Mathematical separation of directly driven and unloading components in the ionospheric equivalent currents during substorms

Abstract

This paper attempts to separate objectively the directly driven and unloading components in substorm processes by applying the method of natural orthogonal components (MNOC). A time series of the ionospheric equivalent current function with time resolution of 5 min during March 17–19, 1978 is calculated on the basis of six meridian chains magnetometer data during the International Magnetospheric Study in order to obtain the fundamental orthogonal basis set. The first and second natural components of the set thus obtained dominate over the rest of the natural components. The first natural component is found to have a two‐cell pattern, which is well known to be associated with global plasma convection in the magnetosphere. It is enhanced during the growth phase and expansion phase of substorms and decays during the recovery phase of substorms. Further, it is in fair correlation to the ϵ parameter with time lag of 20–25 min. This can be identified as the directly driven component. The second natural component reveals itself as an impulsive enhancement of the westward electrojet around midnight between 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 component. It is shown that the directly driven component tends to dominate over the unloading component except for a brief period soon after substorm onset. This is the first clear determination of the time profile of the unloading component.