Radio Echo Research Articles

Amplitude Profiles of Solar Type IIId Radio Bursts with Echo Components and the Global Dislocation of Their Elementary Quasi-Monochromatic Sources

We prove experimentally the possibility of direct observations of natural radio echoes on the Sun. The generally simple echo hypothesis on the origin of the time splitting in solar type IIId radio bursts sustained the crucial experimental test. These conclusions are based on both the new positional data and previous information obtained using the UTR-2 antenna system in different years since 1973. In summer 1992, we were able to register for the first time a large number of decameter type IIId bursts with pronounced echo-like components using a radioheliograph at frequencies of 20 and 25 MHz. Elementary (quasi-monochromatic) sources of the short-lived bimodal type IIId bursts were observed especially often on July 6. In this case, the time delay of the second peak, which depends on the heliolongitude of the active region, reached a maximum of about 7 s. All these double-peak bursts were localized in the central region of the solar corona, and, similar to the case of single-pulse “limb” type IIId bursts at a given radio frequency f, the observed elementary sources of these events appeared to be nonstationary. Such a dynamic pulsating source can radically change, at least once, its location in the celestial sphere in the field of view of a two-dimensional radio heliograph during the burst lifetime τΣ. While decaying, an echo-like burst formed rather slowly may abruptly jump from one location to another. As far as its short (3- to 4-s) steep-front precursor is concerned, its visible source behaved usually as an ephemeral sub- or ultrarelativistic object moving at a subluminal velocity over a global-scale trajectory. To all appearances, a twice-pulsing, narrow-band (Δ f/f∼ 0.01) radio burst with an intensity-time profile of type IIId is generally a rather long-term (τΣ ≈ 10-20 s) transient response of the solar atmosphere to the short-term flare of harmonic radio emission generated in the real (probably motionless and compact) source. This complex double-peak profile is due not only to the existence of a reflecting surface deep in the corona near the corresponding plasma level but also, to a considerable extent, to some opaque or semi-transparent structures located at heights of the middle corona. In particular, due to the strong influence of such structures, a coronal primary source of a bimodal burst can be seen on the Earth in some indirect rays before the solar radio echo and has the form of an additional delayed pulse.

Penetration of Antarctic subglacial lakes by VHF electromagnetic pulses: Information on the depth and electrical conductivity of basal water bodies

Owing to the high level of absorption of very high frequency radio waves in water, previous investigators of airborne radio echo sounding (RES) data from Antarctica have assumed that the depth of subglacial lakes cannot be measured directly by this method. However, we have identified a number of RES returns from beneath the ice‐water interface at the surface of eight subglacial lakes that we have interpreted as being reflected from the lake floor. The returns allow us to measure the depth of subglacial lakes, since the velocity of radio waves in water (33.4 m μs−1) is relatively unaffected by electrical conductivity. Attenuation of radio waves within water is controlled largely by its electrical conductivity. Consequently, by examining the decay of the radio wave amplitude with depth we can gain information about the conductivity of subglacial water bodies. Our results indicate that the minimum water depths of eight subglacial lakes vary between 8 and 21 m. The lakes from which our depth measurements were taken are distributed widely around the ice sheet. Thus it may be concluded for the first time that Antarctic subglacial water bodies are generally at least several meters in depth. By examining the attenuation of radio waves through subglacial water, the electrical conductivity of the water is estimated to be extremely low (i.e., fresh pure water).

Application of Radio Echo Sounding at the arid Andes of Argentina: the Agua Negra Glacier

The results of ground-based Radio Echo Sounding (RES) of the Agua Negra Glacier of the arid Andes of Argentina are analyzed. The glacier (30°15′S, 69°50′W) extends from 5.200 to 4600 m in altitude, and presents a smooth and convex upward surface without evident crevasses. Most potential crevasses seem to be sealed by the high rate of melting–freezing due to extreme differences between positive (diurnal) and negative (night) energy peaks. Seismic methods suggest the existence of a thick unit of debris or altered rock at the base of the glacier. Higher than normal seismic velocities indicate a very compact ice system (ice+voids), very dense near the glacier terminus. An impulse transmitter of 12 MHz was used for the RES survey, with 4 m half-length antennas, and an antenna separation of 30 m. Three profiles were surveyed and isolated measurements were taken in addition, covering about a tenth of the total glacier surface. Very clear bottom echoes occurred in most cases, indicating a maximum ice thickness of 50 to 55 m and an ice volume beneath the surveyed area of 2.0×10 6 m 3. The total glacier volume is estimated as 10 times higher, representing an important water resource for this region. It is also possible that an older ice body lies beneath the debris that forms the present glacier base. The multistorey structure of local glaciers may be an inheritance of past glacial cycles, modulating the evolution of a complex glaciolithic system. These preliminary results, suggest the RES method works well on local glaciers, provided one take into account problems such as debris saturation, low-scale relief producing scattered echoes and low ice thickness. Its application should contribute to studies of the water reserves in this largely glacier-dependent arid region.

Method for measuring the direction and wave mode of the radio echoes that are detected by a satellite radio sounder in the Earth's magnetosphere

A method based on measuring the electric polarization of a wave has been devised to measure the direction and wave mode of the echoes that are detected by a satellite radio sounder in the Earth's magnetosphere. Except for directions that are almost exactly perpendicular to the magnetic field, this method is found to work at frequencies above about 20 kHz with an angular error of less than about 1°. A second method that is based on measuring the Faraday rotation of the axis of polarization has also been devised to measure the echo direction when the ordinary and extraordinary echoes that are produced by a radio sounder overlap to produce an elliptically or linearly polarized wave. These two methods will then be used to analyze the measurements of the Image radio sounder that is scheduled to be launched in the year 2000, as described in the feasibility analysis for the Image sounder that was published in 1995.

Nature of radio echo layering in the Antarctic Ice Sheet detected by a two‐frequency experiment

A two‐frequency radio echo sounding experiment was carried out at Dome Fuji, the second highest dome in East Antarctica, and along a 1150‐km‐long traverse line from the dome to the coast. The goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions. From the two‐frequency (60 MHz and 179 MHz) radio echo responses at various sites, we distinguished four zones. Each of the zones is characterized by a dominant cause of radio echo internal reflection as follows. In the “PD” zone, changes in dielectric permittivity are mainly due to density fluctuations; in the “PCOF” zone, changes in dielectric permittivity are mainly due to changes in crystal‐orientation fabrics; and in the “CA” zone, changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions. In each of these three zones, the changes occur commonly along isochrones. In addition, a basal echo‐free zone, the fourth zone, was found to appear always below the PCOF zone. These four zones and their distribution suggested variations of the physical conditions within the ice sheet.

Latest improvements for the echo sounding system of the Italian radar glaciological group and measurements in Antarctica

Radio echo sounding is an active remote-sensing method using electromagnetic wave penetration in a particular medium such as ice and permits the measurements of its thickness, the level of the bedrock and inhomogeneities i.e. of the internal layering of glaciers. The Italian Radar Glaciological Group has implemented technical improvements on the system operating during the 1995-1996 Antarctic expeditions, increasing the horizontal and the vertical resolution by means of a new acquisition ensemble. The new system was used during the Antarctic expedition in 1997-1998. The present paper presents the latest improvements of the radar acquisition system and radar profiles extracted from the 1995-1996 and the 1997-1998 Antarctic expeditions are presented.

Reanalysis of Clementine bistatic radar data from the lunar South Pole

On April 9, 1994, the Clementine spacecraft high‐gain antenna was aimed toward the Moon's surface, and the resulting 13‐cm wavelength radio echoes were received on Earth. Using these data, we have found that the lunar surface generally follows a Lambertian bistatic scattering function σ0 = KD cosθi cosθs with KD∼0.003 for the opposite (expected) sense of circular polarization and KD∼0.001 for the same (unexpected) sense. But there are important deviations (of up to 50% in some parts of the echo spectrum) from this simple form. Based on an earlier analysis of these same data, Nozette et al. [1996] claimed detection of an enhancement in echoes with the same sense circular polarization from regions near the South Pole in a near‐backscatter geometry. Such behavior would be consistent with presence of perhaps large quantities of water ice near the pole. We have been unable to reproduce that result. Although we find weak suggestions of enhanced echoes at the time of South Pole backscatter, similar features are present at earlier and later times, adjacent frequencies, and in the opposite circular polarization. If enhanced backscatter is present, it is not unique to the South Pole; if not unique to the pole, then ice appears less likely as an explanation for the enhancement.

Construction of a high-resolution DEM of an Arctic ice cap using shape-from-shading

Digital elevation models (DEMs) of ice sheets and ice caps are usually constructed from elevation data acquired from airborne or satellite-borne altimetric systems. Consequently, the DEMs have a spatial resolution of about 1km which limits their use for most glaciological and remote sensing studies. In this paper we investigated the possibility of using a shape-from-shading technique, applied to a Landsat MSS image, to create a high spatial resolution DEM of Austfonna, an ice cap in Svalbard. A high correlation (coefficient of determination = 0.85) was observed between Landsat pixel brightness values, acquired during winter, and the surface slope component parallel to the solar azimuth. This relationship was used to create a DEM by calculating surface elevation profiles across the ice cap, using low spatial resolution radio echo sounding data as tie points. The resulting DEM had an estimated rms error of about 14m, with the error occurring mostly at low spatial frequencies. Shape-from-shading produces less accurate DEMs than interferometric SAR (InSAR) techniques. Nevertheless, in scenarios for which InSAR cannot be used to construct a DEM, shape-from-shading provides an acceptable alternative.

Seasonal behavior of middle-latitude short-wave mesospheric radio echoes

Using the radar of the “Sura” facility, we studied the diurnal and seasonal behavior of mesospheric middle-latitude short-wave radio echoes. It was, found that the diurnal dependence of the intensity of reflected signals is mainly determined by the solar illumination of the mesosphere, i.e., by photoionization intensity. We revealed an almost permanent and season-independent presence of a thin turbulent layer at altitudes of 82 to 84 km whose radio echoes are maximal in summer and winter and much weaker in spring and fall. Sporadic summer radio echoes with increased intensity, which are observed at somewhat greater (by 2 to 4 km) altitudes, apparently correspond to the phenomenon of mesospheric summer echo (MSE) observed in the VHF range.

Unusual surface morphology from digital elevation models of the Greenland Ice Sheet

In this study of the North Greenland ice sheet, we have used digital elevation models to investigate the topographic signatures of a large ice flow feature discovered in 1993 and a unique surface anomaly which we believe has not been observed previously. The small scale topography of the flow feature is revealed in striking detail in a high‐pass filtered elevation model. Furthermore, ice penetrating radar show that the sub‐stream bed is rough with undulation amplitude increasing downstream. The new feature consists of two large depressions in the ice sheet connected by a long curving trench. The low driving stresses in the region and the increased power of radio echo from the ice/bed interface over the feature clearly show presence of basal water. The shape of one of the two depressions and its location above a bedrock trough is indicative of a sub‐glacial lake but we also see evidence suggesting that water transport through a basal hydrological system may be occurring.

Ice flow between the Greenland Ice Core Project and Greenland Ice Sheet Project 2 boreholes in central Greenland

The ice flow between the Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) boreholes in the Summit region of central Greenland is modeled with a steady state finite element model. The model calculates the free ice sheet surface and the coupled ice flow and temperature fields along the flow line between the boreholes. Three‐dimensional effects are expressed in terms of the divergence of the flow and included in the coupled stress and temperature fields. In addition to ice‐core data, several geophysical surface programs have provided data that are used to constrain the model. The modeled isochrones resemble the bedrock structure along the flow line but rise at the divide. The rise at the divide is not seen in internal layers found by surface radio echo sounding. An ice particle, originally deposited at the surface, moves through different stress and temperature regimes. A set of trajectory lines are followed from the surface to the boreholes, in order to follow the variation of stress deviators, temperature, and deformation along with the ice movement. These variations along a trajectory line constitute a history of the corresponding ice layer. The difference between the history of ice layers found in the GRIP and GISP2 boreholes is mainly due to the shear stress, which is 3–4 times higher for GISP2 ice than for GRIP ice. Furthermore, the GRIP ice experiences a nonzero longitudinal stress deviator much longer than the GISP2 ice.

Airborne radio echo sounding of outlet glaciers in Greenland

We used a coherent radar depth sounder operating at 150MHz to collect ice thickness data on outlet glaciers in northwestern Greenland. The radar data were collected in conjunction with laser surface elevation measurements and were tagged with GPS information for accurate geolocation. The radar signals were corrupted by multiple echoes between the aircraft and the ice surface, as well as between the ice surface and the ice-bedrock interface. We applied the homomorphic deconvolution technique to remove multiple echoes successfully and have identified the grounding line of a long ice shelf in northwestern Greenland.

Forward Modeling of the Internal Layers in Radio Echo Sounding Using Electrical and Density Measurements from Ice Cores

The measured density and conductivity of a 181 m ice core are used to calculate a profile of complex reflection coefficients. It is confirmed that the amplitude reflection coefficients due to the conductivity variations are negligible in comparison to the amplitude reflection coefficients due to the density variations. Using the magnitudes of these reflection coefficients we obtain a profile of the expected power being returned to the surface. The modeled power return is compared with the results of a short pulse electromagnetic reflection survey near the drill site. We also convolve the profile of complex reflection coefficients with a depth invariant input wavelet to produce a synthetic radargram for the top part of the ice sheet. The phase change given to wavelets reflecting from the conductivity variations has a negligible effect on the synthetic radargram. Using this simple model we are unable to match the positions of the internal layers in the synthetic radargram with those in the reflection survey.

Subglacial Preservation of Valley Morphology at Amundsenisen, Western Dronning Maud Land, Antarctica

On the high altitude polar plateau of Amundsenisen, western Dronning Maud Land, East Antarctica, a subglacial valley, with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta, was studied by radio echo sounding. In addition, a small, probably glacial, valley was mapped within the same subglacial massif. Basal ice temperatures were calculated using field data on precipitation, air temperature and ice sheet thickness. Discoveries of old landforms which have been preserved more or less intact beneath the former Fennoscandian and Laurentide ice sheets have received increasing attention during the last decade. The aim of this study is to investigate whether preservation of landforms occurs under the East Antarctic Ice Sheet, and to discuss under that climatological and glaciological circumstances preservation may take place. The results show that the ice sheet covering the investigated localities is frozen to bed, and therefore has an insignificant erosional capability. The observations suggest that a large-scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold-based ice sheet marginal zone for a long time period. The process of glacial preservation, recognized for bedrock features and tentatively observed for sediment accumulations, should act on similar large-scale landforms under any cold-based ice sheet, present or past. On the basis of existing studies of the age and stability of the East Antarctic Ice Sheet, a Middle Pliocene age is suggested for the preserved landforms. The presence of the presumed sediment-filled valley further indicates that no prolonged periods of basal melting have occurred at the Amundsenisen study area during the ice sheet history, which includes the Quaternary glaciation periods. Finally, calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation. © 1997 by John Wiley & Sons, Ltd.

Ice flow and mass continuity of Shumsky Glacier in the Djungarski Alatau Range of Kazakhstan, Central Asia

The glaciological monitoring program on the Shumsky Glacier in the Djungarski Alatau Range of southeastern Kazakhstan has included measurements of horizontal surface ice flow velocity, net balance, and emergence velocity at a network of more than 200 stakes during 1967–1991, as well as mappings of the ice surface topography by double‐theodolite triangulation and determination of ice thickness and of bedrock topography by radio echo sounding in 1984. These observations are here evaluated together to deduce the long‐term average dynamic behavior of Shumsky Glacier. The glacier extends between 3100 and 4400 m with a total area of 3 km2 and length of nearly 4 km. Maximum surface velocity is 20 m yr−1 and maximum volume flux 9 × 105 m3 yr−1, located near the equilibrium line altitude around 3680 m. The ice‐volume‐equivalent net balance is +0.5 m yr−1 in the average over the accumulation area, changing to about −1.5 m yr−1 near the terminus. The submergent flow in the accumulation area changes to emergence of 1 m yr−1 in the lower glacier. The rate of ice thinning is less than 0.1 m yr−1 in the middle glacier, increases to 2.5 m yr−1 near the terminus, and averages 0.12 m yr−1 for the glacier as whole. In fact, the Shumsky glacier differs little from equilibrium, being in a more favorable location that the other glaciers of the Djungarski Alatau. However, trends toward slower ice flow, more negative net balance, and faster rate of ice thinning indicate response to a progressively adverse climatic environment.

A three-wave FDTD approach to surface scattering with applications to remote sensing of geophysical surfaces

A major difficulty in physical interpretation of radio wave scattering from geophysical surfaces is the lack of detailed information on the signatures of geologically plausible discrete objects. Although the aggregate response will never be dominated by any single object, differences in the population of discrete objects on or near the surface (their sizes and shapes, for example) can change the character of a radio echo markedly. When the average surface is modelled as a flat, homogeneous half-space, the field that "drives" the scattering process is a composite consisting of the incident plane wave and the reflected and transmitted plane waves, all of which are known quantities; the total field can then be defined as the sum of the driving field and the scattered field. When a discrete object is near the surface, the total field can be calculated using finite-difference time-domain (FDTD) techniques, and the scattered near field can be calculated accordingly. The Green's functions for electric and magnetic currents above and below the surface, obtained by Sommerfeld theory and employed in conjunction with Huygens' principle, transform the local scattered fields to the far field. The FDTD implementation accommodates discrete lossy dielectric and magnetic scatterers in the vicinity of a dielectric surface; extension to a lossy half-space is straightforward. Two-dimensional results for scattering from perfectly conducting circular cylinders above and below a dielectric surface agree with moment method solutions within a few percent. Results for scattering from a dielectric wedge exhibit expected forward diffraction and internal reflection phenomena.

Variable radio echoes from a jet in QSO 0957+561

Time delay determinations in astrophysics are used most often to find time delays between flux density variations of different spectral bands and/or spectral lines in AGNs and different images of gravitationally lensed QSOs. Here we consider a new algorithm for a complex case, when the time delay is itself a linear function of time and the intensity of echo response is power function of the delay. We apply this method to investigate optical-to-radio delay in the double quasar 0957+561, which is a generally accepted case of gravitational lensing.

On the Determination of Velocities of Radar Meteors

AbstractThe results of testing the classical method of velocity determination based on the positions of the extrema of Fresnel diffraction patterns (FDP) in radio echo amplitude, is presented. This technique is compared to the method of nonlinear least squares fitting (NLSF) of the theoretical patterns to the observed those. It is shown that while the classical method can fail in yielding correct values for the velocity of radar meteors, the NLSF method always yields good results.

Non destructive assessment of pavement design and new build quality : Ballard, G S Non-destructive Testing in Civil Engineering, Liverpool, (United Kingdom), 14–16 Apr 1993. Vol. 1, pp 391–404. Edited by J H Bungey. The British Institute of NDT (1993)

Pulsed Radio Echo Sounding (Ground Radar) is used to assess the structural composition of pavements and to provide information on thickness, bond and density for correlation with and correction of surface strength measurements. The construction of an experimental pavement, built to assess the benefits of tapered design in prolonging pavement life, is examined: continuous assessment by radar shows wide deviation from the design specification, and faults of bond and compaction in the pavement itself, both in the experimental tapered sections and the standard parallel layer sections. The effect of these defects on apparent pavement strength and projected life is considered, and the resultant effect on the pavement experiment itself. Assuming that these faults will be repeated in any standard production road, the use of continuous non-destructive pavement assessment is discussed, as a Quality Control tool to avoid costly early life repairs. (A) For the covering abstract see IRRD 861706.

Focusing-defocusing phenomena of radio echo signal after passage of the morning solar terminator

Campaign results of high-repetition ionospheric soundings, performed at the ionospheric station of Rome using a Digisonde 128P, are presented. We introduce an index derived from run rapid ionograms that can be used to detect oscillations of isodensity surfaces ofF ionospheric region started by morning solar terminator.