Waveguide model analysis of VLF wave propagation at 16 kHz

Preliminary results of measurement of phase and amplitude of 16 kHz VLF signals propagated in the Earth-ionosphere waveguide are presented. From the measured values of diurnal phase, amplitude changes and sunrise fadings the relevant waveguide parameters are estimated.     

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.     

The effect of multi-duct structure on whistler-mode wave propagation

Ducting of whistler-mode waves is investigated by ray-tracing in a magnetospheric model in which multiple and complex duct structures are superimposed on a smooth magnetospheric plasma distribution. When two or more ducts are present, propagation through a duct in which the ray does not become trapped is found to result in little deviation of the ray path, showing that upgoing waves can traverse several ducts before becoming trapped in one that is suitably positioned.The presence of two ducts situated in the same meridian plane and close together in L-value (ΔL ~ 0.07) is found to enable a double-duct trapping mode. This has the special property of guiding waves with frequency above half the local electron gyrofrequency across the equatorial plane in such a way that they become retrapped in the duct from which they had previously escaped at its local detrapping frequency. This may explain the observation of whistlers with a particularly high ratio of cut-off frequency to nose frequency (Bernhardt, 1979).Ducting is also investigated for a more complex duct structure in which fine structure is superimposed on a broader larger enhancement main duct. Here, it is found that rays which are first trapped to propagate in the main duct at low altitude can be further trapped to be ducted inside fine structure enhancements at higher altitude. This can result in certain components of multipath whistlers always being excited together and also having a common exit-point in the lower ionosphere. This is shown to be consistent with experimental observations.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.     

The propagation of the mesospheric two-day wave in the southern hemisphere

The results of an analysis of mesospheric wind data collected simultaneously at Grahamstown (South Africa) and Adelaide (South Australia) during the summer appearances of the quasi-two-day wave over a period of eight years are presented and discussed. It is concluded that the apparent zonal wave-number k which characterises the westward travelling wave is significantly less than the value of 3 suggested by current theories. Differences in local conditions and/or observing techniques are found to be unlikely causes of the discrepancy. It is shown that these results, as well as certain global features observed by satellite, could arise as a result of the superposition of modes with different k but similar frequencies.     

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.

     

Electromagnetic wave propagation in a field-aligned-striated cold magnetoplasma with application to the ionosphere

The problem of wave propagation in a cold magneto-ionic medium with plane or circular cylinder small scale random density fluctuations is considered. By including second or terms in the perturbation it is possible to derive a refractive index for a “mean” wave in the medium. It is shown that the striations cause new cutoffs and resonances to occur. The validity of the theory for a warm plasma will depend on the plasma temperature and on the spatial spectrum of fluctuations. Possible applications are considered.     

Gravity wave activity in the upper mesosphere over Urbana,Illinois: lidar observations and analysis of gravity wave propagation models

We analyze 375 h of Na Wind/Temperature lidar measurements of the mesopause region (≈ 80–105 km) Na density and temperature profiles on 57 nights distributed over 2 yr at Urbana, Illinois. These observations yield a high-resolution seasonal data set of gravity wave activity in the upper mesosphere. From this data, we present measurements of the Brunt-Väisälä period, the relative atmospheric density perturbations and their spectra, and the parameters of 143 quasi-monochromatic gravity waves. The direct measurement of the Brunt-Väisälä period allows accurate calculation of the horizontal velocity perturbations and vertical displacement perturbations from the density measurements. The horizontal velocity and vertical displacement vertical wave number spectrum magnitudes and indices show considerable seasonal and nightly variability. The gravity wave amplitudes, wavelengths, and observed periods exhibit systematic relationships similar to those found in previous studies, and are consistent with the MU radar measurements of intrinsic gravity wave parameters. Here, we present a detailed analysis of the observations in terms of Diffusive-Filtering Theory models of gravity wave propagation. The magnitudes of the vertical wave number spectrum, the form of the joint vertical wave number and frequency spectrum, and the systematic relationships between the monochromatic gravity wave parameters are consistent with the Diffusive-Filtering model. We compare these results with a variety of radar, lidar, and airglow observations from other sites. This observational study suggests that the complex nonlinear interactions of the gravity wave field may be modeled successfully as a diffusive damping process, where the effective diffusivity is a function of the total wave variance.     

The suppressing effect of field-aligned currents on VLF emissions in the magnetosphere

The growth rate of whistler-mode waves is calculated analytically for a bi-Maxwellian plasma in the presence of a beam of cool electrons. This beam is moving in the same direction as the gyroresonant electrons and in the opposite direction to the waves which are considered to propagate parallel (or anti-parallel) to the imposed geomagnetic field. A somewhat surprising result is found. This is that even if the anisotropy is greater than a critical value, which is strongly frequency dependent, the beam reduces the growth of the waves near half the electron gyrofrequency. For a field-aligned current density ~ 1 μA m⁻², this mechanism can explain the lack of signals near 1.4 kHz on auroral (return current) flux tubes. It can also explain the observed absorption of signals at half the electron gyrofrequency, around 7 kHz on L = 4 flux tubes, near the equatorial plane and just outside the plasmapause.     

New results on the dip equator and the equatorial electrojet in India

The geophysical implications are examined of the continuing southward migration of the magnetic dip equator in India since 1965, its precise ground location in 1971, and thereafter its drift at 1–6 km/yr accelerating to 7 km/yr in the mid-1980s near its mean central position in the 80-yr secular oscillation, estimated to be about 10 km south of Trivandrum. Simultaneously its drift northwards near the antipodal point at Huancayo Observatory, in Peru (South America), is also observed.The ground projection of the mean axis of the equatorial electrojet for 1980 is clearly delineated about 55 km to the north of the dip equator in India, with positive Sq(Z) values of 25 nT recorded right on the dip equator, based on the ground geomagnetic survey 1971 and the magnetometer array experiment of 1980. The half-width and midday peak total current intensity of the Indian electrojet are determined from the H data recorded at Trivandrum, Annamalainagar and Hyderabad for the solar minimum year 1976 (146 ± 46 km, 137 ± 25 Amp/km) and the maximum year 1980 (169 ± 39 km, 203 ±49 Amp/km), assuming a uniform west-east current band model at a height of 107 km centred on its newly discovered axis. These new results are quite different from those of earlier determinations. Severe induction anomalies observed in the region due to subsurface geological bodies are also appropriately incorporated.     

Dependence of seasonal variation of absorption on solar activity in the equatorial ionosphere

From data for the absorption of radio waves at oblique incidence in Lagos, and at vertical incidence at Colombo, the seasonal variation of absorption at the two sites are examined. It is shown that, if subsolar absorption be assumed to depend upon sunspot number, the cos X law gives the same index for both the diurnal and the seasonal variation of absorption.     

Regional agency and constitution of new paths: a study of agency in early formation of new paths on the west coast of Norway

ABSTRACT

This article analyses regional agency in the early phases of new path constitution. We argue that the early stages of new path constitution can be explained by both structural factors and the strong presence of agency. With a specific focus on agency, this article contributes to the literature by providing a study of the role of agency at different stages of path constitution. The study shows that two types of agency operate together. Public policy agency is carried out through a common thrust for policy tools that can enhance the room to manoeuvre together. In addition, strong entrepreneurial agency functions as a locomotive for other firms which are important trigger points, and pushes the process forward. These two forms of agency need to be interrelated to constitute new paths.     

Some effects of geomagnetic activity on spread-F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread-F occurerence and h′F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated.The results show that for interval A significant suppressions of spread-F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′F variations suggest that h′F is suppressed at times of spread-F suppression. For interval B spread-F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread-F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.     

声频应力波法检测土遗址加固用玻璃纤维锚杆锚固质量初探

土遗址锚杆锚固质量一直是土遗址加固工程中关注的重点问题。目前,土遗址领域均采用具有破坏性的拉拔试验对其进行检测。基于土遗址的文物属性,本研究尝试利用无损检测技术评价土遗址加固用玻璃纤维锚杆的锚固质量。通过锚杆无损检测仪与拉拔仪分别对甘肃省红沙堡遗址与永泰城址加固工程中的玻璃纤维锚杆锚固系统进行检测。比较实际测量杆长与仪器测量杆长结果,得出杆长指标可以判断无损检测仪所测结果的真实性。无损检测仪得出的检测波形衰减规律显著,对比相应锚杆的拉拔试验的评价结果,证实了声频应力波法在土遗址玻璃纤维锚杆锚固系统无损检测中的适用性。本研究结果将为评价土遗址加固用玻璃纤维锚杆锚固质量提供可靠的依据。     

Strongly aligned coastal boulders on Ko Larn island (Thailand): a proxy for past typhoon‐driven high‐energy wave events in the Bay of Bangkok

Recent studies of elevated carbonate boulder deposits on several rock islands near Bangkok have indicated that Thailand's capital city may not be as protected from typhoon strikes as previously thought. Here, new evidence is presented for past high‐energy wave (HEW) events in the form of statistically significant patterns of boulder alignment on exposed rocky shorelines of Ko Larn island. The long‐axis orientations of 193 coastal sandstone boulders were analysed across four study sites. Several scenarios for the unimodal, bimodal, and polymodal patterns found can be envisaged. Either the most recent HEW event was the strongest—in which case most clasts were rearranged unimodally (one observation site), or the strongest HEW event was earlier and subsequent weaker ones realigned only smaller boulders to produce bimodal or polymodal patterns (three observation sites). Inferred northeastward or eastward onshore flow directions are consistent with palaeo‐typhoons penetrating into the Bay of Bangkok on northwestward curving tracks. The calculated minimum flow velocities required to transport all sampled boulders are 5.5–7.8 m s^?1, similar to other findings throughout the Asia‐Pacific region. It was observed that the absence of a fitted boulder geomorphology lends credence to the earlier proposed time frame of 150–200 years between typhoon phases in the upper Gulf of Thailand. The current work has provided additional insights into the characteristics of past HEW events that have a possibility of reoccurring again at some time in the future. Our findings continue to raise awareness for a reassessment of the risks of coastal hazards for the Chao Phraya River delta and densely populated Bangkok, for which storm surge modelling should be an urgent priority, so as to give better perceptions on how typhoon‐driven marine incursion would impact the city.