Chemical processes involving helium ions and the behavior of atomic nitrogen ions in the upper atmosphere

Abstract

The purpose of this note is, first, to suggest that the chemical loss process for helium ions in the upper atmosphere may affect the distribution of N+ above an altitude of 300 km and, second, to give an upper limit for the rate coefficient of the reaction between helium ions and molecular nitrogen consistent with atmospheric data. Recent laboratory experiments indicate that the most important chemical loss processes for helium ions are the dissociative charge transfer reactions Measurements of reactions 1 and 2 at room temperatures (∼300°K) by Ferguson et al. [1964], as well as those at higher energies obtained recently by Stebbings et al. [1965], lead to extrapolated rate coefficients at ionospheric temperatures k1 ≈ k2 ≈ 10-9 cm3 sec−1. Because of the virtual equality of k1 and k2, process 1 would be the most important loss process for helium ions in the upper atmosphere since the atmospheric concentration of N, exceeds that of O2,. There seems to be little doubt about the accuracy of the laboratory measurements for reactions 1 and 2; however, the application of the laboratory rate coefficient for reaction 1 to the ionosphere leads to a serious dilemma. To be consistent with observed concentrations of He+ in the upper atmosphere [cf. Hanson, 1962; Taylor et al., 1963; Pokhunkov, 1963], a rate coefficient as high as k1 = 10−9cm3 sec−1 would require a helium ion production two orders of magnitude higher than currently considered as resulting from photoionization of neutral helium. (A rate coefficient k2 = 10−9 cm3 sec−1 is not necessarily inconsistent with atmospheric data, because of the lower O2, concentration). In view of actual measurements of the neutral helium concentration [Reber, 1964], together with our knowledge of the cross section and the flux of ionizing radiations for He, even an error of one order of magnitude in the production rate of He+ is excessive, and, therefore, this possibility seems unlikely. The other alternative is to assume that the laboratory values for reaction 1 are really not applicable to the ionosphere. Stebbings et al. [1965] have suggested the possibility that the cross section for (1) may be quite different when excited nitrogen molecules are involved, and if this were the case, the rate coefficient applicable to the ionosphere may well be lower.