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The MIPAS Fourier transform spectrometer on board Envisat has measured global\uddistributions of the six principal reactive nitrogen (NOy) compounds\ud(HNO3, NO2, NO, N2O5, ClONO2, and HNO4) during\ud2002–2012. These observations were used previously to detect regular polar\udwinter descent of reactive nitrogen produced by energetic particle\udprecipitation (EPP) down to the lower stratosphere, often called the EPP\udindirect effect. It has further been shown that the observed fraction of\udNOy produced by EPP (EPP-NOy) has a nearly linear relationship with\udthe geomagnetic Ap index when taking into account the time lag\udintroduced by transport. Here we exploit these results in a semi-empirical\udmodel for computation of EPP-modulated NOy densities and wintertime\uddownward fluxes through stratospheric and mesospheric pressure levels. Since\udthe Ap dependence of EPP-NOy is distorted during episodes of strong\uddescent in Arctic winters associated with elevated stratopause events, a\udspecific parameterization has been developed for these episodes. This model\udaccurately reproduces the observations from MIPAS and is also consistent with\udestimates from other satellite instruments. Since stratospheric EPP-NOy\uddepositions lead to changes in stratospheric ozone with possible implications\udfor climate, the model presented here can be utilized in climate simulations\udwithout the need to incorporate many thermospheric and upper mesospheric\udprocesses. By employing historical geomagnetic indices, the model also allows\udfor reconstruction of the EPP indirect effect since 1850. We found secular\udvariations of solar cycle-averaged stratospheric EPP-NOy depositions on\udthe order of 1 GM. In particular, we model a reduction of the EPP-NOy\uddeposition rate during the last 3 decades, related to the coincident decline\udof geomagnetic activity that corresponds to 1.8 % of the NOy\udproduction rate by N2O oxidation. As the decline of the geomagnetic\udactivity level is expected to continue in the coming decades, this is likely\udto affect the long-term NOy trend by counteracting the expected increase\udcaused by growing N2O emissions

atmospheric chemistry and physics

A semi-empirical model for mesospheric and stratospheric NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;y&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; produced by energetic particle precipitation