An investigation of the polar electrojet current system using radio wave heating of the lower ionosphere

The HIgh Power Auroral Stimulation (HIPAS) heating facility has been used to modulate D-region ionospheric currents at high latitudes, producing extremely low frequency (ELF) radio wave emissions. The behavior of these ionospheric currents can be deduced from a comprehensive study of the ELF signals received at a local field site. This document examines the relationship between the ELF magnetic field strength measured on the ground and the intensity of an overhead electrojet current. The mapping of the polar electrojet current from the E region down through the D region, where it can then be modulated by the heater beam, is investigated. A finite difference solution to the electrojet mapping problem is presented in which arbitrary conductivity profiles can be specified. Results have been obtained using a simple Cowling model of the electrojet. These results indicate that for an electrojet flowing at an altitude of 110 km with a scale size in excess of 100 km, the mapping of the horizontal current density can be completely characterized in terms of the Pedersen and Hall conductivities. This indicates that the mapping becomes independent of scale sizes which exceed 100 km. The downward mapping of the electrojet current and associated electric field in the presence of ionospheric conductivity irregularities has been studied. It was demonstrated that D region conductivity irregularities do not significantly change the downward mapping of ionospheric electric fields. However, these conductivity irregularities were found to have a major influence upon the downward mapping of ionospheric currents. A promising new diagnostic technique, for studying ionospheric D region currents, has been implemented using the HIPAS facility. This technique involves HF beam steering for localized ELF generation in the mapped region below electrojets. Beam steering has been used to deduce the strength and current distribution of the polar electrojet, and for charting the movements of overhead currents.