Technique for determining midlatitude O+/H+ transition heights from topside ionograms

The midlatitude O⁺/H⁺ transition height (where the H⁺ and O⁺ number densities are equal) is of importance because it is where the topside altitude distribution of the dominant ions transitions from the ionospheric F region into the plasmasphere, which is considered the inner region of the magnetosphere. This transition height can be determined by fitting ionospheric topside sounder–derived electron density (Ne) profiles to analytical H⁺ and O⁺ functions. There are four variables involved in this process, two involving ion number densities and two involving the electron temperature Te. The density variables are the H⁺ density at the height of the satellite and the O⁺ density at the base of the profile (taken as 400 km). The temperature variables have been treated using different approaches. In an earlier investigation, diffusive equilibrium ion density profiles, based on an earlier Titheridge height-varying electron temperature function, were used to fit the Ne profiles in which the electron temperature Te and the Te altitude gradient at a base height of 400 km, denoted by T0 and G0, respectively, were free variables. In the present work, T0 and G0 are constrained by using a later Titheridge empirical temperature model. Alternatively, when in situ Langmuir probe Te determinations are available, the problem reduces to one with only one free temperature variable. All three of these approaches, using Titheridge's revised height-varying electron temperature function, are applied to a sequence of midlatitude ISIS 2–derived Ne profiles obtained during a period of prolonged high magnetic activity. The results indicate that in the inner plasmasphere, where the transition height is slowly varying, the approach based on the two free Te parameters (T0 and G0) agrees well with the one using the Langmuir probe input. In the region where the transition height is rapidly increasing, however, the approach based on the empirical temperature model produced the most consistent results for the O⁺/H⁺ transition height.