Auroral electrojet AL index and polar magnetic disturbances in two hemispheres

Earlier we found that the auroral electrojet AL index, indicating substorm activity in the northern hemisphere, in local summer months correlates much better with geomagnetic activity not in the nearby polar cap but in the opposite polar cap; we explained this effect as a result of interhemispheric field-aligned currents, which suppress substorm field-aligned currents in the summer hemisphere but increase these currents in the winter hemisphere. In the present paper, we took into account this effect and examined a method for reliably monitoring the substorm auroral electrojet, measured with the auroral electrojet AL index, by using hourly averages of geomagnetic field from two polar observatories (Thule and Vostok) in two hemispheres. We tested this method for 3 years. The correlation between the predicted and actual AL indices for these years was stable and very high, and it showed no significant dependence on season and a relatively weak UT variation. The correlation coefficient between the predicted and actual AL indices for these three years was about 0.89. The proposed method, based on using magnetic field data from two polar geomagnetic observatories in two hemispheres, not only significantly improves the reliability of monitoring the westward auroral electrojet in the northern hemisphere but it may also be used for monitoring the westward auroral electrojet in the southern hemisphere where no AL index is available because a significant portion of the southern auroral zone is located over the oceans. The results of this paper show that measurements of geomagnetic field in two hemispheres are of high importance for reliably monitoring the geomagnetic activity and related events in each hemisphere.