Altitude dependencies in the solar activity variations of the ionospheric electron density

In this study, 7 years (1986-1992) of measurements from the Japanese middle and upper atmosphere (MU) radar have been analyzed to investigate the sola r activity variations of the ionosphere. The observations show strong altit ude dependencies in the solar activity variations of the electron density. Below 300 km altitude, the electron density increases nonlinearly with F10. 7, with the rate of increase being much lower when F10.7 is greater than 15 0. This nonlinear variation becomes weaker with increasing altitude. Above 450 km altitude, the electron density increases almost linearly with F10.7 when F10.7 lies in the range 100 to 250. For values of F10.7 less than 100, the electron density at low altitudes increases with increasing F10.7, whi le at higher altitudes (above about 400 km) the electron density remains al most constant. Mechanisms to explain the observed behavior have been invest igated using the Sheffield University Plasmasphere Ionosphere Model. The mo del calculations show that while the variations with F10.7 of the solar EUV flux and neutral gas densities play important roles in the nonlinear varia tions of the electron density with F10.7, the correlation of the plasma los s rate with temperature being negative at low temperatures and positive at high temperatures is an important mechanism for the nonlinear variations at low altitudes. Model results also suggest that vibrationally excited Na st rengthens the nonlinear variations of electron density with F10.7. The disa ppearance of the nonlinear variations at high altitudes, for values of F10. 7 above 100: results from the altitude dependencies of the neutral gas vari ation on F10.7 and of the relative importance of plasma loss, production, a nd diffusion processes. At high altitudes, the plasma loss processes, which play an important role in the nonlinear variations at low altitudes, are u nimportant when compared with the effects arising from plasma production an d diffusion. The altitude dependencies of the electron density variations w ith F10.7, for values of F10.7 less than 100, are due mainly to the altitud e dependencies of the neutral gas densities with F10.7.