Decay of the Dst field of geomagnetic disturbance after substorm onset and its implication to storm-substorm relation

In order to investigate the causalrelationship between magnetic storms and substorms, variations of themid-latitude geomagnetic indices, ASY (asymmetric part) and SYM (symmetricpart), at substorm onsets are examined. Substorm onsets are defined by threedifferent phenomena; (1) a rapid increase in the mid-latitudeasymmetric-disturbance indices, ASY-D and ASY-H, with a shape of so-called`mid-latitude positive bay'; (2) a sharp decrease in the AL index; (3) an onsetof Pi2 geomagnetic pulsation. The positive bays are selected using eyeinspection and a pattern-matching technique. The 1-min-resolution SYM-H index,which is essentially the same as the hourly Dst index except in terms of thetime resolution, does not show any statistically significant development afterthe onset of substorms; it tends to decay after the onset rather than todevelop. It is suggested by a simple model calculation that the decay of themagnetospheric tail current after substorm onset is responsible for the decay ofthe Dst field. The relation between the IMF southward turning and thedevelopment of the Dst field is re-examined. The results support the idea thatthe geomagnetic storms and substorms are independent processes; that is, thering-current development is not the result of the frequent occurrence ofsubstorms, but that of enhanced convection caused by the large southward IMF. Asubstorm is the process of energy dissipation in the magnetosphere, and itscontribution to the storm-time ring-current formation seems to be negligible.The decay of the Dst field after a substorm onset is explained by amagnetospheric energy theorem