOE) instrument onboard the UARS satellite covering the years 1991 - 2005, to investigate mesospheric NOxproduction during more than one solar cycle. Furthermore the effect on other species e.g. HCl, which is an inactive reservoir for Cl, and its contribution to stratosphericozone loss is investigated as well. A decrease of HCl during the SPE in July 2000 could be observed for the first time. Comparisons with the UBIC model, developedat the University of Bremen, showed good correlation with the HALOE HCl data set.Furthermore, an increase of temperature in the thermosphere and a possible decreaseat altitudes of the upper mesosphere have also been observed during the SPE in July2000.Investigation of the EEP direct effect showed that highly energetic electrons do notaffect the NOx production below 80 km. The EEP indirect effect (IE), in contrast,was found to play a major role in terms of stratospheric ozone loss causing stronginter annual changes during polar winter in the southern as well as in the northernhemisphere.Further, data of highly energetic electron flux measurements of the POES and GOESspacecraft were compared to the Ap-index to find the best proxy for investigations ofparticle events affecting the polar regions.