EVOLUTION OF Dst AND AURORAL INDICES DURING SOME SEVERE GEOMAGNETIC STORMS

The evolutions of the geomagnetic Dst and auroral indices AU, AL, AE were examined in relation to interplanetary parameters for thirteen severe storms (Dst change exceeding 250 nT) during 1980-2003. Dst changes occurred every time Bz component of interplanetary magnetic field B turned negative. Among the auroral indices, changes in AU (representing an eastward auroral electrojet) were generally much smaller than changes in AL (representing a westward auroral electrojet), and both started increasing almost simultaneously with increases of interplanetary magnetic field B, even though its Bz component may not have turned negative. Hence AU and AL changes may precede Dst changes by a few hours and could have a prediction potential. AU and AL changes were not correlated well with each other; their inter-correlation was almost zero. Whenever Bz was negative, AL indices were invariably high, but the reverse was not true. Large AL changes could occur even when Bz was not negative (or even when positive). AL values showed many peaks during the storm and these were not invariably related to Bz changes. Successive AL peaks had separations in a wide range of 40-160 minutes. Spectral analysis of hourly values showed frequent periodicities around 2-4 hours, 6 hours, and 8-9 hours.