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.     

Association of whistlers with lightning discharges on the Earth and on Jupiter

The association between whistlers and lightning discharges has been reviewed on the basis of terrestrial ionospheric satellite observations of VLF radio noise. Evidence indicating that the observed low-latitude radio noise is associated with thunderstorms includes (1) amplitude distribution and noise properties, (2) geographical location, (3) diurnal variation in activity, and (4) diurnal variation of frequency spectrum. Corresponding studies on the propagation of sferics in the ionosphere and the excitation of whistlers recently carried out for Jupiter are presented here and compared with the terrestrial studies.     

ELF and VLF wave generation by modulated HF heating of the current carrying lower ionosphere

This paper presents further experimental results on ionospheric current modulation, using powerful amplitude modulated HF waves produced by the new heating facility at Ramfjordmoen near Tromsø, Norway. As a result of the current modulation, waves in the ULF, ELF and VLF range can be efficiently generated. The experiments discussed here cover the range from low ELF up to 7 kHz. The observed signal strengths are of the order 1 pT. Decomposition of the received ELF/VLF waves into R- and L-mode shows that both modes are usually of comparable strength. The signal strength as a function of modulation frequency shows pronounced maxima at multiples of approximately 2 kHz. The paper also presents a brief theoretical discussion of the processes involved in the generation of ELF/VLF waves by HF induced current modulation.     

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.     

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.     

VELOX: a new VLF/ELF receiver in Antarctica for the Global Geospace Science mission

VELOX (VLF/ELF Logger Experiment), a new facility for systematically studying the characteristics of magnetospherically generated ELF/VLF radio noise received at a high-latitude ground station (Halley, Antarctica, 76°S, 26°W, L = 4.3), measures continuously at 1 s resolution the absolute power (peak, mean, and minimum), arrival azimuth, and polarisation ellipticity in 8 logarithmically spaced frequency bands ranging from 500 Hz to 9.3 kHz. All filtering etc. is done in real time using Digital Signal Processing (DSP) techniques. Key parameters (1 kHz and 3 kHz power channels only, at 1-minute intervals) for each day are extracted and regularly transferred to the Global Geospace Study Central Data Handling Facility. Data from the first year of operation (1992) show that, whilst the upper channels (6 kHz and 9.3 kHz) are dominated by thunderstorm (spheric) noise, which is strongest at night and repeatable from day to day, magnetospheric chorus and hiss emissions are more important in the 1–4 kHz range of high attenuation in the Earth-ionosphere waveguide. They are highly variable in intensity from below system noise level (15–20 dB above the reference level 10⁻³³ T² Hz⁻¹) up to a maximum of 60–70 dB. Three classes of event are usually observed during specific local time sectors: substorm-related chorus events in the midnight-dawn sector, dawn chorus, and hiss-like events in the afternoon; all may occasionally be completely absent on quiet days. The substorm chorus events are shorter (typically 10–20 minutes) and more narrow-band than dawn chorus. Both upper and lower cut-off frequencies rise rapidly (∼ 100 Hz/min), consistent with the energy dispersion of resonant electrons as they drift eastward from injection near midnight, and with the inward drift, driven by substorm-enhanced electric fields, of whistler ducts which support propagation to the ground. Afternoon emission events are often punctuated by sudden deep fading, to noise level within 1–2 minutes, usually followed by complete recovery after a few minutes. All frequencies in the emission band are affected simultaneously. The explanation for this effect is unknown, though it could be a cut-off of propagation through the ionosphere to the ground by irregularities or gradients tilting the wave-normals out of the transmission cone. A similar system to VELOX will be deployed on a network of Automatic Geophysical Observatories extending to higher latitudes, south of Halley.     

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.

     

Wave form and spectral distribution of the electromagnetic field of lightning currents

In a resonant wave guide model of lightning currents, two impulse type standing waves can exist: aperiodic waves of the Bruce-Golde form (type 1), and damped oscillations (type 2). The electromagnetic waves generated by these two types of lightning currents are calculated for various distances and compared with observations. It is shown that the measured wave forms of sferics at distances smaller than about 300 km of return strokes (R strokes) as well as of intracloud strokes (K strokes) are generated mainly by type 2 lightning currents. The channel parameters like channel length and channel diameter derived from the observed sferics are 19 km and 4.6 cm, respectively, for the average R stroke, and 4 km and 1.6 cm for typical K strokes. The large values of the lengths probably correspond to the real lengths of oblique and tortuous channels, rather than to their vertical elevation. The finite electric conductivity of the earth modifies the high frequency component of the wave forms. With decreasing conductivity and/or increasing distance, the rise times of the radiation component to its first maximum increases and the maximum amplitude decreases.Typical rise times for R-strokes are about 3 μs consistent with the observations if the electric conductivity of the earth is of the order of 10⁻³ S/m. The spectral functions of the wave forms are also calculated. The spectral amplitude of the average type 2 R-stroke has its maximum near 4 kHz, and that of the type 2 K-stroke maximizes near 35 kHz. Within the high frequency region at frequencies greater than about 300 kHz. the spectral amplitudes decay proportional to the reciprocal third power of the frequency. The radiation component in the far field contains 7% (16%) of the total electromagnetic energy of type 2 R (K) strokes.     

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.     

Radio wave propagation from antennae at satellite altitudes into the Earth-ionosphere waveguide

The problem of the excitation of the earth-ionosphere waveguide by a short linear antenna or by a small circular one at satellite altitudes is considered. The formulation allows for a spherical regular wave guide as well as for a radially inhomogeneous anisotropic ionosphere. A method for the solution is based on the use of the reciprocity theorems for anisotropic media. Numerical techniques have been developed. Some results for VLF are given. To gain some physical interpretations, the fields of sources at low ionospheric heights were investigated.     

A test of the Hines dispersion equation for atmospheric gravity waves

Simultaneous observations of an ionospheric wave by two incoherent scatter facilities and three Faraday-rotation polarimeters have provided measurements of the frequency, vertical wavelength, horizontal wavelength and direction of propagation of the wave. These measured values confirm the Hines dispersion equation for atmospheric gravity waves.     

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.     

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.     

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.     

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.     

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.     

Possible radioindications of anthropogenic influences on the mesosphere and lower thermosphere

From an analysis of the variations of various ionospheric characteristics influenced by global anthropogenic effects, it is shown that the collision frequency v_en is the parameter that changes most when the ratio between the carbon dioxide plus methane and the other components is changed. Since this collision frequency is directly involved in the formation of ionospheric absorption, the latter is recommended to be regarded as the most sensitive ground-based indicator of the global ‘cooling’ of the near-Earth space. Expressions are obtained for the estimation of man-made influences by absorption measurements. For medium latitudes, we recommend the frequency range from 400 to 800 kHz as a sensitive range for these measurements.     

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.     

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.