The Green Emissions of STEVE: Optical Evidence of an Energy Cascade

Abstract:

STEVE (Strong Thermal Emission Velocity Enhancement) is a mid-latitude optical phenomenon connected with supersonic (>5 km/s) sub-auroral ion drifts (SAIDs). STEVE consists of two distinct components in true-color imagery: a pale purple-white arc extended in the magnetic east-west direction, and a region of dynamic green emission adjacent to this arc (i.e., the “picket fence”). This work employs high-resolution image sequences by citizen scientists in a critical examination of fine scale features in the green emission region. Of particular interest are narrow sub-kilometer “streaks” forming underneath field-aligned features comprising the picket fence. The streaks propagate in curved trajectories toward STEVE from the poleward side. Their elongation is along the direction of motion, suggesting a drifting point-like excitation source, with the elongation due to a combination of motion blur and radiative lifetime artifacts. The common coloration with co-located green features in these regions suggests a common optical spectrum, dominated by the oxygen 557.7-nm emission. The source mechanism is most likely direct excitation of ambient oxygen by superthermal electrons generated by plasma turbulence in the SAID channel. Intriguing connections between the streaks and field-elongated features above them are also observed, inviting comparisons with plasma expansion observed in Barium ion release experiments at high latitudes. These observations — extracted from an ad-hoc network of commercial cameras deployed by amateur and professional photographers — point to an exciting new diagnostic capability for examining magnetosphere-ionosphere interactions at mid-latitudes.