Universal time variation of the Dst index and the relationship between the cumulative AL and Dst indices during geomagnetic storms

[1] The nonuniform distribution in longitude of the four Dst stations provides an unexpected opportunity to examine the storm-substorm relationship. This is because any asymmetry of the ring current with the longitudinal scale size less than or comparable to the spacing between the two closely located stations, Honolulu and Kakioka, 62degrees, would result in UT variation of the Dst index. In this paper, prominent UT variations in Dst are found with the index being 23% lower than the daily mean around 0700 UT and 28% higher around 1200 UT during the main phase of magnetic storms. Since the midpoints between the two stations are around dusk (1800 MLT) and midnight (2330 MLT) at 0700 and 1200 UT respectively, the deviations from the daily average Dst can be interpreted as the development of a partial ring current around dusk and a substorm wedge current in the midnight sector. Interestingly, the UT variation is only noticeable during the main phase of a storm, thus suggesting a possibility that substorm expansive activity is closely associated with storm development. The causal relationship between substorm and storm is further investigated by comparing the AL and Dst indices, assuming that the AL index carries partly the substorm expansive signature. Admitting that a substorm is a transient phenomenon while a storm is a longer period one, an integrated index over a given interval is helpful in examining the contribution of substorms to the development of the storm-time ring current. For this purpose, the cumulative AL and Dst indices are devised. The two indices thus defined are highly correlated with each other regardless of storm phases. More importantly, it is shown that for a given level of Dst, almost twice of the cumulative AL index is needed during the main phase than during the recovery phase. This indicates that a higher level of substorm activity in terms of the cumulative AL index is required to maintain a given level of the Dst index before the maximum phase of a storm than afterwards. It may also have an important implication in understanding the causal relationship between magnetospheric substorms and geomagnetic storms.