A Substorm Triggered by a Sudden Drop in Dynamic Pressure

In traditional substorm theories the growth phase is driven by an extended period of southward IMF, which transfers large amounts of re-connected magnetic flux into the magnetotail. The subsequent substorm onset may be triggered by a northward turning of the IMF. We investigate the possibility that variations in dynamic pressure may produce these same effects for some substorms lacking a clear IMF Bz signature for both the growth phase and the onset. A sustained increase in dynamic pressure may raise the energy stored in the compressed magnetosphere system (similar to the growth phase), while a subsequent decrease in dynamic pressure may allow some of this stored energy to be released, possibly triggering a substorm. We present a global magnetospheric simulation of such a substorm, which was also imaged by the Ultraviolet Imager (UVI) on the Polar spacecraft. The plasma density rapidly doubled about 90 minutes before the substorm onset with relatively little change in the solar wind velocity and the IMF during this period. Likewise, a sudden decrease in the density almost back to the original levels occurred close to the time of substorm onset, again with no significant change in solar wind velocity or IMF, and thus this dynamic pressure drop may have triggered the substorm. We investigate whether the resulting dynamic pressure increase in the simulation produces a growth phase, a feature that is very successfully modeled by global magnetospheric simulations. We also search for the signature of substorm onset in diagnostics such as the field-aligned currents and the position of the neutral line, if any, in the magnetotail.