A Mechanism for the STEVE Continuum Emission
Brian J. Harding, S. B. Mende, Colin Triplett, Yen‐Jung Wu
Abstract
Abstract We describe a mechanism to explain the subauroral emission feature called STEVE (Strong Thermal Emission Velocity Enhancement), with a focus on its continuum spectrum. Spacecraft observations show that emissions co‐occur with typically invisible plasma flows known as subauroral ion drifts. If these flows are fast enough, nitrogen molecules are vibrationally excited by collisions with ions, overcoming the activation energy of the N 2 +O→NO+N reaction. The resulting NO combines with ambient O, producing NO 2 and spectrally broad light. Importantly, this mechanism also produces N, which reduces the lifetime of NO from hours to seconds and thus explains why the emission is confined to a discrete arc. The predicted emission altitude ( 130 km) and occurrence conditions ( 4‐km/s flows) match well with observations. We simulate this mechanism using a simple photochemical model to demonstrate its validity. This mechanism is initiated by fast ion flows and is thus distinct from auroral and airglow processes.