Large-scale vortex dynamics in the evening and midnight auroral zone: Observations and simulations

We use Canadian Auroral Network for the OPEN Program Unified Study All-Sky Imager (ASI) and Meridional Scanning Photometer (MSP) data as the basis for a study of the dynamics of large-scale (hundreds of kilometers) auroral vo rtices. We consider 28 events corresponding to a range of auroral activity levels. Three of these are presented in detail, one corresponding to growth phase, one to pseudo-breakup and one to expansive phase onset. We show tha t vortex formation starts from a discrete are with half thickness delta of the order of 20 km. This are intensifies near the poleward boundary of enha nced proton aurora, as seen in the Hydrogen beta (H beta) MSP data and beco mes azimuthally structured. This structuring is in the form of vortices wit h wavelength of the order of similar to 2 pi delta. The vortices intensify and extend radially, leading to broadening of the initial arc. While the si xes and growth rates of the vortices vary, the overall scenario of vortex e volution is similar for all of the events. Structures that develop during t he growth phase saturate at latitudes matching the poleward boundary of H b eta emissions and pseudo-breakup structures saturate further poleward. Expa nsive phase onset vortices expand poleward in a similar fashion, but we do not observe any saturation stage, presumably due to limitations imposed by the ASI field of view. We present results of shear flow ballooning vortex m odeling in which we used initial conditions and parameters consistent with our observations. On the basis of our model results, we speculate that all of these experimentally observed vortices are the result of shear flow ball ooning instability in the hot proton region in the near-Earth plasma sheet.