A study of the forenoon ionospheric F-2 layer behavior over the middle andupper atmospheric radar

The middle and upper atmosphere (MU) radar incoherent Scatter observations of electron density N-e profiles for 1986-1995 at Shigaraki (34.85 degrees N, 136.10 degrees E) show a forenoon density depression (bite-out) in summe r bo th on individual days and in statistical averages. The depression take s place generally between 0700 and 1200 LT, above the F-2 peak for low sola r activity and around the F-2 peak for high solar activity. The maximum ele ctron density NmF2 shows, in both percentage and time duration, a greater d epression for low solar activity than for high solar activity. With increas ing altitude, the forenoon N-e depression develops earlier for low solar ac tivity. In combination with the MU radar observations of ion drifts and pla sma; temperatures, our modeling indicates the following: (1) Transport proc esses such as the enhancement of electron temperature T-e, the poleward neu tral wind, and the reduced upper boundary ionization density help to reduce N-e and result in a negative partial derivative N-e/partial derivative t i n the morning hours in summer. Among those processes the morning enhancemen t of T-e is found to be most important for the presence of the forenoon bit e-out, since it greatly increases the O+ diffusion such that ionization out flow is significantly intensified. (2) The forenoon bite-out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O]/[N-2] in summer leads to a smaller daytime background N-e and thu s to an enhanced T-e, and the after-sunrise photochemical contributions (ma inly the O+ production) are so small that partial derivative Ne/partial der ivative t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the for enoon bite-out can be explained basically in terms of the low-electron-dens ity background. (4) The height of formation of the forenoon bite-out is als o related to the sunrise effect on photoionization.