Two‐dimensional structure of auroral poleward boundary intensifications

We investigate the two‐dimensional structure of auroral poleward boundary intensifications (PBIs). PBIs are a nightside auroral intensification that has been studied primarily with ground‐based meridian scanning photometers (MSPs). They have a signature that in the MSP data, appears as an increase in intensity at or near the magnetic separatrix and is often seen to extend equatorward. They are also associated with fast flows in the tail and are thus important to the dynamics of the plasma sheet. MSP data provide information about the temporal evolution of the aurora in one spatial dimension, in this case roughly along a magnetic meridian. This paper is motivated by a desire to determine the physics of PBIs that is revealed by their two‐dimensional structure. To do this, we have identified a number of PBI events in the CANOPUS Rankin Inlet and Gillam MSPs that occurred at times when high‐resolution, two‐dimensional images of the aurora over the same region were also available. The two‐dimensional images used in this study were obtained by the Freja UV imager, from October 1992 to January 1993, and by the CANOPUS Gillam all‐sky imager during the winter viewing season of 1996–1997. We find that PBIs, as observed by the MSPs, are either equatorward extending or non‐equatorward extending. Equatorward extending PBIs are either north‐south aligned structures or east‐west arcs propagating mostly equatorward, but we were not able to determine without doubt which type is the most prevalent. We suggest that equatorward extending PBIs may be the auroral footprint of two major modes of energy transfer in the plasma sheet: multiple, narrow, earthward fast‐flow channels in the plasma sheet and sequences of azimuthally broad and primarily earthward propagating phase fronts initiating near the separatrix. Nonequatorward extending PBIs are found to mostly be a series of multiple bead‐like intensifications along the poleward boundary of the aurora zone. Such PBIs may be evidence for shear instabilities at the separatrix boundary on the flanks of the magnetotail.