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.     

Coherent-array HF Doppler sounding of traveling ionospheric disturbances: I. Basic technique

We present an introduction to the use of phase-coherent, multi-receiver HF Doppier sounding arrays for measuring the horizontal velocity of traveling ionospheric disturbances (TID's). The point of departure is the theorem of Pfister (1971, J. atmos. terr. Phys. 33, 999) relating ray Doppler to ray zenith angle for a monostatic full reflection sounder. Retaining the simple model of a specular, smooth ionospheric reflector which is deformed by a propagating undulation, we first generalize the theorem to bistatic sounding geometry and then include the effects of amplitude in addition to phase. Next, these results are cast into an algorithm for treating multi-receiver phase sounders containing many diverse baselines, in order to obtain an accurate and unambiguous solution in the plane of wave slowness (inverse of velocity). The point spread function of this solution is controlled by process bandwidth and by array geometry. We illustrate the coherent-array approach using data from an eight receiver array during passage of a TID.     

Coherent-array HF Doppier sounding of traveling ionospheric disturbances: II. Assessment of errors

We examine errors which are likely to occur in the compact-array HF sounding technique for determining the velocity of traveling ionospheric disturbances. The errors arise from ionospheric irregularities (either large-scale specular facets, or fine-scale scatterers) and from the presence of multiple disturbance undulations propagating simultaneously with different velocities. The errors are either systematic (that is, not causing internal inconsistencies within the array velocimetry data) or random (that is, causing quantifiable inconsistencies between alternative velocity estimates furnished by the array). The random errors can be directly assessed using array data, as is illustrated for actual data from a 16-channel array.     

Ionospheric plasma modification in the vicinity of a spacecraft by powerful radio pulses in topside sounding

A mechanism of strong turbulence is proposed for interpretation of the resonances observed by a wide-band receiver during topside sounding. The turbulence is created in the vicinity of the spacecraft due to the striction modulation instability. Experimental results obtained with the aid of a wide-band receiver on board the Intercosmos-19 satellite are discussed in terms of strong wave-wave and/or wave-particle interaction, namely electron acceleration in Langmuir cavitons and non-linear generation of the electron Langmuir plasma waves and Tonks-Dattner resonances.     

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.     

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.     

Passage of a powerful HF radio wave through the lower ionosphere as a function of initial electron density profiles

The results of numerical modelling of powerful HF radio wave propagation through the ionosphere plasma are presented. Comparison of the heating wave parameters with those of a low powerwave gives the possibility to study the self-action of the powerful HF wave. At low altitudes the ‘translucence’ of the ionosphere plasma takes place. At high altitudes the wave absorption sharply increases. Both self-action effects lead to the reduction of the altitude range of the heated region. The dependence of self-action effects and the resulting electron temperature profiles on the initial electron density profiles are studied.     

Equatorial HF radio wave absorption measurements and the IRI

A modification to the International Reference Ionosphere (IRI-79) electron density profile between altitudes of about 80 km and the peak of the E-region is proposed for compatibility with equatorial HF absorption measurements for Thumba, India. This is tested against independent absorption data for Colombo, Sri Lanka, and Ibadan, Nigeria.     

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 use of the phasor display in studying ionospheric radio echoes

The phase and amplitude of a radio pulse reflected from the ionosphere usually vary during the pulse. It is convenient to observe these variations using the X-Y mode of an oscilloscope to display the phasor of the echo. The variations are then seen as an oval or spiral shape traced out by the end point of the phasor. These shapes provide a sensitive method of detecting the presence of more than one echo, and are useful as a measure of dispersion.     

A scintillation theory of fading in long distance HF ionospheric communications

Scintillation theory is used to study the fading of radio waves returned from the ionospheric F-region in the HF band at oblique incidence. For both low and high penetration frequencies calculations are made as a function of wave frequency, but emphasis is given to behaviour near the maximum usable frequency for one-hop F-transmission over distances up to 4000 km. Estimates are made of

• 1.(i) the fluctuations of phase both for long-term (~ 1h) and for short-term (~ the fading correlation time),
• 2.(ii) the fading correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the twinkling correlation distance,
• 5.(v) the quasi-period of twinkling,
• 6.(vi) the angular departure of the arrival azimuth from the mean and
• 7.(vii) the correlation bandwidth.

The slow fluctuations often experienced in long distance HF radio communications are a manifestation of twinkling, rather than fading. Under normal conditions in the F-region the correlation bandwidth is so large that the bandwidth of transmission at HF is controlled by the dispersive properties of the ionosphere, rather than by the scattering properties. The reverse is true in the presence of spread-F. For sufficiently strong spread-F the correlation bandwidth for long distance HF radio communications is of the order of only 10 Hz, thereby creating the phenomenon known as flutter fading. Theoretical estimates of the scintillation parameters are compared with experience in long distance HF ionospheric communications over the last 60 years. Agreement is promising, but future experiments should be designed in the light of theory; existing observational data do not provide some of the information needed.     

Multiple beam observations of mid-latitude ionospheric disturbances by the MU radar

The pulse-to-pulse beam steerability of the M U radar of Kyoto University enables us to observe multiple beam positions simultaneously. Based on 560 h of this type of data, we present two typical patterns of mid-latitude ionospheric disturbances and their horizontal traveling characteristics. Wavy structures have not been found in large-scale disturbances. Isolated disturbances travel primarily southward (equatorward) in disturbed conditions, while no preferred direction is observed in quiet conditions.     

The effect of travelling ionospheric disturbances on the group path,phase path,amplitude and direction of arrival of radio waves reflected from the ionosphere

A fixed frequency amplitude modulated transmission was reflected from the ionosphere and changes in the group path, phase path, signal amplitude and directions of arrival of this transmission caused by travelling ionospheric disturbances were measured. These measurements enabled approximate determinations of the horizontal wavelength, period and horizontal phase velocity of the disturbances, which were compared with the theory of disturbances for atmospheric waves. A simple model is proposed to explain the phase relationships between the TIDs observed in the group and phase paths, and the faster decrease in power of the phase path than the other measured parameters indicated by spectral analysis.     

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.     

Ionospheric modelling in support of single station location of long range transmitters

Position estimates derived from a large data base of bearing and elevation angles of signals from distant HF transmitters have been analysed, with a view to comparing the validity of available ionospheric models and to examining ionospheric limitations to the accuracy of single station location of such transmitters. In general, the accuracy of the position estimates is almost entirely controlled by a limited ability to model in sufficiently accurate detail the ionospheric effects on the signal propagation. Median miss distances for those cases with a reliable identification of the propagation mode were about 7% for both E and F2 propagation for all models considered. Difficulties were encountered with the International Reference Ionosphere, which failed to support the observed propagation in half the F2 propagation cases. Standard deviations of the bearing errors were about 0.5° for E modes and 0.7° for F2 modes and were largely attributable to the effects of the ionosphere and not to instrumental errors     

Detecting travelling ionospheric disturbances in incoherent scatter data using the maximum entropy method

Two different methods of determining autoregressive coefficients in Maximum Entropy Method (MEM) were compared, investigating power spectra of synthetic signals consisting of three sinusoids and white noise of relatively high level. Spectral line shifts due to the initial phases changes were evaluated. Burg's method was chosen for determining power spectra of experimental signals. Simulating the incoherent scatter data obtained with EISCAT, it was shown that the increasing noise and wave attenuation effects are difficult to separate. Integrating the ME spectral density it was found that, in many cases, it is impossible to reproduce the true power of each harmonic of the signal.     

Solar cycle and seasonal variations in the efficiency of solar flares in producing sudden ionospheric disturbances

The seasonal variation of the efficiency of solar flares in producing sudden field anomalies (SFAs) on 164 kHz, reported by Letfus and Nestorov (1977), has been investigated for the complete 20th solar cycle. This variation persists with one exception of the solar cycle minimum, when the efficiency considerably decreases. The results of Mitraet al. (1964) giving threshold solar radio flux for an SID occurrence have been found to be incorrect.