DECAY OF THE DST FIELD OF GEOMAGNETIC DISTURBANCE AFTER SUBSTORM ONSET AND ITS IMPLICATION TO STORM-SUBSTORM RELATION

In order to investigate the causal relationship between magnetic storm s and substorms, variations of the mid-latitude geomagnetic indices, A SY (asymmetric part) and SYM (symmetric part), at substorm onsets are examined. Substorm onsets are defined by three different phenomena; (1 ) a rapid increase in the mid-latitude asymmetric-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 onset of Pi2 geomagnet ic pulsation. The positive bays are selected using eye inspection 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 the time resolution, does not show any statistically significant developm ent after the onset of substorms; it tends to decay after the onset ra ther than to develop. It is suggested by a simple model calculation th at the decay of the magnetospheric tail current after substorm onset i s responsible for the decay of the Dst field. The relation between the IMF southward turning and the development of the Dst field is reexami ned. The results support the idea that the geomagnetic storms and subs torms are independent processes; that is, the ring-current development is not the result of the frequent occurrence of substorms, but that o f enhanced convection caused by the large southward IMF. A substorm is the process of energy dissipation in the magnetosphere, and its contr ibution to the storm-time ring-current formation seems to be negligibl e. The decay of the Dst field after a substorm onset is explained by a magnetospheric energy theorem.