Conclusion

A large NO2 increase and O3 depletion was observed by GOMOS in the Northern polar stratosphere after the strong 28 October 2003 SPE. During such an event, NOx is formed in the upper stratosphere/mesosphere in the polar cap which coincides approximately to the polar vortex during the winter period. The total ozone column in the polar vortex was decreased by about 2% north of 70°N, about twice model estimations (Jackman etal., 2000). The NO2 perturbation was observable until mid-December when the polar vortex was destroyed by an upper stratospheric warming. The impact of an SPE on the stratosphere is very dependent on the season and on the meteorology of the polar stratosphere. The impact is important if NOx is transported in the polar night downwards into the middle stratosphere where it can stays during several months without being destroyed. A large NO2 increase was also observed in June-July 2003 in the south polar stratosphere with some indications of a simultaneous ozone decrease. The origin of this NO2 increase is still not clear. It may be related to the production of NOx by energetic particle precipitation as proposed by Randall et al. (2005). We propose here another mechanism. A SPE with protons of moderate energy occurred on 29 May 2003. Most of the proton energy was confined in the band 0.8-4 MeV which means that NOx production is limited to the mesosphere. However, it occurred during a period where the south polar vortex is very stable and in the polar night and the diabatic descent inside the vortex is very strong. In these conditions, one can expect NOx to be rapidly transported into the upper stratosphere without being photo-chemically destroyed. Further studies are needed to evaluate the efficiency of this mechanism. If it is confirmed, it means that the impact of SPEs on the stratospheric chemistry may be strongly underestimated by the models taking into account only the strong energy events.

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