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

Abstract

Abstract. We use NO, NO 2 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 NO 2 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.

NO<sub><i>y</i></sub> production, ozone loss and changes in net radiative heating due to energetic particle precipitation in 2002–2010

“…Only one strong solar proton event is captured by MIPAS observations during this time period (October 2003). In the Southern Hemisphere, NO y during and after this event is overestimated by the models in the lower mesosphere (1-0.1 hPa), indicating an overestimation of proton ionization rates there.…”

“…In a few summers, this mid-stratospheric summer ozone loss can reach values of 5-10 %, in particular in early 2004 and early 2005. In these summers, the continuing ozone loss from the indirect effect seems to be strengthened by strong solar proton events occurring in early spring (October 2003) or during summer (January 2005).…”

“…Solar proton events in October 2003, January 2005and September 2005 lead to instantaneous ozone loss from the mid-stratosphere to the upper mesosphere in all models. However, the annual variation of the stratospheric ozone loss due to the indirect effect looks distinctly different.…”

“…Observations from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS, Fischer et al, 2008), which also covered the polar night region, show that NO x produced by energetic particle precipitation (called EPP NO x in the following) reaches down to altitudes below 30 km in both hemispheres, in nearly all winters observed (Funke et al, 2014a). Particularly high values of EPP NO x are observed in Northern Hemisphere late winters after the strong sudden stratospheric warming events in winters (Randall et al, 2006Funke et al, 2014a;Sinnhuber et al, 2014;Funke et al, 2017). These warmings were followed by long-lasting downwelling in the mesosphere and upper stratosphere enabled by a strong polar vortex re-forming after the event.…”

Ionization and NO production in the polar mesosphere during high-speed solar wind streams: model validation and comparison with NO enhancements observed by Odin-SMR

“…Direct changes in middle-atmosphere composition down to 40-50 km heights have been demonstrated during solar proton events (see e.g. Jackman et al, 2014;Sinnhuber et al, 2014, and references therein). Since solar proton events occur primarily close to the maximum of the solar sunspot cycle, it has been suggested that these could be a source of climate forcing in phase with the solar cycle.…”