Abstract
STEVE (Strong Thermal Emission Velocity Enhancement) is an optical
phenomenon of the sub-auroral ionosphere arising from extreme ion drift
speeds. STEVE consists of two distinct components in true-color imagery:
a mauve or whitish arc extended in the magnetic east-west direction, and
a region of green emission adjacent to the arc, often structured into
quasi-periodic columns aligned with the geomagnetic field (the “picket
fence”). This work employs high-resolution imagery by citizen
scientists in a critical examination of fine scale features within the
green emission region. Of particular interest are narrow “streaks” of
emission forming underneath field-aligned picket fence elements in the
100–110-km altitude range. The streaks propagate in curved trajectories
with dominant direction toward STEVE from the poleward side. The
elongation is along the direction of motion, suggesting a drifting
point-like excitation source, with the apparent elongation due to a
combination of motion blur and radiative lifetime effects. The
cross-sectional dimension is <1 km, and the cases observed
have a duration of ~10–30 s. The uniform coloration of
all STEVE green features in these events suggests a common optical
spectrum dominated by the oxygen 557.7-nm emission line. The source is
most likely direct excitation of ambient oxygen by superthermal
electrons generated by ionospheric turbulence induced by the extreme
electric fields driving STEVE. Some conjectures about causal connections
with overlying field-aligned structures are presented, based on coupling
of thermal and gradient-drift instabilities, with analogues to similar
dynamics observed from chemical release and ionospheric heating
experiments.