Isolated disturbances such as earthquakes, tsunamis, and solar eclipses, as well as explosions from volcanoes, nuclear detonations, and meteor air bursts can offer discrete tests for models of atmosphere-ionosphere coupling and variability (

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.

Retrieval of the properties of the middle and upper atmosphere can be performed using several different interferometric and photometric methods. The emission-shape and Doppler shift of both atomic and molecular emissions can be observed from the ground and space to provide temperature and bulk velocity. These instantaneous measurements can be combined over successive times/locations along an orbit track, or successive universal/local times from a ground station to quantify the motion and temperature of the atmosphere needed to identify atmospheric tides. In this report, we explore how different combinations of space-based wind and temperature measurements affect the retrieval of atmospheric tides, a ubiquitous property of planetary atmospheres. We explore several scenarios informed by the use of a tidally forced atmospheric circulation model, an empirically based emissions reference, and a low-earth orbit satellite observation geometry based on the ICON mission design. This capability provides a necessary tool for design of an optimal mission concept for retrieval of atmospheric tides from ICON remote-sensing observations. Here it is used to investigate scenarios of limited data availability and the effects of rapid changes in the total wave spectrum on the retrieval of the correct tidal spectrum. An approach such as that described here could be used in the design of future missions, such as the NASA DYNAMIC mission (National Research Council, Solar and space physics: a science for a technological society, 2013).

Measurements of turbulence and waves were made as part of the Mesosphere‐Lower Thermosphere Turbulence Experiment (MTeX) on the night of 25–26 January 2015 at Poker Flat Research Range, Chatanika, Alaska (65°N, 147°W). Rocket‐borne ionization gauge measurements revealed turbulence in the 70‐ to 88‐km altitude region with energy dissipation rates between 0.1 and 24 mW/kg with an average value of 2.6 mW/kg. The eddy diffusion coefficient varied between 0.3 and 134 m2/s with an average value of 10 m2/s. Turbulence was detected around mesospheric inversion layers (MILs) in both the topside and bottomside of the MILs. These low levels of turbulence were measured after a minor sudden stratospheric warming when the circulation continued to be disturbed by planetary waves and winds remained weak in the stratosphere and mesosphere. Ground‐based lidar measurements characterized the ensemble of inertia‐gravity waves and monochromatic gravity waves. The ensemble of inertia‐gravity waves had a specific potential energy of 0.8 J/kg over the 40‐ to 50‐km altitude region, one of the lowest values recorded at Chatanika. The turbulence measurements coincided with the overturning of a 2.5‐hr monochromatic gravity wave in a depth of 3 km at 85 km. The energy dissipation rates were estimated to be 3 mW/kg for the ensemble of waves and 18 mW/kg for the monochromatic wave. The MTeX observations reveal low levels of turbulence associated with low levels of gravity wave activity. In the light of other Arctic observations and model studies, these observations suggest that there may be reduced turbulence during disturbed winters.

Abstract:The meteorological control of gravity wave activity through filtering by winds and generation by spontaneous adjustment of unbalanced flows is investigated. This investigation is based on a new analysis of Rayleigh LiDAR measurements of gravity wave activity in the upper stratosphere-lower mesosphere (USLM, 40-50 km) on 152 nights at Poker Flat Research Range (PFRR), Chatanika, Alaska (65 • N, 147 • W), over 13 years between 1998 and 2014. The LiDAR measurements resolve inertia-gravity waves with observed periods between 1 h and 4 h and vertical wavelengths between 2 km and 10 km. The meteorological conditions are defined by reanalysis data from the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The gravity wave activity shows large night-to-night variability, but a clear annual cycle with a maximum in winter, and systematic interannual variability associated with stratospheric sudden warming events. The USLM gravity wave activity is correlated with the MERRA winds and is controlled by the winds in the lower stratosphere through filtering by critical layer filtering. The USLM gravity wave activity is also correlated with MERRA unbalanced flow as characterized by the residual of the nonlinear balance equation. This correlation with unbalanced flow only appears when the wind conditions are taken into account, indicating that wind filtering is the primary control of the gravity wave activity.