The electric current associated with polar magnetic sub-storms

Abstract

A study is made of the records of polar magnetic sub-storms recorded at Lerwick and two neighbouring temporary stations with a view to determining the associated ionospheric current system. The measurements of the horizontal and vertical components of the disturbance vectors and their gradients in the direction of the horizontal component are compared with theoretical values computed for a band of current with a parabolic distribution of intensity across the width, taking into account the effects of induced currents and of return flow at a lower latitude. The main conclusion is that on the average the sub-storms can be accounted for by a system in which the primary current is a polar electrojet flowing westward and it is this intense westerly electrojet which causes the negative bays of the sub-storms occurring between 22 hr and 05 hr U.T. whereas the positive bays of the sub-storms occurring between 15 hr and 20 hr are due to the weaker easterly return flow. Estimates of the height, latitudinal extent and strength of the current are obtained.