Coupling From the Middle Atmosphere to the Exobase: Dynamical Disturbance Effects on Light Chemical Species. Issue 10 (12th October 2020) - Jones, M.
Plain Language Summary: Sudden stratospheric warmings (SSWs) and mesospheric coolings (MCs) are episodic polar middle atmospheric (∼12–80 km or ∼7–50 miles altitude) dynamical weather events driven by increased wave forcing from the troposphere. These events are known to have global effects on the meteorology of the upper atmosphere (i.e., the thermosphere and ionosphere). Observational and modeling evidence presented in this study demonstrate that SSWs and MCs in the middle atmosphere act to drive changes in the chemical composition of the upper atmosphere through an intricate series of processes, set in motion by SSW/MC enhancements in lower and middle atmospheric wave forcing. Our results show that SSW/MC driven changes in particularly light species like hydrogen extend well into the transition region between Earth's atmosphere and outer space. This implies that middle atmospheric weather may have an impact on the plasma populations several Earth radii above Earth's surface (∼10, 000 miles or more away). Key Points: Decreases in helium and atomic hydrogen concentration at high northern latitudes occur at the onset of mesospheric cooling events Small‐scale GW and tidal activity drives enhanced meridional transport which extends light species decreases up to the exobase in TIME‐GCM Accompanying decreases in O + and H + during middle atmospheric dynamical events have implications for the inner magnetosphere Abstract: This paper characterizes the impacts of sudden stratospheric warmings (SSWs) and mesospheric coolings (MCs) on the light species distribution (i.e., helium [He], and atomic hydrogen [H]) of the thermosphere using a combined data‐modeling approach. Performing a set of numerical experiments with a general circulation model whose middle atmospheric dynamical and thermodynamical fields were constrained using a numerical weather prediction system, we simulate the effects of SSWs and MCs on light chemical species, and via comparisons with two data sets taken from the mesosphere and thermosphere, we quantify the associated variability in light species abundances and mass density. Large depletions in the observed and modeled polar H abundance in the mesosphere and lower thermosphere (MLT) occur with MC onset, as opposed to SSW onset. Depletions in all light thermospheric species at high northern latitudes extend up to the exobase in our model simulations during the January 2013 SSW/MC period, with the largest depletions simulated for the lightest species. Further, our modeling work substantiates the paradigm of increased mixing in the MLT driven by a meridional residual circulation during SSWs resulting from enhanced small‐scale gravity wave and migrating semidiurnal tidal forcing; the former being the primary driver and the latter of secondary but notable importance in our model simulations. SSW/MC induced light species variability then gets projected upward into the thermosphere through molecular diffusion. Modeled light species variability during the January 2013 SSW/MC event suggests SSW/MC signatures could be present in the topside ionosphere and plasmasphere.