Transequatorial Propagation Research Articles

Arrival Angle and Travel Time Measurements of HF Transequatorial Propagation for Plasma Bubble Monitoring

AbstractA system to measure travel times of high‐frequency (HF) radio waves was developed based on a digital receiver technique. Travel times of HF transequatorial propagation (HF‐TEP) of signals of Radio Australia from Shepparton (36.2°S, 145.3°E), Australia to Oarai (36.3°N, 140.6°E), Japan, were measured by the system. Directions of arrival of the signals were simultaneously observed by a HF Direction Finder located at Oarai (Oarai Direction Finder: ODF). When the ODF observed signals arriving from great circle direction, constant multiple discrete values of propagation times were observed. After sunset, propagation times longer than those expected from the great circle propagation were observed and the ODF observed corresponding signals from off‐great‐circle directions (off‐great‐circle propagation). The observed maximum azimuth angle deviation (∼50°) was much larger than the angle that would have been obtained when a single side reflection in HF‐TEP was assumed. This result supports the multireflection model of off‐great‐circle propagation of HF‐TEP proposed by Tsunoda et al. (2016a, https://doi.org/10.1002/2015JA021695). The propagation time measurements are useful to determine the propagation paths, when they are used with HF‐TEP observations by a HF Direction Finder system. Ray tracing analysis with the propagation times and arrival angles will contribute to monitoring locations of plasma bubbles as well as understanding their structures and the physics behind them. The technique can also be applied to determine the propagation paths in the normal great circle propagations and other geophysical phenomena, such as midlatitude trough, deep traveling ionospheric disturbance modulation of the ionospheric height due to Perkins instability and sporadic E layer irregularities.

Off‐great‐circle paths in transequatorial propagation: 1. Discrete and diffuse types

AbstractThere is mounting evidence that plasma structure in nighttime equatorial F layer evolves from large‐scale wave structure (LSWS) in the bottomside F layer. This process cannot be ignored because equatorial plasma bubbles (EPBs) arise from large‐amplitude LSWS; and, because intense radiowave scintillations are associated with EPBs, understanding the LSWS‐to‐EPB process is a crucial step toward reliable Space Weather Forecasting. In this regard, the transequatorial propagation (TEP) experiment appears to be the most useful among available research instruments. After a lapse of 30 years, the TEP experiment has been resurrected; a goal of this research is to understand TEP measurements well enough so that they can be used to diagnose the LSWS‐to‐EPB process. Toward this end, new results are presented in two companion papers. Herein (P1), off‐great‐circle (OGC) propagation paths are shown to consist of two types, discrete and diffuse. The new findings include the following: (1) a generalized multireflection model that can explain most of the observed properties; (2) the discrete type is supported by multireflections from an unstructured upwelling, (3) the diffuse type is supported by reflections from plasma structure in EPBs; and (4) the observed east‐west (EW) asymmetry can be explained in terms of a distorted upwelling or plasma structure along the west wall of an upwelling. In Paper 2 (P2), a second form of observed EW asymmetry is explained in terms of plasma structure, which is not aligned with the geomagnetic field. The findings strongly confirm a close relationship between upwellings, ESF patches, and OGC paths.

Off‐great‐circle paths in transequatorial propagation: 2. Nonmagnetic‐field‐aligned reflections

AbstractThere is considerable evidence that plasma structure in nighttime equatorial F layer develops from large‐scale wave structure (LSWS) in bottomside F layer. However, crucial details of how this process proceeds, from LSWS to equatorial plasma bubbles (EPBs), remain to be sorted out. A major obstacle to success is the paucity of measurements that provide a space‐time description of the bottomside F layer over a broad geographical region. The transequatorial propagation (TEP) experiment is one of few methods that can do so. New findings using a TEP experiment, between Shepparton (SHP), Australia, and Oarai (ORI), Japan, are presented in two companion papers. In Paper 1 (P1), (1) off‐great‐circle (OGC) paths are described in terms of discrete and diffuse types, (2) descriptions of OGC paths are generalized from a single‐reflection to a multiple‐reflection process, and (3) discrete type is shown to be associated with an unstructured but distorted upwelling, whereas the diffuse type is shown to be associated with EPBs. In Paper 2 (P2), attention is placed on differences in east‐west (EW) asymmetry, found between OGC paths from the SHP‐ORI experiment and those from another near‐identical TEP experiment. Differences are reconciled by allowing three distinct sources for the EW asymmetries: (1) reflection properties within an upwelling (see P1), (2) OGC paths that depend on magnetic declination of geomagnetic field (B), and (3) OGC paths supported by non‐B‐aligned reflectors at latitudes where inclination of B is finite.

A correlation study of solar activity index and amplitude of VLF trans-equatorial propagation

Diurnal and seasonal behavior of amplitude of NWC 22.3 kHz signal transmitted from Australia (22049/S, 114023/E), and received at Kolkata (22034/N, 88024/E), India have been investigated The signal amplitude is remarkably low in October. The variations of daily maximum and daily minimum of signal amplitude have been analyzed. The signal on the present propagation path showed non-correlation with A p index, but showed a moderate but negative correlation with 10.7 cm solar flux.

Observations of small- to large-scale ionospheric irregularities associated with plasma bubbles with a transequatorial HF propagation experiment and spaced GPS receivers

The results from simultaneous observations of the nighttime transequatorial propagation (TEP) of HF radio waves between Australia and Japan and the GPS scintillation measurements in south China and Vietnam are presented in this paper. The results showed that there was good correspondence between the nighttime eastward traveling off-great circle propagation (OGCP) of broadcasting waves of Radio Australia from Shepparton, Australia, measured at Oarai, Japan, and the scintillations in GPS radio waves at Hainan, China. This shows that the nighttime eastward traveling OGCP in HF TEP is caused by a large- scale ionospheric structure associated with a plasma bubble. The zonal drift velocities of the large- scale ionospheric structure estimated by the change in the direction of arrival of the OGCP were similar to those of the small- scale irregularities associated with plasma bubbles measured by the GPS scintillation spaced- receiver technique. Our results show that the HF TEP measurement is quite useful for monitoring the plasma bubble occurrence over a wide area and for forecasting the arrival of the plasma bubble at places located to the east of it.

A note on the semidiurnal non-migrating tide at polar latitudes

Abstract In the Antarctic upper mesosphere and lower thermosphere around 90 km, meteor radar observations at the South Pole have detected a significant semidiurnal wind component in summer which is found to be non-migrating with zonal wavenumber s = 1. It has been surmised that this component might possibly be excited through the non-linear interaction of the migrating semidiurnal tide with stationary planetary waves with zonal wavenumber s = 1 prevailing at stratospheric heights. The Kyushu University GCM has been successful in elucidating very unambiguously this conjecture. In the present paper, linearized steady tidal modeling is carried out in this connection to reproduce, by a fairly simplified but explicit model, trans-equatorial propagation of non-migrating semidiurnal tide forced in the opposite winter hemisphere and to compare latitudinal structures of migrating and non-linearly excited intermittent tides in view of polar latitudes where non-migrating tide tends to dominate over migrating tide. It is also shown that the re-analysis meteorological data for almost 10 years clearly supports the well-known N-S asymmetry in stationary planetary wave activity in winter polar stratospheric regions, possibly due to the difference in surface topography between two hemispheres. We suggest that there might be significant asymmetry in the summertime enhancement of semidiurnal non-migrating tide between both polar regions. This phenomenon may be in a context similar with the N-S asymmetry of gravity wave activity in the polar regions, possibly giving rise to N-S disparity of cold summertime mesopause temperature. Clarification awaits intensive bi-polar studies by coordinated radar and optical observations which are running both in the Arctic and Antarctic regions.

Transequatorial propagation of the Pc1 emission on 23 October 1997

A Pc1 emission event with uncommon propagation characteristics was revealed in the study of the search‐coil magnetometer data from two Russian midlatitude observatories separated by 4000 km. Dynamic spectra of the emission showed pronounced structure with a repetition period of about 130 s. By comparing the wave form plots for the two locations we found an unusually large phase difference as much as nearly 180 between the plotted envelopes. We discuss a most probable model assuming that the signal received at one of the two observatories must propagate from the source region in the opposite hemisphere.

Preliminary results of the ultralong-range sounding of ionospheric irregularities using the ducted mode

We present preliminary experimental results concerning transequatorial propagation (TEP) of HF waves upon chirp sounding over the 11950-km path alice Springs (Australia)-Yoshkar-Ola (Russia). The measurements were made in August, 1998. Two anomalous signals with delays of 3.0 and 4.5 ms with respect to the main mode were observed during night time (21:30–23:00 UT). The maximum observed frequencies (MOF) of these signals were 2–3 MHz greater than the main-mode MOF. Simulations allowed us to identify these signals as the ducted signals trapped in theFE interlayer duct due to radiowave refraction on a negative gradient of the electron density and that escaped from the duct due to the scattering by small-scale field-aligned irregularities of the subpolar ionosphere. We discuss radiophysical and geophysical aspects concerning localization of the irregularities responsible for scattering and perspectives of using the ducted mode for over-the-horizon diagnostics of the inhomogeneous structure of the ionosphere with a global network of chirp sounders and HF radars.

Tunneling modes in ionospheric waveguides

The importance of tunneling rays in ionospheric structures is examined by considering VHF transequatorial propagation in field‐aligned equatorial plasma bubbles. It is found that tunneling rays can cause a significant decrease in the amount of VHF power capable of being guided by bubbles, therefore indicating that allowance should be made for tunneling modes when modeling this sort of propagation. While the power loss associated with tunneling rays decreases with a decrease of frequency, it increases with a decrease in the radius of curvature of a guide's cross section. Consequently, the effect of these modes on HF ducting within guides of small cross section, similar to those observed on topside ionograms, is also investigated. Results from this analysis also indicate a substantial amount of power loss due to tunneling modes, which must be accounted for in any detailed analysis of experimental HF ducting results.

Transequatorial Propagation of TV Signals via Sporadic-E

Field Strengths of video signals by Transequatorial propagation between Kokavil (Sri Lanka) & Madras (India); and Pidurutalagala (Sri Lanka) & Madras (India) were recorded daring March 1982 to June 1983 on VHF band III channels 5 and 8 ie picture carriers frequencies of 175.25 MHz and 196.25 MHz respectively. The observed signal strength on both of these channels indicated strong dependence on the occurrence of blanketing sporadic E layers (Es-b) of the ionosphere near the magnetic equator. A comparison between the video field strengths and critical frequency blanketing sporadic-E, fbEs, led to the conclusion that the transequatorial propagation is mainly due to the presence of sporadic-E rather than the actual values of fbEs The observed video signal strengths plotted vs foEs, critical frequency of sporadic E show better correlation in distinct frequency ranges 3–5 MHz and 7–11 MHz. On the otherhand, there is no evidence of the influence of the F-region (foF2, the critical frequency of the F2 layer and s...

VHF transequatorial propagation via three dimensional waveguides

The three dimensional model of VHF propagation along ionospheric waveguides or ducts developed by Platt I. G. and Dyson P. L., 1989 [ J. atmos. terr. Phys. 51, 897–910] has been used to study transequatorial propagation (TEP) along equatorial bubble irregularities aligned along the earth's magnetic field. Bubbles with circular and elliptical cross-sections and different apex heights have been used to determine the region illuminated in the conjugate hemisphere by TEP and the power of the signals. The results show that the three dimensional waveguide model can explain many features of TEP, including those not well explained by the simpler waveguide models used previously. The calculations therefore confirm that TEP can be caused by guided propagation along equatorial bubble irregularities aligned along the magnetic field. The results show that a bubble located on the transmitter's longitude can illuminate a wide area of the conjugate hemisphere. A well defined maximum central power region occurs which in most cases is centred at a latitude slightly lower than the conjugate of the transmitter. Bubbles with circular cross-section can illuminate a region more than ten degrees in latitudinal extent and five degrees in longitude. Bubbles which are vertically elongated at their apex illuminate a much narrower region in longitude. TEP between points with relatively large separation in longitude can result from either leaky rays or ducts tilted out of the magnetic meridian plane. This explains some observations of TEP previously thought to be inconsistent with waveguide propagation. The variation of power loss with frequency depends on a number of factors such as bubble shape and height. Co-ordinated experiments involving a transmitter and several receivers in the conjugate hemisphere should enable properties of bubbles to be determined.

VHF propagation modes within ionospheric waveguides

A three dimensional waveguide model of VHF ducting has been developed. The waveguide is aligned along the Earth's magnetic field and has an elliptical cross-section which, based on observations of ionospheric bubble irregularities, is taken to have axes of 10–100 km. Rays propagating to the conjugate hemisphere via the duct undergo relatively few reflections at the duct walls (typically 3–20). Consequently the waveguides are not fully excited and significant power is transmitted by modes which are not fully trapped. A complete analysis of the propagation characteristics of such ducts requires the consideration of trapped and leaky modes, focusing and tunnelling. The cross-section of the guide is defined by specifying the vertical and East-West horizontal axes at the apex of the guide (magnetic equator). Comparison of cases with the same vertical axis shows that, when the vertical axis is larger than the East-West axis, the curvature on the upper surface of the guide is consequently increased and tunnelling becomes more important. On the other hand, when the vertical axis is the smaller, focusing is greater and leaky rays become increasingly important. In some models the region of highest power in the conjugate hemisphere is due almost entirely to rays which have suffered some leakage. Obviously non-trapped modes need to be considered when attempting to explain phenomena such as trans-equatorial propagation at VHF.

A Waveguide Model for the Evening Type Transequatorial VHF Propagation: Comparison with Experiment for the Euro-African Sector

ABSTRACT A waveguide model is proposed for the evening type transequatorial propagation mode. Field aligned irregularities observed in equatorial ionosphere are assumed to form dielectric waveguides. A local Champan type layer electron density distribution is taken inside the guide. The transequatorial wave propagation is treated by using this model and the propagation theory of dielectric waveguides. The total attenuation is computed between two points located in opposite hemispheres. The analytic form of the results permits easy interpretation of the propagation characteristics. A good agreement is observed between the experimental results and the theoretical calculations for the total attenuation. Comparisons have also been done for the propagation delay and the incoming elevation angle at the reception point.

Reflection of high‐frequency radio waves in inhomogeneous ionospheric layers

Formulas are given for the calculation of rays in model ionized layers in which the variation of the square of the refractive index is expressed separately in terms of the horizontal (x, y) and vertical (z) coordinates. Examples of three‐dimensional paths in linear‐parabolic layers are presented showing reflections in the horizontal, as well as the vertical, direction. To illustrate long‐distance transequatorial propagation the equatorial anomaly is simulated by two elliptic cylinders with axes parallel to the equator. Sample calculations show that in this model the ground range can increase as the angle of elevation at the ground increases.

Application of a scattering theory of VHF transequatorial propagation

Numerical application is made of the theory of scattering by long, curved, field-aligned irregularities of ionization density in the F-region developed by Ferguson and Booker. Using an intermediate-scale regime of irregularities with an outer scale equal to the scale height of the F-region and an inner scale equal to the ion gyroradius, combined with a small-scale regime with an outer scale equal to the ionic gyroradius and an inner scale equal to the electron gyroradius, calculations are made corresponding to (i) equatorial spread-F in the VHF and UHF bands, (ii) long-range transequatorial propagation of the type observed by Nielson and (iii) short-range transequatorial propagation of the type observed by Cohen and Bowles. The same ionospheric model yields field-strengths of the right order of magnitude in all three cases. The theory also predicts a focusing phenomenon that should be looked for experimentally.

A scattering theory of VHF transequatorial propagation

Transequatorial propagation in the VHF band can occur when the ionosphere does not support such propagation by refraction. The phenomenon is closely associated with the scattering phenomenon known as spread- F. This scattering is caused by the presence of irregularities in the ionization density of the F-layer. An inverse power-law spectrum of irregularities, with scales [wavelength/(2π)] spread from tens of kilometers down to the electron gyroradius, is employed. A model of scattering from long curved irregularities, aligned along the earth's magnetic field, is developed. For transequatorial propagation, the curvature of the irregularities causes the divergence of rays emanating from the transmitter in the magnetic meridian to be replaced by convergence of the scattered rays in the magnetic meridian in many situations of practical interest. The theory leads to the conclusion that single partial reflections from field-aligned irregularities are all that are required to explain observations of VHF transequatorial propagation, and that multiple total reflections in a field-aligned duct are not needed.

Spatial and temporal distributions of midlatitude ionospheric scintillations observed by low-altitude satellites

Spatial and temporal distributions of ionospheric scintillations have been observed at Kashima (36.0°N, 140.7°E) using VHF and UHF signals from low-altitude satellites. From these observations, three different types of prevailing ionospheric scintillations seen from Japan are identified. Scintillations of type I are rather weak scintillations, occur most frequently during the daytime in summer and are primarily associated with the sporadic E-layer. However, considerable occurrences of type I scintillations are also observed during the night in summer and autumn, not necessarily due to the sporadic E-layer but occasionally due to F-layer irregularities which originate from localized midlatitude processes. Type II scintillations are much stronger than type I and occur near the equatorward horizon during spring, summer and autumn. Their occurrences start after sunset, reach a maximum before midnight and decrease subsequently, with a tendency for negative and positive correlations with the magnetic and solar activities, respectively. It is concluded that type II scintillations are the midlatitude aftermath of equatorial plume-associated irregularities and cause trans-equatorial propagation of VHF waves. From observations of type I and II scintillations, the boundary between midlatitude and equatorial scintillations is clearly identified. Type III scintillations are as strong as type II and appear only during magnetically active periods. They can be regarded as another aspect of the severe scintillation events observed on gigahertz waves from geostationary satellites as reported by Tanaka (1981).

On transequatorial VHP propagation paths

Recent modelling has shown that evening VHF transequatorial propagation can occur by ducting through discrete, equatorial plasma bubbles. The model is used to interpret transequatorial two-way communications reports to evaluate the size and location of the zone in one magnetic hemisphere which may receive VHF radio waves from a single transmitter in the other hemisphere. At 144 MHz the mean zone radius is 987 km and the zone is compressed in latitude and extended in longitude by an axial ratio of about 2:1.

Recent progress in transequatorial propagation—review

Transequatorial propagation of HF and VHF radio waves is placed into three categories according to the physical mechanisms. Specular reflection off the underside of the anomalously dense equatorial F-layer is predictable by ray tracing and limited to frequencies less than about 60 MHz. Multipoint reflection from bottomside irregularities applies for the same radio frequencies but is associated with travelling ionospheric disturbances and spread- F traces on ionograms. This type of TEP may be used as a technique for studying some of the properties of bottomside irregularities. Ducted propagation of VHF waves depends upon high plasma density gradients and occurs along equatorial plasma bubbles during the evening hours. Observations on the ducted VHF mode relate to the behaviour of plasma bubbles.

Transequatorial propagation through equatorial plasma bubbles—Discrete events

The discrete nature of VHF transequatorial propagation path openings is pointed out. These events are shown to be consistent with the concept of guided propagation inside equatorial plasma bubbles. The important prediction of this work is that observations on discrete transequatorial VHF links may be used to track the production and development of equatorial plasma bubbles.