Downcoming Wave Research Articles

Vertical structure of AGW associated ionospheric fluctuations in the E- and lower F-region observed with EISCAT—a case study

The vertical structure of AGW (atmospheric gravity wave) associated fluctuations of ionospheric plasma parameters for the 100–240 km altitude range in the daytime of 7 September 1988 has been investigated by making use of the data provided by the Tromsø measurements in the EISCAT CP1 observation mode. The wave power profile vs height has been studied by integrating the power spectral density in each altitude. The essential feature of the power variation can be explained in terms of the energy conservation of AGWs propagating in a dissipative thermosphere. Intrinsic propagation parameters of the dominant AGW have been successfully estimated with a method based on the retrieval of the Doppler effect due to the horizontal prevailing wind. From the fluctuation structure analysis in a time-altitude frame, a downcoming AGW has been clearly identified. This downcoming wave might have been reflected from a wind shear at the altitude around 200 km, which is inferred from the meridional prevailing wind profile.

Comparison of shielding by infinite and finite planar grids over a lossy half space

The shielding properties of finitely extended and infinitely extended parallel thin-wire grids over a lossy half space are considered when illuminated by a downcoming plane wave whose electric field vector is parallel to the wires. Numerical comparisons of the electric field in the illuminated, transition, and shadow regions for the finitely extended case are made with the electric field in the shadow region for the infinitely extended case. The field responses of both grids in the shadow region are shown to be similar. Edge effects, however, may be significant if the field is observed near the truncation of the grid of finite extent. It is demonstrated that a reasonable approximation for the current distribution in the current region of the finite grid is the computed value determined for the infinite grid. >

On the frequency dependence of ELF/VLF waves produced by modulated ionospheric heating

We present results of the local field strength and, for the first time, apparent source height as a function of frequency from 200 to 6600 Hz of ELF/VLF waves produced in the auroral D region through modulated RF heating using the ionospheric heating facility near Tromsø, Norway. These data show, besides the normal amplitude peaks near 2, 4, and 6 kHz, the occasional presence of peaks at intermediate frequencies, especially in the right‐hand circularly polarized mode. A comprehensive model has been developed to explain the data, using a full wave solution of the ELF/VLF waves produced by the self‐consistently calculated source currents, and correcting for the effects of finite source current extent and multiple Earth‐ionosphere reflections. The model can reproduce the amplitudes and source heights for typical sweeps. The irregular peaks occasionally seen can also be reproduced by invoking interference between the downcoming wave and a partially reflected whistler mode wave from sharp electron density gradients in the ionosphere such as sporadic E layers. The model seems to work well above about 600 Hz, but below that it increasingly underestimates the measured amplitudes.

Remote sensing of the magnetospheric plasma by means of whistler mode signals

Early in the past decade of U.S. Antarctic research, the whistler method of measuring equatorial electron density was found to agree with in situ satellite electron density measurements by a radio technique. Furthermore, the whistler method of measuring the east‐west component of the convection electric field in the outer plasmasphere was found to agree, under conditions of mapping in a dipole magnetic field, with simultaneous results from incoherent scatter radar. A global model of the east‐west convection electric field in the outer plasmasphere during substorms was developed. The detection of whistlers and their use for magnetospheric diagnostics have been important elements in recent studies of burst precipitation into the ionosphere induced by whistlers and by other transient whistler mode waves propagating in the magnetosphere. Whistlers have also been used to obtain data on the L values and equatorial electron densities associated with the propagation paths of signals from the Siple VLF transmitter. The process of untrapping of downcoming wave energy from ducts in the upper ionosphere and the upward repropagation of portions of the energy following reflection in the lower ionosphere lead to the excitation of adjacent ducts as well as to upward propagation in the nonducted mode. Efficient interduct coupling has been found to occur over north‐south ionospheric distances of >1000 km. Studies of the outer limits of observed ducting revealed dayside path radii in the range 6–8 RE and nightside limits of ∼5.5 RE. Ducted propagation beyond the plasmapause was found to occur regularly in the 0000–1800 MLT time range, but with variable rates and at various locations with respect to the plasmapause position. The special features of this propagation are believed to be related to conditions of lightning excitation, ionospheric penetration, and wave‐particle interactions that are special to the region beyond the plasmasphere. New aspects of Siple wave injection experiments were demonstrated by the application of a new phase measurement method to Siple signals that did not exhibit fast temporal growth during passage through the magnetosphere. This method, a refinement of techniques developed previously by New Zealand workers, is capable of detecting fluctuations in phase path with period of ∼10 s and greater and thus can be used to study magnetospheric convection and coupling fluxes along field lines of propagation as well as pulsations associated with ultralow‐frequency perturbations of the geomagnetic field. Additional topics discussed include results from direction‐finding experiments and evidence of the dependence of whistlers upon magnetospheric wave amplification.

Whistler leakage from narrow ducts

Low altitude satellite observations of a variation in the upper cut-off frequency of whistlers with latitude (at constant altitude) can be convincingly explained in terms of different whistler frequencies leaving a duct at different altitudes. This selective leakage is explained by a downcoming wave escaping from a duct at an altitude where its wavelength becomes approximately equal to the duct width. This altitude is shown to decrease with increasing wave frequency. Leakage according to this mechanism is shown to result in a minimum duct width at a given altitude for ducted guidance of VLF waves. It is found to be consistent with the distribution of duct widths inferred from whistler observations made aboard the low altitude satellite ISIS 1.

Effect of spread F irregularities on whistler wave propagation at low latitudes

By employing small‐scale field‐aligned irregularity models, ray paths are computed for VLF whistler mode waves to examine whether in the absence of true VLF ducts in the low‐latitude ionosphere such small irregularities, which are responsible for spread F, may significantly influence the propagation of VLF waves so that the waves may reach the ground. The results show that an irregularity of 10 km thickness and amplitude 5% brings the wave normal direction of downcoming waves almost vertical at the base of the F region ionosphere such that the waves may penetrate the lower ionosphere and reach the ground.

Detection of stimulated brillouin scattering by the Jicamarca Radar

Observations of weak stimulated Brillouin scattering with the 50‐MHz incoherent scatter radar at Jicamarca are described. The nonlinear interaction of the upgoing radar wave with the downcoming waves, incoherently backscattered from greater heights, causes an asymmetry in the incoherent ‘ion’ backscatter spectrum. For a given antenna aperture this asymmetry increases with increasing power and decreasing frequency of the radar. The theoretically predicted effect was unambiguously verified, using 3‐ms pulses and long integration time. Model calculations based on the theory presented show good agreement with the observations.

A Discussion on the early days of ionospheric research and the theory of electric and magnetic waves in the ionosphere and magnetosphere - Early work in Australia, New Zealand and at the Halley Stewart Laboratory, London

Ionospheric research began in Australia in 1927 after the formation of the Radio Research Board. A. L. Green, by measuring polarization of downcoming waves travelling in the opposite direction to the Earth’s magnetic field confirmed that electrons were the effective particles. Builder, Pulley and Wood designed equipment for the automatic recording of critical frequencies. Martyn & Pulley found evidence for high temperatures at F region levels. Munro discovered travelling ionospheric disturbances. In New Zealand the earliest measurements were made by Munro in 1927—8. The New Zealand Radio Research Board later supported the measurement of critical frequencies, absorption and collisional frequency of Peddie, White, Banwell and Straker. Australian and New Zealand postgraduate students contributed to Appleton’s group at King’s College and at the Halley Stewart Laboratory, London. Builder introduced the pulse technique and took part in the Polar Year (1932-3) expedition to Tromso. Pulley designed the first manual ionogram equipment. Both returned to work in Sydney. White measured reflexion coefficients and collisional frequencies of electrons and later returned to Canterbury University, New Zealand. The early ionospheric researches sponsored by the Australian and New Zealand Research Boards had a profound influence by expanding university research and the training of many postgraduate students.

A study of the power spectra of dPi's and Pi 2's

A study of the power spectra of dPi's observed between geomagnetic latitudes 64° and 54°N shows that they exhibit clear harmonic structure. The fundamental period is predominantly in the range 300–360 sec and linear harmonics are observed up to the 9th or 10th. The 53 events studied showed no latitude dependence of fundamental period, but it was clear in some cases that the spectral energy was redistributed among the harmonics at the lower latitude stations. A similar study of Pi 2's showed much less harmonic structure in their spectra. There is evidence of a relationship between the trigger which initiates the event and Kpindex (by inference solar wind intensity) and the results are interpreted as strong support for the view that dPi's are the complete manifestation of the nighttime impulsive event and that they are triggered, as transverse free oscillations, on the ‘last closed field lines’ in the premidnight sector of the magnetosphere. It is inferred that when they are observed at low latitudes it is due to the down-coming wave (transmitted across the meridian plane) causing forced oscillations of lower latitude lines of force. Secondary resonance at lower latitudes accounts for the change in spectral distributions observed and for the apparently unconnected Pi 2 spectral forms.

The reflection of radio waves from ionospheric irregularities, at vertical incidence

When a radio wave is reflected at vertical incidence from a horizontally stratified ionosphere which contains random irregularities of electron density, the downcoming wave consists of a cone of rays apparently scattered back to the receiving point by the irregularities. However, there has hitherto been no clear description of how the scattering occurs. It is suggested here that the oblique rays are partially or totally reflected in the backwards direction from irregularities situated at or near the apex of the ray trajectories. The width of the angular spectrum of the downcoming wave is determined by the intensity ΔN/ N of the irregularities. ΔN/ N is deduced to be of the order of 0.01. The rays received at the ground appear to come from a virtual diffracting screen located at the virtual height of reflection. p ]When field-aligned irregularities are present, reflection occurs where the rays are perpendicular to the elongation. This restricts the angular spectrum in the meridian plane and produces a north-south elongation of the ground pattern with an axial ratio r = cosec I, where I is the magnetic dip. An explanation is given of why a ground pattern produced by transmission is more elongated than one produced by reflection.

Fading Characteristics of Radio Waves in the E-Region of the Ionosphere over Ahmedabad and Thumba during IQSY

ABSTRACTThe present article describes the characteristics of fading of the radio waves reflected from the ionosphere over Ahmedabad and Thumba during the period January-December 1964. It is found that the rate of fading at Thumba (geog. lat. 8°32'N, geog. long. 76°52'E) is higher than that at Ahmedabad (geog. lat. 23°01'N, geog. long. 72°36'E). The probability distributions of amplitudes at Ahmedabad follow either Rayleigh or Gaussian law whereas at Thumba neither of these is observed; in most of the cases it is exponential. The semi-angle of the cone of down-coming radio waves is larger at Thumba than that at Ahmedabad. In general the fading characteristics at Ahmedabad and Thumba are very different in nature.

Anomalous variations in phase path records

Simultaneous records of amplitude and phase path of ionospherically reflected pulsed radio waves received on an E− W aerial under periodic fading conditions show sudden discontinuities in the phase variations, when the amplitude is a minimum. Under random fading conditions also, the sudden discontinuities, frequently observed in the phase variations, are always accompanied by an amplitude minimum. It is shown that when two signals with a phase path difference changing continuously are received simultaneously, the resultant phase of the combined signal changes rather slowly, and shows a discontinuity, when the difference passes through an angle of 180°. The phase path variations recorded at points separated by about 100 m, sometimes show variations in opposite directions. This has been interpreted as due to the changes in the direction of arrival of the downcoming waves.

Speed of Fading of Radio Waves Reflected from the Ionosphere over Ahmedabad

ABSTRACTFading records obtained on 2·6 Mc/s. by the method of closely spaced receivers have been used to measure horizontal drifts in the ionosphere over Ahmedabad. In this paper the data for the period March 1960 to February 1961 have been analysed to calculate the semi-angle, θ, of the down-coming waves and the approximate size of the ionospheric irregularities. It is shown that a linear relationship between the fading rate and drift speed holds good for E, Es and F region reflections. Histograms showing the statistical distribution of θ are drawn for day-time and night-time observations and for different seasons. These results are compared with results obtained elsewhere.

Statistical analysis of fading of a single down-coming wave

The paper deals with the statistical analysis of fading of a single wave reflected (vertically) from the F-region of the ionosphere. These fading records are usually random and their amplitude distribution has been found by other workers to be Rayleigh, Gaussian or log-normal. The present analysis shows that the amplitude distribution is Rayleigh only in the case of rapid fading, whereas for slow and quasi-periodic fading it is found to represent what can be termed as an M-type. The distribution of successive differences in the amplitudes has also been studied; for a Rayleigh amplitude distribution this time analysis gives rise to a Type-VII distribution of Pearson as originally pointed out by Mitra (1949b) while in the case of M-type amplitude distribution it becomes Gaussian. Following Mitra's analysis, the r.m.s. value of the random velocity, ν 0 of the ionospheric irregularities has been calculated from the time analysis. The value of ν 0 has also been calculated from the autocorrelation coefficient of the amplitude R following Booker et al. (1950). The two values of ν 0 thus obtained are compared and are found to agree well.

Polarization of electromagnetic waves for vertical propagation in the ionosphere

A theoretical study of the variation of the state of polarization of a vertically incident electromagnetic wave while in propagation in the ionosphere has been made on the basis of an approximate solution of the wave equations obtained by Saha, Banerjee, and Guha. It has been shown that the major axes of the polarization ellipses of both the ordinary and the extraordinary waves would lie in the N–E quadrant in the northern hemisphere and in the N–W quadrant in the southern hemisphere. A new method has been outlined for the determination of the electron density and the collision frequency in the ionized layers from the value of the tilt-angle and the ratio of axes of the elliptic patterns. An analysis of the characteristics of the experimentally observed polarization patterns indicates that in E layer the value of v is 1.7×106 per second. They further show that the polarization of the downcoming waves corresponds to their respective reflection levels, rather than a limiting region below the E layer.

The Effects of Anisometric Amplitude Patterns in the Measurement of Ionospheric Drifts

In certain observations of a downcoming radio wave, made for the purpose of measuring horizontal drifts of ionospheric irregularities, time shifts between fading curves obtained at three spaced receiving points are used to calculate the velocity and direction of drift of the amplitude pattern over the ground. Two simplifying assumptions about the pattern are frequently made, namely (i) the detail remains constant as it moves, and (ii) it is statistically isometric, i.e. the separation of two points for which the amplitudes have a given fixed correlation is independent of direction. The errors that arise if assumption (i) is wrong have been considered in a published paper. But a more general analysis, not relying on either assumption may be applied to the three fading curves, and the result expressed (either analytically or graphically) as the error of a calculation which makes the two assumptions. One important result is that if assumption (ii) is not valid the error in direction of drift may be quite serious. Thus, in an application to amplitude patterns produced by vertical reflection at 2.4 Mc/s (principally from the E layer), errors in individual results averaged 15°. Because the structure of the pattern varied from sample to sample, the error was not, in this case, systematic. But, in some applications, a preferred direction of the pattern structure could, if ignored, lead to wrong conclusions about preferred directions of drift.

The ionospheric propagation of radio waves of frequency 16 kc/s over short distances

The paper describes experiments carried out at Cambridge on waves of frequency 30?65 kc/s reflected from the ionosphere at steep incidence. The phases and amplitudes of two linearly polarized components of the downcoming wave were measured with reference to the ground wave. The results for frequencies of 30, 43 and 65 kc/s are considered in turn, and compared with the results for 16 kc/s. The main conclusions were as follows: (a) The day-to-day variations of the downcoming wave were greater at the higher frequencies. (b) The change of height of reflection in passing from day to night was about the same for all the frequencies. (c) In summer, the amplitudes by day and by night were very different, except at 16 kc/s. (d) The polarization at all the frequencies was approximately circular, left-handed and constant. The height of reflection of waves of frequency 30 kc/s was determined by using the frequency-change method of Appleton and Barnett.

The ionospheric propagation of radio waves of frequency 30–65 kc/s over short distances

The paper describes experiments carried out at Cambridge on waves of frequency 30–65 kc/s reflected from the ionosphere at steep incidence. The phases and amplitudes of two linearly polarized components of the downcoming wave were measured with reference to the ground wave. The results for frequencies of 30, 43 and 65 kc/s are considered in turn, and compared with the results for 16 kc/s. The main conclusions were as follows:(a) The day-to-day variations of the downcoming wave were greater at the higher frequencies.(b) The change of height of reflection in passing from day to night was about the same for all the frequencies.(c) In summer, the amplitudes by day and by night were very different, except at 16 kc/s.(d) The polarization at all the frequencies was approximately circular, left-handed and constant.The height of reflection of waves of frequency 30 kc/s was determined by using the frequency-change method of Appleton and Barnett.

Magneto-ionic multiple splitting determined with the method of phase integration

The method used is a first-order W.K.B. approximation with an integration path in the complex height plane. The refractive index versus height is represented on a Riemann surface with four corresponding to upgoing and downcoming waves and ordinary and extraordinary types of polarization. The branch points connecting the streets are characterized as reflection-points or ascoupling points. Suitable paths of integration in the Riemann surface determine the reflection coefficient for the various magneto-ionic components. Five possible fundamental components have been found, and an infinite number of additional components due mainly to multiple reflections between the different reflection points. Numerical computations have been carried out for ionospheric model with a Chapman distribution of electrons and an exponential decrease of collision frequency located in a moderate magnetic latitude. The results are represented in a sweep frequency picture of virtual height and absorption for five fundamental modes of propagation.

The ionospheric propagation of radio waves of frequency 16 kc/s over distances of about 540 km

The paper describes an investigation of the ionospheric propagation of waves of frequency 16 kc/s over a distance of 535 km. It represents an extension of other investigations in which the propagation over distances of 90 and 200 km was studied.1, 3, 4, 13A long series of measurements of signal amplitude covering the period 1940–44 has been used to study regular diurnal and seasonal variations, and anomalous effects associated with sudden ionospheric disturbances and severe magnetic disturbances. In another series of experiments from May to October, 1949, measurements of phase were made by using the method, previously employed at 200 km, of sending a reference signal from transmitter to receiver by land line. In this way the phase and amplitude of the downcoming wave could be deduced separately.The observations showed that reflection took place from an apparent height of about 74 km by day and 92 km at night; the downcoming wave was of approximately linear polarization during morning twilight, the plane of polarization being rotated clockwise from the normal through an angle of 40°, as seen by an observer looking in the direction of propagation; the effective reflection coefficient in the summer of 1949 was about 0.27 by day and 0.55 at night; the sunrise effects on the height of reflection and the amplitude of the downcoming wave regularly occurred when the zenith distance of the sun was 97° 50′. The effects of sudden ionospheric disturbances on the signal amplitude were explicable in terms of a change of reflection height with little change in the amplitude of the downcoming wave. During and after severe magnetic storms the diurnal variation of the signal amplitude. was abnormal, the abnormality persisting for as long as 38 days.It is clearly evident that there is a marked change in the propagation characteristics of the reflected wave in passing from short (90–200 km) to long (535 km) distances from the transmitter. At short distances the downcoming wave is of approximately circular polarization with a left-hand sense of rotation, while the reflection coefficient in summer is about 0.15 by day and 0.5 at night. The most striking evidence of the change in propagation characteristics is given by the markedly different ways in which the height of reflection varies with the inclina tion of the sun's rays. These conclusions concerning the change of reflection conditions confirm the deductions made by Weekes5 from measurements on the ground interference pattern.