ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-8009-2014 Unusually strong nitric oxide descent in the Arctic middle atmosphere in early 2013 as observed by Odin/SMR Pérot K. https://orcid.org/0000-0002-4267-8560 1 Urban J. 1 Murtagh D. P. https://orcid.org/0000-0003-1539-3559 1 Chalmers University of Technology, Department of Earth and Space Sciences, Gothenburg, Sweden 12 08 2014 14 15 8009 8015 Copyright: © 2014 K. Pérot 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/14/8009/2014/acp-14-8009-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/8009/2014/acp-14-8009-2014.pdf

The middle atmosphere was affected by an exceptionally strong midwinter stratospheric sudden warming (SSW) during the Arctic winter 2012/2013. These unusual meteorological conditions led to a breakdown of the polar vortex, followed by the reformation of a strong upper stratospheric vortex associated with particularly efficient descent of air. Measurements by the submillimetre radiometer (SMR), on board the Odin satellite, show that very large amounts of nitric oxide (NO), produced by energetic particle precipitation (EPP) in the mesosphere/lower thermosphere (MLT), could thus enter the polar stratosphere in early 2013. The mechanism referring to the downward transport of EPP-generated NO<sub>x</sub> during winter is generally called the EPP indirect effect. SMR observed up to 20 times more NO in the upper stratosphere than the average NO measured at the same latitude, pressure and time during three previous winters where no mixing between mesospheric and stratospheric air was noticeable. This event turned out to be the strongest in the aeronomy-only period of SMR (2007–present). Our study is based on a comparison with the Arctic winter 2008/2009, when a similar situation was observed. This outstanding situation is the result of the combination of a relatively high geomagnetic activity and an unusually high dynamical activity, which makes this case a prime example to study the EPP impacts on the atmospheric composition.

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