Lifetime and production rate of NO_x in the upper stratosphere and lower mesosphere in the polar spring/summer after the solar proton event in October–November 2003

Abstract. We present altitude-dependent lifetimes of NO_x, determined with MIPAS/ENVISAT (the Michelson Interferometer for Passive Atmospheric Sounding/the European Environment Satellite), for the Southern polar region after the solar proton event in October–November 2003. Between 50° S and 90° S and decreasing in altitude they range from about two days at 64 km to about 20 days at 44 km. The lifetimes are controlled by transport, mixing and photochemistry. We infer estimates of dynamical lifetimes by comparison of the observed decay to photochemical lifetimes calculated with the SLIMCAT 3-D Model. Photochemical loss contributes to the observed NO_x depletion by 0.1% at 44 km, increasing with altitude to 45% at 64 km.

In addition, we show the correlation of modelled ionization rates and observed NO_x densities under consideration of the determined lifetimes of NO_x, and calculate altitude-dependent effective production rates of NO_x due to ionization. For that we compare ionization rates of the AIMOS data base with the MIPAS measurements from 15 October–31 December 2003. We derive effective NO_x-production rates to be applied to the AIMOS ionization rates which range from about 0.2 NO_x-molecules per ion pair at 44 km to 0.7 NO_x-molecules per ion pair at 62 km. These effective production rates are considerably lower than predicted by box model simulations which could hint at an overestimation of the modelled ionization rates.