A morphological study of black vortex streets

Abstract

Black aurora has been defined as the lack of auroral emissions in well‐defined regions within diffuse aurora [Royrvik, 1976; Davis, 1978]. A particularly intriguing class of black aurora is the occurrence of black vortex streets (black curls) exhibiting clockwise vorticity [Davis, 1978; Trondsen and Cogger, 1997.] An archive of low‐light level all‐sky and narrow field television camera data was searched, yielding 20 examples of black curls. Black curls are concluded to result from the Kelvin‐Helmholtz instability occurring in regions of positive space charge. Calculated peak electric fields ranged from 80 to 300 mV/m. White curls seen in aurora that was intermediate between diffuse and discrete were sometimes found in association with black curls and even paired with black curls to form newly discovered Karman vortex streets. A spectroscopic measurement indicated mixed proton and electron precipitation. Electric field configurations are proposed for the black and white curl systems and the Karman vortex streets.