MATHEMATICAL SEPARATION OF DIRECTLY DRIVEN AND UNLOADING COMPONENTS IN THE IONOSPHERIC EQUIVALENT CURRENTS DURING SUBSTORMS

This paper attempts to separate objectivity the directly driven and un loading components in substorm processes by applying the method of nat ural orthogonal components (MNOC). A time series of the ionospheric eq uivalent current function with time resolution of 5 min during March 1 7-19, 1978 is calculated on the basis of six meridian chains magnetome ter data during the International Magnetospheric Study in order to obt ain the fundamental orthogonal basis set. The first and second natural components of the set thus obtained dominate over the rest of the nat ural components. The first natural component is found to have a two-ce ll pattern, which is well known to be associated with global plasma co nvection in the magnetosphere. It is enhanced during the growth phase and expansion phase of substorm and decays during the recovery phase o f substorms. Further, it is in fair correlation to the Ε param eter with time lag of 20-25 min. This can be identified as the directl y driven component. The second natural component reveals itself as an impulsive enhancement of the westward electrojet around midnight betwe en 65° and 70° latitude during the expansion phase only. It is much less correlated with the Ε parameter than the first one. Thus, as a first approximation, we identify it as the unloading compo nent. It is shown that the directly driven component tends to dominate over the unloading component except for a brief period soon after sub storm onset. This is the first clear determination of time profile of the unloading component.