On the cause of substorm expansion onset and the processes driving the substorm expansion phase

Abstract

Magnetospheric substorms can be a consequence of an enhanced magnetospheric convection driven by the solar wind. The ionosphere responds gradually to an enhanced magnetospheric convection during the growth phase of a substorm, leading to the substorm expansion onset. The growth phase and the expansion onset are powered by the directly-driven process tapping energy from the solar wind. After the expansion onset, a substorm intensifies explosively during the expansion phase which is powered 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 the unloading process is found to be ~ 1.6–2 times the substorm currents associated with the directly-driven process based on a substorm event on 7 March 1979.