Instabilities near the turbo-pause ( 100 km) indicated by the Richardson number (Ri) measured by the MIGHTI instrument on the ICON satellite.

The vertical flux of heat and constituents near the 100 km region of the lower thermosphere is due to mixing associated with atmospheric instabilities as well as breaking gravity waves. The Richardson number (Ri), which combines the convective state of the atmosphere with wind shear, describes a criterion which leads to Kelvin-Helmholtz instabilities (KHI) which results in turbulence for the condition when Ri<0.25, assuming laminar flow. Parametric instabilities, associated with wave-wave coupling and resonance where -1<Ri<1, also leads to vertical mixing.

The analysis described herein is an intra-annual study of middle to equatorial latitudes of the Brunt-Väisälä frequency (N2), the wind shear squared, and the distribution of the Ri in the upper turbo-pause region. The NASA ICON satellite is instrumented with the MIGHTI (Michelson Interferometer for Global High-resolution Thermosphere Imaging) instruments which measure Doppler winds and independently, rotational temperatures from the O2 A-band. These observations were used to determine the Ri above 98 km, a region where turbulence generated eddy diffusion competes with molecular diffusion, a defining region coupling the O reservoir in the MLT upward, to the thermosphere. This analysis is for the year 2020, where the intra-annual, diurnal, and latitudinal variations for four zones between 12oS and 36oN are determined.