ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-4055-2014 Local impact of solar variation on NO<sub>2</sub> in the lower mesosphere and upper stratosphere from 2007 to 2012 Friederich F. 1 Sinnhuber M. https://orcid.org/0000-0002-3527-9051 1 Funke B. https://orcid.org/0000-0003-0462-4702 2 von Clarmann T. 1 Orphal J. 1 Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain 23 04 2014 14 8 4055 4064 Copyright: © 2014 F. Friederich et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/4055/2014/acp-14-4055-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/4055/2014/acp-14-4055-2014.pdf

MIPAS/ENVISAT data of nighttime NO<sub>2</sub> 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<sub>2</sub> 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<sub>2</sub> 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<sub>2</sub> VMR on geomagnetic activity can be distinguished from the impact of solar radiation. This indicates a direct response of NO<sub>x</sub> (NO + NO<sub>2</sub>) to geomagnetic activity, probably due to precipitating particles. The response is detected in the range between 46 km and 52 km altitude. The NO<sub>2</sub> 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)<sup>−1</sup>. 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<sub>x</sub> at those altitudes in the spring/summer/autumn hemisphere.

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