On the excitation of the magnetosphere by the solar wind

In this note, evidence is presented to show that variations in the direction of V, the solarwind plasma velocity, produce a significant part of the activity that is measured by Kp. The activity produced by these variations in V/V may well be dominant during periods of low to moderate Kp. However, other solar-wind quantities may also be effective in producing geomagnetic activity. Most of the coupling mechanisms between the magnetosphere and the solar wind that have been proposed to date may be separated into two categories. In the first category are the mechanisms that produce the Kp component of geomagnetic activity in such a way that the level of activity depends upon the magnitude of some solar-wind quantity, or some function of more than one such quantity. Presumably, then, somewhere in the region of interaction between the solar wind and the magnetosphere, the transfer of the energy required for geomagnetic activity occurs at a rate determined by the magnitude of the controlling solar-wind quantity. Thus one would expect Kp and the magnitude of this controlling quantity, as measured in near-earth interplanetary space, to be correlated.