Modelling of transequatorial propagation of HF radio waves

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

Simultaneous generation of electron temperature and density fluctuations by high power HF radio waves

The parametric interaction between right-hand circularly polarized electron cyclotron waves as well as non-resonant density and temperature perturbations is considered by taking into account the radiation pressure and the differential Joule heating nonlinearities. A nonlinear dispersion relation, which admits a new class of thermal parametric instabilities for the case in which Joule heating nonlinearity far exceeds the radiation pressure, is derived. It is found that the temperature and density fluctuations are rapidly driven when the pump frequency is close to the electron gyrofrequency. The relevance of our investigation to enhanced density and temperature fluctuations due to the action of high power HF radio waves in the Earth's ionosphere is pointed out.     

EISCAT observations of large scale electron temperture and electron density perturbations caused by high power HF radio waves

In this paper EISCAT observations of the effect of artificial modification on the F-region electron temperature and electron density during several heating experiments at Tromsø are reported. During O-mode heating at full power (ERP = 240 MW) the electron temperature is increased by up to 55% of its ambient value at altitudes close to the heater interaction height. Measurements of the electron density have revealed both enhancements and depletions in the vicinity of the heater reflection height. These differences are indicative of variations in the balance between the transport and chemical effects. These results are compared with a time dependent numerical model developed from the perturbation equations of Vas'kov and Gurevich [(1975) Geomagn. Aeron.15, 51]. The results of numerical modelling of the electron temperature are in good agreement with the EISCAT observations, whereas there is less good agreement with regard to electron density.     

HF Doppler observations of gravity waves during the 16 February 1980 solar eclipse

Observations by the HF Doppier technique of the ionospheric effects of the 16 February 1980 solar eclipse in Africa are presented. Some oscillations which are detected at two stations can be attributed to a travelling coherent structure. Its characteristics are consistent with a gravity wave generated by the eclipse.     

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.     

Analytic properties of the whistler dispersion function

The analytic properties of the dispersion function of a whistler are investigated in the complex frequency plane. It possesses a pole and a branch point at a frequency equal to the minimum value of the electron gyrofrequency along the path of propagation. An integral equation relates the dispersion function to the distribution of magnetospheric electrons along the path and the solution of this equation is obtained. It is found that the electron density in the equatorial plane is very simply related to the dispersion function. A discussion of approximate formulae to represent the dispersion shows how particular terms can be related to attributes of the electron density distribution, and a new approximate formula is proposed.     

Investigation by ray-tracing of the effect of a summer-winter asymmetry on whistler ducting

A ray-tracing model of the inner magnetosphere (L < 6) is constructed for a plasma distribution asymmetric about the equatorial plane, thus representing summer and winter conditions in the two hemispheres. At the reference height of 900 km, the oxygen ion concentration and electron density are taken to vary by factors of ten and two respectively between the hemispheres. The concentrations of hydrogen and helium ions at the reference level are chosen to ensure electron density continuity across the equatorial plane. The altitude at which ducts terminate is modelled to differ between the two hemispheres in accordance with the numerical simulations of Bernhardt and Park (1977).It is shown that the different plasma distributions in the two hemispheres affect the paths of ducted rays and consequently the likelihood of the reception of one-hop whistlers in the conjugate hemisphere. The difference in final latitude between propagation in the symmetric and asymmetric models for the same initial latitude is largest when ducts extend down to 300 km altitude in the conjugate hemisphere. When ducts terminate at greater altitude, the effect of a difference in termination heights between the two hemispheres generally has a larger effect than that of the plasma asymmetry. Both these effects may play a role in determining the seasonal variation of whistler occurrence.     

A scintillation theory of the fading of HF waves returned from the F-region: receiver near transmitter

Scintillation theory is used to study the fading of HF radio waves returned from the ionospheric F-region to a receiver close to the transmitter. Estimates are made of

• 1.(i) the fluctuations of phase both for long term (∼ an hour) and for short term (∼ a fading correlation time),
• 2.(ii) the correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the angular divergence in the direction of arrival around the zenith and
• 5.(v) the correlation bandwidth.

The calculations are made as a function of wave frequency for two ionospheric penetration frequencies representative of high day-time values and low pre-sunrise values. Results are compared with observations of fading made with ionosondes over the past 40 years. Precise comparison is rendered impossible by omissions in the experimental data caused by lack of guidance from scintillation theory. Nevertheless, agreement is promising. When fading is deep but spread-F-region is not well-developed there is a slow modulation of the fading. This is what, for optical propagation in the troposphere, is called twinkling. The slow fluctuations observed by Bramley and Ross in the HF band constitute ionospheric twinkling. Calculated quasi-periods of twinkling range from about an hour down to about a couple of minutes, while calculated quasi-periods of fading range from about a couple of minutes down to about a tenth of a second.     

Characteristics of ionospheric ELF radiation generated by HF heating

This paper describes new observations of the characteristics of ELF generation produced by modulation of the dynamo current system from HF heating of the ionospheric D-region. A model of the ELF antenna structure embedded in the D-region is described and stepped ELF frequency observations are shown to support the model assumptions. Presented are data on the phase height of the ELF ionospheric antenna versus ELF frequency, polarization of the downgoing wave and relationship to the dynamo current direction, correlation of ELF field strength with per cent cross-modulation, power linearity tests and duty cycle results. All observations used the high power heater facility of the Arecibo Observatory.     

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.     

Magnetic pulsation generation by a powerful ground-based modulated HF radio transmitter

The Hall and Pedersen ionospheric conductivity disturbances produced by high power amplitude-modulated radio waves as a function of altitude for different initial electron density profiles are calculated. Disturbances of the integral Hall and Pedersen conductivities as well as the artificial magnetic pulsation amplitudes for different initial electron density profiles and different magnetic pulsation periods are calculated too. Some recommendations for carrying out heating experiments for artificial magnetic pulsation generation are proposed.     

On the influence of a plasma hot component on whistler propagation beyond the plasmapause

Theoretical spectrograms are computed for whistlers propagating beyond the plasmapause. The electron distribution function was modelled as consisting of a hot plus a cold component and an appropriate dispersion equation is used. A collisionless (CL) model is used for the cold electron concentration and for the hot electron component the derived model assumes a bi-maxwellian distribution function with a loss cone at the equator. The results indicate limits on the use of the cold plasma approximation (c.p.a.) in the study of magnetospheric whistler propagation beyond the plasmapause and show that whistler analysis with the c.p.a. may under or overestimate the L value of the path deduced from ground spectrograms, depending on the anisotropy of the hot component.     

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.     

The influence of ionospheric absorption on mid-latitude whistler mode signal occurrence from VLF transmitters

Whistler mode signals from VLF transmitters received at Faraday, Antarctica (65° S, 64° W) during 1986–1991 show an annual variation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of changes in absorption of VLF signals in the D-region, where the high geographic latitude of Faraday plays an important role in producing low attenuation levels during the austral winter. In contrast, very little such variation was observed at Dunedin, New Zealand (46° S, 171° E) in 1991. Nighttime whistler mode signals have start and end time trends that are consistent with the influence of F-region absorption. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absorption is discussed.     

The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.     

HF produced ionospheric electron density irregularities diagnosed by UHF radio star scintillations

High frequency waves incident on an overdense ionosphere (i.e. HF < penetration frequency) are known to produce large-scale irregularities with scale sizes of several hundred meters in the F-region of the ionosphere. Three observations of radio star intensity fluctuations at UHF are reported for HF ionospheric modification experiments performed at the Arecibo Observatory. Two observations at 430 MHz and one observation at 1400 MHz indicate that the thin phase screen theory is a good approximation to the observed power spectra. However, the theory has to be extended to include antenna filtering. Such filtering is important for UHF radio star scintillations since the antenna usually has a narrow beam width. HF power densities of less than 37 μW m⁻² incident on the ionosphere produce electron density irregularities larger than 13% of the ambient density (at 260 km) having scale sizes of ~510 m perpendicular to the geomagnetic field. The irregularities form within 20–25 s after the HF power is turned on. From the observed power spectra driftvelocities of the irregularities can be estimated.     

Ionospheric modification by high power radio waves

Powerful, high frequency (HF) radio waves can be used to temporarily modify the ionosphere. These controlled, active experiments have proven useful both for studies of the natural upper atmosphere through observations of ionospheric response to HF induced perturbations, and for basic physics investigations exploiting the ionosphere as a large, natural plasma laboratory-without-walls. This experimental diversity has attracted the attention and participation of physicists from a wide range of disciplines. As a result, HF ionospheric modification research continues to be strongly motivated by its many applications in the fields of aeronomy, space physics, plasma physics, and telecommunications science.     

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.