An ionospheric conductance gradient driver for subauroral picket fence visible signatures near STEVE events

Reported observations of picket fence signatures associated with subauroral STEVE (Strong Thermal Emission Velocity Enhancement) emission events look strikingly similar to rayed auroral curtains. Rayed auroral curtains are often the visible signatures of tearing-mode-unstable current sheets with precipitating auroral electron current carriers. Picket fence signatures are not located where auroral precipitation explanations apply, and are closely collocated with STEVE emission events. A similar tearing-mode-instability explanation can be invoked with a different source for the originating field-aligned current sheet. In this explanation, the field-aligned current sheet is sourced by ionospheric conductance gradients adjacent to the localized flows of the STEVE event. Geospace Environment Model of Ion-Neutral Interactions (GEMINI) models of the 3D ionosphere near counterstreaming STEVE-associated flow structures show the development of sufficiently strong current sheets for tearing mode instabilities to take hold. These instabilities can locally accelerate ambient ionospheric thermal electrons to the few eV needed for the reported observed green picket fence signatures.