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This work develops a method by which a kilometer-spaced array of Global Navigation Satellite System (GNSS) scintillation receivers can be used to estimate the ionospheric irregularity layer height and thickness and associated uncertainties on those estimates. Spectra of filtered signal power and phase data are used to estimate these quantities by comparing the observed ratio of the log of the power spectrum to the phase spectrum with the Rytov weak scatter theoretical ratio. A Monte Carlo simulation of noise on the input signal and the irregularity drift velocity is used to quantify the error in estimates of height and thickness. The method is tested using data from the Scintillation Auroral Global Positioning System (GPS) Array (SAGA) sited in the auroral zone at Poker Flat Research Range, Alaska. For the 30-min scintillation period studied, the technique identifies ionospheric scattering from a thick F layer, which correlates well with on-site incoherent scatter radar measurements of peak electron density, for an event previously identified in the literature as likely due to F layer. [ABSTRACT FROM AUTHOR]

Electron density irregularities in the ionosphere modify the phase and amplitude of trans-ionospheric radio signals. We aim to characterize the spectral and morphological features of E- and F-region ionospheric irregularities likely to produce these fluctuations or "scintillations". To characterize them, we use a three-dimensional radio wave propagation model—"Satellite-beacon Ionospheric scintillation Global Model of upper Atmosphere" (SIGMA), along with the scintillation measurements observed by a cluster of six Global Positioning System (GPS) receivers called Scintillation Auroral GPS Array (SAGA) at Poker Flat, AK. An inverse method is used to derive the parameters that describe the irregularities by estimating the best fit of model outputs to GPS observations. We analyze in detail one E-region and two F-region events during geomagnetically active times and determine the E- and F-region irregularity characteristics using two different spectral models as input to SIGMA. Our results from the spectral analysis show that the E-region irregularities are more elongated along the magnetic field lines with rod-shaped structures, while the F-region irregularities have wing-like structures with irregularities extending both along and across the magnetic field lines. We also found that the spectral index of the E-region event is less than the spectral index of the F-region events. Additionally, the spectral slope on the ground at higher frequencies is less than the spectral slope at irregularity height. This study describes distinctive morphological and spectral features of irregularities at E- and F-regions for a handful of cases performed using a full 3D propagation model coupled with GPS observations and inversion. [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]

The polar vortices play a central role in vertically coupling the Sun-Earth system by facilitating the descent of nitrogen oxides produced by solar energetic particle precipitation (EPP) in the atmosphere. This EPP "Indirect Effect" involves the downward transport of nitrogen oxides (NOx=N+NO+NO2) from the mesosphere-lower thermosphere (MLT) to the stratosphere inside the winter polar vortex and is particularly impactful in the wake of prolonged Sudden Stratospheric Warming events. This work is motivated by the fact that all state-of-the-art global climate models severely underestimate this EPP-NOx transport in the Arctic. As a step toward understanding the transport pathways by which MLT air enters the top of the polar vortex, here we explore the extent to which Lagrangian Coherent Structures (LCS) impact the geographic distribution of EPP-NOx near the polar winter mesopause. As a proof-of-concept, we show that an LCS formed atop one of the split polar vortex lobes during the 2009 SSW. We then demonstrate that the LCS acted to confine air with elevated NOx to high latitudes as the air descended into the top of the polar vortex. These results present a new conceptual model of transport in the polar winter mesosphere whereby regional-scale but long-lived LCS act to sequester elevated NOx to high latitudes until the air descends to lower altitudes. This work contributes to the ROSMIC science questions, "How is the solar signal transferred from the thermosphere to the troposphere?" and "How does the coupling take place within the terrestrial atmosphere?". [ABSTRACT FROM AUTHOR]

Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the U.S. local area augmentation system are shown. During extreme ionospheric activity, the gradient suffered by a global navigation satellite system user a few kilometers away from a ground reference station may reach as high as 425 mm of delay (at the GPS L1 frequency) per km of user separation. The method of data analysis that produced these results is described, and a threat space that parameterizes these possible threats to user integrity is defined. Certain configurations of user, reference station, global navigation satellite system satellite, and ionospheric storm-enhanced density may inhibit detection of the anomalous ionosphere by the reference station. [ABSTRACT FROM AUTHOR]

This paper describes the design, analysis, and evaluation of new ionospheric measurement error models for high-integrity multiconstellation navigation systems, The robustness of the newly derived models is experimentally evaluated using months of dual-frequency global positioning system (GPS) data collected at multiple locations, In parallel, an integrity analysis is devised to quantify the impact of traveling ionospheric disturbances on user position, Finally, overall carrier phase positioning performance is assessed over continental areas for various combinations of GPS and Galileo and Iridium satellites. [ABSTRACT FROM PUBLISHER]

Ground-based augmentations of the global positioning system demand guaranteed integrity to support aircraft precision approach and landing navigation. To quantitatively evaluate navigation integrity, an aircraft computes vertical and lateral protection levels as position-error bounds using the standard deviation of ionosphere spatial decorrelation. Thus, it is necessary to estimate typical ionospheric gradients for nominal days and to determine an appropriate upper bound to sufficiently cover the differential error due to the ionosphere spatial decorrelation. Both station-pair and time-step methods are used to assess the standard deviation of vertical (or zenith) ionospheric gradients (σvig). The station-pair method compares the simultaneous zenith delays from two different reference stations to a single satellite and observes the difference in delay across the known ionosphere pierce point separation. Because these ionosphere pierce point separations limit the observability of the station-pair method, the time-step method is also used to better understand ionospheric gradients at short distance scales (10-40 km). The time-step method compares the ionospheric delay of a single line of sight at one epoch with the delay for the same line of sight at another epoch a short time (a few to tens of minutes) later. This method has the advantage of removing interfrequency bias calibration errors on different satellites and receivers while possibly introducing an estimation error due to temporal ionospheric gradients. The results of this study demonstrate that typical values of σvig are on the order of 1-3 mm/km for nonstormy ionospheric conditions. As a result, σvig of 4 mm=km is conservative enough to bound ionosphere spatial decorrelation for nominal days and still leave enough margin for more active days and for non-Gaussian tail behavior. [ABSTRACT FROM AUTHOR]