Response of the Middle Atmosphere to Energetic Particle Production

Energetic particles (mainly protons and electrons) precipitate into the Earth’s atmosphere, primarily at higher latitudes. The energetic particles cause ionizations, excitations, dissociations, and dissociative ionizations of the background constituents. Complicated ion chemistry leads to the production of HO_x (H, OH, HO₂) and dissociation of molecular nitrogen (N₂) leads to NO_x (N, NO, NO₂) production. The energetic particle production of HO_x and NO_x constituents can then influence ozone in middle atmosphere (stratosphere and mesosphere).

Early in her career, Susan Solomon contributed ground-breaking research into the atmospheric impacts of these particles. The creation of HO_x by energetic particles was known in the early 1970s, however, the specific amount of HO_x production as a function of energy input was not well quantified. Susan led the effort to develop an elaborate quantitative ion chemistry model to compute the HO_x production from precipitating energetic particles (Solomon et al., 1981). The table in that paper of HO_x production as a function of altitude and energy input from the particles is still being used today.

Among other related breakthroughs regarding energetic particles, Susan and her colleagues computed the coupling of the thermosphere to the middle atmosphere. Solomon et al. (1982) calculated the impact of the thermospheric NO_x produced by auroral particles and then being dynamically coupled to lower altitudes. In particular, they found that substantial amounts of aurorally-produced thermospheric NO_x can reach the stratosphere, particularly at high latitudes during polar winter.

Susan Solomon was involved in a number of other papers on the middle atmospheric influence of energetic particles, primarily in the 1980’s. Recently, however, Susan was involved in a study (Stone et al., 2018), which concluded that solar proton precipitation had a profound effect on ozone trends in the upper stratosphere, high latitude (50–80°N/S) regions. Solar proton events (SPEs) in July 2000 and October 2003 were among the largest events on record and occurred near the time of largest equivalent effective stratospheric chlorine (EESC). Thus, there was found to be a need to account for the SPE impact on the detection of recovery in the upper stratosphere.

I will review some of Susan’s ground-breaking contributions to our understanding of the influence of energetic particle precipitation on the middle atmosphere. This will be supplemented with later pertinent material on the topic.

References:

Solomon, S., Rusch, D. W., Gerard, J.-C., Reid, G. C., and Crutzen, P. J., 1981: The effect of particle precipitation events on the neutral and ion chemistry of the middle atmosphere: II. Odd hydrogen, Planet. Space Sci., 29, 885-892.

Solomon, S., Crutzen, P. J., and Roble, R. G., 1982: Photochemical coupling between the thermosphere and the lower atmosphere: 1. Odd nitrogen from 50 to 120 km, J. Geophys. Res., 87, 7206-7220.

Stone, K. A., Solomon, S., and Kinnison, D. E., 2018: On the identification of ozone recovery, Geophys. Res., Lett., 45, https://doi.org/10.1029/2018GL077955