Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsa21b1547h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SA21B-1547
Other
2487 Wave Propagation (0689, 3285, 4275, 4455, 6934), 3369 Thermospheric Dynamics (0358), 3384 Acoustic-Gravity Waves, 4564 Tsunamis And Storm Surges
Scientific paper
Recent observations of F-region electron densities and of total electron content (TEC) have revealed disturbances that appear to be correlated with tsunamis. These observations show large electron density perturbations (~ 100%) and large TEC fluctuations (~ 30%) propagating at speeds of ~ 200 m/s and with a characteristic horizontal wavelength of ~ 300 km to 500 km. Published simulations of tsunami propagation through the atmosphere and subsequent interaction with the ionosphere have revealed striking similarities with the observations. However, important physical processes known to affect gravity wave propagation were not included in these prior analyses, while the ionospheric perturbation models they included were oversimplified. Here we describe numerical simulations of the upward propagation of a spectrum of gravity waves forced by a traveling deformation of the lower boundary and the interaction of these waves with the F-region ionosphere. The tsunami is assumed to travel as a steady-state disturbance at the lower boundary (z=0) with a vertical displacement described by a modified Airy function in the horizontal direction. It travels at the shallow water wave speed of 200 m/s. The horizontal wavenumber spectrum of the tsunami is first calculated and from this the vertical velocity spectrum at the surface is calculated. This spectrum is used to provide the forcing at the lower boundary of a spectral full-wave model. This model describes the propagation of linear, steady-state acoustic-gravity waves in a non-isothermal atmosphere with the inclusion of eddy and molecular diffusion of heat and momentum, ion drag, Coriolis force, and height-dependent mean winds. A steady-state 1-D ionospheric perturbation model including O+, NO+, O2+ and N2+ and electrons is used to calculate the electron density response to the tsunami. Electron density perturbations as a function of height and the total electron content (TEC) are calculated as a function of horizontal position. We perform simulations for an assumed maximum tsunami amplitude of 5 cm. Our simulations show that the effect of molecular diffusion is to strongly damp the waves in the topside (> 300 km altitude) ionosphere. In spite of this, the F-region response is large, with vertical displacements of ~ 2 to 5 km and electron density perturbations of ~ 100%. Mean winds have a profound effect on the ability of the waves to propagate into the F-region ionosphere. The higher frequency gravity waves in the spectrum are Doppler shifted to even higher frequencies for propagation into a headwind, which inhibits the propagation of the disturbance to F-region altitudes. We summarize our simulations by comparison with some ionospheric observations.
Hickey Michael P.
Schubert Gerald
Walterscheid Richard L.
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