Variability of NO<SUB>x</SUB> in the polar middle atmosphere from October 2003 to March 2004: vertical transport versus local production by energetic particles

We use NO, NO<SUB>2</SUB> and CO from MIPAS/ENVISAT to investigate the impact of energetic particle precipitation onto the NO<SUB>x</SUB> 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 one order of magnitude. Correlations of NO<SUB>x</SUB> and CO show that the unprecedented high NO<SUB>x</SUB> 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; an additional source of NO<SUB>x</SUB> due to local production by precipitating electrons at altitudes below 70 km as discussed in previous publications appears unlikely. In the polar summer Southern Hemisphere, we observed an enhanced variability of NO and NO<SUB>2</SUB> on days with enhanced geomagnetic activity but they seem to indicate enhanced instrument noise rather than a direct increase due to electron precipitation. A direct effect of electron precipitation onto NO<SUB>x</SUB> can not be ruled out, but if any, it is lower than 3 ppb in the altitude range 40-56 km and lower than 6 ppb in the altitude range 56-70 km.