Polar cap potentials and the auroral electrojet indices

Abstract

Electric potentials across the auroral zones and polar cap have been determined by an integration of electric fields measured on the Dynamics Explorer-2 satellite. Potentials from 527 cases, divided between winter and summer seasons in the northern hemisphere, are compared with simultaneous auroral electrojet indices which are measured with ground-based magnetic observatories. The dusk and dawn auroral potentials and the total polar cap potential are compared with the AU, AL, and AE indices respectively. The AE indices are found to be higher when it is summer in the northern hemisphere, due to the higher ionospheric conductivity. The electrojet currents are roughly proportional to the potentials, but the AE indices have a range of possible values for a given potential due to variations in the width and distribution of the electrojets, the ionospheric Hall conductivity, and the neutral wind velocity. The data have been fit to linear and non-linear equations, which may be used to estimate the potentials from the AE indices. Although the non-linear equations give slightly better fits than a straight line, the differences in the errors are not significant. Where possible the data have also been grouped according to the phase of magnetospheric substorms. These results indicate that substorms occur only when the polar cap potential exceeds a threshold of approximately 60 kV.