BASIC PROPERTIES AND GRAVITY-WAVE INITIATION OF THE MIDLATITUDE F-REGION INSTABILITY

Observations of midlatitude spread F irregularities have shown th,t the most violent eruptions are often periodic and may be related to modulations of the F region bottomside. Similar aspects of equatorial spread F have been conclusively shown to be the result of seeding by atmospheric gravity waves. hi this paper, we pursue a nonlinear description of the midlatitude F region instability first proposed by Perkins (1973) and show that, by itself, this instability saturates at approximately 1%. With gravity wave seeding, however, the growth of the instability is significantly enhanced and progresses until it is limited by third- or fourth-order nonlinearities. In the course of the analysis we expand the Perkins linear instability study, particularly with regard to the wave propagation angles suitable for stimulating linearly unstable, and eventually, nonlinear turbulent upwellings. Although gravity waves themselves have limited spatial extent, we also show that gravity waves which propagate perpendicular to the Earth's magnetic field create electric fields which map along the field lines to considerable distances. This transmission of electric fields is dependent on both the particular characteristics of the ionosphere along the field lines and the wavelength of the perturbation, but under certain circumstances, gravity wave-induced large-scale electric fields can map to the F region from either the E or conjugate F region and thus influence F region electrodynamics. Finally, we note that our results axe consistent with observations of midlatitude spread F irregularities by the MU radar in Japan and large-scale perturbations of the F region over Arecibo.