Thermal effects of nonlinear acoustic-gravity waves propagating at thermospheric temperatures matching high and low solar activity

Abstract

Numerical high-resolution modeling of nonlinear acoustic-gravity waves (AGWs) generated at the Earth's surface and propagating to the thermosphere shows that wave characteristics are depending on modifications in the mean density, temperature, molecular dissipation and composition due to variations of solar activity (SA). Amplitudes of temperature wave perturbations are generally larger at high SA at altitudes above 150 km, due to larger mean temperature and smaller molecular heat conductivity. Increasing kinematic coefficients of molecular heat conduction and viscosity result in stronger decreasing AGW amplitudes at altitudes larger 150 km at low SA. Dissipating AGWs generally produce heating at altitudes below 120 km. At larger heights, AGWs generally heat the thermosphere at low SA and cool it at high SA. Wave enthalpy fluxes are mainly upwards below 120 km altitude and downwards above 150 km at high SA, where they may have directions opposite to the upward wave energy fluxes. Downward wave enthalpy fluxes correspond to AGW cooling the upper atmosphere at high SA. Nonlinear dissipating AGWs may produce upward and downward transport of atmospheric mass. These mass flows may produce adiabatic heat influxes in the upper atmosphere. Mainly positive residual wave-induced mass flows at altitudes higher 150 km may contribute to the wave cooling of the upper atmosphere. Wave breaking and interactions between waves and the mean flow in the nonlinear model are stronger at higher amplitudes of AGW excitation at the ground, which lead to bigger energy losses for larger-amplitude waves. At high SA, resulting effects in the thermosphere depend on the balance between, on one hand, increases in wave amplitudes, caused by weaker molecular dissipation and smaller transfer of the wave energy to the wave-induced jet flows, and, on the other hand, decreases in the amplitudes due to higher density and larger AGW reflection. The thermal effects of waves in the upper atmosphere may depend on competitions between heating due to dissipation of the upward wave energy flux and cooling due to divergence of the downward wave entropy (or potential enthalpy) flux. At high SA, larger mean temperatures and larger temperature perturbations might increase magnitudes of downward wave entropy fluxes, which may result in more frequent downward wave enthalpy fluxes and wave cooling of the upper atmosphere.