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.     

The diffraction of VLF radio waves by the Antarctic ice cap

Measurements of the amplitude and phase of VLF radio signals from the Omega transmitters on La Reunion Island and in Argentina have been made on routine Antarctic re-supply nights from Christchurch, New Zealand. It has been found that when the propagation paths to the transmitters cross the Antarctic ice cap, the direct path signals are very rapidly attenuated below the receiver noise level, the dominant signal source then being provided by the radio waves diffracting around the edge of the ice cap. These results have been made possible by the simultaneous use of the phase and amplitude data in a synthetic aperture antenna type analysis.     

ELF/VLF propagation measurements in the Atlantic during 1989

The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N. Results of the amplitude measurements in the Atlantic of natural ELF radio signals and those from the VLF navigation system “Omega” at its lowest frequency of 10.2 kHz are given. Characteristics were obtained of the moving ship as the field-site for the ELF observations. Variations in the ELF radio noise amplitude recorded at tropical latitudes agree with the computed data for the model of three continental centres of lightning activity. The VLF results were obtained by the “beat” technique providing the simplest narrow-band amplitude registration. Range dependencies of the field amplitudes from A (Norway), B (Liberia) and F (Argentina) stations have been analysed. The VLF attenuation factor was estimated for the ambient day conditions along the four cardinal directions. This allowed the detection of a statistically significant attenuation difference between the east-west and west-east propagation paths. The VLF radio signal was also used as a probe to evaluate the effective height of the vertical electric antenna and to calibrate the ELF noise amplitudes.     

VLF propagation parameters derived from sferics observations at high southern latitudes

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectral energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German Antarctic von Neumayer Station from January to June 1983. This analyzer recorded sferics from distant lightning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data are evaluated, and the propagation characteristics of the atmospheric wave guide are determined as a function of azimuth and season. The result is compared with theoretical calculations. It is shown that the difference between west-to-east and east-to-west propagation is much smaller than theory predicts, indicating that the ionospheric D layer at high southern latitudes behaves less anisotropic with respect to VLF propagation than at mid-latitudes.     

Experimental studies of daytime LF radio propagation and their implications for D-region electron densities

Measurements are presented of interference phenomena in amplitude and phase of VLF and LF signals along propagation paths from central England to the Norwegian Sea. The data are interpreted by means of the ‘wave-hop’ propagation theory, incorporating full wave evaluation of ionospheric reflection coefficients with realistic D-region models. No published electron density profiles are found which completely satisfy the experimental data, but modified profiles are presented which provide a better fit to the observations.     

VLF radiation generated by a loop antenna in P2 layer of the ionosphere—2. Interpretation of the measurements

The results of an experiment on radiation and reception of VLF waves with a frequency of 5 kHz in the F2 layer of the ionosphere are compared with theoretical calculations made in the framework of the linear theory of a loop antenna in a plane-stratified plasma. The measurements made outside the Storey cone can be explained in the framework of the linear theory by the proper modification of the ionosphere model. The wave polarization differs from circular polarization on the boundary of caustic cone, due to non-linear effects.     

Experimental estimates of electron density variations at the reflection height of VLF signals

To study the behaviour of the electron concentration at the reflection level of very low frequency (VLF) waves, two years of phase and amplitude records of the 12.9 kHz signals emitted from Omega-Argentina (43.20°S; 294.60°E) and received at Tucumán (26.90°S; 294.70°E) have been used. The experimental results are compared with values derived from the International Reference Ionosphere model (IRI-79). The experimental data show a seasonal variation not predicted by the model. Differences are explained in terms of changes of night-time atomic oxygen concentration, which control the electron density profile at the base of the night-time D-region, not taken into account in the IRI model. Values of atomic oxygen necessary to explain VLF data are comparable with published data.     

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.     

Characteristics and frequency of occurrence of Trimpi events recorded during 1982 at Sanae,Antarctica

‘Trimpi’ amplitude perturbations on VLF signals received at Sanae, Antarctica, have been identified using a new computerised technique. Our survey of 1982 data, taken during magnetically disturbed times, shows that events of short duration (<25 s) constitute 60% of all events detected and that all events found are amplitude attenuations with deviations from quiescent levels ranging up to 90%. It is unusual, at Sanae, to observe the causative whistler with a Trimpi event. This, together with further evidence from Trimpi occurrence statistics, may suggest that the gyroresonant interactions responsible for some of the events occur with non-ducted whistler mode waves. A method for estimating the extent of the precipitation region is presented.     

Lower ionosphere effect observed during the 30 June 1992 total solar eclipse

VLF radio signals (12.9 kHz) transmitted from Ω-Argentina (43°12′S, 65°24′W) were received in Atibaia, Brazil (23°11 'S, 46°33'W) during the total solar eclipse of 30 June 1992. The surface path of the totality crossed the VLF propagation path in the sunrise transition period causing a phase delay of 6.4 μs and an amplitude change of 1.3 dB. The ionospheric response to the Sun's obscuration was compared with the phase delays reported for several solar eclipses that occurred from 1966 to 1979. The results are mainly discussed in terms of the length of VLF propagation path affected. Some similarities between a sudden phase anomaly and a reversed eclipse effect are also raised.     

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.     

Size and location of lightning-induced ionisation enhancements from measurement of VLF phase and amplitude perturbations on multiple antennas

A 600-km array of five Trimpi receivers (“elements”) has been set up in New Zealand broadside to the VLF (22.3 kHz) transmitter, NWC, some 6000 km west, with element separations varying from 8 km to 550 km. Although such a five-element array is inadequate for imaging of lightning-induced ionisation enhancements (LIEs) by VLF holography, or inverse scattering, estimates of LIE size and location can be made if the shape and form of the LIE can be guessed or assumed, with even fewer elements. With five elements, tests of the assumed model can be made as well.Owing to its transform properties, the simplest model to use for scattering inversion is the Gaussian LIE distribution. For this model, and for single mode propagation, an inversion process is derived here for the full range of LIE and path dimensions, ranging from those for which the receiver is in the diffraction far field to those in which “geometric optics” dominate. This inversion process has some validity for small LIEs of other shapes of simple form. For more extreme models, the dominance of geometry or diffraction can usually be established in individual cases which then allows simple scaling procedures to be used in scattering inversion.Some 70 Trimpi events were observed on all five elements during a single night in July. 1991 (late winter). These were used to determine LIE location and size, and to test the applicability of various LIE models. It was found that most LIEs that night occurred over the Tasman Sea near the great circle from the VLF transmitter, NWC, to Wellington, generally some 500 to 2000 km from Wellington, and with north-south dimensions of 100–250 km. Much longer east-west dimensions (oriented towards NWC) are suggested to account for the very strong Trimpis observed. While about half of these LIEs that night could have had a smooth lateral spread (e.g., Gaussian), the remainder required varying degrees of fine structure, from “flat” or Butterworth LIEs to multiple LIEs as might be expected from multiduct whistlers, to explain the observed diffraction pattern exhibiting maxima and minima as well as the wide angular range over which simultaneous Trimpis were observed.     

Use of VLF transmissions in the location and mapping of lightning-induced ionisation enhancements (LIEs)

Lightning-induced ionisation enhancements (LIEs) are usually produced by short (~ 1 s) bursts of energetic electrons precipitated from the radiation belts in the process of amplifying whistlers. During their short life (~ 30 s) LIEs diffract or otherwise modify stable transmissions of VLF waves propagating in the two-dimensional Earth-ionosphere waveguide. This causes perturbations (‘Trimpis’) on the same time scale in the phase and amplitude of these VLF waves. Unless the LIEs are large (> 100 km) and smoothly varying (e.g., Gaussian distribution of ionisation enhancement) in the horizontal directions, the LIEs need not be on the great circle path (GCP) from VLF transmitter to receiver to produce Trimpis. Large and smooth LIEs produce ‘GCP Trimpis’, while small or structured LIEs produce ‘echo Trimpis’. The two can usually be distinguished, if Trimpi phase and amplitude are monitored and if the Trimpis are observed at several frequencies or on two or more spaced receivers simultaneously.If only GCP Trimpis are considered, the causative LIEs can be located and mapped by geometric optics using a network of receivers of sufficient density (spacing ~ 100 km) and a few transmitters. Provided all Trimpis are identified as GCP, their mere detection is sufficient for location. This is equivalent to locating the LIE ‘shadows’ cast onto arrays of spaced receivers by two or more transmitters. If this GCP identification is not made, or is just assumed, location and mapping (size estimation) errors can be quite large. At VLF (λ ~ 15 km) this geometric optics approach cannot be used to study the horizontal fine structure of LIEs since LIEs producing GCP Trimpis have no fine structure.Small or structured LIEs cast a diffraction pattern onto an array of spaced receivers. If both the phase and amplitude perturbation of echo Trimpis are measured at each receiver of the array, holographic techniques can be used to reconstruct the two-dimensional map or image of the causative LIEs. It is shown that, for a single system of one transmitter and a receiver array, this allows high resolution (~ 10 km) in the azimuthal dimension only. Equally high resolution in both horizontal dimensions can be achieved with two orthogonal systems. This technique works equally well on GCP Trimpis to map the causative LIEs (which are large and structureless) without incurring location errors thereby.     

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.     

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.     

Wind and temperature effects on F-region medium-scale gravity waves estimated using a multi-layer atmospheric model

Diurnal variations in the propagation direction of atmospheric gravity waves, and the travelling ionospheric disturbances to which they give rise, have been observed in many experimental observations and several modelling studies have demonstrated that this is primarily due to the corresponding diurnal rotation in the direction of the thermospheric wind. Other variations have been attributed to seasonal or other effects, but the effects of variations in the thermospheric temperature have not previously been analysed in detail. We present results from a study of the propagation of gravity waves through a layered atmosphere in which the thermospheric wind and temperature are derived from a three-dimensional time-dependent model. The analysis has been carried out for a range of wave speeds and periods, and for a range of times, seasons and propagation azimuths. Results suggest that a significant diurnal variation in the transmission coefficient for waves propagating through the thermosphere exists with seasonally dependent maxima. Transmission increases for increasing wave period up to about 50 min, after which it remains approximately constant. Maximum transmission occurs for wave phase speeds around 200–250 m/s and falls to zero for speeds less than about 100 m/s. An exception to this rule occurs for waves with periods less than 40 min and speeds less than 50 m/s for which significant transmission appears to be theoretically possible.     

Computation of whistler wave normals using a combined matched filtering and parameter estimation technique

A new method is presented for computing the wave normal directions of deterministic VLF signals that can be approximated by plane waves. The a priori information about the frequency-time behaviour of the signal is exploited by a matched filtering and subsequent parameter estimation technique. The method was developed basically to determine wave normal directions of whistlers from wave field components measured on-board. After an outline of the method, results obtained for simulated whistlers are presented that demonstrate the superiority of the new method over the cross-product and Means methods.     

Reluctant collaborators: Argentina and Chile in Antarctica during the International Geophysical Year, 1957–58

This paper suggests that Argentine and Chilean participation with the Antarctic section of the International Geophysical Year (IGY) of 1957–58 can be characterized as ‘reluctant collaboration.’ It then reflects on the implications of the nature of South American involvement for the history of the Antarctic section of the IGY, especially as it led to the negotiation of the 1959 Antarctic Treaty. During the initial planning stages of the IGY, both Argentina and Chile worried that proposals for widespread scientific research in the Antarctic continent would interfere with their contested claims to political sovereignty in the region. Yet despite their initial skepticism, Argentina and Chile found themselves swept along by the current of international scientific co-operation of the IGY, which persisted into the Antarctic Treaty negotiations of 1959. Within the limited options available, Chile and Argentina were able to negotiate an outcome that was not altogether unfavorable to their political interests. At the same time, their collaboration helped to legitimize both the IGY and the subsequent Antarctic Treaty.     

Small-scale fluctuations of magnetic and electric components of the ELF and VLF wave fields in the sub-auroral topside ionosphere : stochastic characteristics of the wave field

When the Interkosmos-14 and Interkosmos-19 satellites crossed the region of spatially varying electron concentration in the topside ionosphere adjacent to the high-latitude boundary of the main ionospheric trough, it was discovered that there were simultaneous fluctuations of plasma density, temperature and the amplitudes (H_x and E_y) of the ELF and VLF radio/plasma emissions. The probability characteristics of the naturally perpendicular H_x and E_y fluctuations are analysed. The correlation coefficient R(_H, E_y) turned out to be less than 0.6 at frequencies of F ⩽ 4.65 kHz, while at higher frequencies R increases, up to 0.9 at 15 kHz. The following interpretations are proposed:

1. 1.1. While measuring noise emissions, as a rule a mixture of numerous elementary waves is recorded.
2. 2.2. At frequencies exceeding the local lower hybrid resonance frequency (in our case f_LHR ≈ 5 kHz), a mixture of electromagnetic waves experiencing the influence of the inhomogeneous electron concentration N_e is registered.
3. 3.3. At frequencies which are lower than the local value f_LHR the mixture mainly consists of ELF waves. The wave field has a complicated structure, and the dynamical coherence between electric and magnetic field components is not as simple as at VLF frequencies (f ≈ 15 kHz).
4. 4.4. It is shown that the wave components for a mixture of electromagnetic and electrostatic waves (for instance a mixture of VLF and lower hybrid frequency waves) have a lower correlation coefficient because the electrostatic waves are unrelated to the electromagnetic waves.
5. 5.5. The correlation analysis offers an opportunity to detect the presence of waves of various types in the wave mixture.

     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.