ON THE CAUSE OF SUBSTORM EXPANSION ONSET AND THE PROCESSES DRIVING THE SUBSTORM EXPANSION PHASE

Magnetospheric substorms can be a consequence of an enhanced magnetosp heric convection driven by the solar wind. The ionosphere responds gra dually to an enhanced magnetospheric convection during the growth phas e of a substorm, leading to the substorm expansion onset. The growth p hase and the expansion onset are powered by the directly-driven proces s tapping energy from the solar wind. After the expansion onset, a sub storm intensifies explosively during the expansion phase which is powe red jointly by the directly-driven process and the unloading process. The unloading process is responsible for the explosive release of the stored energy in the magnetotail. Substorm currents associated with th e unloading process is found to be approximately 1.6-2 times the subst orm currents associated with the directly-driven process based on a su bstorm event on 7 March 1979.