A simple theoretical model of the diffraction of VLF electromagnetic waves by the Antarctic icecap

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 attempting to propagate over it. The propagation of Omega navigation signals around this model icecap has then been computed using Kirchhoff diffraction theory. Spherical caps extending to 66.5 and 75.5°S have been found to accurately model the signals from Omega La Reunion and Argentina, respectively, received on flights between Christchurch, New Zealand and Scott Base in Antarctica, up to the boundary of the theoretical icecap. These model icecaps were found to be good fits to the boundary of the Antarctic continent, when measured at the 1–1.5 km contour of ice thickness, in the region where the VLF waves diffracted around the icecap. The good agreement obtained between the experimental field strength data and those computed theoretically, using only simple diffraction theory, suggests that coastal refraction plays at most only a secondary role in circumpolar propagation.     

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.     

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.     

Ionospheric ELF radio signal generation due to LF and/or MF radio transmissions—I. Experimental results

A new non-linear ionospheric effect has been discovered from the analysis of ground based FLF/VLF radio data obtained in Scandinavia during 1979. Apart from signals of natural origin, timing signals (six pips which occurred on the hour) were received. The pips of frequency 1000±0.5 Hz, duration 105 ± 8 ms, and field strength ~ 0.1 pT at Sodankyla, Finland, exhibit a favoured source location ~ 150 km south-south-east of Sodankyla. A close association between the reception of these ELF pips and the auroral electrojet is demonstrated by the positive temporal correlation between ELF pip generation and periods of enhanced local magnetic activity, and also by the spatial correlation between source location and the latitude over F'inland at which riometer absorption is a maximum. In the evening and midnight sectors the latter is interpreted as indicating the electrojet position. The originating signals are shown to emanate from one or more Soviet LF/MF broadcast transmitters, all of which are several hundred kilometres or more from the favoured generation region.     

南极旅游开发与设想

本文提出,随着我国南极长城站和中山站的建立、以及"极地号"和"雪龙号"破冰船的购置,更由于人民生活水平的逐步提高,我国开发南极旅游的时机已经到来。本文认为与低层次的物质享受相比,南极旅游更有助于促进国民素质提高和全国主义、国际主义精神的培养。本文对南极旅游资源作了评价.划分出南极半岛等4个旅游区。本文构思了我国未来南极旅游组织实施的规划和设想,提出可先开辟南极半岛旅游区和环南极旅行两条旅游线路,沿途可开展海外观光、远洋考察和远洋捕捞业,以促进我国旅游业、南大洋考察和远洋捕捞运输业的全面发展。     

Travelling ionospheric disturbances and the effectiveness of powerful HF transmitters in ionospheric modification and radio location of the Moon

Using ray tracing we investigate, on a qualitative level and in the linear approximation, the effects of medium-scale travelling ionospheric disturbances (MS TIDs) arising when powerful HF radio transmitters are operated in conjunction with antenna arrays designed for ionospheric modification (heating) and for radio location of the Moon. It is shown that the HF radio wave focusing effect, arising during the movement of the MS TIDs, can give rise to a strong inhomogeneous and nonstationary modulation of the space-time distribution of the field intensity of a powerful radio transmitter both at heights near the reflection region (in heating experiments) and at the exit from the ionosphere (in radio location of the Moon). The excess of intensity over an unperturbed value for typical parameters of MS TIDs in experiments on ionospheric modification can reach values of hundreds of percent: a ‘spot’ of increased intensity of the wave field can have the size of about 1–10 km, and can move with a velocity close to the MS TID phase velocity.In the case of lunar radio location, the inhomogeneity and nonstationarity of the wave field intensity distribution at the exit from the ionosphere substantially complicates the evaluation of the corresponding distribution on the Moon's surface and the interpretation of the Moon-reflected radio signal characteristics.     

Very-low frequency signals (2–10 kHz) received at Palmer Station,Antarctica, from the 21.4 km dipole antenna at Siple Station, 1400 km distant

Radio signals transmitted from the unique experimental VLF transmitter at Siple Station (76°S, 84°W), Antarctica, as well as VLF signals from communication and navigation systems and waves that propagate in the ionosphere and magnetosphere in the whistler mode, are regularly received and analysed at Palmer Station (65°S, 64°W), Antarctica. The amplitude and polarization properties of the Siple signals are predicted using a ray optics analysis. The amplitude of the signal received from Siple varies with frequency; observed nulls in the signal spectrum, where thesignal amplitude/alls 5–10 dB below what might be expected, are explained by the ray analysis. The amplitude spectrum is observed to be very sensitive to ionospheric conditions. Whereas the arrival bearings of signals from VLF transmitters other than Siple are found to be within 5° of their expected values, which is consistent with their expected vertical polarization and the operation of the DF system, an approximately 90° anomaly in the apparent arrival bearing of the signals from Siple is attributed to the essentially horizontal polarization of the received signal. The anomaly is found to be consistent with the theory of operation of the DF system. Occasional anomalies greater than 90° are explained in terms of a combination of polarization error and a smaller multi-path error. Siple two-hop signals and whistlers propagating on a common magnetospheric path showed arrival bearings and other properties consistent with a path end point within 200km of Siple. This suggests that these signals were received at Palmer with essentially vertical polarization.     

Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere,ionosphere and atmosphere

The physical mechanism of a cyclotron resonance interaction between trapped energetic electrons and whistler-mode waves in the magnetosphere is discussed. Not only do the electrons have their pitch angles reduced in this interaction, so that they may be precipitated into the upper atmosphere, but also the waves can be amplified. Such a flux of precipitating electrons can, either by direct ionisation or via bremsstrahlung radiation, cause a pimple to be produced on the bottom of the ionosphere. That can significantly modify the amplitude and/or phase of very low frequency radio signals propagating in the Earth-ionosphere waveguide. Various experimental observations that demonstrate the reality of such effects are reviewed. The conditions necessary for a positive feedback situation are discussed, and some evidence for its existence assessed.     

Lightning-induced perturbations on VLF subionospheric transmissions

Phase and amplitude perturbations on VLF subionospheric transmissions from transmitter NWC to Dunedin have been studied on both MSK frequencies and at spaced receivers, 9 km apart. In any one event (a ‘Trimpi’) the phase and amplitude perturbation can be expressed in terms of a perturbation phasor. This is generally believed to be the result of lightning-induced electron precipitation (LEP) producing a localized increase in ionization near the normal reflection height for subionospheric (waveguide) VLF waves. Most of the Trimpis received on the NWC-Dunedin path can be best explained if the LEP ionization is sufficiently localized so that it acts as a scattering centre for the subionospheric VLF wave from the transmitter. It is then this scattered wave or echo at the receiver which makes the perturbation phasor. We call these ‘echo Trimpis’. The phase of the echo relative to the direct signal will differ on spaced antennae if the angle of arrival of the two signals differ. Similarly, this relative phase will vary with frequency if the group delay of the signals differ. Thus measurement of these differences allows location of the scattering centres, and so too the LEP. Locations made show a significant grouping in a region where the lightning intensity is high. This and other features strongly suggest that these echo Trimpis originate from local (southern hemisphere) lightning. This and other reasons are suggested to explain the high proportion of echo Trimpis on this path.     

Ground and in situ excitation of waves in the ionospheric plasma

A review is presented of seven papers the contents of which range from ULF to VLF wave excitation in the ionospheric plasma by ground-based radio wave and acoustic wave sources, to in situ plasma wave excitation by satellite- and rocket-borne radio transmitters.     

Equatorial scintillations—a review

The scintillation technique, as is well known, provides an integrated measure of phase and amplitude fluctuations imposed on radio signals over a wide range of frequencies during their propagation through the ionosphere. The large amplitude of equatorial irregularities necessitates the use of frequencies in the GHz band to obtain unambiguously the temporal variation of irregularity intensity and the effect of irregularity anisotropy. Recent observations of equatorial scintillations will be reviewed with an emphasis on GHz measurements. The steep spatial gradients observed in in-situ data and their relationship to intense GHz scintillations will be explored. Co-ordinated measurements of equatorial irregularities by such techniques as radar backscatter, in-situ rocket and satellite, total electron content and 6300 Å airglow will be discussed, insofar as they provide a better understanding of the scintillation phenomena. While it is difficult to critically assess results that are so recent and constantly evolving, we have attempted to focus attention on the outstanding problems that still remain in the field.     

Reduction of the effects of non-stationarity in studies of amplitude statistics of radio wave backscatter

By studying the statistics of fluctuations in amplitude of radio signals backscattered from the atmosphere and lower ionosphere, it is possible to obtain information about the scatterers. This procedure has been applied previously, but often relatively long data series have been used (e.g. 10–30 min) in order to produce reliable amplitude distributions. Unfortunately, the nature of the scatterers can often change considerably over such a time interval and this can distort the amplitude distributions. An alternative approach, applied in this work, is to use much shorter data sets, derive a parameter representative of each data set (e.g. the Rice parameter) and then examine the statistics of this derived parameter over a longer period. Computer modelling was used to examine the statistics of such a parameter and some surprising results emerged, even for relatively long data series. Several examples of the application of this new method will be presented.     

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.     

A simple sounder to measure the properties of ionospherically reflected radio waves

A system which measures the direction of arrival, amplitude, group path and phase path of high frequency radio waves reflected from the ionosphere is described. A CW double sideband modulated signal was used and the measurements were made over a slightly oblique path. Comparisons between the group height determined using this ionospheric sounder and an ionosonde located near the midpoint of the transmission are given and results of fast fluctuations in the measurements are 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.     

Science in Antarctica Intellectual and Physical Adventure

Abstract

Antarctica is a geographical region and Antarctic science is therefore multidisciplinary, the main areas of research being concerned with the continent's geological base, the superimposed ice-cap, the atmosphere above it and their external relationship. Each of these research areas is interdisciplinary: the study of its outstanding problem – the history of the Antarctic ice sheet – requires contributions from meteorology, oceanography, glaciology, geology, chemistry and physics; and its future behaviour will he determined by external events, including the consumption of fossil fuels on other continents. Research in Antarctica is given added piquancy by the continent's unique environment and rigorous climate which retain their attractive character even though modern technological developments have removed many elements of discomfort and danger. The region is also singular in its political nature and scientist have a particularly close relationship with their governments through the Antarctic Treaty.     

Dispersion of chirp pulses by the ionosphere

Nonlinearity of the phase time delay vs frequency of the ionospheric channel results in frequency dispersion. This distorts wideband signals and leads to amplitude reduction and ‘elongation’ of narrow pulses. Its effect on frequency modulated continuous wave signals (FMCW or chirp) is to broaden the width of the compressed pulse and to produce a chirp signal at the output of the detector instead of the ideal sine function. Several workers have studied this distortion and derived expressions which related the width of the compressed pulse to the first order derivative of the group time delay vs frequency. These expressions are limited to the case when the bandwidth of the channel is greater than the bandwidth of the transmitted chirp signal. In this paper a generalized expression for the time-bandwidth product of the chirp signal at the output of the detector is derived. Numerical calculations for the width of the compressed pulse vs its time-bandwidth product for various window functions are presented. The results are then applied to experimental data obtained over a short skywave radio link.     

TID,electron density and ionospheric sounding

For the approximation of a parabolic ionospheric layer, the relation between the fluctuation amplitude of radio phase path in HF vertical sounding and sinusoidal disturbance parameters has been found. Horizontal disturbance propagation has not been considered. The determining factors are the relation of the vertical wavelength of the disturbance to the half-thickness of the layer and between operating and critical frequency.     

A climatology of quiet/disturbed ionospheric conditions derived from 22 years of Westerbork interferometer observations

Knowledge of the quiet and disturbed conditions in the propagation medium is essential for quality control of transatmospheric radio signals. This holds equally for the troposphere and the ionosphere. This paper describes a climatology of ionospheric irregularities obtained from observations of celestial radio sources by radio interferometry, i.e. by the Westerbork Synthesis Radio Telescope (WSRT) in The Netherlands. This instrument is located at geomagnetic mid-latitude. All WSRT calibrator observations in the 22-year period 26 June 1970–31 December 1991 have been checked for manifestations of ionosopheric effects. Although seasonal effects are clear, the occurrence and ‘strength’ of ionospheric irregularities show no dependence on solar activity. Assuming that the frequency of occurrence of ionospheric disturbances in spring and autumn are similar, it is found that ‘ionospheric’ winter starts on day 348 ± 3 and all seasons last for 3 months. Medium-scale travelling ionospheric disturbances (TIDs) occur most frequently during the daytime in winter periods. The occurrence of non-periodic irregularities is, however, not a function of time in the day. The daily variation in the amplitude and frequency of the occurrence of the TIDs suggests that the solar terminator and Joule heating near the electrojets do not contribute substantially to their generation. Generation of gravity waves may be caused by winds and tides in the lower thermosphere-mesosphere. This has to be investigated further.On the basis of the available data, a ‘disturbance measure’, indicating to what extent the ionosphere is ‘quiet’, is proposed. The output of this project may be of immediate use for different ionospheric investigations, such as ionospheric modelling and the study of excitation mechanisms for ionospheric irregularities.     

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.