F-REGION SEASONAL BEHAVIOR AS MEASURED BY THE MU RADAR

We present MU radar ionospheric incoherent scatter results from the period December 1986 to August 1988. We extend our previous 3-month model of the ion vector velocity to a full year and determine seasonal variations. We also present solstitial and equinoctial patterns of electron-density and plasma-temperature behavior. Compared with our previous 3-month model, the full-year velocity model is largely diurnal in nature, the higher harmonic being very variable and tending to average out as noise over a long period. The seasonal behavior of the electron density may be explained largely in terms of the behaviors of the neutral composition and of the timing of the diurnal change of direction of the neutral wind with respect to the time of sunset. According to the neutral composition a high-altitude O+ layer and a low-altitude molecular-ion layer compete for density predominance. The molecular-ion layer often becomes stronger during summer, and also during times of strong magnetic disturbance. The mean equinox daytime F-region electron and ion temperatures (T(e) and T(i)), and particularly their ratio (T(r) = T(e)/T(i)), is controlled largely by the electron density, with other specific time-of-day effects. At night T(r) = 1, and at sunrise T(r) > 1 at all F-region altitudes measured. During midday we have two cases. For a peak F-layer density of about 10(6) cm-3 or greater, T(r) > 1 below 300 km altitude and T(r) = 1 above 300 km altitude; for lower electron densities T(r) > 1 at all F-region altitudes. Near sunset T(r) generally increases as the electron density decreases while photoionization is continuing.