An improved method to deduce the unloading component for magnetospheric substorms

Abstract

Because our earlier analysis [Sun et al., 1998] confirmed that the directly driven component prevails during the growth phase, an attempt has been made 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 substorms, then the correlation method is applied to calculate time variations of the magnitude of the directly driven component. Next, the method of natural orthogonal components is used to analyze the residual part, which equals the difference between the total current and the directly driven component. In the present method, the pattern and the magnitude of the unloading component are much improved. The correction in the magnitude of two components is ∼ 50%. Time variations of the magnitude of the unloading component thus obtained will be crucial in understanding the unloading process. The effect of the variable directly driven pattern, which is ∼ 11% of the magnitude of the directly driven component, also has been estimated quantitatively. This improved method has the advantage of allowing study of individual substorms over the earlier method.