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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]

The storm-time ionospheric-thermospheric (IT) state is of great interest, since the IT dynamics change dramatically as energy is input and dissipated in the upper atmosphere. Lagrangian coherent structures (LCSs), which are objective ridges in time-evolving flows that describe the tendency of neighboring fluid elements to separate, provides a unique opportunity to infer the dynamics in the IT system. In this work, we model IT flows with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model and identify the LCSs. We compare the LCSs in the neutral winds and plasma drifts during quiet times versus during active times. We find that LCSs are largely aligned in the modeled IT flows, with a dawn-dusk asymmetry in their latitudinal position. During a geomagnetic storm, the thermospheric LCSs (T-LCSs) and ionospheric LCSs (I-LCSs) shift equatorward, align more closely with each other, and maintain a dawn-dusk asymmetry. The collocation of T-LCSs and I-LCSs and their analogous response to the geomagnetic storm provide evidence of energy input into the thermosphere and ionosphere simultaneously, and the ion drag is the dominant effect causing LCS alignment during a geomagnetic storm. [ABSTRACT FROM AUTHOR]

Geomagnetic storms produce significant electrodynamics at midlatitudes. Strong ion convection can affect thermospheric neutral wind motion. The converse is also true, such that both fields and winds can drive ionospheric plasma movement. This work adjusts a background modeled high‐latitude electrostatic potential to estimate the storm time electric field based on data‐derived plasma densities and measurements of neutral wind. Electron densities are derived from global navigation satellite system (GNSS) total electron content measurements using Ionospheric Data Assimilation Four‐Dimensional (IDA4D). These are input to Estimating Model Parameters from Ionospheric Reverse Engineering (EMPIRE) to estimate electric potential corrections to the background Weimer 2000 potential model. The EMPIRE basis functions for electric potential are spherical harmonics in dipole magnetic colatitude and longitude, enforced to be constant along a magnetic field line. For the 17 March 2015 storm, EMPIRE electric potential produces westward zonal ion drifts that more closely agree with incoherent scatter radar (ISR) measurements made at Millstone Hill than the background Weimer 2000 model alone, when electric potential and meridional neutral winds are both corrected. Additionally ingesting northward line‐of‐sight neutral wind measurements from a Fabry‐Perot interferometer at Millstone Hill makes little difference in the agreement between zonal ion drift predictions and measurements. Estimation of only electric potential reduces the agreement between the assimilated prediction of the field‐perpendicular zonal drifts and ISR measurements significantly. Plain Language Summary: Charged particles and neutral particles in our upper atmosphere respond to different forces. Plasma drifts in response to electric and magnetic fields; neutrals winds blow due to pressure and temperature variations. Because they coexist in the upper atmosphere, plasma and neutral particles interact in ways we do not always understand, especially during geomagnetic storms. This study investigates how using measurements of charged particle density can help us deduce what the storm time forces were. We use time‐lapse images of the plasma density to try to estimate what the electric field that moved the plasma was during a storm. We also examine how stronger neutral winds influence the estimated effect of the electric field and whether a limited number of measurements of the neutral winds help constrain both the neutral winds and plasma drift better. We find that plasma images alone let us estimate the poleward electric field and north‐south neutral wind during a geomagnetic storm and that neutral wind measurements have a small effect on the estimates. Key Points: EMPIRE estimates global corrections to a background model electric potential by filtering GNSS‐derived electron densities and neutral windsSpherical harmonics potential corrections are constrained to have constant potential along a magnetic dipole field lineEMPIRE zonal ion drift estimates of subauroral convection are consistent with incoherent scatter radar data [ABSTRACT FROM AUTHOR]

Abstract: In the high‐latitude ionosphere, predicting transport of polar cap patches is important because of their impact on radio communications and navigation systems. Lagrangian coherent structures (LCSs) are barriers to transport in nonlinear time‐varying flow fields, found by computing the local maximum finite‐time Lyapunov exponent (FTLE). We propose that LCSs are barriers governing patch formation. In this work, we compute and visualize the LCSs in high‐latitude ionospheric convection by computing the FTLE field with the Ionosphere‐Thermosphere Algorithm for Lagrangian Coherent Structures (ITALCS). The Weimer 2005 high‐latitude electric potential model and the 12th‐generation International Geomagnetic Reference Field (IGRF‐12) are used to generate the E → × B → drift field at each gridpoint in the ionosphere. The E → × B → drifts are used as the input to ITALCS. Time‐varying structures are detected in two‐dimensional ionospheric drifts at high latitudes based on locally maximum forward time FTLE values for both geomagnetically stormy and quiet periods. Typically, the dominant structure is shaped like the letter “U,” or a “horseshoe,” oriented with the curved portion of the “U” on the dayside around local noon. The LCSs during the geomagnetically stormy period have more complex topology and shift equatorward compared to the LCSs during quiet times. Analysis of a polar cap patch observed on 17 March 2015 with the Multi‐Instrument Data Analysis System indicates that a necessary condition for its formation and transport is that storm enhanced density exist poleward of the LCS. [ABSTRACT FROM AUTHOR]

We present analysis methods for studying the structuring and motion of ionospheric irregularities at the subkilometer scale sizes that produce L band scintillations. Spaced-receiver methods are used for Global Navigation Satellite System (GNSS) receivers' phase measurements over approximately subkilometer to kilometer length baselines for the first time. The quantities estimated by these techniques are plasma drift velocity, diffraction anisotropy magnitude and orientation, and characteristic velocity. Uncertainties are quantified by ensemble simulation of noise on the phase signals carried through to the observations of the spaced-receiver linear system. These covariances are then propagated through to uncertainties on drifts through linearization about the estimated values of the state. Five receivers of SAGA, the Scintillation Auroral Global Positioning System (GPS) Array, provide 100 Hz power and phase data for each channel at L1 frequency. The array is sited in the auroral zone at Poker Flat Research Range, Alaska. A case study of a single scintillating satellite observed by the array is used to demonstrate the spaced-receiver and uncertainty estimation process. A second case study estimates drifts as measured by multiple scintillating channels. These scintillations are correlated with auroral activity, based on all-sky camera images. Measurements and uncertainty estimates made over a 30 min period are compared to a collocated incoherent scatter radar and show good agreement in horizontal drift speed and direction during periods of scintillation for which the characteristic velocity is less than the drift velocity. [ABSTRACT FROM AUTHOR]

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]