The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.     

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.     

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.     

Error sources and travel time residuals in plasmaspheric whistler interpretation

In the interpretation of observed whistlers by curve fitting, systematic travel time residuals appeared which were studied by extensive simulations using ray-tracing, numerical integration and curve fitting. The residuals were found to originate from the commonly used approximations in the refractive index and ray path of whistler mode waves, which result in travel time increments or decrements, not accounted for in whistler interpretation. These approximations and the assumed form of the electron density distribution also lead to systematic errors in the diagnostics of plasmaspheric electron density by whistlers. In addition, the effects of other error sources, including random measurement errors, are also reviewed briefly.It is shown that the fine structure of residual curves is connected to propagation conditions. Thus, their study may yield a new research tool for studying whistler trapping, ducting structures and other features of whistler propagation. The application of residual analysis in conjunction with digital matched filtering of whistlers seems to be especially promising for further whistler studies.     

Parametric excitation of whistler waves by HF heater

Possible generation of whistler waves by Tromso HF heater is investigated. It is shown that the HF heater wave can parametrically decay into a whistler wave and a Langmuir wave. Since whistler waves may have a broad range of frequency, the simultaneously excited Langmuir waves can have a much broader frequency bandwidth than those excited by the parametric decay instability.     

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.     

The polarisation of whistlers received on the ground near L = 4

The observed polarisation of the horizontal magnetic components of whistler mode signals received at Halley, Antarctica (L≈ 4.3), is in many cases that expected from a simple model of the transionospheric and sub-ionospheric propagation in the southern hemisphere; i.e. right-hand elliptical (field vectors rotate clockwise, looking towards the source) for ionospheric exit points close to the receiver, tending towards linear for more distant exit points. This suggests it may be possible to use the observed polarisation to estimate the propagation distance. However, in other cases, in certain frequency ranges, left-hand elliptically polarised signals have been observed. More realistic models do predict polarisation reversals at certain frequencies and exit point to receiver distances, but not over such a wide frequency range as has sometimes been observed. Also, in some cases, signals with nearly right-hand circular polarisation have been observed for exit points at large distances where linear polarisation would be expected.     

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.     

Oblique ordinary mode propagation in the magnetospheric plasma

An approximate formula is obtained for ordinary mode refractive index for the case of almost perpendicular propagation in a hot anisotropic plasma with a loss cone. It is pointed out that the influence of electron finite temperature on wave propagation is stronger when the wave frequency is close to the first two harmonics of the electron gyrofrequency. Ordinary mode energy focusing in the direction perpendicular to the magnetic field can occur only in those regions of the magnetosphere, where the electron gyrofrequency is close to the electron plasma frequency. Ordinary mode waves are usually trapped in the vicinity of the magnetospheric equator and so their energy can be concentrated in this region.     

Wave-electron interactions induced by VLF transmissions

The five papers discussed all involve interactions between waves produced by ground-based VLF transmitters and energetic electrons in the magnetosphere. Most of these studies concern the electron-cyclotron interaction or that between whistler mode waves and counterstreaming electrons. The most important consequence of this is electron pitch angle diffusion into the loss cone, and so precipitation. New features studied are finite band input waves, skew wave normal interaction, precipitation flux-time profiles and slow growth instabilities.     

Angle-of-arrival oscillations in the mesosphere as seen by medium frequency (MF) radar

The Saskatoon MF radar operates continuously in the spaced antenna mode with 5 min resolution, primarily for horizontal wind analysis at mesospheric heights. Mean Doppler frequency and antenna-pair phase difference are available as byproducts. Clear oscillations are sometimes seen in the latter. Spectral analysis shows that these also occur in the Doppler and extend over a range of heights. The two cases shown are almost certainly perturbations in specular reflecting layers moving with horizontal velocity close to that of the wind. Downward phase propagation is apparent, and when the oscillations are present, the wind appears to be decelerating. The tentative explanation of a breaking gravity wave decelerating the mean flow is put forward, although the relations between frequency, and horizontal and vertical wavelengths, contradict linear gravity wave theory.     

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.     

Effects of land and sea parameters on the dispersion of tweek atmospherics

The appearance of tweeks on whistler sonograms has been discussed in terms of VLF wave propagation through the land-sea and ionospheric waveguide. It is shown that the conductivity of ground and sea mixed path, forming the lower surface of the waveguide, may provide an estimate of the source distance of whistlers generating atmospherics.     

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.     

Plasma wave evidence for lightning on Venus

Plasma wave data from the Pioneer Venus Orbiter provide the largest body of data cited as evidence for lightning on Venus. These data are also the most controversial, mainly because of the ambiguity in mode identification due to limited spectral information. We review some of the more recent studies of the plasma wave data at Venus, and we demonstrate that the characteristics of the 100 Hz waves are consistent with whistler-mode waves propagating vertically from below the ionosphere. We further show that in situ instabilities are too weak to generate whistler-mode waves, mainly because the thermal pressure is comparable with the magnetic field pressure in the ionosphere of Venus. The lower hybrid drift instability has also been suggested as an alternative source for the 100 Hz waves. However, the wave properties are more consistent with whistler-mode propagation; the lower hybrid drift instability requires very short gradient scale lengths to overcome damping due to collisions. We also note that an apparent association between Langmuir probe anomalies and 100 Hz waves is much lower than previously reported, once we apply a consistent intensity threshold for identifying wave bursts. The lightning hypothesis remains the most probable explanation of the plasma waves detected at low altitudes in the nightside ionosphere of Venus.     

Transmission of large-scale TIDs in the ionospheric F2-region

Large-scale travelling ionospheric disturbances (1.s. TIDs) have been investigated in order to derive the horizontal velocity dispersion by using f₀F2 data from four ionospheric observatories in Japan. It was found that the horizontal phase trace velocity lies between 300 and 1000ms⁻¹ with periods in the range 50 to 150 min. There is evidence that the derived velocity generally increases with increase of wave period. This is consistent with the dispersion predicted by the theory of the internal gravity waves. The azimuthal angles are distributed in ±35° sectors centered around 197° (measured clockwise from north), indicating that 1.s. TIDs may be obtainable when they are excited along the auroral zone of the same sector in longitude as that of the observatories. The average propagation direction shifts by 17° from south towards west. This clockwise shift is consistent with the rotation caused by the Coriolis effect. This means that the Coriolis effect cannot be ignored for the wave propagation of 1.s. TIDs. In addition to the positive correlation between TID speed and geomagnetic activity, the direction of wave propagation is found to be correlated with polar magnetic activity. The propagation direction is mostly southward during the period of large polar magnetic disturbances, while during the period of low magnetic activity the direction scatters considerably.     

Anomalous interference in Omega VLF wave propagation on east-to-west equatorial paths

The phase of Omega Haiku (Hawaii U.S.A.) signals has been measured at 13.6 and 10.2 kHz on-board ship from Tokyo to Fremantle, Australia, during the early parts of the winters of 1979 and 1980. Short-term (∼- 1 h) fluctuations are observed on the phase of the Haiku signals as received around the geomagnetic equator at a distance of 8000 km west of the transmitter. Phase cycle slippings take place frequently in association with the phase fluctuations, the occurrence frequency of which is a maximum at 6 S geomagnetic latitude. These propagation anomalies are a consequence of the passage of the ship through an interference pattern the spacing distance of which is about 11 km at 13.6 kHz —one-half of the wave-length of the transmitted wave. It is concluded that the interference is caused by the signal propagated directly from the transmitter and the long-path signal propagating a distance of 32.000 km in the west-to-east direction. This result implies that the attenruation of the east-to-west propagating Omega wave is anomalously great at the equator, in pood agreement with calculated values based on the Galejs' anisotropic waveguide model.     

The phase relationships of the ionospheric signatures of Pc1 geomagnetic pulsations

In this study we consider the phase relationships between the oscillations of various ionospheric signatures associated with Pcl geomagnetic pulsations. Investigations using a simple analytical method and a numerical model, which has proved successful when applied to longer period pulsations, both suggest that Doppler velocity oscillations should be predominantly in anti-phase with oscillations of the rates of change of group range and echo amplitude. However, observations indicate that the Doppler velocity oscillations are in quadrature with the other two types of oscillations. Possible causes for this discrepancy are suggested.     

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.