Large amplitude quasi-periodic fluctuations associated with a mid-latitude sporadic E layer

For the first time a sounding rocket has been launched into a mid-latitude sporadic E event which was shown to be the source of VHF radar echoes. The layer had a very high peak electron density (∼10⁶ cm⁻³) and was thicker (∼5 km) than most events previously studied by rockets and incoherent scatter radars. The layer was modulated in a remarkable quasi-periodic manner which has not been reported earlier. Twenty cycles of these structures were detected and they seem to be oriented horizontally rather than vertically with periods in the rocket frame in the rage 6–10 s. There is also some evidence that the modulation was detected below as well as above the peak in the electron density, although the bulk of the flight was above the peak. Although the VHF radar echoes were decaying at the time and place where the rocket traversed the E layer, one burst of high amplitude short wavelength fluctuations was detected by the space-borne instruments and had a power spectrum similar to that of a secondary gradient drift mode. This burst occurred at the peak of one of the periodic electron density fluctuations. We discuss two possible sources for the dominant fluctuations: large-scale gradient drift waves and atmospheric acoustic waves. The latter seem most consistent with the data.     

A preliminary experimental test of ionospheric tomography

A preliminary experiment to test the application of computerized tomography (CT) to ionospheric imaging is described. Co-ordinated observations are presented in which radio transmissions from NNSS satellites were used to measure the total electron content required for the tomographic image of electron density. Simultaneous measurements of electron density by the EISCAT ionospheric radar facility were made for comparison. The results show that a large-scale ionospheric electron density gradient reconstructed by the tomographic technique is also seen in the EISCAT observations.     

Electrojet irregularity parameters from daytime equatorial scintillations

The second moment of the complex amplitude or the mutual coherence function (MCF) for transionospheric VHF radio waves transmitted from the geostationary satellite ATS-6 is computed from daytime amplitude and phase scintillations recorded at an equatorial station, in order to study the structure of electrojet irregularities. The shape of the correlation function for fluctuations in the integrated electron content along the signal path is deduced by using a theoretical relationship between this correlation function and the MCF which is based on the assumption that the irregularities are “frozen”. Further, using a power-law spectrum to describe the electrojet irregularities, the outerscale l_o associated with the spectrum as well as the r.m.s. density fluctuation are estimated from theoretical fits to the computed values. The irregularity drift speeds V_o transverse to the signal path, for the scintillation events studied here, are derived from power spectra of weak scintillations. On the basis of a relationship between l_o and V_o suggested by a linear theory of the gradient-drift instability, the effective Hall conductivity is estimated to be about five times the effective Pedersen conductivity in the electrojet region.     

The Eulerian-Lagrangian transformation in two-dimensional random flows

The relation between the Eulerian and the Lagrangian correlation functions is studied in two spatial dimensions. Simple analytical expressions for the full space-time varying Eulerian correlation are derived solely cm the basis of the one-dimensional wavenumber power spectrum of the velocity fluctuations. It is demonstrated that an extension of the arguments giving the foregoing results allows also for derivation of analytical expressions for the Lagrangian autocorrelation function. The results are supported by direct numerical solutions of the non-dissipative Euler equations for the fluctuating velocity. The calculations are initialized by a flow with equilibrium spectral energy distributions. The Lagrangian correlation function is obtained by tracing a large number of passively convected test particles.     

Consistency of rocket and radar electron density observations : implication about the anisotropy of mesospheric turbulence

We report about a quantitative comparison of rocket observations of electron density fluctuations and simultaneous 53.5 MHz radar measurements that were obtained during the MAC/SINE campaign in northern Norway in summer 1987. Out of three rockets launched during the Tur-bulence/Gravity Wave salvo on 14 July 1987, two were flown during conditions that allowed a detailed investigation. For a large part of the data from these rocket flights it is found that the radar reflectivity is about 10 dB, enhanced over what would be expected from the rocket observations in the case of isotropic electron density fluctuations. The observations can be reconciled under the assumption of an anisotropic turbulence. Assuming a simple model spectrum for the electron density fluctuations, we derive a relation between the rocket and radar observations that covers the whole range from isotropic turbulent scatter to Fresnel scatter at horizontal density stratifications. For the observed dataset, an anisotropy which typically corresponds to a ratio of the horizontal to the vertical coherence length of about 10 is consistent with the comparison of rocket and radar observations. A similar anisotropy is found also from the observed aspect sensitivity of the radar echoes. The variation of the anisotropy with height and time shows an anticorrelation with the turbulence level of the mesosphere as deduced from the spectral width of the radar echoes. The anisotropy is found to maximize in heights where the electron density displays deep ‘bite-outs’. These depletions in the electron density were independently observed by a Langmuir and an admittance probe on board two of the rockets.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Intense turbulence in the polar mesosphere : rocket and radar measurements

Simultaneous observations of polar mesospheric summer echoes (PMSE) have been made with two different frequency radars during the launch of a sounding rocket designed to measure the fluctuations in the electron density in the same height range. The cross-section for radar backscatter deduced from the rocket probe data under the assumption of isotropic turbulence is in reasonable agreement with the measured signals at both 53.5 MHz with the mobile SOUSY radar and 224 MHz with the EISCAT VHF radar, which correspond to backscatter wavelengths of about 3 and 0.75 m, respectively. Some controversy exists over the relative roles of turbulent scatter vs specular reflections in PMSE. A number of characteristics of the data obtained in this experiment are consistent with nearly isotropic, intense meter-scale turbulence on this particular day. Since equally compelling arguments for the importance of an anisotropic-type mechanism have been presented by other experimenters studying PMSE, we conclude that both isotropic and anisotropic mechanisms must operate. We have found the inner scale for the electron fluctuation spectrum, which corresponds to the diffusive subrange for that fluid, and have compared it to the inner scale for the neutral gas. The latter was found from the Kolmogorov microscale, which in turn depends on the energy dissipation rate in the gas. We found the dissipation rate from the spectral width of the 53.5 MHz backscatter signal and from the rocket electron density fluctuation data. The diffusive subrange was found to occur at a wavelength a factor of about 10 times smaller than the viscous subrange. This corresponds to a Schmidt number of about 100. High Schmidt numbers have been reported in recent measurements of the diffusion coefficient of the electrons in this height range made with the EISCAT incoherent scatter radar. About 15 min after the rocket flight an extremely high radar reflectivity was found with the SOUSY system. We have been able to reproduce this high level theoretically by scaling the rocket data with an increase in the neutral turbulence energy dissipation rate by a factor of 14 as deduced from the SOUSY spectral width, an increase in the electron density which is consistent with riometer data, and a 33% decrease in the electron density gradient scale length which is hypothesized. We also estimate the radar reflectivity at 933 MHz and conclude that signals in excess of thermal scatter levels would have occurred at the peak of the event studied, provided that the electron fluctuation spectrum decreases as k⁻⁷ in the viscous subrange. If the spectrum has an exponential form, however, a turbulent source cannot explain the enhanced 933 MHz echoes reported by EISCAT.     

Ionospheric tomography: an algorithm enhancement

The Applied Research Laboratories, The University of Texas at Austin (ARL: UT), conducted a computerized ionospheric tomography (CIT) experiment called the “Mid-America CIT Experiment”, or MACE'93, from June through December, 1993. Portions of the data from this experiment have been input into various CIT algorithms to reconstruct a two-dimensional electron density image of the ionosphere. One of the CIT algorithms relies on model ionospheres to incorporate a priori information into basis functions that are used to represent the unknown ionospheric electron density structure. When global ionospheric models are used to generate the basis model ionospheres for this algorithm, the reconstructed ionospheres often underestimate the F₂ peak density and some reconstructions display incorrect latitudinal electron density variation. These erroneous reconstructions are mainly caused by the global ionospheric models incorrectly modeling the ionosphere in terms of the F₂ region thickness and latitudinal variations. This paper identifies an alternate set of basis ionospheres that improves the accuracy of the reconstructed ionospheres. In general, this enhancement has reduced the average per cent difference between actual and CIT reconstructed ionograms to 6% and the average absolute error between measured and CIT reconstructed f₀F₂ to 2.6%.     

Electromagnetic wave propagation in a field-aligned-striated cold magnetoplasma with application to the ionosphere

The problem of wave propagation in a cold magneto-ionic medium with plane or circular cylinder small scale random density fluctuations is considered. By including second or terms in the perturbation it is possible to derive a refractive index for a “mean” wave in the medium. It is shown that the striations cause new cutoffs and resonances to occur. The validity of the theory for a warm plasma will depend on the plasma temperature and on the spatial spectrum of fluctuations. Possible applications are considered.     

Simulation of Spatially Variable Seismic Underground Motions in U-Shaped Canyons

This paper focuses on the study of simulations for spatially variable seismic underground motions in U-shaped canyons. First, a canyon ground cross-coherence function based on commonly used coherence function models of flat terrain, is deduced and presented. To further obtain the underground cross-coherence function, a mathematical expression, including its specific deduction process for describing the relationship between coherence functions of multi-support ground and underground motions, is also given in detail and adopted. Then, the key factors (i.e. canyon underground power spectrum density and canyon underground coherence function) for simulating the spatially variable seismic underground motions are obtained by a two-step transferring method from flat-ground to underground soil. Furthermore, a program is coded for generating the spatially variable seismic underground motions. The effectiveness of the generated seismic motions is further verified. Finally, two numerical examples are taken to validate the proposed simulation approach, illustrating the specific characteristics of canyon coherence function. The analysis results show the apparent differences of the simulated seismic motions using the canyon coherence function from those using conventional coherence function models of flat terrain. The proposed approach provides some insights for anti-earthquake analysis of soil-structure interaction or underground structures in canyon topography.     

Drift analysis of random and quasiperiodic scintillations in the ionosphere

Radio signals in the VHF/UHF range from the geostationary satellite ATS-6 were recorded using a system of three spaced antennas at Slough. Simultaneously, the integrated electron content (TEC) was monitored between the satellite and ground. Full correlation analysis and similar fade techniques were used to deduce the drift velocities of irregularities responsible for random and quasiperiodic (QP) ‘ringing’ scintillations. Similar drift velocities were found for the disturbances responsible for both types of scintillations at the times when QP and random scintillations occurred in a sequential pattern. A southward-drifting disturbance was responsible for rare, multiple QP scintillations which were followed by large scale fluctuations in electron density. In general, QP-scintillation-producing irregularities drifted southward, with velocities whose median magnitude and azimuth were 64 m s⁻¹ and 178°E of N respectively.The sequential occurrence pattern of QP-random scintillations as well as the time delay between occurrences of large fluctuations in TEC and QP scintillations, appear to be consistent with a reflection model of generation of the ringing fading of the signal.     

Acceleration of electrons in the ionosphere under the action of intense radio-waves near electron cyclotron harmonics

The effect of electron acceleration by intense ionospheric plasma turbulence induced by a high-power radio wave is studied theoretically, under conditions when the turbulence frequency is nearly equal to harmonics of the electron-cyclotron frequency. The turbulence located in a thin layer at the region of the pump-wave upper-hybrid resonance gives rise to the formation of an intense electron distribution function tail at high energies. Given the resonantly absorbed fraction of the pump-wave energy and the typical plasma turbulence scale, one can estimate the turbulence energy density and calculate the modified electron distribution function.     

Optical Coherence Tomography for the non-invasive investigation of the microstructure of ancient Egyptian faience

Optical Coherence Tomography (OCT) is a non-invasive subsurface 3D imaging technique based on the Michelson interferometer. The non-invasive nature of OCT and its speed of acquisition makes it possible to image large volumes of intact objects to yield a complete overview of the microstructure. The production methods for ancient Egyptian faience were first investigated using scanning electron microscopy (SEM) imaging of the microstructure in polished sections and microprobe analysis of the composition of the glass phases. These studies were based on original Egyptian faience objects and laboratory reproductions of faience beads made using three different production methods. The microstructure of the same laboratory samples and the Egyptian faience objects from the British Museum Research Laboratory Collection are re-examined using OCT. It is found that OCT virtual cross-section images can be used to group ancient faience objects into three categories on the basis of the morphology of the surface glaze layer and the glaze/core interaction layer. The OCT images correspond well with SEM images of polished sections of ancient faience objects and laboratory reproductions. The virtual cross-sections produced by OCT are somewhat limited by the penetration depth, which is affected by the high absorption coefficient of the material and, therefore, cannot always provide information on the presence or absence of interparticle glass that binds together the quartz particles in the core. Nevertheless, the top two layers are well imaged and since OCT images can be obtained rapidly and without the necessity of removing a sample, the method can be applied to a very much wider range of ancient faience objects than is possible by SEM examination of polished cross-sections. In summary, this paper examines to what extent OCT can assist the investigation of the production techniques of ancient Egyptian faience.     

Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 1. Theoretical model

A simplified full-wave method adapted to the propagation of very obliquely incident LF radio waves is developed. For a selected ionosphere model the wave-field structure is calculated inside a horizontally stratified ionosphere and the peculiarities of the reflected field are clearly described. The penetration of the investigated radio waves in the lower ionosphere at noon-time is found to be restricted to a layer several wavelengths thick. The reflected wave is created entirely by the mechanism of partial reflections and the region responsible for its formation is usually below 70 km. The influence of some typical parameters of the electron density profile, as well as the atmospheric pressure and temperature, on the attenuation of the investigated radio waves is demonstrated. It is also found that the reflection at very oblique incidence depends mainly on the height of the bottom of the ionosphere.     

Correlation between the seasonal trends of planetary F2 layer ion density and intensity of the ring current

The paper reports a high degree of correlation between the seasonal trends of planetary F2 layer ion density and the intensity of the ring current. No interpretation is offered about the reasons or significance of this intriguing correlation in the present paper, but its possible implications are discussed.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

MF and HF propagation characteristics of ionospheric ducts

The propagation of MF and HF radio waves along ionospheric ducts is studied in detail by developing a waveguide model of the ducts and determining power levels of ducted echoes. The Earth's magnetic field is included, making the propagation medium anisotropic, and influencing the ray paths through field curvature. Electron density gradients, along and transverse to the field lines are considered, and both circular and elliptical duct cross-sections are treated. Propagation characteristics of the ducts are determined by ray tracing using Haselgrove's equations. Comparison with analytical solutions for simple waveguides is used to elucidate the effects of field line curvature and focussing. Distributions of returned echo power have been calculated for ducts with various cross-sections and different depletions in electron density. It is found that field aligned ducts with diameters of the order of several kilometers behave as effective waveguides for both direct and conjugate ducting modes. However, the percentage depletion required for guiding is higher than previous calculations using simplified theory. The field line curvature causes most power distributions to be shifted upwards from the duct centre. It also causes ducting to be confined to the upper boundary of ducts, especially if they are elongated in the vertical direction. The variation of power across a duct can be quite complicated and distinctive. For direct ducting, there is a clear relationship between the total integrated power across a duct and the electron density gradient and propagation frequency.     

Polychromic structures and pigments in Guangyuan Thousand-Buddha Grotto of the Tang Dynasty (China)

This paper presents a multi-analytical study of the polychromy in the Guangyuan Thousand-Buddha Grotto. Samples taken from 15 locations in No. 512 and No. 689 caves are prepared as cross-sections for the analysis by optical microscopy (OM) and scanning electron microscopy coupled with backscattered electron imaging (SEM-BSE). The cross section analysis indicates that some paintings were repainted in the past. The chemical composition of each painting layer is obtained by scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX). The main pigments for 15 samples including green, red, blue, white and black paint layers, are identified by the combination of polarized light microscopy (PLM), Fourier Transform infrared spectrum (FT-IR), Raman spectroscopy (RS) and X-ray diffraction (XRD) analysis. The integrated analytical results reveal that the green pigments are malachite and atacamite; the red color is attributed to minium, hematite and cinnabar; the blue pigments are lazurite and organic blue materials; the white color is ascribed to anglesite and gypsum; and the black surface of polychrome is the accumulation of longtime smudging by folk burning incense in the devotional practice or the soot deposition resulting from burning bonfires in the caves. Some arsenic-containing pigment is detected in the green samples. This case study also demonstrates the advantages and the limitations of every analytical technique for the pigment identification, confirming the necessity of the integrated analytical techniques approach. The present results are not only useful to assist in the authenticity of the used pigment materials and history of the polychromic in the past, but also aimed at guiding the conservation scientists in taking into account materials and methods utilized in the past.     

D-region electron densities observed by incoherent-scatter radar during auroral-absorption spike events

Electron density profiles in the night-time auroral ionosphere were obtained with the incoherent-scatter radar at Chatanika, Alaska, during short duration precipitation events characterized by riometer data as spike events. The measurements show exceptionally large electron densities in the D-region during spike events, the electron density typically exceeding 10⁶ cm³ at 90 km altitude for a short time. The existence of a steep horizontal gradient, particularly on the poleward edge of the event, is inferred. The altitude and thickness of the absorbing layer are deduced. It is shown that 20–40 keV electrons make the greatest contribution to an absorption spike and that the spectrum of electrons producing such an event is probably softer than that producing a more slowly varying absorption peak. These absorption layers are too high for their altitudes to be measured by the technique of multi-frequency riometry.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.