Abstract
Abstract. We previously considered various aspects of gravity wave penetration and effects at mesospheric and thermospheric altitudes, including propagation, viscous effects on wave structure, characteristics, and damping, local body forcing, responses to solar cycle temperature variations, and filtering by mean winds. Several of these efforts focused on gravity waves arising from deep convection or in situ body forcing accompanying wave dissipation. Here we generalize these results to a broad range of gravity wave phase speeds, spatial scales, and intrinsic frequencies in order to address all of the major gravity wave sources in the lower atmosphere potentially impacting the thermosphere. We show how penetration altitudes depend on gravity wave phase speed, horizontal and vertical wavelengths, and observed frequencies for a range of thermospheric temperatures spanning realistic solar conditions and winds spanning reasonable mean and tidal amplitudes. Our results emphasize that independent of gravity wave source, thermospheric temperature, and filtering conditions, those gravity waves that penetrate to the highest altitudes have increasing vertical wavelengths and decreasing intrinsic frequencies with increasing altitude. The spatial scales at the highest altitudes at which gravity wave perturbations are observed are inevitably horizontal wavelengths of ~150 to 1000 km and vertical wavelengths of ~150 to 500 km or more, with the larger horizontal scales only becoming important for the stronger Doppler-shifting conditions. Observed and intrinsic periods are typically ~10 to 60 min and ~10 to 30 min, respectively, with the intrinsic periods shorter at the highest altitudes because of preferential penetration of GWs that are up-shifted in frequency by thermospheric winds.
Highlights
There has been evidence of, and interest in, gravity waves (GWs) in the thermosphere and ionosphere (TI) for many years
We have focused in this paper on how refraction due to Doppler shifting and variable thermospheric temperatures, and dissipation due to kinematic viscosity and thermal diffusivity, influence the horizontal and vertical wavelengths and observed periods of GWs arising from sources in the lower atmosphere and penetrating to the highest altitudes in the thermosphere
This does not restrict the spatial scales that can occur in the thermosphere, nor does it mean that larger scales cannot arise in the lower atmosphere
Summary
There has been evidence of, and interest in, gravity waves (GWs) in the thermosphere and ionosphere (TI) for many years. Modeling and theoretical studies suggested that GWs arising from deep convection could penetrate to mesospheric and lower thermospheric (MLT) altitudes and have corresponding TI responses (Alexander et al, 1995; Piani et al, 2000; Lane et al, 2001; Lane and Clark, 2002; Horinouchi et al, 2002; Vadas and Fritts, 2004, 2006, hereafter VF06; Vadas, 2007, hereafter V07) Studies such as these, and correlations of GW source regions with ionospheric effects such as equatorial spread F and plasma bubbles, led to recurring suggestions that GWs may play a role in seeding these dynamics (Anderson et al, 1982; McClure et al, 1998). A more recent theory accounting for kinematic viscosity and thermal diffusivity and their variations with altitude assuming a localized, but temporally-varying, GW packet was advanced by Vadas and Fritts (2005, hereafter VF05) This has allowed more complete assessments of GW structure, thermospheric penetration, and momentum transport accompanying increasing dissipation for a wide range of GW scales and propagation conditions (VF06; V07).
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