Ionospheric Current Structure as Determined from Ground-based Magnetometer Data

Abstract

Summary The magnetic perturbations from a three-dimensional current system commonly used to describe magnetospheric substorm variations have been inverted, by using the linear inverse theory of Backus and Gilbert, to obtain estimates of the (height-integrated) ionospheric current density producing the observations. Under the assumption that electrojet intensity is not a function of longitude it has been found that displacement of stations from the central meridian yields no significant change in resolution of the current density so long as the observation sites remain within the longitudinal limits of the ionospheric current; hence a linear array can provide as much resolution as a two-dimensional one. The maximum resolution possible is shown to be ∼ 1° latitude or about one ionospheric height unit. An investigation of the dependence of resolution (of a linear array) on station spacing shows that the optimum spacing is dependent upon the number of stations and the desired standard deviation of the estimate of the current density; nevertheless, the rule of thumb that stations should be separated by 1° of latitude is shown to be a good compromise between maximizing resolution and simultaneously minimizing the number of stations. Inversion results using data from a polar magnetic substorm of 1970 June 15 indicate that the magnitude of the westward current density was at least 0·90 ± 0·15 A m−1 and that the current density cannot be approximated by a constant value over widths much greater than 1° of latitude. Published magnetic data from the equatorial region have also been inverted to obtain estimates of the current density in the equatorial electrojet.