The Impact of Energetic Particle Precipitation on the Chemical Composition of the Middle Atmosphere: Measurements and Model Predictions

We investigate the impact of energetic particle precipitation on the chemical composition of the middle atmosphere by developing models, and combining model results with observations of the chemical response to particle precipitation events. We show that in the upper stratosphere and lower mesosphere, negative ion chemistry plays a role in addition to the well-known NOx and HOx production due to positive ion chemistry, releasing chlorine from its reservoir, and re-partitioning NOy. Model results also show a large direct impact of energetic electron precipitation on the chemical composition of the upper stratosphere and mesosphere, both during large solar events and during and after geomagnetic storms. Observations show that the indirect impact of energetic electron precipitation events on the middle atmosphere composition can be much larger than the impact of even large solar particle events. However, observations have not shown clear evidence for a direct impact of energetic electron precipitation at altitudes below 80 km so far; if there is a direct impact of energetic electron precipitation on the lower mesosphere and upper stratosphere as suggested by the model results, then it is small compared to the direct contribution of large solar events, or to the indirect impact of energetic electron precipitation due to downward propagation of mesospheric or thermospheric air during polar winter.