A Mechanism for the STEVE Continuum Emission

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.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.

The Mysterious Green Streaks Below STEVE

“…The mauve-white emission of the main STEVE channel is likely related to the continuum nightglow emission that has been known for decades (Bates, 1993). Our understanding of its origin remains incomplete, but STEVE has attracted renewed attention to the topic (Harding et al, 2020). The green features accompanying STEVE often exhibit elongation in the magnetic field direction, suggesting that they are produced by precipitation of magnetospheric electrons (Nishimura et al, 2019).…”

“…Thus far, nothing about the optical phenomenology associated with STEVE is conveniently explained in terms of our historical understanding of auroras and airglow (Gillies et al, 2019; Harding et al, 2020; Mende et al, 2019). The reason for this may be generally traced to the extreme nature of the driving electric fields and attendant supersonic ion drifts.…”

The Apparent Motion of STEVE and the Picket Fence Phenomena

“…The pending question is how the SAID interacts with the Earth's atmosphere to excite the chemiluminescence airglow continuum that is characteristic of STEVE (Gillies et al, 2019; Liang et al, 2019). Harding et al (2020) proposed that the nitrogen molecules can be excited to high vibrational levels via the collision with fast‐drifting ions in a SAID and may thus overcome the energy barrier in reacting with oxygen atoms to produce NO, which then react with O to generate the NO 2 continuum (Sternberg & Ingham, 1972) that accounts for the STEVE airglow continuum. Even though the neutral response and the airglow production may have a finite growth time in response to the SAID ions' passage, assuming the growth time is nearly constant, the successive appearance of STEVE structures at different longitudes would still essentially follow the SAID speed.…”

Morphological Characteristics of Strong Thermal Emission Velocity Enhancement Emissions

“…The two layers show localized enhancements simultaneously at some longitudes, suggesting that they likely have the same driver and source region. The existence of the low layer STEVE would have interesting implications for the proposed mechanisms because of the difficulty in accelerating the ions in a regime of increased collisions (Harding et al., 2020). The localized enhancements are not necessarily associated with a green picket fence where higher flux of superthermal electron precipitation is expected.…”