Ionospheric variability from an incoherent scatter radar long‐duration experiment at Millstone Hill

An incoherent scatter radar experiment at Millstone Hill covering 30 consecutive days in September 2005 has enabled this study of day‐to‐day ionospheric variability. This was a period of low solar activity with few magnetically disturbed periods. Our discussion focuses on ionospheric variability during quiet magnetic activity in the 100–500 km height range, with emphasis on its height variation at noon. (1) Very large midday variability is present for the ion temperature Ti near 120 km, which is verified by two other 30‐d experiments at Millstone Hill. This is not apparently associated with solar flux and magnetic activity. The percentage variability in the midday electron density Ne changes with height, being smaller between 150 and 250 km and larger in the topside. (2) With increasing solar flux, Ne decreases between 170 km and the F 2 peak and increases elsewhere, being essentially unchanged near the F 2 peak. With increasing magnetic activity, Ne decreases between 160 and 325 km. Ti increases with solar flux and magnetic activity, in particular in the F 2 region. (3) There is a time lag of ionospheric responses, varying with height, to changes in solar‐geophysical conditions: In the E region, the lag is almost zero; above the F 2 peak, both Ne and Ti respond to F 10.7 with a 2–3‐d delay. The delay in response to 3‐hourly ap index changes for Ne above the F 2 peak can be 9–12 h and between 160 km and the F 2 peak can be 0–3 h. The time delay for Ti is 6–9 h. (4) We estimate that the majority of the topside variability in Ti and Ne can be explained in terms of solar flux F 107 and magnetic activity ap effects. Near the F 2 peak, Ne variability seems to be complicated, and nearly one half (10%) of it cannot be ascribed directly to F 10.7 and ap effects.