This project will investigate the sources of observed perturbations to the F region ionosphere due to upward propagating gravity waves at two incoherent scatter radar sites, Poker Flat and Arecibo. A by-product of the research will be the extraction of time-varying F region neutral winds. The research activities include making gravity wave observations using a new technique at the two radars, modeling of secondary gravity wave excitation as well as the propagation and dissipation of these gravity waves, and reverse ray tracing studies. The F region of the atmosphere frequently exhibits medium-scale traveling ionospheric disturbances that are thought to be gravity waves propagating upwards from the lower atmosphere, but their sources are generally unknown. Of particular interest is determining the role of secondary gravity waves in producing the ionospheric disturbances. Two types of secondary wave generation will be investigated in this project: those excited by mountain wave breaking in the wintertime thermosphere at high latitudes and those excited by the breaking of gravity waves in the mesosphere. One of the tools used to address the identification of gravity wave sources is reverse ray tracing. The essential information needed for the ray tracing approach is: 1) an accurate gravity dispersion relation, and 2) knowledge of the gravity wave's horizontal and vertical wavelengths and its period. Recently, an accurate viscous gravity wave dispersion relation which accounts for realistic thermospheric dissipation was derived and incorporated into a 3D ray trace model under a previous grant. Meanwhile, information on gravity wave wavelengths and period have recently become available from new multi-beam experiments at the Poker Flat and Arecibo radars. As part of this project, observations of electron density perturbations associated with the passage of gravity waves will be made at the two radars. These measurements will be the basis for the modeling studies, which emphasize gravity wave (GW) coupling between the lower atmosphere and the thermosphere. The specific tasks and objectives include: determining wavelengths and periods from the density and velocity perturbation profiles for selected gravity waves at Poker Flat and Arecibo; extracting the temporal evolution of the neutral winds and accelerations in the F region from these profiles using the viscous gravity wave dispersion relation; calculating the secondary gravity wave spectra excited from breaking mountain waves, and characterizing the scales and amplitudes of the GWs in the F-region via ray tracing; determining the lower-atmospheric sources of the gravity waves observed in the thermosphere for three case studies at Poker Flat and one or two case studies at Arecibo; testing the accuracy of the viscous GW dispersion relation by comparing the extracted neutral winds with the measured neutral winds along the magnetic field at Arecibo.
A perturbed fluid will radiate waves; in the atmosphere, a major source of variability is that caused by propagating waves under the influence of gravity (gravity waves). Such waves can propagate large vertical and horizontal distances. In general, gravity waves grow exponentially with increasing altitude to conserve energy, and in doing so cause variability far from their source regions and act as a mechanism for energy transfer between atmospheric regions. The goal of this project was to investigate experimentally and theoretically the properties and source regions of gravity waves in the upper atmosphere, in particular by using their ionospheric manifestation as traveling ionospheric disturbances (TIDs). To do this, we utilized the multi-beam capabilities of the Poker Flat Incoherent Scatter Radar (PFISR) in Alaska and the Arecibo Observatory in Puerto Rico to determine the fundamental properties of TIDs. Our work included measuring vertical wavelengths, horizontal wavelengths, periods, and propagation directions, so that we could investigate details of the source regions of these waves. Specific investigations included detailed study of the dynamics of individual gravity wave events. These included the study of long-period, large-scale, inertia-gravity waves that are generated in the lower atmosphere, and propagate long distances to the mesosphere / lower thermosphere, where they become saturated by instabilities and interact with the background fluid. These interactions accelerate the background fluid, causing a dynamical and measurable change in the local wind conditions. These investigations allowed us to quantify the effect of individual, nearly monochromatic waves on the dynamics of the region in great detail. Additional investigations included the derivation of theoretical relationships relating the phases and amplitudes of gravity waves as they dissipate in the thermosphere. These relationships have numerous applications, and will help scientists extract and quantify gravity wave parameters from measurements of atmospheric and ionospheric perturbations. By applying these theoretical expressions to satellite and ground-based case studies, we showed that we can get unique and fundamental information about propagating waves from distributed and often disparate measurements.
