An improved method to deduce the unloading component for magnetospheric substorms

Because our earlier analysis [Sun ed al., 1998] confirmed that the directly driven component prevails during the growth phase, an attempt has been mad e in this paper to deduce the unloading component as accurately as possible . First, the directly driven pattern is determined by taking the average of patterns of the equivalent current function during the growth phase of sub storms, then the correlation method is applied to calculate time variations of the magnitude of the directly driven component. Next, the method of nat ural orthogonal components is used to analyze the residual Dart, which equa ls the difference between the total current and the directly driven compone nt. In the present method, the pattern and the magnitude of the unloading c omponent are much improved. The correction in the magnitude of two componen ts is similar to 50%. Time variations of the magnitude of the unloading com ponent thus obtained will be crucial in understanding the unloading process . The effect of the variable directly driven pattern, which is similar to 1 1%, of the magnitude of the directly driven component, also has been estima ted quantitatively. This improved method has the advantage of allowing stud y of individual substorms over the earlier method.