Global auroral responses to abrupt solar wind changes: Dynamic pressure, substorm, and null events

[1] Global auroral images are used to investigate how specific types of solar wind change relate to the resulting type of auroral-region disturbance, with the goal of determining fundamental response types. For not strongly southward IMF conditions (Bz ^ 5 nT), we find that IMF changes that are expected to reduce the convection electric field after^30 min of negative IMF Bz cause typical substorms, where expansion phase auroral activity initiates within the expected location of the Harang electric field reversal and expands in 10 min to cover 5 hours of MLT. For not strongly southward IMF conditions, solar wind dynamic pressure (Pdyn) enhancements compress the entire magnetosphere, leading to a global auroral enhancement with no evidence for substorm bulge-region aurora or current wedge formation. Following prolonged strongly southward IMF (Bz ] 8 nT), an IMF change leading to convection electric field reduction gives a substorm disturbance that is not much different from substorms for less strongly southward IMF conditions, other than the expansion phase auroral bulge region seems to expand somewhat more in azimuth. However, under steady, strongly southward IMF conditions, a Pdyn enhancement is found to cause both compressive auroral brightening away from the bulge region and a Harangregion substorm auroral brightening. These two auroral enhancements merge together, leading to a very broad auroral enhancement covering 10–15 hours of MLT. Both current wedge formation and compressive energization in the inner plasma sheet apparently occur for these events. We also find that interplay of effects from a simultaneous IMF and Pdyn change can prevent the occurrence of a substorm, leading to what we refer to as null events. Finally, we apply the plasma sheet continuity equation to the IMF and pressure driven substorm responses and the null events. This application suggests that solar wind changes cause substorm onset only if the changes lead to a reduction in the strength of convection within the inner plasma sheet.