HAO Colloquium - Gary Swenson, University of Illinois

Combined Chemical and Dynamical Variations in the Mesosphere and Lower Thermosphere and the Intra-Annual Variations of kzz and O
The MLT (Mesosphere Lower Thermosphere, 80-120 km) region contains a chemical ‘soup’, where atomic oxygen (O) combines with excited O2 states to form tri-atomics, (e.g. CO2, HO2, NO2, etc.). Meteors oblate injecting metals (Na, Fe, Si, K, etc.) to the region. Photo-dissociation of O2 by the FUV (135-210 nm) and all molecules by the EUV (<120 nm), further alters the composition. Collisional processes, photochemistry, and dynamics combine to challenge the modeling of composition and their density in the MLT, the chemical reservoir of the thermosphere.
The constituents are distributed globally (meridionally), by wave (planetary, tides, gravity) action. The residual circulation, driven by dissipating GWs, up-wells in the mid and high latitude summer and down-wells in winter in the lower MLT. This seasonal process combines with eddy diffusion transport driven by breaking GWs (see Garcia and Solomon, 1985). Recent studies of TIMED SABER (2002-2018) and ENVISAT SCIAMACHY (2002-2012) measurements of MLT O density reveal the global means, and intra-annual (IAV) variations of the parameterized vertical mixing (kzz) in the MLT. A focus on the IAV Annual Oscillation (AO) at mid-latitudes and the Semi-Annual Oscillation (SAO) at low latitudes will be described with some surprising effects in the O density variation.
Recent LIDAR observations of turbulence support the consideration that the 97-105 km altitude region, kzz is predominantly driven by KHI (Kelvin Helmholtz Instabilities, Larsen (2002)), whereas below (80-96 km), breaking GWs dominate. The respective regions likely have unique kzz climates.
Date and time: 
Wednesday, June 16, 2021 - 2:00pm to 3:00pm