On the Existence and Excitation of Eastward Propagating Quasi-Two Day Waves in the MLT

MLT temperature fields measured by TIMED SABER over 60 day periods have revealed the presence of eastward propagating quasi-two day wave components with zonal wavenumbers 1-4, persistent throughout the entire year. These eastward quasi-two day waves display strong enhancements in the winter high latitudes around solstice, and in the high latitudes of both hemispheres around the equinoxes extending from the stratosphere into the mesosphere and lower thermosphere. The low latitude response is dominated by Kelvin modes extending from the MLT into the lower thermosphere. Daily observations from the South Pole meteor radar system show that eastward two-day wave activity occurs in sporadic bursts, peaking around winter solstice, but also occurring with smaller amplitudes at equinox. The results of numerical experiments conducted using the NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM), and the linear mechanistic Global Scale Wave Model (GSWM) are presented in order to understand the excitation mechanisms and time evolution of the eastward propagating quasi-two day waves. The eastward quasi-two day waves are found to be excited by mechanism of mean wind instability at high latitudes, both during the winter solstice and at equinox, which are manifested as the high latitude enhancements found in the observations and model results. This mechanism can excite Kelvin modes in the low latitude MLT, thus influencing the circulation of that region. Taken together, our findings point to the eastward quasi-two day waves as a persistent global scale phenomena not limited only to solstice conditions as previously thought.