Characteristics of the development of the westward electrojet during the expansive phase of magnetospheric substorms

By use of high-, mid-, and low-latitude magnetograms it is concluded that the westward expansion of the substorm westward electrojet is not continuous but takes place as a series of discrete steps or jumps. The substorm is pictured as consisting of the sequential development of a series of westward electrojets, which we have labeled a ‘substorm sequence.’ The westward electrojets develop in succession at intervals of about 10 min in such a way that each new electrojet appears to the northwest of the previous one. Associated with the westward jumps of substorm activity are enhancements in the growth rate of a ring or cross-tail current. These enhancements are concurrent with the onset of the westward electrojets and occur to the west of that sector which is presently undergoing its initial onset of substorm activity. Each substorm intensification is accompanied by a response in the adjacent sector to the west, consistent with the signatures of growth suggested by McPherron (1972) and Iijima and Nagata (1972). We suggest that growth may be stimulated by the same mechanism which triggers the expansion phase and that the energy responsible for ensuing substorm intensifications in the evening sector is made available in this fashion.