We propose a model of the substorm current system in terms of coupled radial and azimuthal current loops. The polarization electric field is shown to be greater than the Cowling field, indicating that the net current in the radial current loop must be driven by the azimuthal pressure gradient in the central plasma sheet. The current in the radial current loop is shown to decrease or unchanged by an enhanced magnetospheric convection. On the hand, the current in the azimuthal current loop is shown to increases with the enhanced convection. The decrease of the current in the radial current loop combined with the increase of the current in the azimuthal current loop associated with an enhanced magnetospheric convection can lead to the the expansion phase of substorms. ¿ American Geophysical Union 1990