A study of polar magnetic substorms

The changing distribution of magnetic disturbance vectors of a polar magnetic substorm that occurred on December 16, 1964, is examined in detail by analyzing records from more than 70 stations in the northern hemisphere. The distribution of the observed magnetic disturbance vectors at about the maximum epoch of the substorm is found to be similar to that obtained by Silsbee and Vestine, except for some details in the auroral latitudes. Since magnetic observations made by satellites strongly suggest, however, that the equivalent current system for substorms, such as that constructed by Silsbee and Vestine, cannot be interpreted as being ionospheric, a model current system that is similar to that proposed by Alfvén and Fejer is considered. In the model, extraionospheric currents flow along the field lines (the field-aligned current), into the auroral latitude, inward in the morning sector and outward from the evening sector, after flowing along the auroral ionosphere (the Cowling current). A model calculation, which is a slightly modified version of one by Kirkpatrick, shows that at the maximum epoch of the substorm the model current system can reasonably reproduce the observed distribution below dipole latitude 55°. However, it is pointed out also that this model current system is too simple to reproduce the observed distribution in the auroral latitude and the polar cap. A large-scale Hall current system, as well as the Cowling current, is likely to be generated in the ionosphere, as pointed out by Fejer, Swift, and Atkinson. It is concluded thus that both the field-aligned Cowling and Hall currents should be considered together to account for the distribution of magnetic disturbance vectors over the entire earth's surface and in space.