Showing papers in "Radio Science in 2001"

International Reference Ionosphere 2000

Abstract: The International Reference Ionosphere (IRI) is the international standard for the specification of ionospheric densities and temperatures. It was developed and is being improved-updated by a joint working group of the International Union of Radio Science (URSI) and the Committee on Space Research (COSPAR). A new version of IRI is scheduled for release in the year 2000. This paper describes the most important changes compared to the current version of IRI: (1) an improved representation of the electron density in the region from the F peak down to the E peak including a better description of the F1 layer occurrence statistics and a more realistic description of the low-latitude bottomside thickness, (2) inclusion of a model for storm-time conditions, (3) inclusion of an ion drift model, (4) two new options for the electron density in the D region, and (5) an improved model for the topside electron temperatures. The outcome of the most recent IRI Workshops (Kuhlungsborn, 1997, and Nagoya, 1998) will be reviewed, and the status of several ongoing task force activities (e.g., efforts to improve the representation of electron and ion densities in the topside ionosphere and the inclusion of a plasmaspheric extension) will be discussed. A few typical IRI applications will be highlighted in section 6.

Vertical electron content from ionograms in real time

Abstract: A new technique for calculating the vertical total electron content (TEC) from ground-based ionosonde measurements is introduced. The ionogram provides the information to directly calculate the vertical electron density profile up to the peak of the F2 layer. The profile above the peak is approximated by an α-Chapman function with a scale height that is derived from the profile shape around the F2 peak. The ionosonde TEC, or ITEC, is then calculated as the integral from 0 to ∞ over the entire profile. ITEC values from Digisonde observations at Millstone Hill, Wallops Island, and Jicamarca are compared with incoherent scatter radar and with Faraday and TOPEX satellite TEC measurements, showing very good agreement at middle latitudes and the magnetic equator.

Tracking tropospheric radio occultation signals from low Earth orbit

Abstract: Propagation of radio occultation signals through the tropical lower troposphere with severe refractivity gradients results in significant spreading of the signal spectrum. Under such conditions a signal acquisition technique which tracks large random troposphere-induced phase accelerations more reliably than a generic phase-locked loop has to be applied. This paper discusses the results of simulations of open loop tracking of radio occultation signals that were generated with data from high-resolution tropical radiosondes. The signal has to be down-converted in real time in the receiver on orbit to a low mean residual frequency by use of a phase (Doppler) model based on predicted orbits and refractivity climatology. The down-converted complex signal is then low-pass filtered and sampled. The phase in excess of the phase model must be reconstructed from the sampled and down-linked signal in postprocessing. This may require an additional down-conversion to eliminate (minimize) aliasing of harmonics in the spectrum. Then the accumulated phase can be reconstructed by resampling the signal at a higher rate to resolve the cycle ambiguities. A fast algorithm for prediction of the Doppler based on the refractivity climatology and an algorithm for the detection of Doppler mismodeling based on sliding window spectral analysis of the down-converted signal are developed and tested. The accuracy of the Doppler modeling, ±(15-20) Hz, the required filter bandwidth, 100 Hz, and the sampling rate, 50-100 Hz, are estimated.

Characteristics of radio transmitter fingerprints

Abstract: In this paper, the characteristics of radio transmitter fingerprints will be examined by analyzing both the amplitude and phase information obtained from complex envelope recordings of transmitter turn-on transients. The interest in the analysis of such transients is related to the identification of malfunctioning or illegally operated radio transmitters in support of radio spectrum management practices. Of the 28 VHF radios considered in this study, many were found to produce fingerprints having uniquely distinctive features which could be used for identification purposes. Unfortunately, some of these radios were found to have fingerprints that were virtually indistinguishable from each other, making the identification process more difficult, if not impossible. Details of the equipment, analyses, and data collection procedures will be presented along with a discussion of the experimental results. The merits of this technique over others currently in use will also be presented.

Fading timescales associated with GPS signals and potential consequences

Abstract: The effect of equatorial ionospheric scintillations on the operation of GPS receivers is investigated, with special attention given to the effect of scintillation timescales on the code division multiple access (CDMA) protocol used by GPS. We begin by examining the timescales of scintillation fades modeled as a horizontally drifting pattern whose timescales are determined by the Fresnel length and the drift speed. The model is tested by comparing the speed, determined by dividing the Fresnel length by the autocorrelation time (width), with the speed estimated using spaced receivers, and the two independent estimates of speed are shown to possess a linear relationship. Next we show that the scintillation pattern drift speed is given by the difference of the ionospheric drift and the speed of the GPS signal F region puncture point. When the ionosphere and GPS signal puncture point speeds match, the fade timescales lengthen. Additionally, if the fade depth is adequate, during periods of longer fade times the loss of receiver lock on GPS signals is more likely, as shown in several examples; that is, both larger fade depths and longer fade timescales are required to produce loss of tracking. We conclude by demonstrating that speed matching or resonance between the ionosphere and receiver is most likely when the receiver is moving from west to east at speeds of 40–100 m/s (144–360 km/h). This is in the range of typical aircraft speeds.

Modeling and inverting radio occultation signals in the moist troposphere

Abstract: Accurate modeling of radio occultation signals is performed by solving the Helmholtz equation with the use of a multiple-phase-screen technique. Refractivity is assumed spherically symmetric, and vertical profiles are reproduced from high-resolution tropical radiosondes. As a result, the characteristics of the signals, which are important for their tracking in low Earth orbit, are evaluated: the spectral bandwidth, ∼50 Hz, and the random phase acceleration, ∼1000 Hz/s. The complex signals are inverted with the use of two radio holographic methods: back propagation and sliding spectral (radio optics). For the back propagation method, finding the position of the auxiliary trajectory which provides an unambiguous bending angle function of impact parameter appears to be a problem. For the sliding spectral method a simple technique, which takes into account the whole spectral content of the signal without identification and selection of local spectral maxima, is introduced and tested. The sliding spectral method allows for the stable reconstruction of bending angles and refractivity with vertical resolution of ∼0.5 km. The small-scale laminated structure of refractivity results in propagation of radio occultation signals down to significantly lower observation altitudes than in the case of smooth refractivity. Information content of radio occultation signals at those low altitudes is important for the radio holographic inversions.

High‐frequency bistatic cross sections of the ocean surface

Abstract: An analysis leading to the first and second-order bistatic cross sections of the ocean surface in the context of high-frequency ground wave radar operation is presented. Initially, a pulsed dipole source is introduced into the previously derived electric field expressions for the bistatic reception of vertically polarized radiation scattered from rough surfaces that do not vary with time. To make application to the ocean, a time-varying surface is introduced via a three-dimensional Fourier series with two spatial variables and one temporal variable. The surface randomness is accounted for by allowing the Fourier coefficients to be zero-mean Gaussian random variables. Fourier transformation of the autocorrelations of the resulting fields gives the appropriate power spectral densities. The latter are used in the bistatic radar range equation to produce the cross sections. The features of the bistatic case are seen to reduce to the well-known monostatic results when the appropriate geometry is introduced. Illustrative comparisons of monostatic and bistatic reception are presented.

Validation of line‐of‐sight water vapor measurements with GPS

Abstract: We present a direct comparison of nonisotropic, integrated water vapor measurements between a ground-based Global Positioning System (GPS) receiver and a water vapor radiometer (WVR). These line-of-sight water vapor observations are made in the straight line path between a ground station and a GPS satellite. GPS double-difference observations are processed, and the residual line-of-sight water vapor delays are extracted from the double-difference residuals. These water vapor delays contain the nonisotropic component of the integrated water vapor signal. The isotropic component is represented by the zenith precipitable water vapor measurement and can be scaled to a specific elevation angle based on a mapping function. The GPS observations are corrected for station-dependent errors using site-specific multipath maps. The resulting measurements are validated using a WVR which pointed in the direction of the observed satellites. The double-difference technique used to make these water vapor observations does not depend on accurate satellite clock estimates. Therefore it is especially well suited for near-real-time application in weather prediction and allows for sensing atmospheric structure that is below the noise level of current satellite and receiver clock errors. This paper describes the analysis technique and provides precision estimates for the GPS-measured nonisotropic water vapor as a function of elevation angle for use with data assimilation systems.

Total solar eclipse effects on VLF signals: Observations and modeling

Abstract: During the total solar eclipse observed in Europe on August 11, 1999, measurements were made of the amplitude and phase of four VLF transmitters in the frequency range 16–24 kHz Five receiver sites were set up, and significant variations in phase and amplitude are reported for 17 paths, more than any previously during an eclipse Distances from transmitter to receiver ranged from 90 to 14,510 km, although the majority were 10,000 km Negative phase changes were observed on most paths, independent of path length Although there was significant variation from path to path, the typical changes observed were ∼3 dB and ∼50° The changes observed were modeled using the Long Wave Propagation Capability waveguide code Maximum eclipse effects occurred when the Wait inverse scale height parameter β was 05 km−1 and the effective ionospheric height parameter H′ was 79 km, compared with β=043 km−1 and H′=71 km for normal daytime conditions The resulting changes in modeled amplitude and phase show good agreement with the majority of the observations The modeling undertaken provides an interpretation of why previous estimates of height change during eclipses have shown such a range of values A D region gas-chemistry model was compared with electron concentration estimates inferred from the observations made during the solar eclipse Quiet-day H′ and β parameters were used to define the initial ionospheric profile The gas-chemistry model was then driven only by eclipse-related solar radiation levels The calculated electron concentration values at 77 km altitude throughout the period of the solar eclipse show good agreement with the values determined from observations at all times, which suggests that a linear variation in electron production rate with solar ionizing radiation is reasonable At times of minimum electron concentration the chemical model predicts that the D region profile would be parameterized by the same β and H′ as the LWPC model values, and rocket profiles, during totality and can be considered a validation of the chemical processes defined within the model

First observations of HF heater‐produced airglow at the High Frequency Active Auroral Research Program facility: Thermal excitation and spatial structuring

Abstract: HF heater-produced artificial airglow emissions at 630.0 nm were detected at the High Frequency Active Auroral Research Program (HAARP) ionospheric research facility near Gakona, Alaska (62.39° N 145.15° W), for the first time in March 1999. HF excitation was identified by virtue of two measurements: A region of emissions enhanced 50-60 R above the background of ∼100 R was observed in the approximate region illuminated by the HF heater beam; and the rise and decay of the airglow followed the HF transmitter on/off cycle with time constants of approximately 80 s and 50 s, respectively. The observations were made in close proximity to the natural aurora, which gradually moved southward over the site during the experiment and eventually overwhelmed the much weaker artificial airglow. Significant structure in the airglow region was observed, including an overall equatorward displacement relative to the nominal transmitter beam, preferential occurrence of intensity maxima along the overhead field line up to and including magnetic zenith, and periods of elongation of the airglow region along the magnetic meridian and later parallel to the encroaching auroral zone. We discuss potential sources of this structuring of the emission shape including auroral plasma transport and neutral winds. Transmitter frequencies of 3.1–3.3 MHz matched the ionospheric plasma frequency near the F region peak at high altitudes of 300–350 km. We identify the primary mechanism behind excitation of the 630.0 nm airglow as thermal electron excitation on the basis of this evidence, along with thermal balance arguments, further substantiated by the observed asymmetry in rise and decay times and a lack of detectable emissions at 557.7 nm.

Scintillation modeling for GPS‐Wide Area Augmentation System receivers

Abstract: A scintillation signal model and a Global Positioning System (GPS)-Wide Area Augmentation System (WAAS) receiver model are developed. The scintillation signal model is based on a Nakagami-m distribution for intensity and a Gaussian distribution with zero mean for phase. The GPS-WAAS receiver model includes Link 1 (L1) GPS and WAAS carrier-and C/A-code-tracking loops, as well as semicodeless Link 2 (L2) carrier and Y-code tracking capabilities. The results show that noncoherent delay locked loops (DLLs) typically used for code tracking are very robust to both amplitude and phase scintillation. Carrier-phase-tracking loops are much more susceptible to scintillation, and the signal-to-noise threshold for reliable carrier tracking is very dependent on the scintillation strength. Fortunately, it appears that the worst case scintillation encountered at midlatitudes, including the United States, does not significantly impact L1 carrier-tracking performance. Semicodeless tracking of the L2 carrier is shown to be very fragile. Even weak scintillation can cause loss of L2 carrier lock for low-elevation satellites.

On the Laws-Parsons distribution of raindrop sizes

Abstract: A new analytical function is given for the distribution density of raindrop diameters implied by the measured Laws-Parsons distribution of the volume fraction of water in rain. It yields results that differ importantly from those obtained via the Marshall-Palmer exponential function of drop diameter in the millimeter range of radar wavelengths.

Volume‐integral‐equation‐based analysis of transient electromagnetic scattering from three‐dimensional inhomogeneous dielectric objects

Abstract: A novel technique for analyzing transient electromagnetic scattering from three-dimensional inhomogeneous dielectric targets is proposed. An integral equation for the electric flux density throughout the scatterer is constructed by invoking the electromagnetic volume equivalence principle. This equation is solved using a marching-on-in-time scheme in which the electric flux density is expanded in space by volumetric rooftop basis functions defined on a tetrahedral mesh and in time by piecewise polynomials. The proposed method is validated for representative dielectric structures by comparison via Fourier transformation of scattering data obtained with this method and various frequency domain techniques.

Microwave Radiometer for Cloud Carthography: A 22-channel ground-based microwave radiometer for atmospheric research

Abstract: A 22-channel Microwave Radiometer for Cloud Carthography (MICCY) for the profiling of tropospheric temperature, humidity, and cloud liquid water has been developed. The radiometer has 10 channels along the high-frequency wing of the 22.235 GHz water vapor line, 10 channels along the low-frequency side of the 60 GHz oxygen complex, and 2 channels at 90 GHz. Two features make the radiometer a unique tool for the observation of small-scale structures related to cloud processes: (1) Atmospheric brightness temperatures are measured simultaneously at all channels with an integration time of 1 s. (2) A Cassegrain system including a 90 cm off-axis parabolic mirror leads to a spatial resolution better than 1° full width at half maximum for all frequencies. The necessity of these features for cloud observations is demonstrated. Algorithms for the retrieval of integrated water vapor, integrated cloud liquid water, and the profiles of temperature and humidity were developed on the basis of artificial neural networks. Measurement examples, including comparisons with radiosondes, demonstrate the systems capabilities for high-resolution atmospheric monitoring.

Diurnal and seasonal variability of turbulence parameters observed with Indian mesosphere‐stratosphere‐troposphere radar

Abstract: Seasonal and diurnal variation of turbulence parameters such as refractivity structure constant Cn2 and eddy dissipation rate e is presented using the data collected with the Indian mesosphere-stratosphere-troposphere (MST) radar over 3 years. The log Cn2 values estimated from signal-to-noise ratio (SNR) are found to be in the range of −17 to −19 m−2/3 in the height range of 7.5–21 km. Monthly mean values of log Cn2 show a maximum variation below 12 km with a magnitude of 12–15 dB during the course of annual cycle. A maximum variability of ∼7–10 dB is observed below 12 km in seasonal mean values of log Cn2. The diurnal variation of log Cn2 at different heights is also given. Cn2 is found to be more in the region of strong shears, generally observed in the boundaries of jet streams. Different methods for the estimation of eddy dissipation rate and their limitations are discussed. For the present study, spectral width method is used after correcting the observed spectral width from beam and shear broadening effects. The observed median log e is on the order of −3 to −4 m2 s−3. Monthly variation of log e is found to be ∼5–7 dB. Below 10 km the magnitude of e is more in the postmonsoon than that observed in other seasons. The interannual variation of e is less in winter than in other seasons. The diurnal variation of log e is found to be small in postmonsoon at most of the heights. To facilitate a comparison with the other results, we have estimated the eddy diffusivity K and the inner and outer scales of turbulence. The observed values of Cn2, e,K, and the inner and outer scales of turbulence are largely consistent with the results available in the literature.

An evaluation of range accuracy in the Super Dual Auroral Radar Network over-the-horizon HF radar systems

Abstract: The authors thank the director and staff of EISCAT for the operation of the Tromso heater facility. EISCAT is an international facility funded collaboratively by the research councils of Finland (SA), France (CNRS), the Federal Republic of Germany (MPG), Japan (NIPR), Norway (NFR), Sweden (NFR), and the United Kingdom (PPARC). The SuperDARN Hankasalmi and Pykkvib•er HF radars are deployed and operated by the University of Leicester and funded by the PPARC (grant PPA/R/R/1997/00256), the Finnish Meteorological Institute, and the Swedish Institute for Space Physics. D.M.W. is supported on PPARC grant PPA/G/O/1997/000254.

Electrical parameters of soils in the frequency range from 1 kHz to 1 GHz, using lumped‐circuit methods

Abstract: For studying electrical properties of soil samples with various compositions and water contents, we applied a lumped-circuit approach. The extension of this method up to 1GHz was made possible by using a coaxial sample holder. The complex electrical parameters of soils, such as the relative permittivity , conductivity , and resistivity , were obtained by measuring the magnitude Z and phase ϕ of the sample impedance . The experimental setup is described in our previous paper [Levitskaya and Sternberg, 2000]. The relative real permittivity e′ and imaginary permittivity (dielectric losses) e″ for high-loss soils from Arizona decrease with frequency and increase with water content. Regression equations, derived for the relative permittivity e′ versus water content at a given frequency, can be used to determine the water content in soil from e′ data. The third-degree polynomial equations, which relate the relative permittivity to the volumetric soil moisture content, are different for various frequencies. The complex electrical resistivity components ρ′ and ρ″ reveal a time-dependent polarization process at frequencies above 1 MHz, which shifts to higher frequencies with increasing water content. The propagation parameters, such as attenuation constant α, phase velocity Vp, and penetration depth P, which we calculated from the electrical parameters, also depend on soil wetness. Our comparison of the electrical and propagation parameters for different soils shows that the high-loss soil samples from Avra Valley, Arizona, have higher values e′ and e″, higher attenuation constant α, and lower penetration depth P than the low-loss soils from Brookhaven, New York. For example, at 500 MHz, a high-loss soil (Avra Valley) with volumetric moisture content of ∼10%, exhibits an attenuation of 43 dB/m, whereas for a low-loss soil (Brookhaven) with the same wetness the attenuation constant is only 4 dB/m. We also note that very dry, clean sand in a sheltered “sand box,” which is a favorite medium for testing ground-penetrating radar (GPR), is usually not representative of natural conditions. Therefore, GPR data from such “sand box” experiments must be used with considerable caution because they yield unrealistically large penetration depths and unnatural target responses.

Rapid numerical simulation of axisymmetric single‐well induction data using the extended Born approximation

Abstract: We derive an extended Born approximation for electromagnetic scattering due to rock formations that exhibit axial symmetry around a well bore. An alternative formulation is also developed to improve the accuracy of the approximation when scattering takes place in close prroximity with the source. The resulting equations are solved via a numerical procedure that is as efficient as the first-order Born approximation but substantially more accurate. This conclusion follows from studies carried out on a variety of source-receiver configurations and frequencies typical of single-well induction measurements.

More total electron content climatology from TOPEX/Poseidon measurements

Abstract: We have used the TOPEX satellite data set to compare climatologies of total electron content (TEC) measurements from the dual-frequency altimeter with the Doppler orbitography and radiopositioning integrated by satellite (DORIS) TEC results and with the international reference ionosphere (IRI) and Bent model predictions corresponding to the satellite measurements. We have used the TOPEX measurements from launch in 1992 through 1997 to build a database that includes time, geographic and geomagnetic coordinates of the measurement, geomagnetic indices (Kp, previous Kp, hemispheric power, and integral of hemispheric power over the previous 36 hours), solar index (F 10.7), TOPEX and DORIS TEC measurements, and empirical model (IRI and Bent) results corresponding to the TOPEX measurements. We have binned the measurements in a magnetic local time, magnetic latitude coordinate system to produce global maps of TEC. We present climatological differences between TOPEX TEC and DORIS measurements as well as between TOPEX and the two empirical models. The maps were constructed using all appropriate TOPEX TEC data, DORIS TEC measurements, and the corresponding Bent and IRI model results for solar minimum conditions.

Polar cap convection relationships with solar wind

Abstract: Relationships between the speed of polar cap antisunward convection and the southward component of the interplanetary magnetic field (IMF Bz negative), and also the induced solar wind electric field (VswBz) are derived from polar cap convection measurements using Canadian Advanced Digital Ionosondes at Eureka (CGM latitude 88.67°N) and Resolute Bay (CGM latitude. 83.55°N). The relationships are similar for Eureka and Resolute Bay, which are separated by ∼600 km, showing that as a first approximation, one can assume uniform polar cap convection. Resolute Bay, however, appeared to show some saturation at IMF Bz values less than approximately −10 nT. The average relationships are as follows: Vconv (m/s) ≈ 294 – 66Bz (nT), and Vconv (m/s) ≈ 293 – 166VswBz (mV/m). The convection speed did not show a significant dependence on the IMF By and Bx components. The amplification factor between the induced solar wind electric field and ionospheric electric field is ∼8.

Atmospheric radar imaging using multiple‐receiver and multiple‐frequency techniques

Abstract: Atmospheric radar imaging techniques have shown promise in revealing the fine-scale structure of the atmosphere within the resolution volume of the radar. Enhanced resolution can be obtained in both angle and range by using spaced receivers and shifted frequencies, respectively. The distinct techniques have been termed coherent radar imaging (CRI) for angular resolution enhancement and range imaging (RIM) for radial resolution improvement. Because of the mathematical similarities between CRI and RIM it is possible to derive a generalization of both techniques. In this work, the three-dimensional (3-D) imaging technique, which uses multiple receivers and multiple frequencies simultaneously, is developed for the first time. Three-dimensional imaging has the advantage of mitigating the limitations of beam width as well as pulse width of a conventional radar to simultaneously improve both angular and range resolution. It is shown that CRI and RIM are special cases of 3-D imaging. The mathematical problem is formulated as an inverse problem with solutions provided by the Fourier, Capon, and maximum entropy (MaxEnt) methods. These three 3-D imaging methods are verified and statistically tested through numerical simulations.

Reciprocity theorems for electromagnetic or acoustic one‐way wave fields in dissipative inhomogeneous media

Abstract: Reciprocity theorems have proven their usefulness in the study of forward and inverse scattering problems. Most reciprocity theorems in the literature apply to the total wave field and are thus not compatible with one-way wave theory, which is often applied in situations in which there is a clear preferred direction of propagation, like in electromagnetic or acoustic wave guides and in seismic exploration. In this paper we review the theory for one-way wave fields (or bidirectional beams), and we extensively discuss the symmetry properties of the square root operator appearing in the one-way wave equation. Using these symmetry properties, it appears to be possible to derive reciprocity theorems of the convolution type and of the correlation type for electromagnetic or acoustic one-way wave fields in dissipative inhomogeneous media along the same lines as the usual derivation of the reciprocity theorems for the total wave field. The one-way reciprocity theorem of the convolution type provides a basis for representations of scattered one-way wave fields in terms of generalized Bremmer series expansions or generalized primaries. The one-way reciprocity theorem of the correlation type finds its application in reflection imaging based on inverse one-way wave field propagators.

Fast inhomogeneous plane wave algorithm for the analysis of electromagnetic scattering

Abstract: The fast inhomogeneous plane wave algorithm has been developed to accelerate the solution of three-dimensional electromagnetic scattering problems in free space. By expanding the kernel of the Green's function using the Weyl identity and choosing a proper steepest descent path, the diagonalization of the translation matrix is achieved after the interpolation and extrapolation techniques are applied. The proposed algorithm is implemented on top of the scalable multipole engine, a portable implementation of the dynamic multilevel fast multipole algorithm for distributed-memory computers. The computational time per matrix vector multiplication is reduced to O(NlogN) and the memory requirement is reduced to O(N), where N is the number of unknowns in the discretized integral equation. The algorithm is validated by applying it to the solution of the electromagnetic scattering from the perfect electric conducting scatterers. This approach can be easily extended to more general problems with complicated Green's function expressed in terms of the plane wave spectral integrals, such as the ones encountered in the multilayered medium studies.

Polarization observations of broadband VHF signals by the FORTE satellite

Abstract: Coherent very high frequency (VHF) radio observations with the pair of orthogonal log-periodic array antennas of the FORTE satellite allow us to study thoroughly the polarization properties for a received signal. Eighty-one broadband VHF pulses that were generated by the Los Alamos Portable Pulser (LAPP) have been analyzed. The data are analyzed by computing the Stokes parameters in the time-frequency domain. We first examine the LAPP pulses at high time resolution so as to separate the ordinary and extraordinary ionospheric modes. The two modes have been found to be mirror images of each other in terms of polarization, as would be expected. For each mode the polarization degrades from circular toward elliptical as the nadir angle increases. Antenna pattern effects on this observation are discussed. The tilt of the detected polarization ellipse is found to be tightly associated with the azimuthal direction of the pulse source. The same set of data are then examined with much lower time resolution to intentionally mix together the two split modes, so that the ionospheric Faraday rotation can be detected. With the known geomagnetic field the total electron content (TEC) is computed, which shows good agreement with the TEC computed by dechirping the signal. A case study of an impulsive lightning emission shows that it is highly polarized, indicating that the associated breakdown processes are highly coherent and organized. Finally, we discuss the potential use of the polarization observations for locating terrestrial radio signals.

Middle atmosphere dynamical studies at Resolute Bay over a full representative year: Mean winds, tides, and special oscillations

Abstract: Wind motions in the 80–98-km altitude region above Resolute Bay, Nunavut, Canada (75°N, 95°W), are presented for a representative year. The measurements were made using an interferometric meteor radar during 1997, 1998, 1999, and 2000. Results presented include a comprehensive study of monthly mean winds, using all of the available data from these 4 years, as well as running tidal studies and power spectra over the time frame 1998–1999 and studies of special persistent oscillations which appear from time to time. All studies are accompanied with a detailed consideration of the effects of the spectral windows used. A superposed epoch analysis is used to investigate a dominant 15-hour oscillation which appeared in November 1999, and evidence of period changes, associated with changes in resonance conditions, is demonstrated. Other periodic motions are also seen, especially at periods in the range 9–12 hours.

Concepts, observations, and simulation of refractive index turbulence in the lower atmosphere

Abstract: Advances in computers and in computational techniques now allow the calculation of electromagnetic (EM) wave propagation through simulated refractive index turbulence in the lower atmosphere. Such applications call for instantaneous turbulence fields, not turbulence statistics, the traditional focus of the turbulence community. We clarify their important differences and review what is known about key statistics of refractive index turbulence. We discuss the calculation of EM propagation with a parabolic equation model that uses composite refractive index fields, the larger scales being calculated with a dynamical mesoscale model and the smaller scales being calculated through large-eddy simulation. The locally, instantaneously sharp top of the atmospheric boundary layer can have a profound effect on forward scatter of EM waves. This top appears to be even sharper than is revealed by conventional measurements, particularly in the convective boundary layer.

Resolute Bay VHF radar: A multipurpose tool for studies of tropospheric motions, middle atmosphere dynamics, meteor physics, and ionospheric physics

Abstract: A VHF radar has been established at a site near Resolute Bay in Nunavut, Canada (75°N, 95°W), which has the capability to make a variety of measurements relating to the atmospheric and ionospheric environment in the polar regions. The site is very close to the north geomagnetic pole, and therefore the radar is well situated to make some unique measurements. The system is a multipurpose instrument with good remote control capabilities. It can be used as a wind profiler radar to study the lower troposphere, as a mesospheric radar to study polar mesosphere summer echoes (PMSE) in summer, as a meteor radar to determine winds in the altitude region of 80–100 km, and as an ionospheric radar to study 3 m scale irregularities in the E and F regions. The radar has some unique design features, partly dictated by the rough terrain in which it is sited. In this paper, the radar system is described, including description of some unusual approaches to deal with special conditions at the site, and then some key early results are presented. Important findings include error determinations for tropospheric wind measurements, detection of PMSE, correlations between PMSE and atmospheric temperatures at 86 km altitude, measurements of mean winds and tidal characteristics over a full year, and detection of various normal modes of oscillation in the 80–100 km region, especially in nonsummer months. Some of these features will be discussed here, but more detailed discussions will be left to related papers in this issue.

Lifetime of a depression in the plasma density over Jicamarca produced by space shuttle exhaust in the ionosphere

Abstract: When the space shuttle orbiting maneuver subsystem (OMS) engines burn in the ionosphere, a plasma density depression, or “hole,” is produced. Charge exchange between the exhaust molecules and the ambient O+ ions yields molecular ion beams that eventually recombine with electrons. The resulting plasma hole in the ionosphere can be studied with ground-based, incoherent scatter radars (ISRs). This type of ionospheric modification is being studied during the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) series of experiments over ISR systems located around the globe. The SIMPLEX 1 experiment occurred over Jicamarca, Peru, in the afternoon on October 4, 1997, during shuttle mission STS 86. An electron density depression was produced at 359 km altitude at the midpoint of a magnetic field line. The experiment was scheduled when there were no zonal drifts of the plasma so the modified field line remained fixed over the 50 MHz Jicamarca radar. The density depression was filled in by plasma flowing along the magnetic field line with a time constant of 4.5 min. The density perturbation had completely vanished 20 min after the engine burn. The experimental measurements were compared with two models: (1) SAMI2, a fully numerical model of the F region, and (2) an analytic representation of field-aligned transport by ambipolar diffusion. The computed recovery time from each model is much longer than the observed recovery time. The theory of ambipolar diffusion currently used in ionospheric models seems to be inadequate to describe the SIMPLEX 1 observations. Several possible sources for this discrepancy are discussed. The SIMPLEX 1 active experiment is shown to have the potential for testing selected processes in ionospheric models.

GPS normalization and preliminary modeling results of total electron content during a midlatitude space weather event

Abstract: On November 22–23, 1997, a geomagnetic storm occurred during a period of excellent viewing conditions over the Arecibo Observatory in Puerto Rico. Here we explore the total electron content (TEC) registered by Global Positioning System (GPS) receivers located close to the Cornell All-Sky Imager (CASI) at the Arecibo Observatory. The storm began with the equatorward surge of a very high (100% increase) TEC enhancement stretching for many hours of local time on the dayside. At dusk the TEC over the Caribbean remained elevated with levels equal to the noontime monthly averages. During the event the TEC was highly structured and clearly correlated with high and low airglow emission levels. In one fortuitous instance a common ionospheric penetration point (15 km apart), shared by two GPS satellites viewed from two receiving stations, registered an 8 TEC unit difference during the active period. We show that a GPS station can be calibrated using the pseudorange method and a reliable data-driven technique during quiet conditions and still have absolute TEC capability within 2 TEC units (RMS) 5 days later. We compare the observations to a climatological model which, although reasonable for quiet times, is very poor during the storm period. We also present an independent evaluation of the GPS TEC. This study is an initial step toward quality control of this database, needed before it is used in an assimilation model.

Use of GPS for estimation of bending angles of radio waves at low elevations

Abstract: The paper compares three methods of calculating the bending angles of radio waves propagated from space to a ground-based receiver: (1) from refractivity climatology corrected for refractivity at the receiving antenna, (2) from radiosonde refractivity profiles, and (3) from the refractivity at the antenna and the measured Doppler frequency shift of the GPS signals. The methods are tested with the use of radiosonde and GPS observations collocated in space and in time. We analyzed seven cases during October-November 1999 where GPS satellites were observed to below 0.5° elevation from Point Loma, California, and which coincided closely in time with radiosonde launches from the nearby Miramar station. In all cases the bending angles calculated from Doppler and from radiosondes agree fairly well at all elevations, but in a number of cases both differ significantly at low elevations from the bending angles calculated from climatology corrected for the refractivity at the antenna. Thus GPS has the potential of being used for the correction of radar observations at low elevations instead of (or complementary to) radiosondes. The differences between the bending angles calculated from climatology corrected for the refractivity at the antenna and those calculated from the Doppler frequency shift indicate anomalies in the refractivity profile in the lower troposphere and can thus be used as an indicator of ducting conditions.