Abstract
Precipitation of energetic particles into the atmosphere greatly disturbs the chemical composition from the upper stratosphere to the lower thermosphere. Most important are changes to the budget of atmospheric nitric oxides (NOx = N, NO, NO2) and to atmospheric reactive hydrogen oxides (HOx = H, OH, HO2), which both contribute to ozone loss in the stratosphere and mesosphere. The impact of energetic particle precipitation on the chemical composition of the atmosphere has been studied since the 1960s, and there are a number of observations as well as model studies concerning especially the auroral impact and large solar particle events. Changes to the NOx budget due to energetic particle precipitation can be quite long-lived during polar winter and can then be transported down into the lower mesosphere and stratosphere, where NOx is one of the main participants in catalytic ozone destruction. Energetic particle precipitation can also affect temperatures and dynamics of the atmosphere from the source region down to the stratosphere and possibly even down to the surface, due to a coupling of chemical composition changes affecting atmospheric heating and cooling rates, the mean circulation, and wave propagation and breaking. Thus, energetic particle precipitation impacts have been implemented in chemistry-climate models reaching from the surface up to the mesosphere or lower thermosphere. However, there are still a number of open questions in the theoretical description of the energetic particle precipitation impact; the most important are uncertainties in the formation rate of different NOx species due to energetic particle precipitation, and the complex coupling between chemical changes, atmospheric heating and cooling rates, and atmospheric dynamics.
Highlights
Energetic particles—protons, electrons, and heavier ions—that precipitate into the atmosphere come from different sources: directly from the Sun in large solar particle events (SPEs), from the aurora and the radiation belts during geomagnetic storms and substorms, or from outside the solar system
The impact of energetic particle precipitation on the chemical composition of the atmosphere has been studied since the 1960s, and there are a number of observations as well as model studies concerning especially the auroral impact and large solar particle events
Solar particles come from the solar wind, a continuous source of plasma outflow from the solar polar regions or solar coronal holes modulated in strength throughout the solar cycle, or from large eruptions in the solar atmosphere related to high solar activity, so-called solar coronal mass ejections
Summary
Energetic particles—protons, electrons, and heavier ions—that precipitate into the atmosphere come from different sources: directly from the Sun in large solar particle events (SPEs), from the aurora and the radiation belts during geomagnetic storms and substorms, or from outside the solar system. The good coverage of a number of trace gases with remote sensing observations from space in the middle atmosphere in the last decade has made the discoveries of new chemical processes due to energetic particle precipitation possible, showing, for example, enhancements of N2O (Funke et al 2008a, b; Semeniuk et al 2008) and HNO3 (Kawa et al 1995; de Zafra and Smyshlyaev 2001; Lopez-Puertas et al 2005a; Stiller et al 2005; Orsolini et al 2005, 2009) correlated to geomagnetic activity and large solar events, or a decrease of HCl (Winkler et al 2009, 2011) and increases in reactive chlorine (von Clarmann et al 2005) It has been discussed in a number of publications whether precipitation of relativistic electrons from the radiation belt can have a large impact on the composition of the stratosphere and mesosphere comparable to SPEs (e.g., Baker et al 1993; Callis et al 1998a, b, 2001).
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