Details

Autor(en) / Beteiligte

• Harding, Brian J.
• Mende, Stephen B.
• Triplett, Colin C.
• Wu, Yen‐Jung Joanne

Titel

A Mechanism for the STEVE Continuum Emission

Ist Teil von

• Geophysical research letters, 2020-04, Vol.47 (7), p.n/a

Ort / Verlag

Washington: John Wiley & Sons, Inc

Erscheinungsjahr

2020

Link zum Volltext

Quelle

Wiley Online Library Journals Frontfile Complete

Beschreibungen/Notizen

• 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 N2+O→NO+N reaction. The resulting NO combines with ambient O, producing NO2 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. Plain Language Summary Citizen scientists and night‐sky photographers have been capturing pictures of a peculiar type of polar light for many years but only recently has the scientific community explored its significance. This narrow purple/white arc stretches east‐west across the sky and has come to be known as Strong Thermal Emission Velocity Enhancement (STEVE). Although its appearance is suggestive of aurora, it is not caused by fast electrons from the magnetosphere, and it is dominated by a broad spectrum (mostly white light). Most auroral and airglow emissions are caused by electronic transitions of atmospheric constituents initiated by electron or photon impact, producing spectrally discrete light. The physical processes producing the light in STEVEs are unknown, particularly the chemical mechanism that produces light that could appear white. In this work we describe a candidate mechanism where fast‐moving ions cause vibrational excitation of nitrogen molecules, which then undergo chemical reactions to produce spectrally broad light. These fast‐moving ions are known to co‐occur with STEVEs. This hypothesis is supported by a simple chemical simulation, but observational validation is needed. Key Points The NO2 continuum can explain the broadband emission of STEVEs N2 is vibrationally excited via impact by ions inside SAIDs, which enhances NO production and thus the NO2 continuum This process requires 3 eV of translational ion energy, which explains why STEVEs occur only above 130 km and only for the fastest SAIDs