Planetary waves in the thermosphere-ionosphere system

In the limit of an isothermal atmosphere without dissipation, the equations governing atmospheric motions admit solutions corresponding to ''free'' (unforced) oscillations for discrete zonal wavenumbers and periods. Global oscillations belonging to the rotational subclass are referred to as free Rossby waves or Rossby normal modes. Despite the existence of mean winds, nonisothermality, and dissipation, atmospheric manifestations of these waves apparently exist. To date the study of such oscillations has been confined mainly to the troposphere and stratosphere (0-50 km), which includes the source region for the disturbances. However, there is increasing evidence that the effects of free Rossby waves extend into the mesosphere/lower thermosphere (ca. 80-150 km) and ionosphere (ca. 90-400 km). The present work draws upon recent numerical modeling results and data analyses as a framework for discussing the penetration of planetary wave effects into the upper regions of the atmosphere, especially the ionospheric dynamo region (ca. 100-170 km) and the electrodynamic interactions which ensue there. Attention is primarily focused on the westward propagating normal modes of wavenumber s = 1 (periods of 5, 10 and 16 days), and the so-called ''2-day wave'' which is normally associated with the mixed Rossby-gravity mode with s = 3.