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Magnetotail earthward‐propagating fast plasma flows provide important pathways for magnetosphere‐ionosphere coupling. This study reexamines a flow‐related red‐line diffuse‐like aurora event previously reported by Liang et al. (2011, https://doi.org/10.1029/2010ja015867), utilizing THEMIS and ground‐based auroral observations from Poker Flat. We find that time domain structures (TDSs) within the flow bursts efficiently drive electron precipitation below a few keV, aligning with predominantly red‐line auroral intensifications in this non‐substorm event. The diffuse‐like auroras sometimes coexisted with or potentially evolved from discrete forms. We forward model red‐line diffuse auroras due to TDS‐driven precipitation, employing the time‐dependent TREx‐ATM auroral transport code. The good correlation (∼0.77) between our modeled and observed red line emissions underscores that TDSs are a primary driver of the red‐line diffuse‐like auroras, though whistler‐mode wave contributions are needed to fully explain the most intense red‐line emissions. Plain Language Summary: Fast plasma flows in the magnetotail, traveling earthward at several hundred kilometers per second, transport energetic particles and magnetic flux into the inner magnetosphere. Upon braking near Earth's high magnetic flux regions, they trigger plasma instabilities and waves, leading to increased electric currents and particle precipitation in the polar regions. This precipitation, depending on its driver, results in either diffuse auroras from electron pitch‐angle scattering, or discrete auroras from field‐aligned electron acceleration and currents. Our case study highlights the important role of time‐domain structures in diffuse‐like aurora generation during flow braking. This reveals a new aspect of magnetosphere‐ionosphere coupling: the generation of diffuse auroras through electron scattering by time‐domain structures in braking flow bursts. Key Points: Predominantly red‐line auroras are linked to flow bursts, TDSs, and <1 keV electron precipitationFor the first time, red‐line diffuse‐like auroras have been forward‐modeled using TREx‐ATM with time domain structure (TDS) inputsA good correlation between forward‐modeled and observed red‐line emissions suggests that TDSs are a major driver [ABSTRACT FROM AUTHOR]

We present a binary hypothesis test for detecting clear sky in auroral all‐sky images based on single‐wavelength keograms. The coefficient of variation c, the ratio of the sample standard deviation to the mean over elevation angle along the meridian, is the test statistic. After image‐correcting keograms and excluding dark sky intervals, detection performance is compared to true conditions as determined by Advanced Very High Resolution Radiometer satellite imagery. The cloud mask, an index of cloud cover, is selected at the corresponding nearest time and location to the site of a meridian spectrograph at Poker Flat Research Range. With training data from 2014 to 2016, theoretical Rayleigh distributions fit to the histograms show a decision threshold of 0.40 could yield an accuracy of about 80%. Separately, we numerically compute the false alarm and missed detection statistics of the greenline 557.7 nm emission and of the redline 630.0 nm emission. We find a threshold of 0.25 for the greenline c maximizes the percent of events correctly identified at 76%. Applied to testing data from 2015 to 2017, the 0.25 threshold yields an accuracy of 68%. Diffuse aurora can have coefficient of variation around 0.2 to 0.5, which would be included by the numerical minimum, but partly excluded by the theoretical model obtained. Numerical results are a few percent worse for the redline emission. Plain Language Summary: Clouds in the sky are a problem for scientists trying to view space beyond. For upper atmospheric scientists, clouds can obscure or scatter auroral light in scientific auroral cameras, making it hard to identify, locate, and track auroral shapes. This paper shows a way to simply and automatically detect clouds using a north‐to‐south line scan of a single color of light from the sky over time, known as a keogram. We compute the ratio of the variation in pixel intensity to the average pixel intensity, for each north‐to‐south scan. Excluding dark sky periods, a large ratio means that the sky is clear, and a small ratio that the sky is cloudy. We find the method works with about a 70% correct rate using red or green auroral light. With this method we can eliminate data during cloudy conditions for any auroral studies that require clear sky conditions. Key Points: Auroral keogram coefficient of variation is used to decide if the sky is cloudy or clear, verified with NOAA satellite data from 2014 to 2017The coefficient of variation distinguishes clear from cloudy auroral conditions with as much as 75% accuracyThis method is intended for subsequent automation of auroral all‐sky image analysis [ABSTRACT FROM AUTHOR]

This paper surveys six years of Global Positioning System (GPS) L1 and L2C ionospheric scintillation in the auroral zone and, with a collocated incoherent scatter radar, hypothesizes the ionospheric irregularity layer. The Scintillation Auroral GPS Array of six scintillation receivers is sited at Poker Flat Research Range, Alaska, as is the Poker Flat incoherent scatter radar (PFISR). Scintillation intervals are identified across at least four receivers of the array using S4 and sigma phi (σϕ) indices at 100 s cadence. Classification as "amplitude," "phase," or "both‐phase‐and‐amplitude" scintillation is performed by analyzing common time intervals of elevated S4 and σϕ. Scattering of Global Navigation Satellite System (GNSS) waves by refractive or diffractive effects is hypothesized to occur in the E or F layer, or a transition layer in between, based on the PFISR peak density altitude at the time of the scintillation event. We analyze the statistics of the irregularity layer from 2014 to 2019, spanning solar maximum to solar minimum. We find fewer scintillation events per day with the waning solar cycle, nearly all of them phase scintillations. We also find that the percentage of events hypothesized to be caused by irregularities in the E layer increases with the declining solar cycle. The local time dependence of phase scintillations is primarily at night and in the E layer. Phase scintillation events occurring during daytime occur at solar maximum and are nearly all in the F layer. The majority of the events containing amplitude scintillations are daytime F layer at solar maximum (2014). Key Points: A survey of a half solar cycle of auroral Global Navigation Satellite System scintillation data is conductedScintillation occurrence rate decreases with the waning solar cycleFor phase events, the hypothesized irregularity layer altitude decreases with the waning solar cycle [ABSTRACT FROM AUTHOR]

We use the Geospace Environment Model of Ion‐Neutral Interactions (GEMINI) to create three‐dimensional, time‐dependent simulations of auroral ionospheric parameters in the localized, several 100 km region surrounding auroral arcs observed during a winter 2017 sounding rocket campaign, resolving three‐dimensional features of fine‐scale (km) flow structures in the vicinity of an auroral arc. The three‐dimensional calculations of GEMINI allow (with sufficient driving data) auroral current closure to be investigated without idealizing assumptions of sheet‐like structures or height integrated ionospheres. Datamaps for two nearly sheet‐like arcs are reconstructed from replications of the Isinglass sounding rocket campaign data, and combined with camera‐based particle inversions into a set of driving inputs to run the 3D time‐dependent model. Comparisons of model results to radar density profiles and to in situ magnetometry observations are presented. Slices of volumetric current, flow, and conductance structures from model outputs are used to interpret closure currents in an auroral arc region, and are compared to original in situ measurements for verification. The predominant source of return current region field aligned current closure for these slow time variation events is seen to be from the conductance gradients, including the Hall. The importance of the ∇ΣH versus ∇ΣP terms in the determination of the current structure provides a more complicated picture than a previous GEMINI study, which relied predominantly on the divergence of the electric field to determine current structure. Sensitivity of data‐driven model results to details of replication and reconstruction processes are discussed, with improvements outlined for future work. Key Points: Three‐dimensional physics‐based Geospace Environment Model of Ion‐Neutral Interactions (GEMINI) ionospheric model driven with two‐dimensional reconstructions of auroral data for non‐idealized view of auroral current closureConductance gradients are shown to be significant for current closure in these examples, including Hall conductance gradients in some casesExploration of sensitivity of GEMINI output to fidelity of data reconstruction input drivers at radar and in situ resolutions [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Turbulence remains an outstanding challenge in understanding coupling, energetics, and dynamics of the atmosphere. However, turbulence is recognized as a critical component in our models of terrestrial and space weather. Obtaining routine and accurate measurements of turbulence continues to be a major challenge. We present new rocket‐borne measurements of turbulence in January 2015 at Poker Flat Research Range, Alaska. These rocket‐borne measurements were coordinated with a suite of ground‐based instruments. The rocket‐borne instruments captured the small‐scale structure of the turbulence. The ground‐based measurements documented the meteorological and space weather conditions. We find low levels of turbulence coinciding with a disturbed atmosphere where wave activity is reduced. These finding suggest that there may be systematically low levels of turbulence in the Arctic middle atmosphere, as the Arctic middle atmosphere is routinely disturbed in winter. Key Points: Low levels of turbulence in the upper mesosphere are observed in a disturbed Arctic atmosphere following an SSWLow levels of turbulence coincide with low levels of gravity wave activityTurbulence is found in both the topside and bottomside of MILs [ABSTRACT FROM AUTHOR]