Influence of extremely large solar proton events in a changing stratosphere

Two periods of extremely large solar proton events (SPEs) occurred in the p ast 30 years, which forced significant long-term polar stratospheric change s. The August 2-10, 1972, and October 19-27, 1989, SPEs happened in stratos pheres that were quite different chemically. The stratospheric chlorine lev els were relatively small in 1972 (similar to 1.2 ppbv) and were fairly sub stantial in 1989 (similar to 3 ppbV). Although these SPEs produced both HOx and NOy constituents in the mesosphere and stratosphere, only the NOy cons tituents had lifetimes long enough to affect ozone for several months to ye ars past the events. Our recently improved two-dimensional chemistry and tr ansport atmospheric model was used to compute the effects of these gigantic SPEs in a changing stratosphere. Significant upper stratospheric ozone dep letions >10% are computed to last for a few months past these SPEs. The lon g-lived SPE-produced NOy constituents were transported to lower levels duri ng winter after these huge SPEs and caused impacts in the middle and lower stratosphere. During periods of high halogen loading, these impacts resulte d in interference with the chlorine and bromine loss cycles for ozone destr uction. This interference actually led to a predicted total ozone increase that was especially notable in the time period 1992-1994, a few years after the October 1989 SPE. The chemical state of the atmosphere, including the stratospheric sulfate aerosol density: substantially affected the predicted stratospheric influence of these extremely large SPEs.