Influence of solar variability on gravity wave structure and dissipation in the thermosphere from tropospheric convection

Abstract

We investigate the role convection plays in the thermosphere using deep convection and ray trace models with a general dissipative anelastic GW dispersion relation. In the absence of dissipation, a GW's vertical wavelength λz increases by √T̄/T̄0, or greater if its intrinsic frequency ωlr is close to the smaller thermospheric buoyancy frequency N = √T̄0/T̄ N 0. Here, T̄ and T̄0 are the asymptotic temperatures in the thermosphere and lower atmosphere, respectively, and N 0 is the buoyancy frequency in the lower atmosphere. In the presence of dissipation, λz also increases in the thermosphere by a factor of ∼2-3 when ωlr > 0.2N0 and λz > 25 km during active solar conditions. GW dissipation altitudes and maximum vertical wavelengths, which increase as T̄ increases, are displayed for small-scale and midscale GWs. GWs excited from deep convection encounter horizontal shears, which impose anisotropy on the spectrum. Along with dissipative filtering, momentum flux divergence and body forces result. The thermospheric body forces resulting from our convection model achieve maximum accelerations at z ≃ 180-200 km, extend down to z ≃ 130 km, last for the duration of deep convection, are ∼600 km × 600 km × 40-80 km in the x, y, and z directions, and are very strong with accelerations ≃0.5-0.75 m s-2 and ≃0.25-0.4 m s -2 during extreme solar minimum (T̄ = 600 K) and active solar conditions (T̄ = 2000 K), respectively. During extreme solar minimum, there is negligible forcing above z ≃ 230 km, whereas the forcing extends up to z ≃ 360 km during active solar conditions. These horizontal, thermospheric body forces may be a new source of large-scale, long-period secondary GWs and induced TIDs (traveling ionospheric disturbances) at high altitudes in the thermposphere. Copyright 2006 by the American Geophysical Union.