Phase relations between polar magnetic substorm fields at the surface of a finitely conducting Earth

Most past studies of polar electrojet morphology have not included consideration of the finite electrical conductivity of the earth. Here we present theoretical calculations based on realistic earth models and show that magnetic disturbance field components measured on the ground are expected to be offset in phase from one another by 10–20 min and from the electrojet by up to 10 min at a period of about 2 hours. These phase shifts are large enough to lead to misinterpretation in studies of spatial motions of the electrojet as well as to affect the temporal correlation between ground-based records and in situ measurements of ionospheric processes. The analysis of a standard magnetogram supports our results.