Computation of the ion/neutral density ratio in the presence of wave and chemical effects

Abstract

We examine in this paper the relationship between ion and neutral density fluctuations due to gravity wave motions in the presence of ion source and sink terms. Our results indicate that the ion/neutral density ratio in the middle atmosphere ranges between two very different limits, depending on the characteristics of the wave motion and the ion production and recombination rates. For wave motions with frequencies much larger than ion chemistry rates, the ion/neutral density ratio approaches the value appropriate for adiabatic motions in the absence of chemical effects, with ion and neutral fluctuations negatively correlated and a magnitude dependent on the ion and neutral density and pressure scale heights. For wave motions with lower frequencies, the ion/neutral density ratio approaches a value consistent with chemical equilibrium, with ion and neutral density fluctuations positively correlated and an amplitude ratio of 1 2 . Our results also indicate that, for wave parameters and ion chemistry time-scales appropriate for the middle atmosphere, significant phase shifts between ion and neutral fluctuations relative to the adiabatic or chemical equilibrium limits may result. This has important implications for the inference of wave and turbulence fluctuations in the neutral atmosphere from measurements of ion densities using rocket-borne ion probes or other comparable instrumentation.