Theories of the geomagnetic storm main phase

Abstract

The main phase of a geomagnetic storm may be explained in terms of ring currents caused by drifts of trapped ions; D st requires a symmetrical ring and DS an unsymmetrical ring. Alternatively both components may be caused by a geomagnetic tail; its field lines may link with the Earth and be open or closed. Other field lines may be created from the interplanetary field. A comparison of these theories is first made on a theoretical basis. Some objections to the unsymmetrical ring current are raised; the symmetrical ring current and tail are both likely to occur, the latter being maintained by gas which becomes attached to the field lines. The theories are compared with sea-level geomagnetic storm data, both averaged and for individual storms and sub-storms. It is concluded that the main phase D st component is caused by the combined effects of a ring current and a tail, in varying proportions. DS is caused partly by magnetospheric motions which result from twist deformations of the field lines and partly from a tail into which plasma remains frozen. Weak, moderate and great storms all result from solar winds of the same average intensity and duration. The different main-phase intensities result from differing degrees of frictional interaction, the difference perhaps depending on the interplanetary magnetic field strength and direction. Magnetometers and ion counters in spacecraft and ionospheric observations tend to confirm the above conclusions. The diurnal variation in latitude of geomagnetically trapped electrons (Injun I) is explained in terms of a geomagnetic tail as are other observed diurnal effects.