Is there a direct solar proton impact on lower-stratospheric ozone?

Abstract

We investigate Arctic polar atmospheric ozone responses to solar proton events (SPEs) using MLS (Microwave Limb Sounder) satellite measurements (2004 now) and WACCM-D (Whole Atmosphere Community Climate Model) simulations (1989 2012). Special focus is on lowerstratospheric (10 30 km) ozone depletion that has been proposed earlier based on superposed epoch analysis (SEA) of ozonesonde anomalies (up to 10% ozone decrease at 20 km). SEA of the satellite dataset provides no solid evidence of any average SPE impact on the lower-stratospheric ozone, although at the mesospheric altitudes a statistically significant ozone depletion is present. In the individual case studies, we find only one potential case (January 2005) in which the lower-stratospheric ozone level was significantly decreased after the SPE onset (in both model simulation and MLS observation data). However, similar decreases could not be identified in other SPEs of similar or larger magnitude. Due to the input proton energy threshold of 300MeV, the WACCM-D model can only detect direct proton effects above 25 km, and simulation results before the Aura MLS era indicate no significant effect on the lower-stratospheric ozone. However, we find a very good overall consistency between WACCM-D simulations and MLS observations of SPE-driven ozone anomalies both on average and for the individual cases including January 2005.