Estimation of the Dst index based on the AL index

Abstract

Under the assumption that the substorm expansive activity energizes the storm-time ring current belt, the Dst index is estimated from the auroral electrojet index, AL, an indicator of the substorm activity. For this purpose we employ the Dst and AL indices obtained during the past 20 years from 1966 to 1987. During this interval a total of 220 geomagnetic storms are analyzed. To estimate the Dst index, the injected energy into the ring current, Q( t), is assumed to be α · AL ( t − Δ t), where α is a kind of the energy injection efficiency and can be calculated from the correlation between the cumulative AL index and the Dst minimum value (Dst min). The α’s thus estimated for the main and recovery phases are found to be 0.0067 and 0.0030, respectively. The time delay, Δ t, is assumed to be an hour. On the other hand, the decay time of the storm-time ring current, τ, is estimated from the correlation between the duration time of the recovery phase and the Dst min for 68 geomagnetic storms, which show fast and monotonous recovery characteristics. The fast decay time of the recovery phase of storms can be expressed by τ = −0.006 × Dst min + 7.3. Since the decay process is also in progress even during the main phase, the same decay time is used for the main phase. By applying α and τ thus obtained, it is found that there is a high correlation between the observed and estimated Dst indices.