Dynamic pressure enhancements as a cause of large‐scale stormtime substorms

Intense substorm disturbances, which typically have an unusually broad auroral enhancement, occur in association with magnetic storms. We have investigated the cause of these intense and broad disturbances using interplanetary observations, global auroral images from the WIC imager on the IMAGE spacecraft, geosynchronous energetic particle observations, and mid/low‐latitude dayside ground H observations for 28 large stormtime auroral disturbances. We find evidence that the magnetosphere was impacted by a substantial P dyn increase at the onset of 19 of these disturbances. For disturbances with evidence of such an impact, we found that auroral brightenings extended over about twice as broad an MLT range as did the auroral brightening of disturbances that did not have evidence for a P dyn increase at onset. We thus conclude that P dyn increases at onset offer a feasible explanation for many of the larger‐scale aurora disturbances that often occur during magnetic storms. Our results indicate that it is the combination of the global compression response to a P dyn increase and the nightside substorm response that gives a much broader response for many stormtime disturbances than is seen from the substorm alone. We also found some indications that a substantially shorter than typical growth phase may be sufficient for there to be a triggered substorm onset if the preceding IMF ∣B y ∣ or negative B z , and/or a pressure increase at onset, is unusually large.