Oxygen and hydrogen ion densities above Millstone Hill

Abstract

Measurements of the vertical flux of oxygen ions, when combined with simultaneous measurements of electron density and of electron and ion temperatures, present a unique opportunity to examine conditions in the topside ionosphere. The measured fluxes and densities may be used directly to evaluate terms in the O + continuity equation without requiring any assumptions to be made about neutral winds, electric fields, or the ambipolar diffusion coefficient. From observations at Millstone Hill we have, in this way, derived the rate of loss of O + in the charge-exchange reaction with hydrogen, the rate of photoionization of atomic oxygen, and the rate of loss of O + in reactions with N 2 and O 2. In combination with laboratory and theoretical results, these rates can be interpreted to yield number densities of the corresponding neutral species. We have been able, moreover, to demonstrate that the upward flux of hydrogen ions at the time in question was about 10 8 cm −2sec −1, which is close to the critical flux for these conditions. When the measured fluxes are examined for consistency with measured density gradients and temperature profiles, we find clear evidence of wind-induced or electrodynamic vertical drifts larger than 60 m sec −1. The magnitude of the non-diffusive drift can not be uniquely determined, however, because of uncertainty in the atomic oxygen density and hence in the ambipolar diffusion coefficient.