Daily NO number density, retrieved from measurements of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) from 2002 to 2012 for polar summer in the mesosphere, is used to investigate the response of NO to geomagnetic activity, as expressed by the auroral electrojet (AE) index. Applying the superposed epoch analysis, we observe a clear response of NO to strong geomagnetic forcing at geomagnetic latitudes 55–75°N/S and altitudes above 66 km. The 27 day solar rotation cycle is observed, indicating that some of the observed geomagnetic events are related to solar coronal holes. We find a linear relationship between anomalies of AE and NO at geomagnetic latitudes 55–70°N/S and 70–74 km altitude. A clear auroral oval‐like structure is observed on days of strong geomagnetic forcing in both hemispheres, with small longitudinal inhomogeneities, which might be related to the South Atlantic Anomaly or the magnetic local time. The NO lifetime and production rate per AE anomaly has been derived from a least squares fit to the observations. Comparisons of results from a simple model using this empirical NO production and a lifetime varying from 1.2 days in summer to 10 days in winter to SCIAMACHY observations show good agreement. In particular, the strength and interannual variability of the wintertime maximum is well captured. This suggests that direct production of NO in the upper mesosphere above 72 km contributes substantially to the so‐called energetic particle precipitation indirect effect.

Abstract. MIPAS/ENVISAT data of nighttime NO 2 volume mixing ratios (VMR) from 2007 until 2012 between 40 km and 62 km altitude are compared with the geomagnetic Ap index and solar Lyman-α radiation. The local impact of variations in geomagnetic activity and solar radiation on the VMR of NO 2 in the lower mesosphere and upper stratosphere in the Northern Hemisphere is investigated by means of superposed epoch analysis. Observations in the Northern Hemisphere show a clear 27-day period of the NO 2 VMR. This is positively correlated with the geomagnetic Ap index at 60-70 • N geomagnetic latitude but also partially correlated with the solar Lyman-α radiation. However, the dependency of NO 2 VMR on geomagnetic activity can be distinguished from the impact of solar radiation. This indicates a direct response of NO x (NO + NO 2 ) to geomagnetic activity, probably due to precipitating particles. The response is detected in the range between 46 km and 52 km altitude. The NO 2 VMR epoch maxima due to geomagnetic activity is altitude-dependent and can reach up to 0.4 ppb, leading to mean production rates of 0.029 ppb (Ap d) −1 . Observations in the Southern Hemisphere do not have the same significance due to a worse sampling of geomagnetic storm occurances. Variabilities due to solar variation occur at the same altitudes at 60-70 • S geomagnetic latitude but cannot be analyzed as in the Northern Hemisphere. This is the first study showing the direct impact of electron precipitation on NO x at those altitudes in the spring/summer/autumn hemisphere.

Abstract. We present altitude-dependent lifetimes of NOx, 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 NOx 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 NOx densities under consideration of the determined lifetimes of NOx, and calculate altitude-dependent effective production rates of NOx 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 NOx-production rates to be applied to the AIMOS ionization rates which range from about 0.2 NOx-molecules per ion pair at 44 km to 0.7 NOx-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.

We present altitude dependent lifetimes of NO_x, determined with MIPAS/ENVISAT, for the southern polar region after the solar proton event in October–November 2003. Varying in latitude 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 photolysis. We infer dynamical lifetimes by comparison of the observed decay to photolytical lifetimes calculated with the SLIMCAT 3-D Model. Photochemical loss contributes to the observed NO_x depletion by 10% at 44 km, increasing with altitude to 35% at 62 km at a latitude of –63° S. At higher latitudes, the contribution of photochemical loss can be even more important.

In addition, we show the correlation of modeled 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 for the whole Austral polar summer 2003/04. 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 0.9 NO_x-molecules per ion pair at 54 km at a latitude of –63° S. At –73° S, the NO_x-production rate ranges from about 0.2 NO_x-molecules per ion pair at 44 km to 1.0 NO_x-molecules per ion pair at 60 km. These effective production rates are considerably lower than predicted by box model simulations which could hint at an overestimation of the modeled ionization rates

MIPAS/ENVISAT data of nighttime NO 2 volume mixing ratios (VMR) from 2007 until 2012 between 40 km and 62 km altitude are compared with the geomagnetic Ap index and solar Lyman-α radiation. The local impact of variations in geomagnetic activity and solar radiation on the VMR of NO 2 in the lower mesosphere and upper stratosphere in the Northern Hemisphere is investigated by means of superposed epoch analysis. Observations in the Northern Hemisphere show a clear 27-day period of the NO 2 VMR. This is positively correlated with the geomagnetic Ap index at 60-70 • N geomagnetic latitude but also partially correlated with the solar Lyman-α radiation. However, the dependency of NO 2 VMR on geomagnetic activity can be distinguished from the impact of solar radiation. This indicates a direct response of NO x (NO + NO 2 ) to geomagnetic activity, probably due to precipitating particles. The response is detected in the range between 46 km and 52 km altitude. The NO 2 VMR epoch maxima due to geomagnetic activity is altitude-dependent and can reach up to 0.4 ppb, leading to mean production rates of 0.029 ppb (Ap d) −1 . Observations in the Southern Hemisphere do not have the same significance due to a worse sampling of geomagnetic storm occurances. Variabilities due to solar variation occur at the same altitudes at 60-70 • S geomagnetic latitude but cannot be analyzed as in the Northern Hemisphere. This is the first study showing the direct impact of electron precipitation on NO x at those altitudes in the spring/summer/autumn hemisphere.