A nonlinear maximum entropy method for spectral estimation applied to incoherent scatter radar measurements: application to synthetic incoherent scatter data

The EISCAT measurements are based on the autocorrelation function technique. An alternative approach is to derive the ionospheric parameters from an estimate of the power spectrum for the received radar signal. We have used a nonlinear maximum entropy method to deduce parameters from the power spectrum of a generated signal. A comparison has been made with parameters obtained by the ACF method. This preliminary study suggests that the spectrum method could become useful, especially for determining the position of peaks in the incoherent scatter spectrum.     

Reconstruction of travelling ionospheric disturbances parameters by a tomographic method

A tomographic method is proposed to reconstruct parameters of the inhomogeneous ionosphere on the basis of model representations using angles of arrival or total electron content. With such an approach, determining the function of the inhomogeneous ionosphere implies finding a set of parameters of TIDs. Suitable signals can be generated by satellites.     

Mid-latitude range spread and travelling ionospheric disturbances

This paper first discusses some early results (most hitherto unpublished) on off-vertical reflections which result from tilted isoionic contours associated with the passage of travelling ionospheric disturbances (TIDs). Azimuth of arrival and zenith angle information on these returning signals is discussed together with the role of these signals in producing both resolved and unresolved range spread on ionograms. Some ray tracing through a model ionosphere which incorporates wavelike structures is shown to predict fixed-frequency patterns (on recordings of virtual height vs. time) of converging and diverging satellite traces similar to those observed in practice. New experimental evidence is then presented to suggest (from the N(h) analyses performed) that TID wavetrains of several cycles are effective in producing spread traces on ionograms by specular reflections from the tilted surfaces of each cycle of these wavetrains. Ionograms from a modern ionosonde show that range spread consists primarily of discrete satellite traces even though at times these traces overlap and vary in intensity as a function of frequency thus creating a diffuse range spread ionogram.     

The effects of travelling ionospheric disturbances on ionograms

The numerical synthesis of ionograms by ray-tracing in an analytic two-dimensional ionosphere is simplified by a technique which avoids the usual wastage of ray tracings. The technique is particularly suitable for obtaining ionogram sequences corresponding to a moving ionospheric disturbance. It is applied firstly to a moving tilted ionisation increase, which gives the familiar travelling-cusp records. Ionograms do not give a good measure of the vertical distribution of ionisation, and in the case considered lead to an overestimate of the horizontal size of the increase by a factor of about 2. For a travelling ionisation decrease or trough, the ionograms show an additional U-shaped trace overlapping the main trace. Calculated ionograms agree closely with some observations. It is difficult to obtain any measure of the size of the electron-density decrease; critical frequency scalings may give no indication of the passage of an intense disturbance. Similar results are obtained for a wavelike perturbation in an exponential topside ionosphere. Topside ionograms then show multiple ‘nose’ traces, following in general the curve for the unperturbed ionosphere. It is suggested that the multiple echoes frequently observed on topside ionograms may, in some cases, be due to refraction in large disturbances rather than the commonly-assumed ducting mechanism.     

Horizontal velocity dispersion of medium-scale travelling ionospheric disturbances in the F-region

Daytime observations of the horizontal velocity dispersion of medium-scale travelling ionospheric disturbances (TID) near the F2 peak height have been carried out using an array of HF Doppler sounders in central Japan. Cross-correlation analysis of sample records has shown that the horizontal trace velocity is a decreasing function of the period of fluctuations in the range 13.3–40 min. The theoretical dispersion of the atmospheric gravity waves is also calculated using Klostermeyer's (1974) method. Comparison between the observed and the calculated results suggests the possibility that the components of the lower period of the observed velocity dispersion may be a remnant of the quasi-evanescent mode pertinent to lower-height levels.     

The spatial coherence of travelling ionospheric disturbances at mid-latitudes

A multifrequency HF Doppler sounder and four spaced receivers were operated near Alma-Ata to form a three-dimensional array of reflection points of HF radio waves. The spacings of reflection points ranged from 5 to 80 km in the vertical and from 30 to 65 km in the horizontal. The purpose of the experiment was to estimate the spatial coherence of travelling ionospheric disturbances (TIDs). Estimation of the coherence length (the distance at which the coherence falls to e⁻¹) in both vertical and horizontal planes is carried out. The coherence often shows peaks at frequencies exceeding the Brunt-Väisälä frequency. Measurements of the slant coherencies have given the opportunity to study the coherence as a function of orientation.     

HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52°32′N 1°8′W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360° in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the northern hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130–140° relative to the wind. A periodic variation of the direction of wave propagation with respect to the anti-wind direction is evident, which may indicate that lower atmospheric winds can have a greater influence on waves at thermospheric heights than previously supposed.A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at mid-latitudes.     

Source regions of medium scale travelling ionospheric disturbances observed at mid-latitudes

The characteristics of medium scale travelling ionospheric disturbances (MSTIDS) have been determined from observations carried out between 1972 and 1975 at Leicester U.K. (52°32′N, 1°8′W) using the HF Doppler technique. By reverse ray tracing through a model atmosphere an estimate of the source locations of these waves can be obtained. Auroral sources do not appear to represent an important generation mechanism for MSTIDs observed at mid-latitudes. The majority of waves originate at tropospheric altitudes at ground ranges of less than 1500 km from the observation point, and a moderate correlation is found between the occurrence frequency of MSTIDs and the intensity of the meteorological jet stream.     

Tropospheric gravity waves and their possible association with medium-scale travelling ionospheric disturbances

Analysis of pressure fluctuations observed over a period of several days using an array of microbarographs has shown the existence of long trains of gravity waves with two or more waves often present simultaneously. Meteorological data from radiosonde ascents indicates that many of the waves have a velocity which matches that of the background wind at some level within the troposphere. Generally this height corresponds to that of a frontal zone marking the transition between air masses and it is suggested that the waves may have been generated by shear flow instability within the frontal layer. Theoretical considerations, based on a three-layer model troposphere, show that some of the observed waves could have been ducted in or near the frontal zone. Some evidence is found to indicate that a non-linear wave-wave interaction between pairs of waves occurring simultaneously in the frontal zone could yield secondary waves with the characteristics of the gravity waves which had been observed in the thermosphere at appropriate times and whose group paths were traced to source regions in the troposphere in the general vicinity of the microbarograph array.     

On medium scale travelling ionospheric disturbances as seen through total electron content power spectra

Modulation phase of 140 MHz with respect to 360 MHz signals from ATS 6 satellite recorded at Slough (51.5°N, 0.6°W geographic latitude and longitude; 54.3° geomagnetic latitude) at one-minute interval are power spectral analysed to derive dominant periodicities corresponding to Travelling Ionospheric Disturbances. From the significant peaks in the spectra, an occurrence peak at periods between 10 and 15 min and a secondary peak at 60–65 min are seen. From cross-spectral analysis of the same records from three stations separated by a few hundred kilometers for a short period, the speed and azimuth of the propagating disturbances are determined. During the day-time, most of the waves in the period range 30–100 min are seen to propagate at azimuths of 90–160°. At night-time they propagate poleward. Theoretical computations of the azimuth response of TEC to typical gravity waves, including the effect of neutral winds, show that the observed azimuths of propagation are in reasonably good agreement with theoretical predictions.     

A comparison of ionization densities determined from spacecraft and incoherent scatter radar data

By comparing direct measurements taken from onboard Atmosphere Explorer spacecraft (AE), in eccentric orbit, with incoherent scatter radar (ISR) measurements taken from the ground, we illustrate both the merits and the difficulties involved in such comparisons. Five altitude profiles of ionization determined from AE, in near coincidence with ground stations making ISR measurements, compared favorably with the ISR data so long as the AE measurements were properly analyzed for the effects of variations in latitude and solar zenith angle along the spacecraft orbit.     

Refraction distortions of transionospheric radio signais caused by changes in a regular ionosphere and by travelling ionospheric disturbances

This paper generalizes experimental data on variations of the angles of arrival of transionospheric radio signals caused by changes in a regular ionosphere and by effects of medium-scale travelling ionospheric disturbances (TIDs). The data are based on radio astronomical observations of discrete sources and compact active features on the Sun as well as on angular measurements of signals from artificial Earth satellites with geostationary and circular orbits.The experimental data are interpreted through calculations of refraction corrections using a Gaussian model of a regular ionosphere disturbed by a three-dimensional travelling wave (the TID model) as well as an adaptive model of a regular ionosphere. Some possibilities of correcting refraction distortions with the use of appropriate models and ionospheric diagnostic tools are discussed.     

Simultaneous observations of travelling ionospheric disturbances by two-dimensional radio interferometry and the differential Doppler technique applied to satellite signals

The main object of the campaign reported here was to compare TID characteristics obtained from two essentially different observation techniques: (1) observation of the apparent angular position shifts of Virgo A by the Nançay radioheliograph (47.33°N, 2.15°E) gave azimuths and periods of travelling ionospheric disturbances (TIDs); (2) differential Doppler shifts of signals from NNSS-satellites recorded simultaneously at Tours (47.35°N, 0.70°E), Nançay and Besançon (47.32°N, 5.99°E) provided azimuths and latitudinal wavelengths. Observations were made during the period 10–30 November 1987, between 6 and 12 h UT. It is found that azimuths obtained from the two techniques are consistent if sufficient averaging over wave trains is performed: averaging over several hours for radio interferometry and averaging over the whole satellite trace for the differential Doppler technique. Averaging is necessary because of (1) the intrinsic dispersion in wave azimuth, (2) the broadness of observed wave spectra and the dispersive properties of gravity waves, and (3) the spatial separation of ionospheric points for the two techniques. Good agreement between the azimuths was achieved by setting the altitude of the TIDs, which is used in the differential Doppler analysis, to about 250 km, appreciably lower than the maximum in electron density (about 350 km). The mean azimuth of observed TIDs was 12° East from South with a standard deviation of about 30°. The dominant period and horizontal wavelength of the observed TIDs were 40 min and 450 km. The East-West coherence length of the TIDs was found to be only of the order of 200 km.     

An incoherent scatter experiment for the measurement of particle collisions

An incoherent scatter experiment is suggested where a VHF-and an UHF-radar simultaneously measure in an identical scatter volume. This paper discusses possible applications and, in particular, the estimation of collision frequencies.     

Gravity wave detection in the lower thermosphere with the French incoherent scatter facility

Very few gravity-wave measurements are available in the 100 to 120 km altitude range of the atmosphere. This paper illustrates the capability of the French multistatic incoherent scatter facility to fill this altitude gap in medium scale gravity-wave studies. We have been able to detect a particular gravity-wave and to compute its horizontal characteristics: horizontal wave-length: 51km, azimuth: 294°, period: 15 min. Its associated vertical energy flux at 105 km was equal to 0.4 erg cm⁻² s⁻¹. This flux shows the importance of medium scale gravity-waves in the lower thermosphere equilibrium.     

Range ambiguity effects in Barker-coded multipulse experiments with incoherent scatter radars

A Barker-coded multipulse is the best of the classical modulations when high spatial resolution is needed in incoherent scatter measurements. Unfortunately, this type of modulation generates range ambiguities which reduce the quality of data. In this paper, range ambiguity functions for an EISCAT experiment, ESLA-T4, are displayed. A method of eliminating the ambiguities in data analysis is developed and its applicability is demonstrated. It is found that the method can remove the effects of the range ambiguities in the lower part of the E-region, but above 120 km altitude the correction is not sufficient. The residual of the fit is reduced by an order of magnitude. The changes in the temperatures are a few Kelvin at the lowest altitudes and a few tens of Kelvin between 110 and 120 km.