MU radar observations of H+ ions in the topside ionosphere

Abstract

We report the detection of H+ ions in the topside ionosphere above the middle and upper atmosphere (MU) radar in Japan, the first such detection by a mesosphere‐stratosphere‐troposphere radar extended to perform as an incoherent scatter radar. Owing to the very limited signal‐to‐noise ratios of ∼1% achievable in the topside ionosphere with this radar, long integration times of 45 min and statistical analysis of 10 years of data are employed to yield useful results. This sensitivity restriction limits the top altitude of observation to only ∼400 km during solar‐minimum winter but up to ∼650 km during solar‐maximum summer. These measurements are possible only at night, when we may assume that the electrons and ions have the same temperature. While the data scatter is large, climatological averages constructed for various solar‐activity, seasonal, and magnetic activity cases show less than ∼5% H+ at most under conditions of detect ability, although it is exactly those conditions of undetectability which are expected to correspond to the highest proportions of H+. At solar maximum we see no detectable amounts of H+ up to 650 km altitude, except perhaps for a faint amount in winter. At solar minimum the data are good up to ∼550 km during summer and equinox but to only ∼400 km during winter. The H+ profiles for solar‐minimum summer and equinox closely resemble each other and reach ∼5% at 550 km altitude. We see no clearly discernible H+ in winter up to our 400 km altitude limit of detectability; we expect that there is substantial H+ present above 400 km, but the total ion density is so low that we cannot measure it. We see no detectable change in H+ between magnetically quiet and disturbed periods. The H+ proportions seen above the MU radar are much smaller than those seen above Arecibo, which has a similar magnetic latitude but much lower geographic latitude, and are much more comparable to those amounts seen by the higher‐latitude radars at Saint Santin and Millstone Hill. Comparisons with models suggest that at these low altitudes, H+ may well be near its charge‐exchange‐equilibrium value.