Energetic Particle Precipitation and the Chemistry of the Mesosphere/Lower Thermosphere

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, NO 2) and to atmospheric reactive hydrogen oxides (HOx = H, OH, HO 2), 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.

Solar forcing for CMIP6 (v3.2)

“…The production of HO x relies on complicated ion chemistry that takes place after the initial formation of ion pairs (Swider and Keneshea, 1973;Frederick, 1976;Solomon et al, 1981;Sinnhuber et al, 2012). Solomon et al (1981) computed HO x production rates as a function of altitude and ion pair production rate.…”

“…Each ion pair typically results in the production of around two HO x constituents in the stratosphere and lower mesosphere. Sinnhuber et al (2012) have shown that HO x is formed as H and OH in nearly equal amounts, with small differences of less than 10 % due to different ion reaction chains. In the middle and upper mesosphere, one ion pair is computed to produce less than two HO x constituents per ion pair because water vapor decreases sharply with altitude there, and is no longer available as a source of HO x .…”

“…Verronen and Lehmann, 2013;Nieder et al, 2014) is encouraged. Similarly, if atmospheric models include detailed cluster ion chemistry of the lower ionosphere (D region), then the ionization rates should be used to drive the production rates of the primary ions (N + 2 , N + , O + 2 , O + ) and neutrals (N, O) produced in particle impact ionization or dissociation (Sinnhuber et al, 2012). Since such a comprehensive treatment of EPP effects on minor species may introduce more sensitive composition changes via chemical feedbacks, it would be important to document the adopted approaches.…”

“…In particular, the production of odd nitrogen and odd hydrogen species causes changes in ozone abundances via catalytic cycles, potentially affecting temperature and winds (see, e.g., the review by Sinnhuber et al, 2012). Recent model studies and the analysis of meteorological data have provided evidence for a dynamical coupling of this signal to the lower atmosphere, leading to particleinduced surface climate variations on a regional scale (e.g., Seppälä et al, 2009;Baumgaertner et al, 2011;Rozanov et al, 2012;Maliniemi et al, 2014).…”

Analysis and parameterisation of ionic reactions affecting middle atmospheric HO<sub>x</sub> and NO<sub>y</sub> during solar proton events

“…Energetic particle precipitation affects not only the ion and electron densities but also the neutral composition of the middle atmosphere through particle impact ionization, dissociation, and ion chemistry (see Sinnhuber et al, 2012, for a recent review). Precipitating particles are mostly protons and electrons, which are released from the Sun in, for example, coronal mass ejections and arrive in the near-Earth space with the solar wind.…”