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.     

The effect of vertical drift on the equatorial F-region stability

Owing to the high conductivity along magnetic field lines, the stability of the night-time equatorial F-region is determined by magnetic field line integrated quantities. However, slow vertical diffusion near the magnetic equator plus the rapid increase in ion chemistry rates at lower altitude combine to give a very small positive scale height for the electron concentration on the bottomside of the region. As a result, the field line averaged quantities are reasonably approximated by their equatorial values, provided that the E-region does not contribute significantly. The time-dependent behavior of the growth rate for the Rayleigh-Taylor gravitational instability on the F-region bottomside is examined here as a function of the vertical E × B drift velocity using reasonable chemistry to obtain approximate equatorial vertical profiles of ionospheric parameters. It is found that the growth rate exceeds the chemical recombination rate over most of the bottomside F-layer even without vertical drift, but that a realistic E × B drift can result, after about 1 h, in an increase of this growth rate by an order of magnitude. The absolute growth rate is so small (< 10⁻³ s⁻¹) with zero vertical drift that a seeding mechanism would probably be required for the formation of bubbles. The rapid appearance of bubbles shortly after sunset appears likely only after a period of upward drift, as is observed.     

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.     

Characteristics of ionospheric irregularities capable of producing quasi-horizontal traces on ionograms

This paper presents simulated ionograms calculated for a parabolic ionospheric layer containing irregularities in the form of small amplitude waves. With small amplitudes, perturbation techniques can be used enabling results for the irregular ionospheres to be calculated from the results for smooth ionospheres. This approach is relatively straightforward and avoids having to ray trace new paths each time the irregularity parameters are changed. It is, however, restricted to irregularities which do not cause multiple echoes. Irregularities with vertical wavelengths of up to a few kilometres can produce significant changes in the ionosphere over height intervals smaller than those involved in reflecting a single pulse. Consequently, in the simulation procedure, it is essential to consider not just the carrier frequency but the complete frequency spectrum of the pulse. Irregularities with vertical wavelengths of the order of 10 km or more can produce ripples in an ionogram trace. These will, of course, be more evident on ionograms with high frequency resolution. Irregularities with vertical wavelengths of up to several kilometres and amplitudes up to a few per cent can produce significant pulse spreading and splitting. The actual effects depend not just on the irregularity properties but also on the ionosonde pulse width, gain and frequency and height resolutions. Some simulations show trace splitting and quasi-horizontal traces similar in many respects to effects observed by Bowman (1987, J. atmos. terr. Phys. 49, 1007) and Bowmanet al. (1988, J. atmos. terr. Phys. 50, 797). Consequently it is suggested that, at least in some cases, small amplitude (≤3%) and small scale (≤4 km) irregularities produce the spread-ifF reported by these authors.     

Spread-F-like irregularities observed by the Jicamarca radar during the day-time

Until now the presence of F-region irregularities responsible for spread-F (sp-F) traces in ionograms has been considered as a purely night-time phenomenon extending sporadically to the early morning hours. We herein report that, on two occasions (26 March 1974 and 1 February 1984) similar irregularities were observed between 1400 and 1600 hours local time with the Jicamarca radar. These irregularities caused enhancements in the power of the radar echo of as much as two orders of magnitude, were found over a region of a few hundred kilometers on the topside of the F-region extending from around 600 to 1000 km altitude, and persisted for 1–2 h. The irregularities were aspect sensitive (aligned with the magnetic field) and produced echoes with a fading rate of the order of one to a few seconds. The background zonal electric field, inferred from the vertical drift velocity, was fairly constant in altitude, with values smaller than 0.1 mV m⁻¹. During the duration of the events, zonal components of both signs occurred, with the component passing through zero several times. We have no information on the vertical component of E. These irregularities could not be observed with ground-based ionosondes, since they are on the topside of the F-region. They may be related to fossil bubbles that are responsible for HF ducting observed by satellites.     

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.     

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.     

Satellite observations of signals from a Soviet mid-latitude VLF transmitter in the magnetic-conjugate region

The results of the Intercosmos-19 satellite experiment which receives VLF signals arriving via a magnetospheric path have been examined. The reception zone in the magnetically conjugate region (MCR) has been shown to be centred near the L-value (2.6) of the transmitter of 15 kHZ radio waves. The received signals arrive at the MCR with wave normal angles to the geomagnetic field, ψ, far from the resonance cone. These results indicate an effective amplification of the signal in the magnetosphere by 10–15 dB and effective ducting of VLF waves across the equatorial plane of the magnetosphere.     

On ELF transmission in the Earth-ionosphere waveguide

The propagation constant for ELF (extremely low frequency) propagation in the Earth ionosphere waveguide is determined analytically. The derivation carried out for a planar model, with the Earth's surface impedance Z_g> 0, confirms the important result obtained earlier by Greifinger and Greifinger [(1978), Radio Sci. 13, 831] in the limitZ_g = 0. The present method avoids the use of auxiliary potentials.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.     

Pulsing hiss,pulsating aurora and micropulsations

Simultaneous conjugate observations between the GEOS 2 geostationary satellite, an all-sky TV camera operated at Andenes, Norway and a magnetometer at Tromsø, are presented. A close correlation was found between the occurrence of pulsing ELF hiss on GEOS 2, pulsating aurora observed with the TV camera and the ground detected micropulsations. However, a one-to-one correlation between the individual pulses of all three phenomena was generally not found. Ten events were chosen for a case study where the optical pulsations could be classed into two types (either classical pulsating patches or propagating forms). Classical pulsations were seen to be associated with Pi(b) micropulsations while propagating forms showed a one-to-one correlation with Pi(c) micropulsations. Type matching between the optical pulsations and two different types of ELF hiss, agreed well on a statistical basis, but the exact relationship for individual events was not so clear. On occasions, the micropulsations possessed major frequency components whose periods were longer than the periods of the other two phenomena. This is shown to imply that the observed micropulsations were of ionospheric origin in these cases.     

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.     

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.     

Calculation of auroral Bahner volume emission height profiles in the upper atmosphere

Energetic protons entering the atmosphere will either travel as auroral protons or as neutral hydrogen atoms due to charge-exchange and excitation interactions with atmospheric constituents. Our objective is to develop a simple procedure to evaluate the Balmer excitation rates of H_α and H_β, and produce the corresponding volume emission rates vs height, using semi-empirical range relations in air, starting from proton spectra observed from rockets above the main collision region as measured by Reasoneret al. [(1968) J. geophys. Res.73, 4185] and Søbraaset al. [(1974) J. geophys. Res.79, 1851]. The main assumptions are that the geomagnetic field is parallel and vertical, and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. Calculations show that the largest energy losses occur in the height interval between 100 and 125 km, and the corresponding volume emission rate vs height profiles have maximum values in this height interval. The calculted volume emission rate height profile of H_β compares favorably with that measured with a rocket-borne photometer.     

Solar activity effects on equatorial plasma bubble zonal velocity and its latitude gradient as measured by airglow scanning photometers

Using a new mode of scanning 630-nm photometer operation the zonal velocities of ionospheric plasma depletions were measured over Cachoeira Paulista in Brasil in two east-west planes tilted 30°N and 30° S with respect to zenith. The measurements cover a time period of approximately 2 years, from January 1988 to January 1990, a period marked by significant increase in solar activity of the ongoing cycle. The results have permitted a rather detailed evaluation of the local time and latitude variations in the zonal plasma bubble velocity as a function of solar activity. Although the mean trend in the velocity local time variation is a decrease from early evening to post-midnight hours, a strong tendency for velocity peaks is observed near 21 LT and midnight. The velocities as well as their height (latitude) gradients show perceivable increases with solar activity represented as sunspot numbers. The present results are compared with the ambient plasma velocities measured using the Jicamarca radar by Fejer el al. (1985), J. Geophys. Res. 90, 12249, with that measured on board the DE 2 satellite on the equatorial latitudes by Coley and Heelis (1989), J. geophys. Res. 94, 6751, and with various theoretical calculations, in an attempt to bring out the salient features of the plasma dynamics of the equatorial ionosphere.     

Analytic calculation of the ordinary (O) and extraordinary (X) mode nose frequencies on oblique ionograms

On oblique ionograms, the maximum frequencies of the ordinary and extraordinary modes are referred to as nose frequencies. The difference in the nose frequencies depends on parameters such as the length of the propagation circuit and the direction of propagation. In this paper, the difference in nose frequencies is studied using the frequency scaling technique of Bennettet at. [(1991) Appl. Comput. Electromagn. Soc. Jl6, 192]. For long paths, an explicit formula is obtained which gives the difference approximately as a function of the local magnetic dip and azimuth of propagation at the ray mid point. For shorter paths, it is shown how analytic ray tracing can be used to determine the difference.     

Transformation of deep‐water methane bubbles into hydrate

This study focuses on the mechanics of methane bubble phase behavior in the gas hydrate stability zone. The transformation of deep‐water methane bubbles into solid hydrate was investigated in Lake Baikal in situ. After being released from the lake bottom, methane bubbles were caught by different traps with transparent walls. When bubbles entered the internal spaces of the traps, the bubbles could be transformed into two different solid hydrate structures depending on the ambient conditions. The first structure was hydrate granular matter consisting of solid fragments with sizes on the order of 1 mm. The second structure was a highly porous solid foam consisting of solid bubbles with sizes on the order of 5 mm. The granular matter did not change as it was brought up to the top border of the gas hydrate stability zone, whereas in the solid foam, free methane rapidly exsolved from the sample during depressurization. We conclude that the decrease in depth and the decrease in the bubble flux rate were key factors in the formation of the hydrate granular matter, whereas the increase in the depth of bubble sampling and the increase in the bubble flux rate facilitated the conversion of bubbles into a highly porous solid hydrate foam.     

Radiative perturbation due to the eruption of El Chichón: Effects on ozone

The ozone depletion over the Antarctic region is now attributed to processes involving heterogeneous chemistry on polar stratospheric clouds. Similar mechanisms are probably working also in the Northern hemisphere high latitudes [Douglass and Stolarski (1989) Geophys. Res. Lett. 16, 131] and may be important in explaining the secular trend of ozone in the last twenty years above 50° North [Pitari and Visconti (1991) J. geophys. Res. 96, 10,931]. Hofmann and Solomon [(1989) J. geophys. Res. 94, 5029] have shown that the local observed decrease in the ozone amount following the eruption of El Chichón could be explained in terms of heterogeneous chemistry on the volcanic aerosol surface. In this paper we use a two dimensional model to study the effects on ozone introduced by the El Chichón aerosols through a perturbation in the radiation field; both the temperature and the photolysis rates are affected. We show that up to half of the observed decrease may be attributed to radiative effects at mid latitudes.     

Topside and intelhemispheric ion flows in the mid-latitude plasmasphere

A modelling study has been carried out of field-aligned ion flows in the topside ionospheres of conjugate hemispheres under solstice conditions at mid to low latitudes. In the model calculations coupled time-dependent O⁺, H⁺ and electron continuity, momentum and heat balance equations are solved along dipole magnetic field lines at L = 1.5 and 3.0 Sunspot medium and sunspot minimum atmospheric conditions are considered.It has been found that thermal coupling between conjugate hemispheres gives rise to strong flows of O⁺ in the topside ionosphere of the summer hemisphere that are directed upwards at conjugate sunrise and directed downwards at conjugate sunset. At conjugate sunrise in the winter hemisphere there is a small upward-directed signature in the O⁺ field-aligned flux; there is no observable signature in the O⁺ field-aligned flux in the winter hemisphere at conjugate sunset. There are strong upward and downward flows of O⁺ at local sunrise and local sunset, respectively, in both the summer and winter hemispheres.At both L = 1.5 and 3.0 the 24 h time-integrated interhemispheric H⁺ flux is in the direction summer hemisphere to winter hemisphere. At L = 1.5 its magnitude is in good agreement with the magnitude of the 24 h time-integrated plasma (O⁺ + H⁺) field-aligned flux at 1000 km altitude; there are no such agreements at L = 3.0.A study of the roles played by the individual terms of the O⁺ momentum equation has demonstrated the complex structure of momentum balance. Certain of the terms may be orders of magnitude greater than the combined total of the individual terms, i.e. the O⁺ field-aligned flux.