A study of F2 region night-time vertical ionization fluxes at millstone hill

Vertical fluxes of ionization in the F2 region have been measured by the incoherent scatter technique over Millstone Hill in 1969. The results obtained near midnight for the region above h_maxF2 have been examined to determine whether there is a significant flux of ionization from the magnetosphere to the ionosphere that serves to maintain the F-layer. It is found that H⁺ ions are a minor constituent over the altitude range in which useful measurements can be made, so that any conclusion must rest upon properly interpreting the observed O⁺ fluxes. By selecting periods when the layer did not appear to be decaying rapidly it was hoped to find cases where the O⁺ flux did not vary with altitude in the range 500 h 800 km (i.e. where losses are unimportant), since this would imply that the flux is of magnetospheric origin.

While three cases exhibited this behaviour, the majority exhibited a decrease in the O⁺ flux with height, indicating that the layer was descending. Attempts to correct for this were made, and the average flux from the magnetosphere was estimated as 3 × 10⁷ el/cm²/sec. This is in fair agreement with other recent estimates, and implies that at this latitude the ionosphere is not maintained solely by the magnetospheric flux. Moreover, large increases in flux that could give rise to nocturnal increases in the total content of the layer do not appear to have been seen.