On the cause of geomagnetic storms

A study of the solar wind and interplanetary magnetic field during four geomagnetic storms shows that strong southward interplanetary magnetic fields are associated with the development of each main phase. Weak southward magnetic fields do not necessarily lead to an increase in the ring current even though such southward fields persist. Such behavior is consistent with the existence of a threshold for the initiation of strengthening of the ring current. These data also suggest that if the threshold is exceeded and the southward field attains a new constant level, Dst will decrease until some saturation value is reached. This apparent threshold for the interplanetary magnetic field and the saturation level for Dst can be explained by a ring current injection process, which is a function of the interplanetary magnetic (or electric) field, and by a ring current dissipation process, which is a function of the strength of the ring current.