Atmospheric Effects of Energetic Particle Precipitation after Sudden Stratospheric Warmings (Invited Presentation)

The impacts of energetic particle precipitation (EPP) on the atmosphere can be divided into two categories, direct and indirect effects. The direct effect refers to the local production of reactive odd nitrogen (NO_x = N+NO+NO₂) and odd hydrogen (HO_x = H+OH+HO₂) by reactions that are triggered by EPP. The indirect effect refers primarily to the transport of EPP-produced NO_x (EPP-NO_x), which is relatively long-lived in the mesosphere and lower thermosphere (MLT) during the polar winter. Of particular interest is the transport of EPP-NO_x from the MLT into the stratosphere, where it can interfere with ozone catalytic cycles. There is now abundant evidence that after sudden stratospheric warmings (SSWs), the EPP indirect effect is augmented because descent in the mesosphere is enhanced. This presentation will summarize the current state of understanding regarding how SSWs alter the EPP indirect effect. Evidence for the indirect effect will be shown, and comparisons of observations and model results between seasons with and without SSWs will be described. Outstanding questions and future directions will be discussed.