UFKW Propagation in the Dissipative Thermosphere

Ultra-fast Kelvin waves (UFKWs) serve as a mechanism for coupling the tropical troposphere with the mesosphere, thermosphere and ionosphere. Herein, solutions to the linearized wave equations in a dissipative thermosphere in the form of Hough Mode Extensions (HMEs) are employed to better understand the vertical propagation of the subset of these waves that most effectively penetrate into the thermosphere above about 100 km altitude; namely, UFKWs with periods ?4 days, vertical wavelengths (lambda(z)) ?30 km, and zonal wavenumber s = -1. Molecular dissipation is found to broaden latitude structures of UFKWs with increasing height while their vertical wavelengths (lambda(z)) increase with latitude. Collisions with ions fixed to Earth's magnetic field (ion drag) are found to dampen UFKW amplitudes, increasingly so as the densities of those ions increase with increased solar flux. The direct effect of ion drag is to decelerate the zonal wind. This leads to suppression of vertical velocity and the velocity divergence, and related terms in the continuity and thermal energy equations, respectively, that lead to diminished perturbation temperature and density responses. Access is provided to the UFKW HMEs analyzed here in tabular and graphical form, and potential uses for future scientific studies are noted. Plain Language Summary In earth's atmosphere, Kelvin waves (KWs) are eastward-propagating oscillations with periods of days to weeks that are centered on the equator and confined to low latitudes. They are forced by the spatial-temporal variability of the heat of condensation (latent heating) that is released when rising moist air forms rain droplets, mainly in the tropics. As with many atmospheric waves, they propagate vertically and grow exponentially with height in concert with the decrease in atmospheric density and pressure. The KWs that survive the trip from near the surface to about 100 km altitude are called ultra-fast Kelvin waves, or ultra-fast Kelvin waves (UFKWs) . Just above 100 km, they reach maximum amplitudes where their exponential growth is curtailed by the viscosity of this part of the atmosphere. Here they interact with ionized particles (the ionosphere) and generate electric fields that ultimately drive ionospheric variability at higher altitudes (> 200 km), thus presenting an element of space weather to navigation and communications systems. In this paper we model a set of UFKWs to better understand how their amplitudes, vertical and latitudinal structures change as they propagate above 100 km, and in so doing advance our knowledge of the physical processes underpinning near-earth space weather.