Gravity wave interactions with mesospheric planetary waves: A mechanism for penetration into the thermosphere-ionosphere system

This research is motivated by observations of oscillations in the thermosph ere and ionosphere at periods close to those associated with planetary wave s (PW) or "normal modes" of the lower atmosphere. Herein a 2-D linear stead ystate numerical model is used in conjunction with a hybrid gravity wave (G W) parameterization to examine the role of GW interactions in the penetrati on of westward propagating planetary waves from the mesosphere into the low er thermosphere. Results are presented for the quasi 2-day and quasi 16-day waves previously studied using the same model without explicit parameteriz ation of GW influences. It is shown that the horizontal winds associated wi th the PWs result in modulation of upward propagating internal gravity wave s, providing an insitu source of periodic forcing in the mesopause region. This forcing is shown to produce separate amplitude peaks in the PW horizon tal wind fields in the lower thermosphere apart from the primary response i n the mesosphere. Horizontal winds associated with the GW-modified 2-day wa ve extend well into the thermosphere up to 140 km, primarily at lower latit udes, whereas the GW-modified 16-day wave is enhanced in the 80-120 km alti tude range. The phase structure of the modified waves are consistent with t he concept of gravity wave filtering by a longitudinally varying planetary wave, with an anticorrelation between the phase of the mesospheric and ther mospheric amplitude peaks. The roles of wave stress and eddy diffusion are analyzed and variations of the strength of the meridional GW source are con sidered.