Tidal effects on D and E region ion chemistries

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Abstract
Numerical simulations are presented which demonstrate that diurnal and semidiurnal oscillations in temperature and O, O2, and N2 densities can produce asymmetries of D and E region electron concentrations about noon similar to those observed experimentally. In the D region it is assumed that NO+ is the precursor ion in a chain which involves three‐body formation of the intermediary cluster ions NO+(H2O)m−1 (X) (m=1–3), where X can be N2, O2, H2O, or CO2, switching reactions which convert these weakly bound clusters to hydrates of NO+, and reaction of the third hydrate of NO+ with H2O to initiate the chain to form H+(H2O)n (n=1–7). The rates of three‐body and thermal breakup reactions are affected by tidal oscillations in the ambient temperature and total density. For instance, lower (higher) temperatures enhance (inhibit) the formation of clusters and inhibit (enhance) their thermal breakup, thus reducing (increasing) the electron concentration, since the recombination coefficients of cluster ions increase with cluster size and are all large in comparison with that of NO+. In the E region, ion production is affected by variations in the attenuation of solar flux due to optical depth changes of the overlying atmosphere, as well as local changes in O, O2, and N2 densities. A noon bite‐out in D region electron concentrations, which is sometimes observed experimentally, could not be simulated with the present model. Further, a noon bite‐out at mid‐latitudes cannot be produced via modulation of the electron concentrations by any reasonable tidal variation in electron‐neutral collision frequency (it is the product of these quantities which is actually inferred from the measurements). It is suggested that such an effect might be produced by temperature oscillations associated with a gravity wave propagating through the region, or a time variation in NO concentrations due to vertical transport by tides, gravity waves, or changing eddy mixing rates.

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