Comparison of auroral electrojet indices in the Northern and Southern Hemispheres

Abstract

The auroral electrojet (AE) index is traditionally calculated from a set of about 10 to 13 ground magnetometer stations located around the typical northern auroral oval location. Similar coverage in the Southern Hemisphere does not exist, so the AE calculation has only been performed using Northern Hemisphere data. In the present paper, we use seven southern auroral region ground magnetometers as well as their conjugate Northern Hemisphere data to calculate conjugate AE indices during the Northern Hemisphere winter using the standard method. The correlation coefficient between the northern and southern AE indices for many of the intervals is above 0.7, but in one interval, it is close to 0. We compare our conjugate AE indices with the standard AE index and find a number of asymmetries because of station coverage gaps both in our conjugate indices and in interplanetary magnetic field (IMF) conditions. When the southern AE index is compared with the standard AE index, we find that for most intervals the correlation is less than 0.7, and in one interval, the correlation is about 0. The correlation between the conjugate northern AE index and the standard AE index is somewhat better, suggesting true interhemispheric asymmetries. The mean difference between the southern and northern AE indices is largest during southward IMF and for large values of IMF ∣By∣ (>5 nT). This is most likely due to the increased activity levels during southward IMF and the greater twisting of the magnetic field lines during strong IMF By. IMF Bx seems to have no effect on the interhemispheric differences of the AE index. The mean differences between the southern and conjugate northern H component are of the order of ∼35 nT, with the largest differences occurring in the midnight magnetic local time (MLT) sector. We suggest that these differences may be related to seasonal effects, ionospheric effects, and MLT effects.