Ionic mobility of the middle atmosphere

Positive ion mobilities are calculated for 40–75 km by computing the theoretical positive ion composition and combining it with laboratory-determined mobilities. Theoretical determinations for mobility appear to be especially apt for the 40–65 km region since oxonium ions are observed to be the principal positive ions and they should be subject to thermodynamic equilibrium. We compute a mean reduced mobility of 2.1 ± 0.1 cm²/V-s for 40–65 km using the 1976 U.S. Standard Atmosphere and a water vapor mixing ratio of 5 ppmv. The results are compared with atmospheric data for mobility. Observations of a lower mobility from about 35 km down to ground level are qualitatively compatible with the onset of the so-called non-proton hydrate ions at about this altitude and extending to lower heights. We note that the laboratory determined mobilities for oxonium ions average about 12 % less than the theoretical Langevin values. The total positive ion conductivity is determined also and compared with in-situ results.