(June 1981)

This paper describes in detail how we are led to the first approximation expression for the solar wind-magnetosphere energy coupling function ɛ, which correlates well with the total energy consumption rate U T of the magnetosphere. It is shown that ɛ is the primary factor which controls the time development of magnetospheric substorms and storms. The finding of this particular expression ɛ indicates how the solar wind couples its energy to the magnetosphere; the solar wind and the magnetosphere constitute a dynamo. In fact, the power P generated by the dynamo can be identified as ɛ by using a dimensional analysis. Furthermore, the finding of ɛ indicates that the magnetosphere is closer to a directly driven system than to an unloading system which stores the generated energy before converting it to substorm and storm energies.