Latitudinal effect in semiannual variation of geomagnetic activity

Abstract

The study of the semiannual variation in both geomagnetic activity indices and raw geomagnetic field data at a large number of geomagnetic observatories showed a dramatic decrease in the relative magnitude of this variation from low to high latitudes. While the semiannual variation in the Dst index at low latitudes is about 3 times, for the midlatitude Ap and Am indices it is only about 20–30%, and the auroral electrojet AE index and polar cap PCN index show a very small or no semiannual variation. The study of auroral electrojet AU and AL indices showed that the semiannual variation is evident in AL index and not observed in AU index. Although relative magnitudes of the semiannual variation in geomagnetic activity indices are strongly reduced at high latitudes, absolute magnitudes of these variations in the Dst, Ap, Am, and AL indices are approximately the same. The semiannual variation in geomagnetic field H component consists of two equinoctial minima probably caused by the increasing westward ring current and substorm westward auroral electrojet in equinoctial months. The additional analysis of raw geomagnetic field data from geomagnetic observatories in two hemispheres confirmed the results inferred from the study of geomagnetic indices. Meanwhile in the polar caps, the semiannual variation in raw geomagnetic data showing a strong decrease in geomagnetic field H component reveals a strong increase in the Z component which also might be caused by increasing the ring current and westward auroral electrojet during enhanced geomagnetic activity occurring in equinoctial months. It was also found that the amplitude of the semiannual variation at all latitudes increased significantly with increasing geomagnetic activity.