ALBERT

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-14, Ahead of Print.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-16, Ahead of Print.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-11, Ahead of Print.

Description: This paper presents an analysis of the parallel plate waveguide, based on a hybrid mode formulation. The nonideal metallic conductors of the waveguide are treated as a media characterized by an equivalent permittivity. The frequencies of interest in the presented analysis are at the terahertz band (from 300 GHz to 30 THz), and appropriate models are used for the correct characterization of metallic conductors at these frequencies. The behavior of the electromagnetic field of the fundamental mode is studied in a wide frequency range. At low frequencies (microwave regime) the fundamental mode is the well-known transverse electromagnetic (TEM) mode; as frequency increases, the electromagnetic field changes significantly and a surface wave or surface plasmon polariton (SPP) behavior is observed at the highest frequencies of the terahertz band. This paper shows a unified formulation that explains this transformation in the electromagnetic field behavior.

Description: We have used a relatively long, contiguous VHF observation of a bright cosmic radio source (Cygnus A) with the Very Large Array (VLA) through the nighttime, midlatitude ionosphere to demonstrate the phenomena observable with this instrument. In a companion paper, we showed that the VLA can detect fluctuations in total electron content (TEC) with amplitudes of ≤ 10−3 TECU and can measure TEC gradients with a precision of about 2 × 10− 4 TECU km−1. We detail two complementary techniques for producing spectral analysis of these TEC gradient measurements. The first is able to track individual waves with wavelengths of about half the size of the array (∼20 km) or more. This technique was successful in detecting and characterizing many medium-scale traveling ionospheric disturbances (MSTIDs) seen intermittently throughout the night and has been partially validated using concurrent GPS measurements. Smaller waves are also seen with this technique at nearly all times, many of which move in similar directions as the detected MSTIDs. The second technique allows for the detection and statistical description of the properties of groups of waves moving in similar directions with wavelengths as small as 5 km. Combining the results of both spectral techniques, we found a class of intermediate and small scale waves which are likely the quasi-periodic (QP) echoes that have been observed to occur within sporadic-E (Es) layers. We find two distinct populations of these waves. The members of one population are coincident in time with MSTIDs and are consistent with being generated within Es layers by the E–F coupling instability. The other population seems more influenced by the neutral wind, similar to the predominant types of QP echoes found by the Sporadic-E Experiments over Kyushu (SEEK). We have also found that the spectra of background (i.e., isotropic) fluctuations can be interpreted as the sum of two turbulent components with maximum scales of about 300 km and 10 km.

Description: This paper presents an altimetric method for ocean monitoring by remote sensing. It uses carrier observations of reflected GNSS signals. The method is illustrated in a simulation study and applied to a long term experiment yielding an ocean tide spectrum. The altimetric concept is based on residual observations of Doppler frequency. A linear relation between Doppler residuals f0 and height departures ΔH from the surface level is derived. In contrast to existing phase-based methods which are constrained by smooth ocean conditions, the frequency-based retrieval here described holds good for rougher ocean conditions. Two retrievals of Doppler residuals are distinguished: Tracking Retrieval and Spectral Retrieval. A simulation study investigates the performance of Spectral Retrieval for a rough ocean surface with a noise-like sea level deviation ξ(t). Simulation settings were adjusted to reflection events in coastal experiments with an elevation range of [5…15] deg. In this range Tracking Retrieval tolerates a surface standard deviation σξ 〈 5 cm, whereas Spectral Retrieval tolerates σξ ≤ 30 cm. These limits correspond to significant wave heights of 20 cm for Tracking Retrieval and 1.2 m for Spectral Retrieval. The simulation results are confirmed by applying the altimetric method to the experimental data. The recovery of continuous phase tracks in experimental data is onerous and Tracking Retrieval only works for a period of smooth ocean conditions (162 events). By contrast, Spectral Retrieval yields altimetric estimates throughout the whole experiment (2607 events). The altimetric time series extends over more than 60 days and results in a tide spectrum that resolves diurnal (K1) and semidiurnal (M2, S2) constituents. The formal precision for these estimates lies in the decimeter range.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-19, Ahead of Print.

Description: The capability of current and future sensors to make accurate measurements of polarized microwave radiation allows for the investigation of particle shape and orientation. Additionally, the dichroic properties of media consisting of particles with preferential alignment will alter the polarization state of radiation emitted and reflected by the surface below a cloud boundary. Therefore, a deep understanding of the influence of particle orientation and shape upon radiation are required for remote sensing of both cloud and surface properties. In this study, we compute the scattering properties of three horizontally aligned snow crystals: two dendrites and an hexagonal plate. Additionally, we create two approximations using cylindrical plates. One uses a plate with a diameter equal to the maximum dimension of the respective snow crystal, utilizing an effective dielectric model which assumes the disk to be a matrix of ice with air inclusions. The other approximation uses a cylindrical plate of equal mass, with a radius chosen to conserve mass. To simplify the analysis, all particles have equal thickness. The results show a strong polarization response, particularly in the Q element of the Stokes vector. This polarization response is captured well by the two approximations. While the approximations are applicable for certain cases, there are discrepancies between the scattering properties of the idealized snow crystals and the two cylindrical plate models that may limit the generality of the approximations. Further radiative transfer studies are required to test the full applicability of the crystal approximations.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-12, Ahead of Print.

Description: We observe ionospheric perturbations caused by the Tohoku earthquake and tsunami of March 11, 2011. Perturbations near the epicenter were found in measurements of ionospheric total electron content (TEC) from 1198 GPS receivers in the Japanese GEONET network. For the first time for this event, we compare these observations with the estimated magnitude and speed of a tsunami-driven atmospheric gravity wave, using an atmosphere-ionosphere-coupling model and a tsunami model of sea-surface height, respectively. Traveling ionospheric disturbances (TIDs) were observed moving away from the epicenter at approximate speeds of 3400 m/s, 1000 m/s and 200–300 m/s, consistent with Rayleigh waves, acoustic waves, and gravity waves, respectively. We focus our analysis on gravity waves moving south and east of the epicenter, since tsunamis propagating in the deep ocean have been shown to produce gravity waves detectable in ionospheric TEC in the past. Observed southeastward gravity wave perturbations, seen ∼60 min after the earthquake, are mostly between 0.5 to 1.5 TECU, representing up to ∼5% of the background vertical TEC (VTEC). Comparisons of observed TID gravity waves with the modeled tsunami speed in the ocean and the predicted VTEC perturbation amplitudes from an atmosphere-ionosphere-coupling model show the measurements and models to be in close agreement. Due to the dense GPS network and high earthquake magnitude, these are the clearest observations to date of the effect of a major earthquake and tsunami on the ionosphere near the epicenter. Such observations from a future real-time GPS receiver network could be used to validate tsunami models, confirm the existence of a tsunami, or track its motion where in situ buoy data is not available.

Description: The accuracy of the positioning systems such as GPS, GLONASS or Galileo is heavily affected by the presence of the ionosphere. Ionosphere-free dual-frequency algorithms used for positioning applications remove most of the ionospheric error but do not take into account its higher-order terms. In addition, the raypaths and total electron content (TEC) are assumed to be the same for both frequencies. This leads to centimeter-level range errors that can cause millimeter-level errors in positioning. In this paper an accurate estimation of the higher-order ionospheric errors based on a realistic 3-D electron density model is presented. A numerical homing-in ray-tracing algorithm is implemented to rigorously calculate satellite to receiver ray trajectories. The numerical simulations performed showed that higher-order ionospheric residual range errors may reach several centimeters (up to 5 cm) at low and middle latitudes; however, at high latitudes they hardly exceed several millimeters (up to 1 cm).

Description: A technique for studying inhomogeneous structure and motions in the ionosphere by measurements of the amplitude and phase characteristics of wideband radio pulses is developed and demonstrated in investigations of the HF-pumped ionospheric volume above the Sura facility. For the study of the vertical structure (electron density profile and vertical velocities) with high altitude and temporal resolution, a Tikhonov regularization algorithm for the solution of the inverse problem using phase sounding data is implemented. Horizontal velocity data are obtained by means of a correlation analysis of the reflected signals received by the diversity technique. The application of the technique described allows measurement of the HF pump-induced modifications of the electron density profile, particularly the plasma expulsion from the plasma resonance and upper hybrid resonance regions, with high accuracy.

Description: The electromagnetic properties of active cylindrical coated nano-particle antennas are investigated. It is demonstrated that the active cylindrical coated nano-particle, whether illuminated by a plane wave or an electric Hertzian dipole (EHD) (small current) element, acts as a strong dipole radiator at its resonant frequency. It is shown that the plane wave scattering cross section could be increased by about 40 dBsm, and the maximum peak of the power radiated by an EHD element could be increased more than 65 dB in the presence of the active nano-particle over its value when radiating into free space. An array, constructed with four active cylindrical coated nano-particles and excited by an EHD element located at or near its center is also studied. Large directivity values, more than 8 dB, are obtained for particular array configurations and EHD locations.

Description: This is the second part of a two-part series dealing with complex dipolar waves propagating along the axes of 1D, 2D and 3D infinite periodic arrays of small lossless and lossy permeable spheres. Shore and Yaghjian (2012) provide the theory of the complex waves and the dispersion (k–β) equations for their propagation constants. In this paper we present and discuss representative dispersion diagrams for arrays of magnetodielectric spheres (spheres with appreciable permittivity and permeability), diamond spheres, and silver nanospheres.

Description: An improved method of estimating ground-scattered power using high-frequency (HF) ray tracing techniques that overcomes the limitations of the derivation presented by Bristow and Greenwald (1995) is presented. The improved method is applied toward identifying the effects of an artificial ionospheric density layer on measured ground scatter power. The presence of artificial density layers induced at the High Frequency Active Auroral Research Program (HAARP) station in Gakona, Alaska, are observed through the ground-scattered power received by the Kodiak Super Dual Auroral Radar Network (SuperDARN) HF radar. The location and physical dimensions of the artificial layers are estimated by simulating radar returns using HF ray tracing through a model ionosphere that includes a model artificial density layer. Simulation results of ground-scattered power as a function of range are compared to the measured ground-scattered power as a function of range during a time period when artificial layers were evident in ionogram data. It is shown that a model artificial density layer based on research by Pedersen et al. (2009) produces simulation results that approximate the mean of the measured results.

Description: In this paper we present the first results from measurements of scintillation and total electron content (TEC) from an equatorial station, Lagos (Latitude 6.5°N, Longitude 3.4°E, magnetic latitude 3.03°S), Nigeria, using a Novatel GSV4004B GPS ionospheric scintillation and TEC monitor. Details are presented for data collected between February 2010 and August 2010. The results show that the presence of some large scale depletions of TEC or plasma bubbles may be noted during the evening hours and that TEC depletions correspond to increased rate of change of TEC (ROT). This confirms that plasma bubbles are associated with large scale irregularities. It is also established that enhanced amplitude scintillation (S4) corresponds quite well with TEC depletions and increases in ROT. The diurnal and seasonal percentage occurrence for different levels of scintillation activity has peaks in the equinox months (March and April) at 23:00 LT.

Description: The common occurrence of a marine boundary layer in coastal regions can affect radio wave propagation significantly. Refractivity from clutter (RFC) techniques are aimed at estimating the refractivity profile of the ambient environment based on the received radar clutter. While most previous RFC work has not considered the dependence of RFC inversions on a variable grazing angle, this study investigates incorporation of the grazing angle information into the clutter model. A newly proposed clutter model based on multiple incident angles at each range is used here. The inversion performance of the multiple angle clutter model is compared to that of other models. Synthetic examples of a range-independent surface-based duct and a range-dependent evaporation duct are investigated for a S-band radar. Finally, a comparison of inversions on one set of experimental measurements from the SPANDAR 1998 data set is provided, using single and multiple grazing angle clutter models, and the previously used model based on grazing angle independent sea surface reflectivity.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-9, Ahead of Print.

Description: The Space Environment Group of the National Institute of Information and Communications Technology, Tokyo, Japan, operates four ionosondes at Okinawa, Yamagawa, Kokubunjii, and Wakkanai in the Asian sector. Okinawa is located at the lowest latitude and lies at the northern edge of the northern equatorial anomaly, while Wakkanai is located at the higher latitudes of the midlatitude ionosphere. For this study, ionograms obtained from the Internet are analyzed using the automated Expert System for Ionogram Reduction (ESIR). An anomalous 3 day foF2 enhancement observed by the Wakkanai ionosonde from 9 to 11 October 2009 forms the basis for this study. The scientific question being addressed pertains to the remarkably quiescent geomagnetic activity experienced during the extended solar minimum between cycles 23 and 24 that enables a search for the ionospheric response to weather in the lower atmosphere. The analysis of the ionograms from these four stations using the proprietary ESIR technique provides an extended database of electron density profiles that describes the ionospheric variability as a function of latitude, local time, and season. In addition, independent observations of the ionospheric TEC used by the USU Global Assimilation of Ionospheric Measurements (GAIM) model verify the anomalous ionospheric behavior as well as establishing its extent. Typical solar minimum conditions were seen during this study, with geomagnetic activity restricted to well-characterized corotating interaction region (CIR) events. After eliminating geomagnetic and solar disturbances as drivers of the October 2009 anomaly, the presence of Typhoon Melor is suggested as a possible source mechanism for the ionospheric anomaly.

Description: The effect of strong scintillation conditions on GNSS transionospheric paths of propagation is further investigated employing the most recent update of the Hybrid Scintillation Propagation Model (HSPM). The variation of various parameters including spectral indices and other statistical moments of the field is studied as a function of the severity of the signal fluctuations. The correlation time of the complex amplitude of the field is found to rapidly decrease as the scintillation severity increases, but by contrast, the intensity correlation time stays almost constant over a wide range of S4 showing only slight decrease in the model's range of validity. The dependence of the spectral indices of both phase and amplitude on S4 is also determined, and the spectral index of the phase fluctuations tends to 2 for the most severe scintillation, as expected from both experiment and theory. The effect of “canonical fading” is also studied, when, in the conditions of strong scintillation, fast phase changes occur along with deep amplitude fades. The probability of the effect of the “canonical fading” is studied for the conditions of strong scintillation, and the mean time between cycle slips shows a significant decrease as S4 increases. A comparison is also presented between calculated results of S4, spectral indices, and the correlation radii of the complex field and field intensity, utilizing both the HPSM and equivalent phase screen model for both weak and strong scintillation conditions. These show the differences that can occur which can also depend on the equivalent phase screen height.

Description: Increasingly accurate and detailed global 3-D specification of the Earth's space plasma environment is required to further understand its intricate organization and behavior. For a long time space physics and aeronomy research has been data starved due to the great variety of natural time scales involved in the plasma phenomenology. We have started developing a new approach to the global ionospheric specification called Real-Time Assimilative Mapping (RTAM). The IRI-RTAM will use data from the Global Ionospheric Radio Observatory (GIRO) to smoothly transform IRI's background empirical maps of the ionospheric characteristics to match the observations. Such empirical assimilative modeling will provide a high-resolution, global picture of the ionospheric response to various short-term events observed during periods of storm activity or the impact of gravity waves coupling the ionosphere to the lower atmosphere, including timelines of the vertical restructuring of the plasma distribution. It will also contribute to the challenging task of providing a rapid insight into the temporal and spatial space weather development using the real-time GIRO data streams. The new assimilation technique “updates” the IRI electron density distribution while preserving the overall integrity of IRI's typical ionospheric feature representations. The technique adjusts the coefficients of the spherical/diurnal expansions used by the CCIR and URSI-88 model to obtain the global subpeak electron density distribution. The set of global corrected coefficients can be generated as frequently as every 15 min and easily disseminated using a single real-time RTAM server operated by GIRO.

Description: This is the first part of a two-part series dealing with complex dipolar waves propagating along the axes of 1D, 2D, and 3D infinite periodic arrays of small lossless and lossy permeable spheres. The theory is presented in this paper and numerical results are presented by Shore and Yaghjian (2012). The focus is on the dispersion (k–β) equations relating the array propagation constant, β, to the free-space wave number, k, for dipolar complex waves. The k–β equation for the complex propagation constants of a given array is obtained from the corresponding equation previously obtained for the real propagation constants by rewriting the real propagation dispersion equation in a form that can be analytically continued into the complex β plane. This equation reduces correctly to the real β dispersion equation and enables complex values of β to be found as a function of the array element parameters. By allowing for all the possible branches of the multivalued homogeneous dispersion equation analytically continued into the complex β plane, the propagation constants of all the improper as well as proper complex waves supported by the 1D, 2D, and 3D arrays are found from the homogeneous solutions for these arrays. Green's functions for external sources are not required to find the propagation constants of the complex waves supported by the arrays. For 3D arrays, in certain frequency ranges, it is possible to regard the arrays as media characterized by bulk or effective permittivities and permeabilities. Expressions for these bulk parameters, more accurate than the Clausius-Mossotti expressions, are obtained from quantities readily available in the solutions of the dispersion equations.

Description: The Radio Aurora Explorer (RAX) satellite is the first of several satellites funded under the NSF CubeSat-based Space Weather and Atmospheric Research Program. RAX is a ground-to-space bi-static radar remote sensing experiment designed to measure and understand the causes of meter-scale ionospheric irregularities. Also known as field-aligned irregularities (FAI), such non-thermal, coherent fluctuations of electron density occur in response to strong ionospheric flows or plasma density gradients during geomagnetic disturbances and are considered a space weather concern due to disruption to communication and navigation signals. The RAX CubeSat was launched in November 2010 and conducted a single experiment in coordination with the Poker Flat Incoherent Scatter Radar. Due to geophysical inactivity, e.g., lack of strong ionospheric electric fields and low ionospheric densities, no FAI were expected or observed. However, the radar receiver payload operation was successfully demonstrated, including the capability to sense signals as low as −110 dBm, the capability of transmitter–receiver synchronization and accurate ranging, processing of 1.2 GB of raw radar data on board in less than 1 hour, and the downlink of the science results within three–four passes. Analysis of the payload data shows that the noise level is sufficiently low. Although the interference level is a concern, it does not appear to significantly limit the measurements. Toward the end of December 2010, the solar power system gradually degraded and the mission terminated in early February 2011 after prolonged loss of contact with the satellite. Meanwhile, RAX II was launched in October 2011 to a polar orbit. This paper describes the RAX science and radar system and presents the results from the first experiment conducted.

Description: Drifting structures characterized by inhomogeneities in the spatial electron density distribution at ionospheric heights cause the scintillation of radio waves propagating through. The fractional electron density fluctuations and the corresponding scintillation levels may reach extreme values at low latitudes during high solar activity. Different levels of scintillation were observed on experimental data collected in the Asian sector at low latitudes by means of a GPS dual frequency receiver under moderate solar activity (2005). The GPS receiver used in these campaigns was particularly modified in firmware in order to record power estimates on the C/A code as well as on the carriers L1 and L2. Strong scintillation activity was recorded in the post-sunset period (saturating S4 and SI as high as 20 dB). Spectral modifications and broadening was observed during high levels of scintillation possibly indicating refractive scattering taking place instead of diffractive scattering. A possible interpretation of those events was attempted on the basis of the refractive scattering theory developed by Uscinski (1968) and Booker and MajidiAhi (1981).

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-11, Ahead of Print.

Description: For coherent targets, Huynen then Cameron proposed a set of characteristic parameters deduced from monostatic full polarimetric scattering matrices. Among these parameters, the two Authors have considered two parameters connected to the same physical properties: orientation and symmetry degree. As the Huynen definition seems to be more intuitive, we will try to compare these parameters by simulations with the aim to test the real connection of the Huynen parameters to physical properties. This study will show significant differences. However, it is shown that the Huynen parameters correctly represent the orientation and the symmetry degree for nearly symmetric targets. In some cases (real eigenvalues in particular), the two orientation angles can be equal to an ambiguity of 45°, even for asymmetric targets.

Description: This study examines the impacts of ionospheric scintillations on GPS receivers that are intended for equatorial or transequatorial aviation applications. We analyzed GPS data that were acquired at Ascension Island during the Air Force Research Laboratory (AFRL) campaign of the solar maximum year of 2002. Strong scintillations impacted the receiver-satellite geometry, leading to poor dilution of precisions and positioning accuracy. In addition, deep signal fades (〉20 dB-Hz), leading to navigation outages were observed during most of the nights of the campaign. Under quiescent conditions, the C/No of satellites fluctuated slowly between 50 dB-Hz and 35 dB-Hz baselines for both L1 (1.5754 GHz) and L2 (1.2276 GHz) signals, depending on the satellite's elevation angle. The satellite's elevation angle and the effective scan velocity of the satellite's ionospheric penetration point (IPP) with respect to the magnetic field and plasma drift influenced the rate of fading of satellite signals.

Description: Extreme ionospheric anomalies can pose a potential integrity threat to ground-based augmentation of the Global Positioning System (GPS), and thus the development of ionospheric anomaly threat models for each region of operation is essential for system design and operation. This paper presents a methodology for automated long-term ionospheric anomaly monitoring, which will be used to build an ionospheric anomaly threat model, evaluate its validity over the life cycle of the system, continuously monitor ionospheric anomalies, and update the threat model if necessary. This procedure automatically processes GPS data collected from external networks and estimates ionospheric gradients at regular intervals. If ionospheric gradients large enough to be potentially hazardous to users are identified, manual data examination is triggered. This paper also develops a simplified truth processing method to create precise ionospheric delay estimates in near real-time, which is the key to automating the ionospheric monitoring procedure. The performance of the method is examined using data from the 20 November 2003 and 9 November 2004 ionospheric storms. These results demonstrate the effectiveness of simplified truth processing within long-term ionosphere monitoring. From the case studies, the automated procedure successfully identified extreme ionospheric anomalies, including the two worst ionospheric gradients observed and validated previously based on manual analysis. The automation of data processing enables us to analyze ionospheric data continuously going forward and to more accurately categorize ionospheric behavior under both nominal and anomalous conditions.

Description: We present the results of a multiscale analysis of TEC fluctuations using a roughly five-hour observation of the bright radio source Virgo A with the Very Large Array (VLA) at 74 MHz in its B configuration. Our analysis combines data sensitive to fine-scale structure (∼10 km and

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-16, Ahead of Print.

Description: Data assimilation models like the Utah State University (USU) Global Assimilation of Ionospheric Measurements (GAIM) models use physics-based models of the ionosphere, ionosphere-plasmasphere, or thermosphere and a Kalman filter as a basis for assimilating a diverse set of measurements. With a sufficient amount of data and with multiple data types, the data assimilation models can provide reliable specifications and near-term forecasts. However, for long-term forecasts (5 days or longer) stand-alone or coupled physics-based models are needed. Unfortunately, the various physics-based models contain several uncertain parameters and processes as well as missing physics. Further complications arise for coupled physics-based models because of coupling issues and error propagation from model to model. Some of the problems are associated with the magnetosphere and lower atmosphere drivers, the adopted set of physics-based equations, the parameterization of physical processes, the values adopted for the transport coefficients, the numerical techniques used, the spatial and temporal resolutions adopted, and the uncertainties in the initial and boundary conditions. Examples of the type of problems the space weather community faces in its attempt at long-term ionosphere-thermosphere forecasting are given.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-19, Ahead of Print.

Description: Radio receivers capable of high-rate sampling such as GPS scintillation monitors and the ALTAIR VHF/UHF tracking radar can measure ionospheric phase fluctuations suitable for scintillation modeling using phase screen techniques. For modeling purposes, the phase variations caused by the refractive effects of electron density irregularities encountered along the propagation path are desired. The phase fluctuations measured by ground-based receivers, however, also include the unwanted effects of diffraction (phase scintillations). In this paper, we investigate the effect of phase scintillations on the accuracy of phase screen simulation when using the phase measured on the ground as a proxy for the ionospheric screen. Using stochastic and deterministic (measured) phase screens, we quantitatively assess the accuracy of this approach by cross-correlating the predicted and measured intensity fluctuations. We find that the intensity cross-correlation is less than unity even in the weak scatter limit, due to the presence of weak phase scintillations. This correlation decreases rapidly with increasing irregularity strength once rapid transitions in the phase (strong phase scintillations) develop. We demonstrate that, when using the measured phase on the ground as a proxy for the ionospheric screen, both the temporal structure of simulated fluctuations and their statistics deviate increasingly from those of the measurements as the turbulence strength increases, especially when strong phase scintillations are present. We also demonstrate that back-propagating the complex signal up to ionospheric altitudes prior to the forward propagation calculation yields improved results, but some errors still remain as a consequence of neglecting amplitude fluctuations which develop inside the random medium.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-12, Ahead of Print.

Description: Improved predictions of multipath spread at HF have been obtained from VOACAP by including the effect of probability distribution functions of the signal power rather than just taking the median signal power. A Monte Carlo calculation method has been adopted that is superior in terms of accuracy and computation speed over the brute force method previously employed. The predictions of multipath spread are generally comparable with the values measured over an 1800 km sub-auroral path. The new method will be useful to those planning or operating digitally modulated radio systems in the HF band since these can be adversely affected by multipath spread.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-9, Ahead of Print.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-15, Ahead of Print.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-17, Ahead of Print.

Description: Midlatitude space weather events are being monitored at a location in Western Australia (−26.7° latitude, 116.7° longitude, and −38° magnetic latitude). This location is that of the Murchison Widefield Array (MWA), a new low radio frequency array (80 to 300 MHz) being built with the objective of developing powerful new capabilities for radio astronomy, solar, heliospheric, and ionospheric science. The MWA is a radio interferometer that images large fields of view with unprecedented fidelity and dynamic range at these frequencies. With respect to the ionosphere, it will be able to make important new measurements of ionospheric scintillation and improve our understanding of midlatitude space weather events in the Southern Hemisphere. For example, by studying midlatitude space weather events in the Australian sector, the seasonal and geomagnetic dependencies of storm enhanced density (SED) can be characterized and separated from other forces that may impact its formation elsewhere, such as in the longitudinal sector of the South Atlantic Anomaly (SAA). After full commissioning, the MWA will consist of 2048 dipole antennas arranged as 128 “tiles,” each being a 4 × 4 array of dual-polarization dipoles. Currently, an engineering prototype comprising 32 tiles has been installed and has been used to observe the Faraday rotation from a linearly polarized satellite beacon on the DMSP-15 satellite. We will report on these initial observations of ionospheric Faraday rotation and on results from GPS receivers located at the site and elsewhere in the Australian continent. We will also discuss future space weather products enabled by the MWA.

Description: Phased arrays provide new possibilities for remote sensing with radars. By imposing an azimuthal phase variation, electromagnetic beams that carry orbital angular momentum can be formed. Such beams have a phase structure that appears twisted and as a result an intensity null in the center of the beam cross section. Here we numerically investigate twisted beams for incoherent scatter radars that are used to study the ionosphere. We discuss the possibility of utilizing such radar beams to probe twisted beams of plasma waves and flows transverse to the beam axis, such as associated with auroral arcs. Transverse plasma flows may give rise to a rotational frequency shift of the scatter from a twisted beam and Doppler broadening due to the beam divergence, the latter also occurring with regular beams. Although the angular momentum effects of the considered large scale flows are generally small, sheared and vortical flows transverse to the beam axis can in principle be discriminated from unidirectional flows with beams carrying orbital angular momentum.

Description: The implementation details of a fast direct solver is described herein for solving dense matrix equations from the application of surface integral equation methods for electromagnetic field scatterings from non-penetrable targets. The proposed algorithm exploits the smoothness of the far field and computes a low rank decomposition of the off-diagonal coupling blocks of the matrices through a set of skeletonization processes. Moreover, an artificial surface (the Huygens' surface) is introduced for each clustering group to efficiently account for the couplings between well-separated groups. Furthermore, a recursive multilevel version of the algorithm is presented. Although asymptotically the algorithm would not alter the bleak outlook of the complexity of the worst case scenario, O(N3) for required CPU time where N denotes the number of unknowns, for electrically large electromagnetic (EM) problems; through numerical examples, we found that the proposed multilevel direct solver can scale as good as O(N1.3) in memory consumption and O(N1.8) in CPU time for moderate-sized EM problems. Note that our conclusions are drawn based on a few sample examples that we have conducted and should not be taken as a true complexity analysis for general electrodynamic applications. However, for the fixed frequency (h-refinement) scenario, where the discretization size decreases, the computational complexities observed agree well with the theoretical predictions. Namely, the algorithm exhibits O(N) and O(N1.5) complexities for memory consumption and CPU time, respectively.

Description: Compressed sensing, a method which relies on sparsity to reconstruct signals with relatively few measurements, provides a new approach to processing radar signals that is ideally suited to detailed imaging and identification of multiple targets. In this paper, we extend previously published theoretical work by investigating the practical problems associated with this approach. In deriving a discrete linear radar model that is suitable for compressed sensing, we discuss what the discrete model can tell us about continuously defined targets and show how sparsity in the latter translates to sparsity in the former. We provide details about how this problem can be solved when using large data sets. Through comparisons with matched filter processing, we validate our compressed sensing technique and demonstrate its application to meteors, where it has the potential to answer open questions about processes like fragmentation and flares. At the cost of computational complexity and an assumption of target sparsity, the benefits over pulse compression using a matched filter include no filtering sidelobes, noise removal, and higher possible range and Doppler frequency resolution.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-16, Ahead of Print.

Description: The Earth's geomagnetic field can, in principle, cause significant magnetic-azimuth variations of the lower ionosphere's reflection of 2–150 kHz radio waves emitted by lightning. There has been little published work on this azimuth variation, either modeled or observational. We use broadband emissions from negative cloud-to-ground lightning strokes to study the azimuthal variations systematically. The data are from the Los Alamos Sferic Array, operating in the United States' southern Great Plains during 2005. We compare the observations to a model of lower-ionosphere reflection of radio waves. The model recapitulates the basic features of the time domain reflection waveforms rather well, except at the lowest frequencies. The model transfer function describing the vertical electric field at the receiver is symmetric about 90° magnetic and about 270° magnetic. Two noteworthy features of the azimuth variation are both predicted by the model, and seen in the data: First, at the lowest frequencies ( 50 kHz) there is an opposite enhancement, of the reflection for westward propagation, relative to eastward propagation. The westward enhancement at 〉50 kHz depends sensitively on range and is most evident in nighttime conditions, while the eastward enhancement at

Description: In this work, an extension in latitude range and time span with respect previous studies on Medium Scale Traveling Ionospheric Disturbances (MSTID) propagation, is presented. So far they have been basically studied at mid latitude and for limited periods (less than few years) at solar maximum conditions. This extension has been possible due to the availability of local Global Positioning System (GPS) networks at mid-north hemisphere (California), mid-south hemisphere (New Zealand), high and low latitudes (Alaska and Hawaii), for the last 13, 11, 7 and 4 years respectively. Optimal algorithms specially suitable for mass data processing have been used, such as the Single Receiver Medium Scale Traveling Ionospheric activity index (SRMTID) and the phase difference method for MSTID propagation estimation. The results reveal that several of the main MSTID climatological trends at mid latitude are also shared at low and high latitude, also modulated in intensity also by the Solar Cycle. This is the case for local fall/winter day-time equatorward propagated MSTIDs with typical velocities and wavelengths of 150–250 m/s and 100–300 km respectively. Moreover the comparison of MSTID propagation estimation using different techniques, and their implications in terms of potential origins of MSTIDs, are also discussed.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-3, Ahead of Print.

Description: Mainly based on the channel base current of the 0803 and 0902 flashes detected in the Shandong Artificial Triggering Lightning Experiment (SHATLE) from 2005 to 2010, the current waveforms of six return strokes and an RM process (a process characterized by both a return stroke and an M-component) containing subsidiary peaks were analyzed. The percentage of strokes with subsidiary peak is approximately 17%. The amplitudes of the stroke primary and subsidiary peaks varied from 8.53 to 16.34 kA and 5.40 to 15.86 kA, respectively. The time intervals between the primary and the secondary peak varied from 6.3 to 12.0 μs. The strokes of the 0803 and 0902 flashes have different current waveform characteristics, indicating that the mechanisms generating the subsidiary peaks in the two lightning may be different. To learn more about the causes of forming the subsidiary peaks based on the assumption that channel branches and every peak corresponds to a physical process, a Heidler function model was used to simulate the current waveforms of four strokes and an RM in flash 0902. The simulated results indicated that the Heidler function component could reflect the physical characteristics of the peaks to some extent. Based on simulated results and comparison of waveforms, four possibilities of forming subsidiary peaks were proposed: channel branching, flashover along a triggering wire from a previous unsuccessful launch, the corona current or reflection of the current. Branching and flashover may be the main causes of generating the subsidiary peaks in 0902 and 0803 flashes, respectively.

Description: The high-latitude ionosphere is a dynamic region in the solar-terrestrial system. The disturbances in this region can adversely affect numerous military and civilian systems, and the accurate specification and forecast of its plasma and electrodynamic structures are important for space weather research. Presently, most of the space weather models use limited observations and/or indices to define a set of empirical drivers for physical models forward in time. The empirical drivers have a “climatological” nature, and there are significant physical inconsistencies among various empirical drivers. Therefore, the specifications of high-latitude environment from these space weather models cannot truthfully reflect the weather features. Utah State University (USU) has developed a data assimilation model for the high-latitude ionosphere plasma dynamics and electrodynamics to overcome these hurdles. With a set of physical models and an ensemble Kalman filter, the model can define the drivers that are most truthful to the real space environment by ingesting data from multiple observations. In this paper, we will provide the details on how the model drivers truthful to real space weather are defined in the developed USU data assimilation model and show the space weather variability of the model outputs driven by these model drivers for various seasonal and geomagnetic conditions. Also, we will present preliminary results of validation and comparison studies to demonstrate that the model results with the optimal magnetospheric drivers determined by data assimilation are the better representations of real space environment.

Description: A new method of absolute calibration of sky noise temperature using a three-position switched spectrometer, measurements of antenna and low noise amplifier impedance with a vector network analyzer, and ancillary measurements of the amplifier noise waves is described and compared with other methods of calibration. An initial test of the method has been made using brief observations from 55 to 110 MHz at West Forks, Maine, to estimate the sky noise spectral index. Estimates are made of the accuracy that might ultimately be achieved with observations to detect or set limits on the red-shifted 21 cm line at a radio quiet site. It is concluded that an antenna reflection coefficient better than about −20 dB is required to avoid being limited by the accuracy of the antenna reflection coefficient measurements using a vector network analyzer.

Description: A new method for the calculation of the radiation pattern of corrugated E-plane rectangular horn antennas based on a hybrid uniform theory of diffraction-physical optics (UTD-PO) formulation is presented. The method, which allows for the analysis of horns in which V-shaped corrugations have been considered, has been validated with numerical data obtained through the application of both an electric field integral equation (EFIE) solved by the method of moments (MoM) and the finite element method (FEM)-based full-wave electromagnetic analysis software, HFSS. In this sense, the proposed solution is mathematically less complex and computationally more efficient than existing techniques.

Description: The detection performance of high frequency surface wave radar (HFSWR) and high frequency over-the-horizon radar (OTHR) systems is heavily influenced by the presence of radar clutter. In HFSWR systems, the clutter has its origins in vertical-incidence ionospheric reflections, whereas in OTHR systems, the origin is Bragg backscatter from plasma structures in the auroral zone. This paper models the spreading of the radar clutter signal in the Doppler and angle-of-arrival domains that arises from forward-scattering effects as the radar pulse propagates through regions of ionospheric plasma irregularities. The models use a geometric optics approach to determine the power spectrum of the radar signal phase. This power spectrum is then used to simulate three-dimensional space-time-range radar data cubes. The accuracy of the models is tested by comparing the simulated data to measured data cubes. As an application, the data are then used to evaluate the performance of the newly developed fast fully adaptive (FFA) space-time adaptive processing (STAP) scheme to improve the extraction of target echoes from a clutter background.

Description: Wirelessly coupled coils are crucial for efficient power transmission in various applications. Previous design methods are only eligible for improving the efficiency of circular or square coils. This paper presents a method of characterizing and optimizing rectangular coils used in inductively coupled systems. After setting up a lumped component model for inductive coils, the efficiency can be expressed in terms of geometrical parameters of the coils. Subsequently, the power efficiency can be plotted versus these parameters in Matlab, thus getting the desired coils for optimum power transfer. With this design procedure from mathematical optimization, we eventually designed two rectangular coils spaced 10 mm apart, which achieves a power transmission efficiency of 46.4% at a frequency of 3 MHz. The design methodology is verified by simulation and measurement.

Description: Nowadays a huge amount of data is available for the statistical characterization of rainfall worldwide, although unfortunately not always with the adequate spatial and temporal resolution required for the very high demanding telecommunication applications. On the basis of the NIMROD radar network composite rain maps, first, this paper investigates separately the impact of space or time integration on the spatial correlation of rainfall ρ, a key parameter for most Propagation Impairment Mitigation Techniques (PIMTs), as well as for many prediction models such as time-space rain field generators. Analytical formulations are proposed to model the average trend of ρ with the distance d between two sites as a function of the integration time T or the integration area A, which, in turn, can be used to de-integrate the spatial correlation information estimated respectively from networks of raingauges with long integration time or from radar data with coarse spatial resolution. As an example, the last part of the paper compares the spatial rain decorrelation trends estimated by a database of radar maps collected in Northern Italy with the ones de-integrated from products of meteorological re-analyses (ERA40) or Earth Observation missions (TMPA 3B42).

Description: Since polar cap patches were discovered, their nature, physics, and impact on navigation and communication signals has been repeatedly addressed. Both terminology and inference of physical processes from diverse instruments have introduced confusion. Poleward moving auroral form is a morphological descriptor but cannot be equated to an island of high-density plasma. Particle precipitation produces low-density patches, but high-density patches derive from solar produced plasma. The challenge of patches is finding the dominant mechanism for chopping entering ionization into islands (∼100–1000 km in size). Velocity-dependent recombination physics is valid in principle but not relevant to patches formed in at least the European sector. Most patches are formed by transient magnetic reconnection events. While the plasma is at too high an altitude for the strong velocity shears to erode plasma densities, the shears become the dominant plasma structuring mechanism until the initial magnetic tension force is relaxed. Initial patch structuring is not by gradient drift as believed for decades but rather by the shear driven instability, impacting mitigation techniques. Large-scale shears, driven to 2–3 km/s, impact satellite drag through thermospheric heating. The study here is intended to sharpen understanding of patches for future research and development of techniques for mitigation of their effects on navigation and other systems dependent on receiving radio frequency signals from satellites and understanding reconnection driven impact on thermospheric density and satellite drag.

Description: Equivalent edge currents (EECs) are widely used to asymptotically extract and express the diffraction from the periphery of the scatterers, as in the concept suggested by Young. Authors proposed novel EECs based on the unique concept named as Modified Edge Representation (MER), for surface to line integral reduction of Physical Optics (PO) radiation integral. MER is based upon not only asymptotic approximation but also Stokes' theorem, and it has remarkable accuracy even for small scatterers and is uniformly applicable at the geometrical boundaries. Moreover MER-EECs are clearly defined not only at the edges but also everywhere on the scatterer surface, with the singularity at the stationary phase point (SPP) if any. It comes that the radiation integral in general, consisting of both the diffraction and the Geometrical Optics (GO) components, could be reduced into two sets of line integration of MER-EECs along the periphery and the indentation at SPPs. In this paper, the GO reflection is numerically compared with the MER indentation integral around the reflected point for the dipole scattering from an ellipsoid; the error is empirically derived in analytical form.

Description: Detecting Doppler frequency shifts from ionospheric high-frequency echoes is an important way to study ionospheric disturbances. This paper presents and realizes a new mode for observation of ionospheric disturbances using a combination of coded pulses and echo phase measurement analysis in ionospheric vertical sounding based on the Canadian Advanced Digital Ionosonde (CADI) platform. Experimental results show that the newly developed mode for observation of ionospheric disturbances on CADI can acquire accurate Doppler ionogram (Dopplionogram) and obtain temporal and spatial variations of the velocity of ionospheric disturbances in real time so that it has essential value in observation and research of ionospheric disturbances. The application of the new mode for observation of ionospheric disturbances in ionospheric vertical sounding opens up a new, effective way by which much more ionospheric information can be acquired with existing common ionospheric sounding instruments.

Description: HF Doppler sounders represent a low-cost and low-maintenance solution for monitoring wave activity in the F region ionosphere. HF Doppler sounders together with modern data analysis techniques can provide comprehensive traveling ionospheric disturbance (TID) characteristics, including both horizontal and vertical TID velocities and wavelengths across the entire spectrum from periods of 1 min to over an hour. Atmospheric and Space Technology Research Associates LLC has developed a new system called “TIDDBIT” (TID Detector Built in Texas), and data will be presented from a TIDDBIT system deployed in Virginia. These results reinforce the relationship between atmospheric gravity waves (AGWs) and TIDs. The TID propagation azimuths rotate through 360° in 24 h, mimicking the rotation of the thermospheric winds but with approximately a 90° offset. The rotation of TID azimuths and thermospheric winds in Virginia is similar to that observed previously by other Northern Hemisphere systems and opposite from the direction observed in Antarctica. These results illustrate the filtering effects that thermospheric neutral winds can have on the propagation of AGW. The completeness of the wave information obtained from the TIDDBIT system makes it possible to reconstruct the vertical displacement of isoionic contours over the ∼200 km horizontal dimension of the sounder array. Such information will be relevant for understanding the seeding of irregularities, as well as for several operational needs involving navigation, communication, and surveillance systems.

Description: A rigorous but yet highly accurate and efficient numerical treatment of plane wave scattering by T-shaped planar corrugations through full-wave modal analysis is first presented in this paper, which entails the moment method using parallel-plate waveguide cavity Green's functions and a numerical spectral-domain Green's function for planar stratified media. Investigations in terms of both reflection-phase and dispersion diagrams are conducted. After validating with the commercial software package: CST Microwave Studio, this moment-method is in turn used to verify a formula derived by the transverse resonance technique (TRT) for the surface-wave propagation constant of corrugations in terms of the dispersion diagram. Correspondences between the reflection-phase and dispersion diagrams are then established by relating the high and low impedance frequencies in the former with the pass and stop bands of the latter. With the abovementioned formula, the way is paved for a novel derivation of explicit formulas for the reflection-phase of an incident plane wave impingent on the corrugations as closed-form analytic functions of the various parameters, even for oblique azimuth planes of incidence and for both TE and TM polarizations. Doing so, the high or low surface-impedance (AMC or AEC) properties of such corrugations can virtually be acquired instantly, thus providing incomparable speedup of the process of thorough reflection-phase characterization of AMC or high-impedance surfaces and soft surfaces, thus affording rapid designs of antennas and microwave devices that make use of them.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 1-21, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 45-67, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 23-43, May 2012.

Description: Signal power measurements for a UK-based network of three beacon transmitters and five receiving stations operating on 5.290 MHz were taken over a 23 month period between May 2009 and March 2011, when solar flux levels were low. The median signal levels have been compared with monthly median signal level predictions generated using VOACAP (Voice of America Coverage Analysis Program) and ASAPS (Advanced Stand Alone Prediction System) HF prediction software with the emphasis on the near-vertical incidence sky wave (NVIS) links. Low RMS differences between measurements and predictions for September, October, November, and also March were observed. However, during the spring and summer months (∼April to August), greater RMS differences were observed that were not well predicted by VOACAP and ASAPS and are attributed to sporadic E and, possibly, deviative absorption influences. Similarly, the measurements showed greater attenuation than was predicted for December, January, and February, consistent with the anomalously high absorption associated with the “winter anomaly.” The summer RMS differences were generally lower for VOACAP than for ASAPS. Conversely, those for ASAPS were lower during the winter for the NVIS links considered in this analysis at the recent low point of the solar cycle. It remains to be seen whether or not these trends in predicted and measured signal levels on 5.290 MHz continue to be observed through the complete solar cycle.

Description: Short-period gravity waves associated with the passage of tropical cyclone using mesosphere-stratosphere-troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E) has been discussed. The observed stratospheric gravity wave is found to have a periodicity of ∼42 min, vertical and horizontal wavelength of ∼3.5 km and 14 km, respectively. Maximum amplitude of gravity wave is observed in the upper troposphere and lower stratosphere (UTLS) region due to which periodic updrafts and downdrafts are observed. This weakens the stability of tropopause, which is observed in radar signal-to-noise ratio (SNR). The enhancement of vertical momentum flux of order ∼−0.6 m2/s2 observed in the lower stratosphere is attributed to the cyclone generated gravity waves. The obstacle effect is found to be the generative mechanism for the observed gravity waves associated with the tropical cyclone.

Description: A numerical algorithm based on Fermat's Principle was developed to simulate the propagation of Global Positioning System (GPS) radio signals in the refractivity field of a numerical weather model. The unique in the proposed algorithm is that the ray-trajectory automatically involves the location of the ground-based receiver and the satellite, i.e. the posed two-point boundary value problem is solved by an implicit finite difference scheme. This feature of the algorithm allows the fast and accurate computation of the signal travel-time delay, referred to as Slant Total Delay (STD), between a satellite and a ground-based receiver. We provide a technical description of the algorithm and estimate the uncertainty of STDs due to simplifying assumptions in the algorithm and due to the uncertainty of the refractivity field. In a first application, we compare STDs retrieved from GPS phase-observations at the German Research Centre for Geosciences Potsdam (GFZ STDs) with STDs derived from the European Center for Medium-Range Weather Forecasts analyses (ECMWF STDs). The statistical comparison for one month (August 2007) for a large and continuously operating network of ground-based receivers in Germany indicates good agreement between GFZ STDs and ECMWF STDs; the standard deviation is 0.5% and the mean deviation is 0.1%.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-24, Ahead of Print.

Description: The electromagnetic field excited by an arbitrary current source embedded in an unbounded uniform cold magnetoplasma is investigated in this paper. The Green's function method for solving the radiation equation is used, and the derived solution is valid for any cold plasma parameters. There are always two wave modes excited, and in the far field each mode has the form of a spherical wave. A refractive index for spherical waves is introduced to describe the propagation. The properties of these excited spherical waves are discussed in this paper in comparison with the plane wave representation. It is shown that the energy flow of the spherical wave is always in the radial direction, i.e., parallel to the wave normal.

Description: The Wide Area Real Time Kinematic (WARTK) is an augmentation system concept for multi-frequency users based on precise real-time ionospheric modeling. It is able to provide a high accuracy and integrity GNSS positioning service over continental areas using the infrastructure of a network of permanent ground monitor stations, such as the European Geostationary Navigation Overlay Service (EGNOS) network of Ranging and Integrity Monitoring Stations (RIMS) in Europe. In this way, it allows an additional benefit to be obtained from these reference stations, that is, the network has the potential to support two independent systems: a satellite-based augmentation system, such as EGNOS, and a high-precision positioning service, based on WARTK. Indeed, thanks to the accuracy of the ionospheric corrections provided, WARTK users have available in real-time an extra constraint per satellite between the carrier phase ambiguities, which helps solve them quickly. Once such ambiguities have been solved, the GNSS user obtains navigation accurate to within 20 cm at the 95th percentile (about 10 cm RMS). Moreover, this precise positioning is achieved in a few minutes (with two frequency signals) or in a single epoch, after initial convergence of the tropospheric delay (with three frequency signals), even up to hundreds of kilometers away from the nearest reference station. While previous WARTK research has been devoted to implementing the concept and assessing its feasibility, considering in particular the accuracy achievable, the work reported in this paper focused on consolidating the results by analyzing a large and representative data set, and on deeper analysis of the integrity issue. It was carried out in the context of the Multi-constellation Regional System (MRS) project, within the European Space Agency GNSS Evolution Programme, with the aim of designing a high accuracy service for GPS and/or Galileo. Three months of actual data, from more than 25 permanent GPS stations in Europe, have been processed (some of them as a roving user), for high-, mid- and low-solar cycle conditions (in 2002, 2004 and 2006 respectively). In addition, several ionospheric storms occurred during the selected periods, with Dst values reaching up to −150 nT. Results based on these data show that user domain integrity was maintained for baselines of up to 400 km. At the 95th percentile, the daily horizontal and vertical position errors were 20 and 30 cm, respectively, and the corresponding protection levels were about 1 and 2 m. The convergence time was around 5 minutes with actual GPS constellation data. The benefits of using a multi-constellation system were also studied, with simulated GPS and three-frequency Galileo data, showing that it is possible to reduce the convergence time to a few seconds.

Description: We develop a ray-tracing package for the calculation of path delays of microwave signals in the troposphere based on numerical weather models which we use for the determination of the delays of geodetic Very Long Baseline Interferometry (VLBI) observations. We show results for a two-week campaign of continuous VLBI sessions in 2008 (CONT08), where we apply those ray-traced delays and analyze the repeatability of baseline lengths in comparison to a standard approach with zenith delays and mapping functions. We find improvement in baseline length repeatabilities when no tropospheric gradients are estimated in the analysis. Furthermore, ray-traced delays are applied for Intensive sessions containing the stations Tsukuba (Japan) and Wettzell (Germany) for the determination of Universal Time (UT1). We perform an external validation using GPS-derived length-of-day values and find an improvement for UT1 with ray-traced delays by up to 4.5%.

Description: A high-fidelity HF channel simulation has been developed that is suitable for Over-the-Horizon Radar (OTHR) and HF communication system design studies and test planning. The simulation capability is called HiCIRF, for High-frequency Channel Impulse Response Function. HiCIRF provides simulated HF signals corresponding to transmissions from individual transmitter array elements to individual receiver array elements for propagation through the naturally disturbed or undisturbed ionospheric channel. Both one-way link geometries and two-way radar geometries can be simulated. HiCIRF incorporates numerical ray tracing and stochastic signal structure computations to realistically simulate signal scatter by small-scale ionization structure. Stochastic signal generation is employed to generate signal realizations that can be used for OTHR array design and advanced signal processing studies.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-17, Ahead of Print.

Description: This paper illustrates the importance of understanding and taking into account the presence of various structural features in the polar ionosphere (in particular, patches and arcs of enhanced electron density) when planning and operating HF radio links. These features result in radio waves propagating over paths well displaced from the great circle direction and impact on almost any HF communications system where the signal reflects from the ionosphere within the region poleward of the subauroral trough. The off–great circle mechanisms give rise to propagation at times that are not predicted by current prediction codes and may also suppress propagation at times that are expected. Techniques to account for this type of propagation are therefore required. A ray-tracing model that accurately reproduces many of the direction of arrival features observed in experimental measurements has been developed. Particular attention will be given in this paper to area coverage estimations undertaken by means of the ray-tracing model.

Description: In order to make best use of the opportunities provided by space missions such as the Radiation Belt Storm Probes, we determine the response of complementary subionospheric radiowave propagation measurements (VLF), riometer absorption measurements, cosmic noise absorption, and GPS-produced total electron content (vTEC) to different energetic electron precipitation (EEP). We model the relative sensitivity and responses of these instruments to idealized monoenergetic beams of precipitating electrons, and more realistic EEP spectra chosen to represent radiation belts and substorm precipitation. In the monoenergetic beam case, we find riometers are more sensitive to the same EEP event occurring during the day than during the night, while subionospheric VLF shows the opposite relationship, and the change in vTEC is independent. In general, the subionospheric VLF measurements are much more sensitive than the other two techniques for EEP over 200 keV, responding to flux magnitudes two-three orders of magnitude smaller than detectable by a riometer. Detectable TEC changes only occur for extreme monoenergetic fluxes. For the radiation belt EEP case, clearly detectable subionospheric VLF responses are produced by daytime fluxes that are ∼10 times lower than required for riometers, while nighttime fluxes can be 10,000 times lower. Riometers are likely to respond only to radiation belt fluxes during the largest EEP events and vTEC is unlikely to be significantly disturbed by radiation belt EEP. For the substorm EEP case both the riometer absorption and the subionospheric VLF technique respond significantly, as does the change in vTEC, which is likely to be detectable at ∼3–4 total electron content units.

Description: To investigate ion density depletion along magnetic field lines, we compare in situ-measured ion density fluctuations as seen from C/NOFS and compare them to the field-line-integrated depletion of the whole bubble as inferred from electric field measurements. Results show that, within C/NOFS' range, local measurement of the normalized density depletion, Δn/n0, near the apex may be far less than at other points on the same field line. We argue that the distribution of Δn/n0 is a weighted distribution concentrated at latitudes of the Appleton anomalies and becomes more heavily weighted the closer the field-aligned bubble rises to the peak of the anomalies. A three-dimensional simulation of an ionospheric bubble verifies our arguments.

Description: Over-the-horizon radar (OTHR) uses ionospheric reflection to propagate HF transmissions to long range (∼500–5000 km). The ionosphere acts as a dynamic “mirror” that varies diurnally, seasonally, and with the solar cycle. Geolocation of targets observed by OTHR (Coordinate Registration (CR)) requires accurate real-time ionospheric modeling and HF propagation calculations to convert radar-measured target signal delays and beam steers to geographical position. We merged our backscatter ionogram (BI) leading edge inversion algorithm CREDO with our more advanced ionospheric data assimilation capability, GPS Ionospheric Inversion (GPSII). The combined algorithm produces a dynamic model of electron density for a fixed geographical region. The model is consistent with BI leading edge data, vertical sounding data, as well as with absolute and relative total electron content (TEC) data from a number of GPS/LEO receivers. Incorporation of additional ionospheric data beyond conventional OTHR vertical and oblique backscatter soundings is expected to enhance the fidelity of real-time ionosphere models, resulting in improved OTHR Coordinate Registration metric accuracy. Initial tests of the OTHR CR supported by the new ionospheric inversion algorithm indicate noticeable improvement of CR accuracy in comparison with legacy techniques.

Description: Eliminating the electromagnetic interaction of a device with its background is a topic which attracts considerable attention both from a theoretical as well as from an experimental point of view. In this work, we analyze an infinite two-dimensional planar microstrip antenna, excited by an incident plane wave, and propose its potential operation as a low-profile receiving antenna, by suitably adjusting the parameters of its cloaking superstrate. We impose a semi-analytic integral equation method to determine the scattering characteristics of the microstrip antenna. The method utilizes the explicit expressions of the Green's function of the strip-free microstrip and yields the surface strip's current as the solution of a suitable linear system. Subsequently, the antenna's far-field response is obtained. Numerical results are presented for the achieved low profile of the receiving antenna, by choosing suitably the cloaking superstrate parameters. It is demonstrated that for specific cloaking parameters the scattered field by the antenna is considerably reduced, while the received signal from the antenna is maintained at sensible levels. We point out that the material values achieving this reduction correspond to a superstrate filled with an ϵ-near-zero or a low-index metamaterial. Finally, the variations of the device reaction for various superstrates are depicted, concluding that for optimized superstrate's parameters, the reaction values are significantly reduced, while at distinct scattering angles even approach zero.

Description: Previous studies have quantified the longitude gradients in E × B drift associated with the four-cell tidal structures and have confirmed that these sharp gradients exist on a day-to-day basis. For this paper, we incorporate the Ion Velocity Meter (IVM) sensor on the Communications/Navigation Outage Forecasting System satellite to obtain the daytime, vertical E × B drift velocities at the magnetic equator as a function of longitude, local time, and season and to theoretically calculate the F region ion densities as a function of altitude, latitude, longitude, and local time using the Global Ionosphere Plasmasphere model. We compare calculated ion densities assuming no longitude gradients in E × B drift velocities with calculated ion densities incorporating the IVM-observed E × B drift at the boundaries of the four-cell tidal structures in the Peruvian and the Atlantic longitude sectors. Incorporating the IVM-observed E × B drift velocities, the ion density crests rapidly converge to the magnetic equator between 285 and 300°E geographic longitude, are absent between 300° and 305°, and move away from the magnetic equator between 305° and 340°. In essence, the steeper the longitude gradient in E × B drifts, the steeper the longitude gradient in the equatorial anomaly crest location.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 89-111, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 327-354, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 655-683, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 685-711, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 625-654, May 2012.

Description: Atmosphere-Ocean, Volume 50, Issue 2, Page 156-168, June 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 521-549, May 2012.

Description: Atmosphere-Ocean, Volume 0, Issue 0, Page 1-16, Ahead of Print.

Description: In this study, velocities measured by the Rankin Inlet and Inuvik PolarDARN HF radars over Resolute Bay (RB) are compared with measurements from the RB CADI ionosonde to investigate the consistency of the measurements. Two types of comparisons are performed: 1-D, where each PolarDARN radar line-of-sight velocity is plotted against CADI velocity projected onto appropriate radar beam, and 2-D, where the PolarDARN merged velocity is compared with the full CADI vector. In both cases, velocities were found to statistically agree. For the 1-D comparison, the velocities were comparable in ∼85% of cases. For the 2-D comparison, a minor tendency for larger PolarDARN merge velocities (∼60 m/s) was noticed. The second task performed is a comparison of the SuperDARN cross polar cap potential (CPCP) and the CADI-based CPCP and their dependence on the CADI velocity. Linear dependences were found allowing for inter-conversion between these parameters. For large plasma drifts, the SuperDARN CPCPs were found to be much smaller than the CADI-based CPCPs hinting that the separation between the foci of a large-scale convection pattern is often underestimated in the SuperDARN convection mapping.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 301-326, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 277-300, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 251-275, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 113-139, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 413-439, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 441-465, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 467-494, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 495-520, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 551-568, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 383-412, May 2012.

Description: Annual Review of Earth and Planetary Sciences Volume 40, Page 597-623, May 2012.

Description: Results from simultaneous measurements of vertical air velocity (W), particle fall velocity, and hydrometeor sphericity in stratiform precipitation are reported for the first time. Cases of stratiform precipitation on 8 (case A) and 16 December 2008 (case B) observed at Sumatra, Indonesia (0.2°S, 100.32°E), are described. A 47 MHz wind-profiling radar measured W and reflectivity-weighted particle fall velocity relative to the air (VZ) simultaneously. Upward W above ∼6.0 km altitude in case B (〉0.2 m s−1) was greater than in case A (

Description: Although ionospheric perturbations such as traveling ionospheric disturbances have a strong impact on Global Navigation Satellite Systems (GNSS) and other space-based radio systems, the description of individual perturbations is difficult. To overcome this problem, it is suggested to use a disturbance ionosphere index (DIX) that describes the perturbation degree of the ionosphere in a less specific form as a proxy. Although such an index does not describe the exact propagation conditions at the measurement site, the estimated index number indicates the probability of a potential impact on radio systems used in communication, navigation, and remote sensing. The definition of such a DIX must take into account the following major requirements: relevance to practical needs, objective measure of ionospheric conditions, easy and reproducible computation, and availability of a reliable database. Since the total electron content has been shown in many publications to act as an outstanding parameter for quantifying the range error and also the strength of ionospheric perturbations, we propose a DIX that is based on GNSS measurements. To illustrate the use of the index, recent storms monitored in 2011 and the Halloween storm are discussed. The proposed index is a robust and objective measure of the ionospheric state, applicable to radio systems which are impacted by a highly variable perturbed ionosphere.