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.     

The computation of ELF radio wave fields in the Earth-ionosphere duct

Evaluation of ELF radio wave fields using the mode theory requires the computation of Legendre functions. Four different representations of the Legendre functions in terms of hypergeometric series have been programmed for computation on a microcomputer. By an appropriate choice of series for given values of the propagation constant and distance from the source, only a few (typically eight) terms are needed to calculate the Legendre functions with a precision of six decimal digits.     

VLF scattering from a column of ionization in the Earth-ionosphere waveguide

Choosing a highly idealized model, we analyze the scattering of VLF radio waves from a thin vertical column of ionization within the Earth-ionosphere waveguide. It is shown that mode conversion is produced if the column is limited in height and/or if the ionization is a function of height. However, the relative higher-mode content would be small in some situations such as in the case of the ionization produced by a cloud-to-ionosphere lightning discharge.     

The Siple VLF transmitter as a multi-frequency probe of the Earth-ionosphere waveguide

1983 receptions of subionospheric signals radiated from Siple, Antarctica (L = 4.3) to neighboring stations Palmer (L = 2.3), Halley (L = 4.3), and South Pole (Λ = 74°), each ~ 1500 km from the horizontal (magnetically east-west) VLF transmitting antenna at Siple, were found to be strongly dependent upon azimuth and upon signal frequency. At Palmer, located equatorward in the broadside direction with respect to the antenna, signals near 2.5 kHz were often well defined, while the third harmonic of the transmitted signal, near 7.5 kHz, was not detected. Meanwhile, at Halley, the third harmonic was regularly observed and directionally stable, while the fundamental was often weak or undetectable. The field strength of the third harmonic component at Halley exceeded by ~ 40dB the level of the fundamental, when both were normalized to the same antenna input power. The large size of these effects is attributed in part to antenna properties that favor the endfire direction (toward Halley) at the 3d harmonic of the antenna half wave resonance frequency, and in general provide greater efficiency at higher frequencies. Other factors are high waveguide attenuation in the 2–4 kHz range and azimuth dependent differences in the propagating modes. The observed effects represent a way of extending the effective frequency range of the narrowband Siple antenna system, and also, by using the new crossed dipole configuration at Siple, of selectively probing certain regions of the Earth-ionosphere waveguide.     

The sensitivity of stratospheric photodissociation rates to the solar spectral resolution in the Schumann-Runge bands

Several molecules important in the chemistry of the stratosphere, including N₂O, H₂O, HNO₃, and several chlorofluorocarbons photodissociate in the wavelength interval 1750–2050 Å. The transmission of solar radiation in this spectral region is largely controlled by the Schumann-Runge (S-R) bands of molecular oxygen. The absorption cross sections of oxygen in this region vary by several orders of magnitude as a function of wavelength, resulting in large variations in the magnitude of solar radiation penetrating into the middle atmosphere. It is thus of interest to examine the potential effects that a knowledge of the fine structure of the solar spectrum would have on the accurate calculation of stratospheric photodissociation rates. A U.S. Naval Research Laboratory solar spectrum with a resolution of 0.07 Å has been used to compute high resolution photodissociation rates for several molecular species. Computations have also been performed in which this solar spectrum has been degraded by roughly a factor of 100. These calculations show that knowledge of the fine structure in the S-R region of the solar spectrum produces only a marginal (1–2%) improvement in the computation of the photodissociation rates. This results from the fact that the photodissociation rate is a spectrally integrated quantity, and that there is, on average over the S-R bands, no correlation between the solar spectrum and the oxygen absorption cross sections. This very small improvement should be compared with the much larger uncertainties in important chemical reaction rates, absorption cross sections, and quantum yields, which are often 50–100%.     

青铜病的闭塞孔穴腐蚀特征的研究

为探求符合文物保护要求的青铜器保护处理方法,对青铜病形成的过程和原因进行了模拟小孔腐蚀实验研究。对青铜模拟闭塞电池内的化学和电化学状态的变化,进行了不同Cl~-浓度和不同的pH值溶液条件下的电化学测试。实验结果证明,青铜闭塞孔穴腐蚀能导致蚀孔内Cl~-浓缩,pH值降低,腐蚀电流增大,局部腐蚀的加速,导致青铜病的形成。青铜病的闭塞孔穴腐蚀特征的研究,对进一步探求符合文物保护要求的青铜器保护处理方法具有较好的理论指导意义。     

The relation between ionospheric profiles and ELF propagation in the Earth-ionosphere transmission line

Based on an approximate wave solution, it is shown that, for reflexion of ELF waves from a given ionosphere described by a simple profile of ionization density, the phase-integral method may be used above a certain level, and the ionosphere may be abolished below this level. The height of ELF reflexion thus determined is independent of angle incidence, but not of frequency. The level is the one where the ionosphere passes from rapidly varying to slowly varying behavior, judged in relation to the local wavelength.An approximate solution is obtained to the mode problem in the Earth-ionosphere transmission line in terms of four crossing plane waves, one pair having O wave polarization and the other X wave polarization. The velocity of phase propagation is calculated, and also the rate of attenuation due to leakage of energy into the region above the level of reflexion. The attenuation rate due to collisional absorption below the level of reflexion is also calculated using a method similar to that employed for dielectric loss in an engineering transmission line.As the frequency descends through the ELF band, penetration of the D-region occurs in succession for the O and X waves, leading to reflexion from the E-region at the Schumann resonant frequency and penetration of the ionosphere at micropulsation frequencies. Under quiet day-time ionospheric conditions the penetration frequency-band for the D-region is around 20–60 Hz in middle and high latitudes, but around 75–100 Hz at the equator. At a frequency low enough to be reflected primarily from the E-region under quiet ionospheric conditions, an increase in D-region ionization that is just sufficient to transfer primary reflexion from the E-region to the D-region results in an increase in the rate of attenuation. On the other hand, when once reflexion is firmly established at the lower level, further increase of ionization in the D-region causes a reduction in the rate of attenuation. Similar effects are expected to occur at night in association with a sub-E-region ledge of ionization. Small variations in the ionization profile of such a ledge are the likely cause of night-time fluctuations of transmission at 45 and 75 Hz.     

Radio wave propagation from antennae at satellite altitudes into the Earth-ionosphere waveguide

The problem of the excitation of the earth-ionosphere waveguide by a short linear antenna or by a small circular one at satellite altitudes is considered. The formulation allows for a spherical regular wave guide as well as for a radially inhomogeneous anisotropic ionosphere. A method for the solution is based on the use of the reciprocity theorems for anisotropic media. Numerical techniques have been developed. Some results for VLF are given. To gain some physical interpretations, the fields of sources at low ionospheric heights were investigated.     

Very unusual low latitude whistlers with additional traces of the Earth-ionosphere waveguide propagation effect

The present paper reports very unusual whistlers strongly influenced by the Earth-ionosphere waveguide propagation after emerging from the ionosphere, as observed simultaneosly at our two stations, Sakushima (geomag. lat. 24°) and Kagoshima (20°). These unsual whistlers are characterized by clearly exhibiting additional dispersion effects near the cut-off frequencies of the 1st and 2nd order modes of waveguide propagation and, to our knowledge, they are a new finding. All the subionospheric dispersion is deduced to occur between the ionospheric exit point and the receiver. Detailed spectral analysis, after extracting the small waveguide dispersion effect from the overall spectrum by taking the beat with the appropriate pseudo-whistler, has enabled us to determine the propagation distance of the ionospheric exit region from each station. These distances have then been used to locate the ionospheric exit region, which is found to be about 3000 km east of the stations and in the local sunrise time sector. The generation mechanism of such unusual whistlers is discussed in terms of the joint influences of the ionospheric transmission mechanism (longitudinal gradient of the ionosphere, wave scattering by density irreglarities) and magnetospheric propagation and characteristics of ducts.     

The impact of gaps and spectral methods on the spectral slope of the middle atmospheric wind

The spectral slope of the middle atmospheric wind is an important index of the gravity wave and turbulence processes. Gaps exist in MF radar spaced-antenna winds data because significance criteria are built into the analysis. These cause a smearing of the spectrum and seriously modify the slope, as well as affect the absolute power at high frequencies. A comparison between sites with different gap rates must account for this. Different methods of dealing with these gaps are tested in this paper. The periodogram (with linear interpolation across gaps), the correlogram, and the Lomb-Scargle analyses are compared on synthetic data with known slope, and also with some of the best measured data (less than 20% gaps), both with added gaps to a maximum of 50%. The periodogram is seen to be the best choice. Parallel calculations on real data and synthetic data with the real gaps inserted are used to compare 1992 summer and winter spectral slopes from the Saskatoon MF radar. The latter are also compared with those of winter spectra from the two CNSR (Canadian Network for Space Research) radars which, with Saskatoon, form a ∼ 500 km array. A similar process is used to compare the seasonal variation of absolute power (10–100 min) at the three sites.     

High resolution turbulence observations in the middle and lower atmosphere by the MU radar with fast beam steerability: preliminary results

Refractive index fluctuations or turbulence in the mesosphere, stratosphere and troposphere are observed with the aid of the fast beam steerability of the MU (middle and upper atmosphere) radar which operates at 46.5 MHz with 1 MW peak radiation power and 8330 m² antenna aperture. Morphology of the mesospheric and stratospheric turbulence is studied by making use of the high altitude and time resolutions. Sixteen beam observations based on the fast beam steerability reveal advection properties and spatial variability of echoing regions in the troposphere. These results demonstrate new possibilities for this system in the investigation of three dimensional structures of turbulence.     

Temporal and spatial distribution of the spectral components of foF2

A spectral analysis of almost all available monthly median of the maximum plasmafrequency in the F-region (foF2) was performed. Some evidence for a significant tidal influence on the diurnal and semidiurnal variation has been found. The declination effect (Eyfrig, 1963) appears in our spectral presentation as a dependence of the degree of the seasonal variation and of the phase of the 24 hour period on the magnetic declination. The most peculiar phenomenon is the different variation of the annual component in North America and Eurasia with the sunspot number. There are several indications that the variation of (foF2) is also influenced by the variation of the gravitational potential. Some other results are presented which are believed to be related to dynamic processes in the F-region.     

Commercial conflict resolution across the religious divide in the thirteenth-century Mediterranean

Commercial conflict resolution in the medieval Mediterranean has been treated by a number of scholars in recent years, notably through the use of documents from the Cairo Geniza and the archives of the Italian port of Genoa. Recent research on this subject, and more specifically on contract enforcement, has focused on contract reinforcement within the Mediterranean Jewish community, largely because of the sources available. Parallels drawn with medieval Italian mechanisms of conflict resolution emphasize differences between public- vs. private-order responses, that is, the reliance on personalized groups in the Islamic world rather than on public institutions typical of the Italian port-cities. These studies do not, however, examine how commercial conflicts were resolved across religious and political lines, despite the growing role of Italian merchants in the trade networks of Islamic North Africa, a role that inevitably led to trade disputes and occasional uncollected payments. Through close textual analysis of 14 Latin and Arabic letters exchanged between Islamic Almohad Tunis and Christian Italian Pisa, this article explores how Almohad commercial agents and governmental authorities sought to maintain positive trade relations across the religious divide while protecting the interests of their own clients and citizens when disputes arose over commercial payments and debt collection. Rather than relying on commercial conflict resolution methods specific to one culture or the other, these documents reveal a middle ground of borrowed vocabulary and procedures. Through these letters, Almohad merchants and officials attempted to negotiate through the bonds of personal trust and reputation established with their Italian counterparts. However, they also appealed to Italian sensibilities with hybridized methods recognizable by the legal and public institutions of both cultures.     

Wave form and spectral distribution of the electromagnetic field of lightning currents

In a resonant wave guide model of lightning currents, two impulse type standing waves can exist: aperiodic waves of the Bruce-Golde form (type 1), and damped oscillations (type 2). The electromagnetic waves generated by these two types of lightning currents are calculated for various distances and compared with observations. It is shown that the measured wave forms of sferics at distances smaller than about 300 km of return strokes (R strokes) as well as of intracloud strokes (K strokes) are generated mainly by type 2 lightning currents. The channel parameters like channel length and channel diameter derived from the observed sferics are 19 km and 4.6 cm, respectively, for the average R stroke, and 4 km and 1.6 cm for typical K strokes. The large values of the lengths probably correspond to the real lengths of oblique and tortuous channels, rather than to their vertical elevation. The finite electric conductivity of the earth modifies the high frequency component of the wave forms. With decreasing conductivity and/or increasing distance, the rise times of the radiation component to its first maximum increases and the maximum amplitude decreases.Typical rise times for R-strokes are about 3 μs consistent with the observations if the electric conductivity of the earth is of the order of 10⁻³ S/m. The spectral functions of the wave forms are also calculated. The spectral amplitude of the average type 2 R-stroke has its maximum near 4 kHz, and that of the type 2 K-stroke maximizes near 35 kHz. Within the high frequency region at frequencies greater than about 300 kHz. the spectral amplitudes decay proportional to the reciprocal third power of the frequency. The radiation component in the far field contains 7% (16%) of the total electromagnetic energy of type 2 R (K) strokes.     

The random scatter of spectral parameters of VLF-atmospherics

Two hypothesis are considered for explaining the scatter of spectral parameters of the atmospherics that can be observed by VLF-Atmospherics-Analyzers:

• 1.1. The scatter results from interference between atmospherics of closely subsequent atmospherics,
• 2.2. The scatter is due to the random variations of the time dependence of the return stroke currents.

Quantitative investigations of both hypotheses are carried out which lead to delimiting ranges of random variations of several physical parameters which are introduced in constructing a mathematical model function for the return stroke current.