ALBERT

Description: Periodic black auroral patches were observed in the Canadian postmidnight sector by ground-based all-sky cameras during an auroral substorm. The aurora consisted of several black patches of ∼50 km (∼1° in longitude) scales that appeared along the equatorward edges of the proton auroras over a 3 h local time segment from Gillam to Sanikiluaq ground stations 15 min after the auroral intensification occurred in Alaska. To the best of our knowledge, this type of black auroras has never been reported before, although the surrounding luminous auroras evolve into the well-known torch structures later. In situ observations by the THEMIS satellites, three of which were close to being magnetically conjugate in the plasma sheet, as well as ground-based proton auroral observations, show that a dipolarization front and associated dawnward ion flows propagated eastward at poleward of the black patches coinciding with their appearance. The flow motions of auroras observed poleward of the black patches are possibly generated by decelerations of dawnward ion flow because of the consistency of both durations and positions. We speculate, based on manifestations of high-pressure regions in dipolarized flux tubes, the pressure-driven interchange instability formed a beading shape (periodic black patches) of field-aligned currents. The black auroral patches developed into torch structures after the flow disappearance, indicating the dawnward flows might stabilize and limit the instability within a narrow L shell where the pressure gradient is directed Earthward and flows cannot enter. We conclude that the periodic black patches arise as a consequence of a relaxation process of enhanced plasma pressure near the inner boundary of the plasma sheet during a substorm.

Description: [1]  Observations of a pulsating aurora event occurring on February 11, 2008, using the THEMIS All-Sky Imager (ASI) array, indicate a spatially and temporally continuous event with a duration of greater than 15 hours and covering a region with a maximum size of greater than 10 hours MLT. The optical pulsations are at times locally interrupted or drowned out by auroral substorm activity, but are observed in the same location once the discrete aurora recedes. The pulsations following the auroral breakup appear to be brighter and have a larger patch size than before breakup. This suggests that, while the onset of pulsating aurora is not necessarily dependent upon a substorm precursor, the pulsations are affected and possibly enhanced by the substorm process. The long duration of this pulsating aurora event, lasting approximately 8 hours without interruption as imaged from Gillam station, is significantly longer than the typical 2–3 hour substorm recovery phase, suggesting that pulsating aurora is not strictly a recovery phase phenomenon. This paper is accompanied by a movie of the THEMIS ASI array data, from 0000 to 1715 UT, plotted in mosaic and mapped to geographic coordinates. The mosaic images are superimposed onto a map of North America.