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     

Modelling of transequatorial propagation of HF radio waves

In the geometrical optics approximation, a synthesis oblique ionogram of ionospheric and magnetospheric HF radio wave signals propagating between magnetic conjugate points has been carried out. The magnetospheric HF propagation is considered for a model of the waveguide formed by field-aligned irregularities with depleted electron density. The characteristic peculiarities of the magnetospheric mode have been determined: (i) strong disperion of the group delay with a frequency at 14–18 MHz, from − 1.4 to 0.6 ms/MHz for magnetically conjugate points at geomagnetic latitudes φ = 30°, 40° and 50°, respectively, (ii) spreading ∼ 1–2 ms, and (iii) a possibility of propagation between magnetic conjugates points at moderately low geomagnetic latitudes φ₀ ∼ 30–40° at frequencies exceeding 1.5 times the maximum usable frequency (MUF) of multi-hop ionospheric propagation.     

LHR whistlers and lhr spherics in the outer ionosphere

Special types of VLF signals, which follow whistlers and spherics and have an anomalous dispersion near the lower hybrid resonance (LHR) frequency, have been observed on the low-altitude Intercosmos satellites. These signals have been named LHR whistlers and LHR spherics, respectively. A mechanism is suggested for the formation of their spectra, based on the peculiarities of quasi-resonance wave propagation at frequencies near the LHR frequencies. It is shown that the large dispersion observed may be accounted for by a significant increase in the propagation time of the wave as its frequency approaches the maximum in the LHR frequency profile.     

Very unusual low latitude whistlers with additional traces of the Earth-ionosphere waveguide propagation effect

The present paper reports very unusual whistlers strongly influenced by the Earth-ionosphere waveguide propagation after emerging from the ionosphere, as observed simultaneosly at our two stations, Sakushima (geomag. lat. 24°) and Kagoshima (20°). These unsual whistlers are characterized by clearly exhibiting additional dispersion effects near the cut-off frequencies of the 1st and 2nd order modes of waveguide propagation and, to our knowledge, they are a new finding. All the subionospheric dispersion is deduced to occur between the ionospheric exit point and the receiver. Detailed spectral analysis, after extracting the small waveguide dispersion effect from the overall spectrum by taking the beat with the appropriate pseudo-whistler, has enabled us to determine the propagation distance of the ionospheric exit region from each station. These distances have then been used to locate the ionospheric exit region, which is found to be about 3000 km east of the stations and in the local sunrise time sector. The generation mechanism of such unusual whistlers is discussed in terms of the joint influences of the ionospheric transmission mechanism (longitudinal gradient of the ionosphere, wave scattering by density irreglarities) and magnetospheric propagation and characteristics of ducts.     

D-region electron density diurnal changes observed in association with the PCA event of July 1974

Well defined diurnal phase and amplitude deviations of very low frequency radio signals propagation were detected during the solar proton event that occurred from 3 to 8 July 1974. Phase advances observed on signals transmitted from GBR (U.K.), received at Atibaia (Brazil) and at Inubo Radio Observatory (Japan), were compared with simultaneous NWC (Australia) transmissions received at Atibaia, and at Syowa Station (Antarctica). Emphasis was given to the propagation paths NWC-Atibaia and NWC-Syowa, because both propagate completely in the southern hemisphere, crossing regions where the Earth's magnetic field behaves anomalously. The comparison allowed the determination of parameters typical of the D-region at a given height in the lower ionosphere, for geomagnetic regions defined through the McLlwain parameter(L). An exponential model was adopted to fit the vertical distribution of the diurnal electron density. The experimental results showed a delayed contribution to the total ionization observed which was attributed to a slow precipitation of energetic particles in the South Atlantic Geomagnetic Anomaly region.     

VHF transequatorial propagation via three dimensional waveguides

The three dimensional model of VHF propagation along ionospheric waveguides or ducts developed by Platt I. G. and Dyson P. L., 1989 [J. atmos. terr. Phys. 51, 897–910] has been used to study transequatorial propagation (TEP) along equatorial bubble irregularities aligned along the earth's magnetic field. Bubbles with circular and elliptical cross-sections and different apex heights have been used to determine the region illuminated in the conjugate hemisphere by TEP and the power of the signals. The results show that the three dimensional waveguide model can explain many features of TEP, including those not well explained by the simpler waveguide models used previously. The calculations therefore confirm that TEP can be caused by guided propagation along equatorial bubble irregularities aligned along the magnetic field. The results show that a bubble located on the transmitter's longitude can illuminate a wide area of the conjugate hemisphere. A well defined maximum central power region occurs which in most cases is centred at a latitude slightly lower than the conjugate of the transmitter. Bubbles with circular cross-section can illuminate a region more than ten degrees in latitudinal extent and five degrees in longitude. Bubbles which are vertically elongated at their apex illuminate a much narrower region in longitude. TEP between points with relatively large separation in longitude can result from either leaky rays or ducts tilted out of the magnetic meridian plane. This explains some observations of TEP previously thought to be inconsistent with waveguide propagation. The variation of power loss with frequency depends on a number of factors such as bubble shape and height. Co-ordinated experiments involving a transmitter and several receivers in the conjugate hemisphere should enable properties of bubbles to be determined.     

Numerical simulation of the penetration and reflection of a whistler beam incident on the lower ionosphere at very low latitude

By the full-wave algorithm with Fourier synthesis, 3-D propagation of a whistler beam incident on the pre-dawn lower ionosphere at very low latitude is numerically investigated. Processes of transmission, reflection, and coupling with the Earth-ionosphere waveguide are discussed via the wave energy and polarisation distributions and their dependence on the wave parameters and the ionospheric profile (such as the E_s-layer). It is shown that the dominant wave above 90 km altitude has the propagation characteristics of the magneto-ionic whistler mode, and absorption, spreading, reflection and mode conversion mainly occur at, and are greatly affected by, the bottom of the ionosphere. It is found that the transmitted energy density along the Earth's surface is reduced by 20 dB or more. Beam transmission loss varies asymmetrically with the incident angle, but changes little with the frequency. In the region 150 km (for 5 kHz) away from the ‘exit area’ where whistlers emerge, the bearing measurements using ground-based VLF direction-finders may be in error because direction-finding algorithms assume plane wave propagation. Only a small portion (about −25 dB at 5 kHz) of the incident energy is reflected up to an altitude of 150 km, and major reflection takes place in a small range of altitude at the bottom of the ionosphere with little spreading and lateral shift with respect to the incident beam. Reflection is enhanced considerably at lower frequency. Our results also suggest that an E_s-layer or an ionospheric gradient refracting waves to higher latitudes would be favorable factors for multi-hop echoes to be received on the ground.     

Multi-station VLF direction-finding measurements in eastern Canada

The directions of propagation, in the earth-ionosphere waveguide, of multi-component two-hop whistlers recorded on 10 July 1972 by four VLF goniometer receivers in eastern Canada have been determined. Using the bearings of these great circle paths, triangulation of several whistler exit-points has been accomplished. The L-values of the whistler exit-points determined by this method are systematically lower than those expected from their nose frequencies, by ~ 0.6. Various explanations are discussed for this effect. The most satisfactory is that the whistler waves leave through the side of the ducts (in which they had propagated for most of their path through the magnetosphere) at an altitude of a few thousand kilometres, and then are refracted to lower L-values before exiting from the lower ionosphere. The results are consistent with both the duct termination altitude predicted by Bernhardt and Park (1977) for the appropriate conditions and also with the observed upper cut-off frequency of the whistlers.     

Spatial dependence of day-time vertical polarisation of Pc 3–5 magnetic pulsations in Sri Lanka

The rate of change of the horizontal and vertical components of magnetic pulsations in the period range 20–600 s have been recorded at four stations in Sri Lanka, namely, Vavuniya, Maradankadawela, Maho and Colombo. An analysis of the records shows that the horizontal polarisation of the pulsations is predominantly along the magnetic meridian at all four stations. The vertical polarisation as measured by the ratio ΔZ/ΔH increases with increase in period and for signals in a given periodic band, except for those recorded at Maho ; there is also an increase of vertical polarisation with distance of the recording station from the magnetic equator. At Maho, there is a local decrease of the vertical polarisation at all periods probably due to anomalous electrical conductivity in this region. Although the spatial and period dependence of the vertical polarisation of the pulsations recorded at the other three stations can be explained in terms of an oscillating ionospheric current band of half-width about 105 km flowing east-west in the neighbourhood of the magnetic equator, and its image current in a uniform conducting earth, such a model may not be realistic in that it neglects possible effects due to induced currents in the ocean deflected round the coasts of Sri Lanka. It is suggested that the observed day-time polarisation may represent the effect of a wider equatorial ionospheric current system, such as the equatorial electrojet and additional effects due to induced currents in the ocean.     

Experimental observations of very low latitude man-made whistler-mode signals

VLF signals at 23.4 kHz from NPM in Hawaii (lat 21.5°N) are commonly received at night in Dunedin, New Zealand with very low group delays of between about 75 ms and 160 ms and frequency shifts of several tens of milliHertz or more. The ratio of the frequency shift to the rate of change of group delay generally agrees with the ratio which would be expected from signals which have travelled through the equatorial ionosphere. Normal whistler-mode signals with delays of 0.3–0.6 s are quite frequently observed at the same time.     

Effect of multipath and snow on millimeter wave scintillations on a 4.1-km line-of-sight link

Multipath propagation can occur for various reasons. For example, it can be due to sudden changes in the refractive index of the propagation medium or to reflections from melting snow/ice. We find that dry snow, or even ‘moist’ snow at 0°C, has no measurable effect on the propagation of radio waves up to at least 54.5 GHz. When multipath propagation occurs, the low frequency part of the scintillation spectrum is distorted. The usual theoretical predictions applicable to line-of-sight millimeter wave propagaton through clear air turbulence must be used with caution when the scintillations of the propagating signals are a result of multipath effects. It is also noted that propagation through vegetation greatly distorts the clear-air scintillation spectrum. Saturation of scintillations, which can also dramatically alter the scintillation spectrum, is not expected to be a problem for millimeter wave propagation over line-of-sight paths.     

VLF transmissions observed at anomalously high latitude above the ionosphere in the conjugate hemisphere

Ariel 3 and 4 satellite observations of the GBR 16 kHz and NAA 17.8 kHz transmissions above the ionosphere in the conjugate hemisphere show that their wave-fields generally show a rapid reduction in signal strength for geomagnetic latitudes greater than 55°–60°. Sometimes, however, the signal strength has been observed to be high in the invariant latitude range > 60°. At certain times during these observations, the signal showed clear evidence of amplification, whilst at other times the pattern of signal strength was displaced to higher latitude with the signal strength integrated over latitude being unchanged from that normally observed.It is shown that the plasmapause can guide both the NAA and GBR signals but that the efficiency of this guiding depends on the plasmapause position. The important condition is found that the plasmapause must be situated sufficiently equatorwards that half the equatorial electron gyrofrequency at the plasmapause position is greater than (or approximately equal to) the transmitter signal frequency. Ray-tracing calculations in a realistic magnetosphere model indicate that for the 16 kHz GBR signal, the efficiency of guiding falls off for L_pp, (the L-value of the plasmapause) > 3.0 and guiding effectively ceases for L_pp > 3.5.Guidance by the plasmapause results in a wave-field at higher latitude than for non-guided propagation. This will only occur when, following geomagnetic storms, the plasmapause position is at a sufficiently low L-value. This is in agreement with the experimental observations of anomalously high latitude signal reception following strong magnetic storms (K_p ≥ 4+).     

A simple theoretical model for computing omega navigation errors near Antarctica

The propagation of VLF signals around a model icecap has been computed using Kirchhoff diffraction theory. The Antarctic continent has been modelled as a spherical cap, whose pole is coincident with that of the South Pole, which totally absorbs VLF radio waves propagating over it. Using this simple model, the range errors expected, whilst travelling between Antarctica and New Zealand, on signals from Omega La Reunion and Argentina have been calculated and compared with recently derived measurements. It has been found that in order to model the measured range errors accurately it has been necessary to modify the simple spherical cap model to that of a semi-circular spherical cap, producing what is effectively knife edge diffraction. To ‘best fit’ the data, the northernmost limit of the spherical cap has been found to be 66.1°S for propagation from La Reunion and 75°S for propagation from Argentina. These model icecaps agree well with the northernmost boundary of the Antarctic continent where signals from Omega La Reunion and Argentina graze it tangentially.     

Detection of elliptical polarization and mode splitting in discrete Schumann resonance excitations

Elliptical polarization and mode splitting have been detected in the magnetic component of discrete, well defined Schumann resonance excitations. These ELF excitations, which are large electromagnetic transients of approximately l s duration, are called Q-bursts and typically occur every few minutes. They are believed to be the signature of the impulsive excitation of the Earth-ionosphere cavity by ultra-large lightning currents. In this paper the magnetic polarization and spectral characteristics of four large Q-bursts are examined in detail using a new analysis technique. Two events display right-hand polarization and two display left-hand polarization. The theoretical polarization properties of the central and side multiplets of the Schumann resonances are used to define a local orthogonal coordinate system in the measurement frame in which these components may be separated. Maximum entropy spectrums computed separately for what are identified to be the central and side multiplets in this coordinate system show distinctly different eigenfrequencies for the lowest mode near 7.5 Hz. For the limited number of cases examined the magnitude of the line splitting detected using this technique is roughly 1.4–1.8 Hz, larger by nearly a factor of two than theoretical or observed values of the splitting previously reported. The frequencies of the side multiplets may lie either above or below the frequency of the central multiplet.     

Notes on the diversity of the properties of radio bursts observed on the nightside of Venus

We report on further studies of radio wave bursts detected by the Orbiting Electric Field Detector (OEFD) on PVO in the nightside ionosphere of Venus. We have tested a total of 25 cases of wave burst activity for evidence of whistler-mode propagation to the spacecraft from impulsive subionospheric sources. As in a previous study of 11 of these cases (Sonwalkar et al., 1991), we find at least two distinct classes of events, one, mostly involving bursts at 100 Hz only, that passes certain tests for whistler-mode propagation, and another, mostly involving bursts in two or more of the four PVO narrowband channels (at 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz), that fails to pass the tests. The subionospheric lightning hypothesis continues to be tenable as a candidate explanation for many of the 100 Hz-only events, but its plausibility could be better evaluated if mechanisms could be found to explain the existence of a significant number of 100 Hz-only cases that do not pass all the applicable whistler-mode tests, as well as the existence at a wide range of altitudes of multichannel cases that are clearly not propagating whistler-mode waves. The wideband bursts are often observed at altitudes above 1000 km and frequently occur in regions of locally reduced electron density. Those observed at high altitude (and possibly at low altitude as well) are believed to be generated near the spacecraft, possibly by an as yet unknown mechanism responsible for similar burst observations made near Earth and other planets.     

The Siple VLF transmitter as a multi-frequency probe of the Earth-ionosphere waveguide

1983 receptions of subionospheric signals radiated from Siple, Antarctica (L = 4.3) to neighboring stations Palmer (L = 2.3), Halley (L = 4.3), and South Pole (Λ = 74°), each ~ 1500 km from the horizontal (magnetically east-west) VLF transmitting antenna at Siple, were found to be strongly dependent upon azimuth and upon signal frequency. At Palmer, located equatorward in the broadside direction with respect to the antenna, signals near 2.5 kHz were often well defined, while the third harmonic of the transmitted signal, near 7.5 kHz, was not detected. Meanwhile, at Halley, the third harmonic was regularly observed and directionally stable, while the fundamental was often weak or undetectable. The field strength of the third harmonic component at Halley exceeded by ~ 40dB the level of the fundamental, when both were normalized to the same antenna input power. The large size of these effects is attributed in part to antenna properties that favor the endfire direction (toward Halley) at the 3d harmonic of the antenna half wave resonance frequency, and in general provide greater efficiency at higher frequencies. Other factors are high waveguide attenuation in the 2–4 kHz range and azimuth dependent differences in the propagating modes. The observed effects represent a way of extending the effective frequency range of the narrowband Siple antenna system, and also, by using the new crossed dipole configuration at Siple, of selectively probing certain regions of the Earth-ionosphere waveguide.     

Comparison of Pc3 and Pc4 pulsation characteristics on an East-West mid-latitude chain of magnetometers

We compare the results of analysing Pc3 frequencies on an East-West chain of 4 magnetometers at mid-latitudes with the results of an earlier analysis of the same data at Pc4 frequencies. The Pc3 signals show some remarkable similarities to those at the lower frequencies. Near local midnight, when the higher frequencies are a component of Pi2 pulsations, they share the characteristic of very high coherence across the chain. At other times, Pc3 signals resemble the Pc4 band studied earlier in that the longitudinal wave number is small, and no clear diurnal propagation pattern is systematically observed but at times there is evidence of preferentially sunward phase motion in all daylight hours. By night westward propagation dominates. We conclude that our results are consistent with field line resonance theory, but not with the Kelvin-Helmholtz instability model.     

Mid-latitude spread-F structure

Spread-F has been observed at frequencies of 1.98, 3.84 and 5.80 MHz and multiple angles of arrival have been resolved using an HF radar near Brisbane (27°S, 153°E). The spreading of the ionogram trace has been shown to be due to a spread in angles of arrival of echoes, rather than any ‘vertical’ spreading. The reflection process appears to involve total specular reflection rather than scattering. The previously reported very strong bias for angles of arrival from the north-west at Brisbane is supported. The direction of movement of the reflection points is not radial and therefore, the structure cannot be purely frontal with purely linear movement, as is often supposed. The velocities are much less than for coexisting travelling ionospheric disturbances. The variations of angle of arrival, range and rate of change of range with frequency do not fit previously proposed ideas of the plasma distribution and an alternative is suggested in which the distortions of the isoionic surfaces resemble small, elongated, asymmetrical ‘hills’ or ‘dips’.     

Experimental results on satellite scintillations due to field-aligned irregularities at mid-latitudes

Experiments using multi-station networks receiving signals from the VHF beacon of a geostationary satellite have been carried out in order to clarify the geometrical factor involved in ionospheric intensity scintillations due to field-aligned irregularities. The characteristics of scintillation observed in the daytime agree with the theoretical value expected for weak diffractive scattering by ionospheric irregularities with an elongation of 10 along the geomagnetic field. However, those in the night-time show much marked enhancement along the field-line due to strong refractive scattering by irregularities having the same elongation. Finally, it is shown that the geometrical factor in scintillation at mid-latitudes can be expressed as a function of the propagation angle between the radio path and the geomagnetic field in the ionosphere. The maximum values of the geometrical factor are respectively about 5 in the daytime and 14 at night.     

Doppler shift pulsations on whistler mode signals from a VLF transmitter

Whistler mode signals from the NAA transmitter (24 kHz) received at Faraday, Antarctica are processed to obtain the Doppler shift at a much higher time resolution than has previously been possible. This has allowed the observation of pulsations of about 13 mHz frequency which are believed to be associated with hydromagnetic waves in the magnetosphere. The pulsations are observed separately on signals with a number of discrete group delay features that can be interpreted as individual whistler ducts. Using the measured pulsation phase over the array of ducts the phase velocity and wave normal direction of the hydromagnetic wave in the equatorial plane are estimated. The direction of propagation is consistent with a source on the dayside magnetopause.The association between whistler mode Doppler shifts and hydromagnetic waves has been reported before but not, as far as we are aware, using an experimental technique that allows measurements on individual ducts in order to determine the direction of propagation of the hydromagnetic wave.