Variability of NO_x in the polar middle atmosphere from October 2003 to March 2004: vertical transport vs. local production by energetic particles

Abstract. We use NO, NO₂ and CO from MIPAS/ENVISAT to investigate the impact of energetic particle precipitation onto the NO_x budget from the stratosphere to the lower mesosphere in the period from October 2003 to March 2004, a time of high solar and geomagnetic activity. We find that in the winter hemisphere the indirect effect of auroral electron precipitation due to downwelling of upper mesospheric/lower thermospheric air into the stratosphere prevails. Its effect exceeds even the direct impact of the very large solar proton event in October/November 2003 by nearly 1 order of magnitude. Correlations of NO_x and CO show that the unprecedented high NO_x values observed in the Northern Hemisphere lower mesosphere and upper stratosphere in late January and early February are fully consistent with transport from the upper mesosphere/lower thermosphere and subsequent mixing at lower altitudes. In the polar summer Southern Hemisphere, we observed an enhanced variability of NO and NO₂ on days with enhanced geomagnetic activity, but this seems to indicate enhanced instrument noise rather than a direct increase due to electron precipitation. A direct effect of electron precipitation onto NO_x can not be ruled out, but, if any, it is lower than 3 ppbv in the altitude range 40–56 km and lower than 6 ppbv in the altitude range 56–64 km. An additional significant source of NO_x due to local production by precipitating electrons below 70 km exceeding several parts per billion as discussed in previous publications appears unlikely.