Radio Tomography Research Articles

The CTIP Model with SuperDARN Electric Potential Patterns as Input: First Comparison of Modelled Ion Densities with Radio Tomography Observations

In a recent development of the Coupled Thermosphere Ionosphere Plasmasphere (CTIP) model, electric potential patterns from the SuperDARN radar network are used as the input to drive the high-latitude ionospheric convection. Spatial images of the ionised atmosphere under conditions of IMF Bz negative given by the Aberystwyth University tomography experiments in Scandinavia and the UK are compared with model simulations for assessment of the model development. Results are presented showing broad agreement between the observations and the model.

Fractal structure of artificial ionospheric turbulence

We show the results of the first experimental studies of the multifractal structure of the developed artificial ionospheric turbulence. As a result of the special multifractal analysis of the recorded amplitudes of signals from the orbital satellites, which were obtained during the experiments on radio tomography of the irregularities excited in the ionosphere by the powerful mid-latitude heating facility “Sura,” it is found that the characteristic multifractal structure of these records is caused by the nonuniform spatial distribution of the small-scale fluctuations of the electron number density in the artificial irregularities of the ionospheric plasma. Comparative analysis is performed for the multifractal spectra of fluctuations of both the amplitudes and energies of signals received from the orbital satellites upon radio transmission probing of the region of artificial ionospheric turbulence by these signals at three observation points located near the “Sura” heating facility and spaced apart to a distance of about 100–150 km.

High-latitude ionosphere during magnetic storms of October 26, 2003–November 1, 2003: Tomographic reconstructions and numerical modeling

The height-latitude distributions of the electron density in the European Arctic sector of the subauroral and high-latitude ionosphere have been reconstructed based on the data of satellite radio tomography. The reconstructions have been compared with the results of the numerical modeling obtained using the UAM global numerical model and the IRI-2001 empirical ionospheric model. The goal of the paper was to find the degree of adequacy of these models to the observational data during geomagnetic disturbances. The UAM theoretical model adequately reproduces the dynamics of the main ionospheric trough, in particular, the position of its equatorward wall, and to a certain degree better describes the behavior of the high-latitude ion-osphere than the IRI empirical model. The discrepancies are observed in the regions of increased electron density poleward of the trough. To eliminate such disagreements, it was proposed to correct the model input distributions of precipitating high-energy electron fluxes with an energy of about 0.5 keV; i.e., the inverse problem of determining these distributions has been solved for extremely strong geomagnetic disturbances.

Multi-instrument observations of nightside plasma patches under conditions of IMF <I>B<sub>z</sub></I> positive

Abstract. Results are presented from two multi-instrument case studies showing patches of cold, long-lived plasma in the winter nightside ionosphere during times when the z-component of the Interplanetary Magnetic Field (IMF Bz) was positive. These enhancements were coincident with the antisunward convective plasma drift, flowing from polar to nightside auroral latitudes. In the first case, on 5 December 2005 with IMF By negative, two regions of enhanced electron density were observed extended in MLT in the magnetic midnight sector separated by lower densities near midnight. It is likely that the earlier enhancement originated on the dayside near magnetic noon and was transported to the nightside sector in the convective flow, whilst the later feature originated in the morning magnetic sector. The lower densities separating the two enhancements were a consequence of a pair of lobe cells essentially blocking the direct antisunward cross polar flow from the dayside. A second case study on 4 February 2006 with IMF By positive revealed a single nightside enhancement likely to have originated in the morning magnetic sector. These multi-instrument investigations, incorporating observations by the EISCAT radar facility, the SuperDARN network and radio tomography, reveal that plasma flowing from the dayside can play a significant role in the nightside ionosphere under conditions of IMF Bz positive. The observations are reinforced by simulations of flux-tube transport and plasma decay.

Radio tomography of the ionosphere: Analysis of an underdetermined, ill‐posed inverse problem, and regional application

After analysing the forward and inverse problems of radio tomography, a time varying three‐dimensional imaging method of the ionosphere from GPS slant TEC data is described and applied at regional scale. Our approach is based on local basis parametrisation of electron density, and constrained by NeQuick ionosphere model and its spatial gradients. Our inversion scheme is fundamentally different from the data assimilation approach because it is not based on a physical ionosphere model. Preliminary results obtained with European GPS receiver data validate the stability of our method which is able to retrieve small‐scale ionosphere features in properly resolved volumes. Many improvements are still possible on the algorithm, and a careful validation of inverted models by independent data is still necessary. However, our inversion algorithm is shown to improve the determination of the small‐scale features of a coarse a priori ionosphere model.

The role of the tongue‐of‐ionization in the formation of the poleward wall of the main trough in the European post‐midnight sector

A series of radio tomography reconstructions from the University of Wales Aberystwyth receiver chains in Scandinavia and the UK, imaging the midnight‐dawn sector on 13 December 2001, reveal a persistent large‐scale electron density enhancement, which forms the poleward wall of the main ionization trough. Measurements by the European Incoherent Scatter radar (EISCAT) rule out in situ soft‐particle precipitation as the main source of the higher densities. SuperDARN plasma drift observations and electric potential patterns place the feature in the dawn cell of the high‐latitude convection, leading to the conclusion that the higher density is likely to have originated as photoionization and was convected over the polar cap to the nightside and around toward dawn in a tongue‐of‐ionization (TOI). Suitable runs of the Coupled Thermosphere‐Ionosphere‐Plasmasphere (CTIP) model support this interpretation and also reveal that the formation of the TOI is heavily UT dependent, which would lead to it being most prominent at nighttime in the European sector.

Numerical simulation of the structure of the high-latitude ionospheric F region during meridional HF propagation

A previously developed model of the high-latitude ionosphere is used to calculate the distribution of the ionospheric parameters in the polar region. A specific method for specifying input parameters of the mathematical model, using the experimental data obtained by the method of satellite radio tomography, is used in this case. The spatial distributions of the ionospheric parameters characterized by a complex inhomogeneous structure in the high-latitude region, calculated with the help of the mathematical model, are used to simulate the HF propagation along the meridionally oriented radio paths extending from middle to high latitudes. The method for improving the HF communication between a midlatitude transmitter and a polar-cap receiver is proposed.

Characteristics of the spatial spectrum of plasma irregularities excited at mid-latitudes by the high-power “Sura” facility

We present the results of analysis of the spectra of amplitude scintillations at a frequency of 150 MHz and a difference phase at frequencies of 150 and 400 MHz, which were obtained in the experiment on radio tomography of artificial ionospheric turbulence (AIT) excited by the mid-latitude high-power “Sura” heating facility [1]. We used the data on radio probing of the AIT region at a frequency of 150 MHz by signals from artificial satellites in near-polar circular orbits at altitudes of 1000 km above the Earth’s surface. The signals were received simultaneously at three spaced apart points located at distances of about 100–150 km from each other along the projection of the satellite trajectory onto the Earth’s surface. The analysis of the data shows that in the range of scales smaller than 0.5–1.0 km across the geomagnetic field, the AIT spatial spectrum can be described by the power-law function with the spectral index p⊥ = 1.7–2.5. For irregularities with the same transverse scales, the spectral index in the direction of the magnetic field amounts to p‖ = 5–7.

Two-Dimensional Radio Frequency Tomography

A new formalism for scalar tomographic inversion is developed for two-dimensional radio frequency tomography. A least squares minimization of the difference between predicted and measured data is performed in order to obtain the lossless permittivity distribution. The novelty of the method is twofold. First, a multifrequency near-field two-dimensional scattering forward problem is solved via Richmond's moment method. Second, the tomographic inversion is regularized using a roughness constraint on the model permittivity. The algorithm has been tested with synthetic data. The experimental setup for acquiring the relevant data in a real-world application (viz. in situ testing of lumber quality) is under development. It is anticipated that the algorithm will work equally well on the experimental data. In the tests, the relative lossless permittivity was determined from synthetic scattered multifrequency transverse magnetic data, which had been corrupted by additive Gaussian noise. The method was able to recover the location and magnitude of permittivity anomalies without immersing the target prism in a matching fluid

Characterising borehole radio antenna performance using finite-difference time-domain modelling

Research Article| December 01, 2006 Characterising borehole radio antenna performance using finite-difference time-domain modelling Declan Vogt; Declan Vogt CSIR Division of Mining Technology, PO Box 91230, Auckland Park, 2006, South Africa e-mail: dvogt@csir.co.za Search for other works by this author on: GSW Google Scholar Andy Marvin Andy Marvin Department of Electronics, University of York, Heslington, York, YO10 5DD, United Kingdom e-mail: acm@ohm.york.ac.uk Search for other works by this author on: GSW Google Scholar Author and Article Information Declan Vogt CSIR Division of Mining Technology, PO Box 91230, Auckland Park, 2006, South Africa e-mail: dvogt@csir.co.za Andy Marvin Department of Electronics, University of York, Heslington, York, YO10 5DD, United Kingdom e-mail: acm@ohm.york.ac.uk Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2006 Geological Society of South Africa South African Journal of Geology (2006) 109 (4): 561–570. https://doi.org/10.2113/gssajg.109.4.561 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Declan Vogt, Andy Marvin; Characterising borehole radio antenna performance using finite-difference time-domain modelling. South African Journal of Geology 2006;; 109 (4): 561–570. doi: https://doi.org/10.2113/gssajg.109.4.561 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Radio Tomography (RT) has proven itself as an imaging tool for base metal orebody delineation. To date, theoretical considerations of the imaging technique and inversion algorithms have concentrated on the propagation of energy from the transmit antenna to the receive antenna, while ignoring the antennas themselves.The Finite-Difference Time-Domain technique for modelling antennas has been extended to efficiently model antennas embedded in arbitrary media such as rock. The model is set up with body-of-rotation symmetry to produce models that have three dimensional accuracy, while only having two dimensional computational cost. Wire dipole antennas are efficiently modelled by the addition of a subcell extension for a thin wire coated with a thin layer of insulation.The extended code is used, both to aid in the design of an improved antenna, and to investigate how the performance of the antenna affects the imaging of RT data in particular circumstances. A completely insulated antenna is preferred because its performance is more independent of the surrounding rock. The numerical model aids in the design of an improved antenna, with the optimum combination of performance features in a physically realizable antenna. If the electronics package is placed at the end of the dipole, the electronics package can be housed in a bare metal pressure casing without significantly affecting antenna performance as a function of rock type.The model also shows how the use of RT can be influenced by the geometry of the system and particularly by the use of conductors to suspend the RT antenna: wire cable support is not recommended until full waveform inversion techniques can take into account the presence of the wire. Antenna arrays appear to be viable, but if antennas without insulation are used, the spacing between the antennas should be at least as great as the length of each antenna. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Simultaneous inversion of total electron content and UV radiance data to produce F region electron densities

The High Resolution Airglow and Aurora Spectroscopy (HIRAAS) experiment was launched aboard ARGOS on 23 February 1999. The HIRAAS experiment operated from mid‐May 1999 through March 2002. One of the HIRAAS instruments, the Low Resolution Airglow and Aurora Spectrograph (LORAAS), gathered limb scans over the 750–100 km altitude range of the 911‐Å emission during the daytime and the O I 1356 Å emission at night; these emissions are useful for characterizing the ion density distribution in the F region. The Coherent Electromagnetic Radio Tomography (CERTO) experiment, a coherently emitting radio beacon operating at 150 and 400 MHz, also flew on the ARGOS. The slant total electron content (TEC) between ARGOS and the ground was measured, using the CERTO beacon emissions, by a receiver located at the Naval Research Laboratory during early 2001. During the mission life of ARGOS, the ARGOS and TOPEX/Poseidon satellites occasionally crossed paths, permitting additional validation of the ARGOS measurements against TOPEX vertical total electron content measurements. We present a comparison of the UV‐derived TEC and the radio beacon–derived TEC over the Naval Research Laboratory during January–April of 2001, based on LORAAS and CERTO measurements; we also present the electron densities derived by simultaneously inverting both the UV radiances and the CERTO‐derived TEC. These results are validated against ionosondes. The TOPEX and UV measurements are also inverted tomographically and are used to demonstrate enhanced spatial resolution of the ionospheric reconstructions.

Novel improvements to magnetospheric tomography using MHD

[1] Over the past decade, radio tomographic techniques have seen increasing use within space science, both in practice, with ionospheric reconstruction, and theoretically as a tool for remote sensing magnetospheric plasmas. The promise of this technique for use in the magnetosphere was greatly enhanced with the introduction of combined Faraday rotation and phase or group delay radio tomography. As measurables these techniques provide distributions in space and time of electron density ne and magnetic field . Given these measured quantities, this paper provides a theoretical framework for further derivation of additional parameters, such as plasma temperature, based upon MHD relations. Additionally, we show how using the physical constraint · = 0 to regularize the reconstructions of magnetic field yields superior reconstructions with fewer measurements and provides a physical and effective method for incorporating in situ magnetometer measurements. Specifically, we find that including this regularization often reduces RMS errors in magnetic field reconstructions by a factor of 3. This simple regularization, especially when combined with in situ magnetometer measurements, is so powerful that it may influence future designs of a magnetospheric tomography mission.

EISCAT observations of plasma patches at sub-auroral cusp latitudes

Abstract. A sequence of 3 patches of high-density (1012 m−3) cold plasma on a horizontal scale-size of 300–700 km was observed near magnetic noon by the EISCAT VHF radar above Svalbard on 17 December 2001. The patches followed a trajectory towards the cusp inflow region. The combination of radar and all-sky observations demonstrates that the patches must have been segmented equatorward of the cusp/cleft auroral display, and hence their properties had not yet been influenced by cusp particle showers and electrodynamics on open flux tubes. The last patch in the sequence was intersected by radio tomography observations, and was found to be located adjacent to a broader region of the same high electron density further south. The patches occurred under moderately active conditions (Kp=3) and the total electron content (TEC) of the high-density plasma was 45 TEC units. The train of patches appeared as a segmentation of the tongue of ionization. The sequence of patches occurred in association with a sequence of flow bursts in the dusk cell return flow. It is proposed that reconnection driven pulsed convection is able to create sub-auroral patches in the region where high density mid-latitude plasma is diverted poleward toward the cusp. It is the downward Birkeland current sheet located at the equatorward boundary of the flow disturbance that represents the actual cutting mechanism.

Reconfiguration of polar-cap plasma in the magnetic midnight sector

Abstract. Radio tomography and the EISCAT and SuperDARN radars have been used to identify long-lived, high-altitude, cold plasma in the antisunward convective flow across the polar cap. The projection of the feature to later times suggests that it was reconfigured in the Harang discontinuity to form an enhancement that was elongated in longitude in the sunward return flow of the high-latitude convection pattern. Comparison with a tomographic image at a later time supports the interpretation of a polar patch being reconfigured into a boundary blob. There is also evidence for a second plasma enhancement equatorward of the reconfigured blob, likely to have been produced by in situ precipitation. The observations indicate that the two mechanisms proposed in the literature for the production of boundary blobs are operating simultaneously to form two distinct density features separated slightly in latitude.

New satellite‐based systems for ionospheric tomography and scintillation region imaging

The new constellation of radio beacons called Coherent Electromagnetic Radio Tomography (CERTO) will be available for measurements of ionospheric total electron content and radio scintillations. These beacons transmit unmodulated, phase‐coherent waves, VHF, UHF, and L band frequencies. A fixed radio of 3/8 is used between successive frequencies. Total electron content (TEC) can be measured using the differential phase technique. The range between beacon and receiver is removed from the phase measurements, leaving a differential phase that is proportional to TEC. The three CERTO frequencies cover a wide range for determination of the radio scintillation effects caused by diffraction after propagation though ionospheric irregularities. All of the CERTO beacons are in low Earth orbit with inclinations ranging from equatorial to polar. Each satellite that carries CERTO has other plasma instruments that complement the beacon data. In addition, a Scintillation and Tomography Receiver in Space (CITRIS) instrument will be placed in orbit to detect signals from the CERTO beacons and from the array of 56 Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) VHF/S band radio beacons placed around the word by the French Centre National D'Etudes Spatiales. CITRIS will record ionospheric occultations and radio scintillations with a unique occultation and ground‐to‐space geometry. New algorithms have been developed for the multifrequency CERTO and CITRIS data to provide improved acquisition and analysis of TEC and scintillation data in ionospheric studies. The data from the CERTO constellation of beacons and receivers may be used to update space weather models.

Inclined focusing method in subsurface radar

An oblique scanning method for subsurface radio tomography is proposed that is aimed at reconstructing the inhomogeneity 3D distribution and discovering out-of-level scatterers under a planar interface. The method is based on synthetic-aperture radar technology and uses data of multiposition sensing by omnidi-rectional antennas elevated over the Earth’s surface. The focusing of the synthetic-aperture radar is based on fast Fourier transformation providing a fast algorithm of 3D reconstruction. The method is verified numerically and in real experiments.

An orthogonal projection and regularization technique for magnetospheric radio tomography

A challenging problem in ill‐posed inverse problems is incorporating prior knowledge of the solution into reconstruction techniques. This problem is particularly important in magnetospheric radio tomography where the path integrated measurements of the target region may be sparse. We present in this paper an orthogonal projection and regularization (OPR) technique that incorporates prior knowledge of magnetospheric parameters from existing models or past measurements into a direct reconstruction algorithm. The OPR scheme extracts first an optimal orthonormal basis containing the main features of the unknowns from an ensemble of modeled or measured snapshots through the proper orthogonal decomposition (POD) and then projects the line‐of‐sight equations onto the subspace spanned by the empirical orthonormal basis. The resulting low‐dimensional model is well‐conditioned, its coordinates are uncorrelated, and it contains prior knowledge of the solution. The magnetospheric parameters in the transformed coordinate are reconstructed from the low‐dimensional model and quantities in the physical coordinate are easily recovered from the POD transformation. On the basis of magnetohydrodynamic (MHD) simulations and hypothetical satellite constellations, we demonstrate that the POD‐based method may perform significantly better than the regularized direct method with sparse path‐integrated measurements, combined with a few (5–10) model snapshots.

Radio tomographic imaging of the northern high‐latitude ionosphere on a wide geographic scale

Several chains of receivers, measuring total electron content (TEC) during passes of satellites in the U.S. Navy Ionospheric Measuring System (NIMS), are operated at northern high latitudes by members of the International Ionospheric Tomography Community (IITC). Results are presented here of nearly simultaneous latitude‐altitude images of the ionosphere over Scandinavia, Greenland, and Alaska, generated from IITC data obtained on 20 September 2001 and interpreted in the context of an IMF‐dependent convection model. The images are compared with output from a coupled thermosphere‐ionosphere‐plasmasphere model and also with maps of high‐latitude TEC generated from GPS measurements. With Bz and By both weakly negative, the images and map reveal plasma features of a two‐cell convection pattern rotated slightly to earlier local magnetic times. The tomographic images provide details of map features such as a tongue of ionization created by solar EUV radiation on the dayside and entrained by convection into the polar cap from the western Russian sector. Combining latitude versus altitude tomographic images generated in different longitudinal sectors from polar‐orbiting beacon satellites such as those of NIMS with TEC maps from GPS offers an emerging opportunity for polar aeronomic studies, especially if further related to synoptic convection measurements. The results also demonstrate the potential role of wide‐scale radio tomography in the verification of ionospheric models.

The dayside high-latitude trough under quiet geomagnetic conditions: Radio tomography and the CTIP model

Abstract. The dayside high-latitude trough is a persistent feature of the post-noon wintertime auroral ionosphere. Radio tomography observations have been used to map its location and latitudinal structure under quiet geomagnetic conditions (Kp≤2) near winter solstice. The trough is also a clear feature in the ion density distribution of the Coupled Thermosphere-Ionosphere-Plasmasphere model (CTIP) under similar geophysical conditions. Comparisons of the measured and modelled distributions show that the plasma production equatorward of the trough is mainly controlled by solar radiation, but there are also other processes maintaining the equatorward trough-wall that are open to debate. The poleward trough-wall is produced by particle precipitation, but the densities are significantly overestimated by the model. At the trough minimum the observed densities are consistent with low nighttime densities convecting sunward to displace the higher daytime densities, but this is not borne out by the CTIP model. The study shows the potential of combining radio tomography and modelling to interpret the balance of the physical processes responsible for large-scale structuring of the high-latitude ionosphere, and highlights the role of tomographic imaging in validating and developing physical models.

A flexible and robust direct reconstruction method for magnetospheric radio tomography

We present a flexible and robust direct reconstruction method for magnetospheric radio tomography. We show that for a combined reconstruction of plasma density Ne and magnetic field B, the direct reconstruction method performs as well as popular iterative methods such as algebraic reconstruction technique (ART) and multiplicative algebraic reconstruction technique (MART) for large number of satellites, but it performs significantly better when the number of satellites is small. The main advantages of this method are that extra information, such as in situ measurements, can be easily and flexibly incorporated into the reconstruction; it is relatively robust in the presence of noise; and it is less sensitive than other methods to numerical reconstruction parameters like assumed grid size. We demonstrate the good performance of this method in reconstructing electron density and magnetic field using constellations of relatively few satellites (11 and fewer) in a single orbit in a variety of magnetospheric regions. Although this method is relatively robust to noisy measurements, local measurements can significantly increase the reconstruction accuracy.