Anomalous propagation of VHF radiowaves behind the horizon in the seismic region

A theory of electromagnetic radiation generation by random electric discharges in the troposphere and VHF radiowave scattering by these discharges has been developed. The discharge model, which makes it possible to calculate the spatiotemporal distribution of the discharge channel conductivity depending on the electric current value in this discharge, has been obtained. The electromagnetic radiation spectrum in the troposphere occupied by random discharges has been calculated. VHF electromagnetic wave scattering by random electric discharges in the troposphere has been considered. Equations have been derived, and the method for calculating the average value of the electromagnetic wave field scattered by random discharges has been developed. The calculations indicated that the scattered wave field amplitude is much larger than the diffraction wave field amplitude behind the horizon. The theoretical results agree with the observations of the electromagnetic radiation and VHF transmitter signals behind the horizon relative to the earthquake epicenter during the earthquake preparation.     

Rapid variations in echo power maps of VHF radar backscatter from the lower atmosphere

For the first time, echo power maps of aspect-sensitive VHF backscatter are shown, with good time and spatial resolutions, for angles 0°–7° from zenith. Sequences of power maps show large changes in appearance over timescales of a few minutes and height intervals of a few hundred metres. Often, individual power maps are consistent with tilted and distorted specular-type scattering layers, rather than anisotropic turbulence, and the direction of maximum echo power is sometimes several degrees off-vertical. Nevertheless, after time-averaging the variable echo-power patterns, the average pattern can become almost circular and centred on zenith, as has been assumed in the past. Echo power maps measured in strong windshear beneath the jet stream show a skewing of the echo power distribution. However, some power maps in the lower stratosphere, despite stronger wind shear, appear more constrained and their maximum echo power remains closer to zenith.     

地电场方法在川滇地区地震观测中的应用

利用云南省(5个)和四川省(4个)十五期间布设的地电场观测数据,利用大地电场观测系统中,同一测向长短极距测值的比值方法进行分析,发现能够较好地排除来自大地电场及自然电场远场的变化信息,突出可识别的地震前兆异常信息。研究结果表明:在剔除各种环境和空间电流体系的影响因素后,提取了与地震有关的异常信息,在川滇地区5.0级地震前,多数台站的地电场观测资料比值发生异常变化,异常大多出现在地震前0.5—3个月,其中汶川地震和姚安地震前,研究区的9个台站均出现异常。     

On the relation between high frequency acoustic emissions in near-surface rocks and the electric field in the near-ground atmosphere

A field instrument package was installed for synchronous measurements of acoustic emission in rocks at frequencies of 0.1–10000 Hz and the vertical gradient of electric potential in near-ground atmosphere. These investigations for the first time revealed a relationship between emission disturbances in the kilohertz frequency range due to deformation of near-surface rocks and the electric field. The relationship may be observed both during seismically quiet periods and at the final phase of earthquake precursory periods.     

VHF radar observations of turbulent structures in the polar mesopause region

The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.     

Characteristics of VHF scintillations in the equatorial anomaly crest region in India

The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h‘F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported.     

Oxygen and water vapor absorption of radio waves in the atmosphere

Summary Calculated values of the gaseous atmospheric absorption are presented for the frequency range 100 to 50,000 Mc at elevations above ground up to at least 130,000 feet, for average conditions during February and August at Bismarck, N. D. and Washington, D. C. Total radio path absorptionsare presented for tropospheric forward scatter communication links for distances of 100, 300 and 1000 miles. The total path absorptions were calculated by summing the absorption contributed by each portion of the atmosphere traversed by a radio ray passing from a 60 foot parabolic antenna resting on the ground to the scattering center and then to a similar receiving antenna. A correlation of total path absorption with the surface value of absolute humidity is developed, thus providing estimates of the range of absorption values in different geographic areas. Maps of average absolute humidity for the world are presented. Previous work on rain absorption is then combined with the present study to provide estimates of the radio power loss due to absorption expected to be exceeded 1 per cent of the time.Portions of this paper were presented in preliminary form at the Symposium on Communication by Scatter Techniques-George Washington University, Washington, D. C., November 14 and 15, 1955.     

Excitation of Rossby waves by HF electromagnetic seismic origin emissions in the earth's mesosphere

Interaction of high-frequency seismo-electromagnetic emissions with the weakly ionized gas of the ionospheric D-layer is considered. It is shown that through the earth's ionosphere weakly damped high-frequency electron cyclotron electromagnetic waves can propagate. These new type of waves easily reach the ionospheric D-layer where they interact with the existing electrons and ions. Acting on electrons ponderomotive force is taken into account and corresponding modified Charney equation is obtained. It is shown that only nonlinear vortical structures with negative vorticity (anticyclone) can be excited. The amplitude modulation of electromagnetic waves can lead to the excitation of Rossby waves in the weakly ionized gas. The corresponding growth rate is defined. Depending on the intensity of the pumping waves generated by seismic activity different stable and unstable branches of oscillations are found. Detection of the new oscillation branches and energetically reinforcing Rossby solitary vortical anticyclone structures may be serve as precursors to earthquake.     

Perturbations of the upper atmosphere in the cleft region

In the dayside polar region—loosely referred to as the cleft region—particle precipitation and Joule heating cause significant perturbations of the upper atmosphere. Here Dynamics Explorer-2 satellite data are used to present a synopsis of these disturbance effects. Documented are an increase in electron temperature and a decrease in electron density; increases in ion drift speed and ion temperature; an increase in the upward-directed ion velocity; increases in zonal wind speed and neutral gas temperature; and changes in the neutral gas composition and mass density. It is suggested that the increase in electron temperature is partly controlled by the decrease in electron density; that the ion upflow velocity mainly depends on the electron temperature, less frequently on the ion temperature; and that the observed decrease in thermospheric mass density is due to a decrease in the atomic oxygen density, which in turn is caused by diverging wind flows.     

The atmospheric electric ring current in the higher atmosphere

Summary The atmospheric electric current flow in the ionosphere was discussed in a qualitative way at the UGGI General Assembly at Berkeley, California in 1963. The following picture emerged: The atmospheric electric fair weather current leaves the earth in a radially outward direction. As it enters the higher regions of the atmosphere and the ionospheric it is increasingly influenced by the earth's magnetic field. Because the main part of the current is crowded into the polar regions, the current density over the equatorial belt is small. A circular movement around the earth's axis results in an overall flow pattern tentatively termed, the atmospheric electric ring current. An attempt to calculate this current flow soon made it clear that the generally used simplification of the one-dimensional case with slanted magnetic field lines is not adequate—not even as a first approximation. The same is true for the assumption usually made in magnetohydrodynamics that the current follows approximately the magnetic field lines. An essential feature of the atmospheric electric ring current is that in equatorial regions the flow is forced across the magnetic field lines, the component along the lines being zero. A calculation is discussed that treats the magnetic field lines as those of a true dipole field with the corresponding tensor character of conductivity. The results of the calculation are presented as graphs of the density distribution of the ring current, the space charge distribution, the current flow, and equipotential lines.

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Zusammenfassung Der luftelektrische Stromfluss in der Ionosphäre ist in qualitativer Weise während der UGGI Tagung in Berkeley California, 1963 diskutiert worden. Hierbei hat sich das folgende Bild ergeben: Der luftelektrische Schönwetterstrom fliesst von der Erdoberfläche nach ausswärts in radialer Richtung. Sobald er in die höheren Atmosphärenschichten und dann in die Ionosphäre kommt wird er in zunehmendem Masse vom erdmagnetischen Feld beeinflusst. Der Hauptteil des Stromes wird in die Polarzonen abgedrängt, wodurch die Stromdichte über dem Äquatorgürtel verhältnismässig klein wird. Zu gleicher Zeit wird eine kreisförmige Bewegung um die Erdachse ausgelöst, was ein Strombild ergibt, das versuchsweise der luftelektrische Ringstrom genannt wird.—Bei der Berechnung dieses Stromflusses ergab sich bald, dass die allgemein üblichen Vereinfachungen des eindimensionalen Falles mit homogenem, schräg einfallendem Magnetfeld nicht brauchbar sind, nicht einmal in erster Näherung. Dasselbe gilt für die Annahme, die gewöhnlich in der Magnetohydrodynamik gemacht wird, nämlich dass der Stromfluss angenähert dem magnetischen Felde folgt. Eine wichtige Eigenschaft des luftelektrischen Ringstromes ist es, dass der Strom über dem Äquatorgürtel gezwungen ist quer über die magnetischen Feldlinien zu fliessen, wobei die Stromkomponente in Richtung der Feldlinien gleich 0 ist. In der hier durchgeführten Rechnung wird das magnetische Feld als wahres Dipolfeld behandelt mit dem einer solchen Feldverteilung entsprechenden Tensorcharakter der Leitfähigkeit. Die Ergebnisse der Rechnung werden an Hand von graphischen darstellungen der Ringstrom- und Raumladungsdichte und der Strom- und Äquipotentiallinien diskutiert.

     

On the statistical correlation of over-horizon VHF signals with meteorological radio ducting and seismicity

With the use of a complete 1 year (2005) data on over-horizon VHF signals (transmitted in Sendai and received in Chofu), we have investigated the statistical significance on the correlation of VHF signal characteristics (amplitude and variance) with meteorological radio duct effect and seismicity (earthquakes). It is found that the meteorological radio duct effect is rather predominant in July–September. On the other hand, over-horizon VHF signals are found to be correlated with earthquakes in other seasons. Especially, when we narrow our analysis area just around the middle region of the great-circle path, we have always significant correlation between VHF signal characteristics (average, median intensity or variance) with seismicity in any months of the year, with exceeding the confidence level.     

Planetary waves in coupling the lower and upper atmosphere

The purpose of the paper is to answer the question if planetary waves (PW) are capable of propagating into the thermosphere. First the simplest vertical structure equation of the classic tidal theory accounting for a realistic vertical temperature profile is considered. Analysis and simulation show that the well-known normal atmospheric modes (NM), which are trapped in the lower and middle atmosphere, exhibit a wave-like vertical structure with a large vertical wavelength in the thermosphere. Moreover, the reflection of these modes from the vertical temperature gradient in the lower thermosphere causes appearance of the wave-energy upward flux in the middle atmosphere, and in a linearized formulation this flux is constant above the source region. To investigate a possibility of the NM forcing by stratospheric vacillations and to consider the propagation of different PW up to the heights of the upper thermosphere, a set of runs with a mechanistic Middle and Upper Atmosphere Model has been performed. The results of the simulation show that quasi-stationary and longer-period PW are not able to penetrate into the thermosphere. The shorter-period NM and ultra-fast Kelvin wave propagate up to the heights of the lower thermosphere. However, above about 150 km they are strongly suppressed by dissipative processes. The role of the secondary waves (nonmigrating tides) arising from nonlinear interaction between the primary migrating tides and quasi-stationary PW is discussed. We conclude that PW are not capable of propagating directly up to the heights of the ionospheric F2 region. It is suggested that other physical processes (for instance, the electrostatic field perturbations) have to be taken into account to explain the observed PW-like structures in ionospheric parameters.     

Generation of electrostatic emissions by lightning-induced whistler-mode radiation above thunderstorms

Data from two separate thunderstorm sounding rocket experiments – Thunderstorm-II (T-II), which involved two rockets, and Thunderstorm-III (T-III) – are compared. In both cases, the sounding rockets over-flew active storm cells, but the bottomside of the ionosphere had a lower density and was much more structured for the first experiment than for the second. Electric field measurements on the flight through a structured ionosphere (T-II) show a triggered emission that has a height variation with altitude that seems to track the lower hybrid resonance frequency (LHR). Theories and other experimental data are presented supporting the concept that LH waves can be stimulated by intense whistlers when density gradients are present, and we interpret these T-II data in just such a context. We believe these emissions may be responsible for the irregularities causing the remote-sensing phenomenon called explosive spread F. The T-III flight had no such height-dependent emissions, which we attribute to the smoothness of the medium during that flight. Curiously, long-lasting emissions also occurred during T-III, but at frequencies that were constants with height and harmonics of the cut-off frequency for the earth-ionosphere wave guide. To our knowledge, there is no existing theory against which to test the T-III emissions.     

Radiosonde observations of high-latitude planetary waves in the lower atmosphere

The characteristics of high-latitude planetary waves (PWs) in the troposphere and lower stratosphere (TLS) are studied by using the data from radiosonde observations during 1998 to 2006 at three Alaskan stations in USA (Nome, 64.50°N, 165.43°W; McGrath, 62.97°N, 155.62°W; Fairbanks, 64.82°N, 147.87°W). It is found that strong PWs exist in two regions. One is around tropopause, and the other is in the polar night jet (PNJ) in winter. The PW activities are rather intermittent, and their lifetimes are no longe...     

Estimation of seismic hazard in the Kaliningrad region

The paper discusses problems of seismic zoning of the Kaliningrad region, where a series of perceptible earthquakes occurred in 2004; the strongest event had a magnitude of M _s = 4.3 and produced shakings of an intensity of 6 in the coastal zone of the Sambiiskii Peninsula, classified as a 5-intensity zone. The enhanced seismic effect is shown to be caused by bad ground conditions, long-term action of seismic effects, resonance phenomena, and other factors. To gain additional constraints on the seismic hazard degree in the Kaliningrad region, the paper discusses an improved version of the model of earthquake sources underlying the compilation of normative maps of seismic zoning (OSR-97). Modified fragments of OSR-97 probability maps of the Kaliningrad region are constructed at different levels of probability that the seismic effect indicated in the maps will be exceeded over 50 yr. It is shown that additional seismological investigations should be conducted in this region.     

Influence of the upper section on the amplitude-frequency content of a seismic signal by the example of seismic stations in the Baikal and the Transbaikal region

The amplitude-frequency characteristics of the upper section of all permanent seismic stations in the Baikal and the Transbaikal region were studied. These data were required to solve certain inverse seismological problems and in connection with the fact that the accelerograms recorded at these stations are used for zoning of sections of the Baikal and the Transbaikal region. Frequency characteristics were obtained using computational and experimental methods. Seismic and geoelectric surveys were carried out; the data were used to obtain geophysical profiles of the upper part of the sedimentary column for most of the seismic stations. These profiles allowed estimating the amplitude-frequency characteristics. The H/V spectral ratios were treated as an experimental way to obtain the frequency characteristics. The frequency characteristics of the upper part of the section underneath seismic stations in the Baikal region were found for the first time.     

Three-dimensional Crustal model of the Moravo-Silesian region obtained by seismic tomography

All recent available data sets containing traveltimes of Pg and Sg phases for the Moravo-Silesian region were collected. These data come from several experiments designed mostly in 2D (in-line) configurations. Simultaneous processing of all data is possible only considering the true 3D measurement geometry. The goal of this work is to establish the representative 3D seismic model by means of seismic tomography. The resulting minimum-structure model is presented in a simple and easy to use form on the Internet. The amount of processed data is relatively low but repeating the computations with new experimental data included would be very easy. Important material related to this paper is placed on web pages.