The dynamics of the magnetotail during substorms is controlled by two external influences: the pressure in the magnetospheric boundary layer and the strength of the dawn-to-dusk electric field. Pressure pulses are often used in MHD models of the tail to drive enhanced reconnection while particle models often invoke the dawn-to-dusk electric field to produce the observed particle acceleration. Multiscale length electromagnetic particle simulations are presented which self-consistently investigate the influences on the tail of these processes and their coupling. It is shown that, while both processes can lead to the compression of the current sheet, they do so in different ways. Pressure pulses can produce local pinching of the magnetic field, which can in turn lead to reconnection and the tailward ejection of a plasmoid, similar to MHD models. However, there is no net energy stored in the magnetic field nor any strong earthward flows induced during the growth phase, contrary to observations. On the other hand, increases or surges in the electric field can produce significant storage of energy in the magnetic field as well as strong earthward flows. However, plasmoid formation is unlikely for typical values of the dawn-to-dusk electric field. It is argued that these two processes are not independent but are rather coupled through the conductivity of the system. This conductivity is shown to decrease with increasing magnetic field strength as the current sheet is compressed. Thus when the pressure increases in the magnetospheric boundary layer, the dawn-to-dusk electric field must increase if the current system is to be sustained. When this coupling is incorporated, the model is able to more comprehensively describe the development of substorms, including earthward flows, enhanced energy storage in the magnetic field during the growth phase and the formation and tailward ejection at high speeds of plasmoids in association with the production of high-energy particles and intense field-aligned currents. ¿ American Geophysical Union 1993