EXB INSTABILITY IN THE F-REGION AT LOW-LATITUDE TO MIDLATITUDE

Macroscopic instability in the nocturnal ionospheric F-region at midlatitudes is examined when eastward external electric fields exist. The instability growth rate is calculated considering plasma in the entire flux tube. Ion density distribution is obtained by a modeling approach, in which the ion continuity equations for O+ and H+ are solved. The gravitational Rayleigh-Taylor mode is stable at midlatitudes higher than the crests of the equatorial anomaly, while the E × B mode is unstable at all latitudes when the electric fields are eastward. Net instability growth rate is negative in the narrow region around the magnetic field line which crosses the equator at 1100 km and penetrates the F-layer peak near 18° in magnetic latitude. In lower and higher latitude regions, the ionosphere becomes unstable. In the lower latitude region the instability accounts for generation of plasma bubbles, while in the higher latitude region it accounts for generation of plasma ducts, plasma blobs observed in the topside ionosphere, and midlatitude bottom- side spread F. © 1990.