Incoherent scatter contributions to studies of the dynamics of the lower thermosphere


  • John V. Evans


This paper reviews what has recently been learned by means of the incoherent scatter radar technique concerning the dynamics of the lower thermosphere (100–150 km). In this region, thermal tides generated in situ and propagating upward from below appear to be the principal source of motions at low and middle latitudes. The winds that they establish can drive ions across magnetic field lines, thereby establishing electric currents and polarization electric fields (which in turn modify the motions of the ions). Efforts to understand this region have traditionally been made through calculations of the tidal amplitudes, from studies of magnetometer records, and from vertical wind profiles observed via chemical releases from rockets. The direct measurements of the temperatures, winds, and electric fields made possible by means of incoherent scatter radar probing have confirmed some early ideas and also have revealed considerably more complexity than had been anticipated. Presently available evidence indicates that the interval 100– 125 km is dominated by upward propagating solar semidiurnal tidal modes (e.g., (2, 4) and (2, 5)) generated in the mesosphere where background winds couple energy out of the (2, 2) mode, which is the mode most strongly excited in the stratosphere-mesosphere. These higher-order modes appear to be damped above 120 km by viscous dissipation. In the region 125–150 km the (1, −2) trapped diurnal mode generated by direct absorption of solar EUV seems to be the predominant one at midlatitudes, although some of the energy remaining in the semidiurnal (2, 2) mode appears to leak up to these heights. Nearer the equator it appears that the (2, 2) tide dominates the region below 200–300 km, particularly in summer. These tides create winds that appear capable of explaining a variety of related phenomena including the Sq (solar quiet) dynamo current system, the generation of sporadic E layers and layers intermediate between the E and F layers, and the drift of ions in directions normal to the magnetic field at F region heights (i.e., electric field induced drifts) seen within the plasmasphere during the day.