The Source of Auroral Omega Bands

Abstract:

The auroral wave-like structures called “omega bands” appear within the post-midnight sector auroral oval with shapes resembling the Greek letter omega, and are typically associated with the recovery phase of substorms. Prior work and MHD simulations suggest both high speed earthward flows and post-midnight flow shears are possible omega band source mechanisms. However, what produces omega bands is not well understood. It is most likely that the paucity of concurrent magnetospheric data has limited our ability to understand fully the mechanism responsible for the generation of the omega bands. We have identified about 263 auroral omegas in 6 different THEMIS all sky cameras over a 10 year period. A fraction appear to form from north-south streamers, but some appear to form from east-west auroral arcs. About 50 of the 263 omegas also have conjugate or near conjugate THEMIS and GOES spacecraft data. There is evidence in about 80% of these events that high speed earthward flows have occurred prior to or at about the same time as the auroral omega observation. This evidence consists of magnetic field dipolarizations, particle injections into the inner magnetosphere, and auroral streamers. Eight events also show a dawnside shear in the plasma flow data, but for 5 of these events the Vy component is inconsistent with the auroral omega direction of motion. The remaining three Vy flow shear events also show evidence of magnetic field dipolarizations, particle injections into the inner magnetosphere, and auroral streamers. Our observations suggest high speed earthward flows are more likely to be the source of auroral omega bands.