Journal of Geophysics
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Converted phases from the mantle transition zone observed at European stations
Get PDFConverted phases from the mantle transition zone have been observed as precursors about 1 min before the main S-wave phases S, SKS and ScS in long-period records of the Grafenberg array (GRF) and a few European WWSSN stations, at epicentral distances from 70-90°. The 23 earthquakes used were located along the west coast of America from Alaska to Ecuador, with a concentration of events in Central America, and in East Asia from the Aleutians through Japan to Sumatra. Relatively strong converted energy was observed for the American earthquakes, except for two events in the South Mexico/Guatemala region. The East Asian earthquakes produced significantly less precursor energy. The interference of conversions from P to SV below the focus and from SV to P below the stations is studied with theoretical-seismogram calculations. Due to interference precursors are normally stronger on the horizontal-radial than on the vertical component; this is in agreement with the observations. In special cases with either maximum or minimum P radiation towards the station conversion takes place only on one side, and precursor observations can be related directly to structure either below the focus or the station. The data set includes such favorable cases. The interpretation of observed precursors in terms of the fine structure in the conversion zone is difficult, even in favorable cases, because of the low resolving power of long-period converted phases. Nevertheless, the following conclusions can be drawn from the observations presented. Most of the precursor observations for the American events are compatible with typical models of the transition zone between upper and lower mantle, having two discontinuities at depths of about 400 and 670 km. Such a structure applies for western Europe and for the Caribbean Sea/Gulf of Mexico region, in the latter case with a possible local interruption by a smoother transition zone. A relatively smooth transition zone below East Asia from about Korea to the Sea of Okhotsk can also explain the lack of precursor energy for a few earthquakes in and close to Japan. These results indicate large-scale lateral variations in the sharpness of the mantle transition zone.
ARK: https://n2t.net/ark:/88439/y031913
Permalink: https://geophysicsjournal.com/article/265
 
Ocean tides and periodic variations of the Earth's rotation
Get PDFWe consider the periodic acceleration of the Earth's rotation rate caused by the oceanic M2-tide and the corresponding cumulative effect in universal time UT1. The necessary information stems from a hydrodynamical model of the world's oceans. The time variations of the contributions to the relative angular momentum balance and of the solid Earth's center-of-mass are provided. As for the theoretical aspects, two hypotheses are examined: the 'quasi-isolated-Earth' -hypothesis, which is basic for the derivation of the Δω- and ΔUT1-effects, is acceptable at least as a first-order approximation, whereas the idea of 'locked oceans' is not. The main result is a ΔUT1-contribution with a total range of 0.05 ms, thus nearly detectable by means of modern observational techniques: this effect essentially originates from the tidal currents.
ARK: https://n2t.net/ark:/88439/y059561
Permalink: https://geophysicsjournal.com/article/154
 
Experimental data on electric field and electron density dependence of auroral E-region drift turbulence and radar backscatter
No full textHaldoupis et al. (1982), Uspensky et al. (1983) and Starkov et al. (1983) have recently found that the 140 MHz backscatter amplitude in the auroral ionosphere depends mainly on the mean electron density (height-integrated conductivity) in the E–layer. A similar relationship for the 46 MHz band was also found by Leinonen et al. (in press, 1983). In principle, this can be explained by only a slight dependence of the relative level of E-region drift turbulence < (∆N/ N)2 > in the auroral ionosphere on ionospheric parameters, viz. electric field and mean electron density. However, up to now, quantitative evidence for such a backscatter amplitude — electron density relationship is restricted to the few events which have so far been analyzed. Therefore, the aim of this short note is to add some more data to that data set. Furthermore, we will compare measurements made in the evening and morning sectors, where the conditions for irregularity excitation might be different.
ARK: https://n2t.net/ark:/88439/y063050
Permalink: https://geophysicsjournal.com/article/196
 
Spatial variations of ionospheric conductivity and radar auroral amplitude in the eastward electrojet region during pre-substorm conditions
Get PDFThe dependence of auroral backscatter amplitude on different ionospheric parameters (conductivity, current density, electric field) is studied by means of data recorded by STARE (Scandinavian Twin Auroral Radar Experiment), the two-dimensional Scandinavian Magnetometer Array (SMA), and auroral all-sky cameras. The observations were made on 16 March 1978 during pre-substorm conditions in the region of the eastward polar electrojet. The paper shows that in this event the auroral backscatter amplitudes in the 140 MHz frequency band were controlled mainly by spatial variations in the electron density or conductivity inside the back-scatter volume. To a certain extent also a linear relationship between backscatter amplitude and ionospheric current density was found but it is regarded as a special case of a more general relationship between electron density and backscatter amplitude. A stable discontinuity in the Hall conductivity over the most equatorward auroral arc was deduced from the data: On the equatorward side the conductivity was 3-5 times higher than on the nearby poleward side. Our conclusions are discussed in the light of some previously published results on the same subject.
ARK: https://n2t.net/ark:/88439/y000016
Permalink: https://geophysicsjournal.com/article/262
 
Recent ISEE observations of the magnetopause and low latitude boundary layer: A review
Get PDFISEE-1 and 2 satellite observations at the dayside magnetopause have enabled important progress to be made on the solar wind/magnetosphere coupling problem. The ISEE results have emphasized the significance of reconnection and have indicated that the process can occur both in a quasi-steady form and in an unsteady (flux transfer event (FTE)) manner. The detection of FTEs on open field lines within the magnetosphere, the discovery of reverse polarity FTEs, and the observation that FTEs can be associated closely with intervals of quasi-steady reconnection flow, all support further the view that FTEs are reconnection phenomena. Indeed, a sharp distinction need not exist between FTEs and the type of reconnection event described as quasi-steady.
ARK: https://n2t.net/ark:/88439/y013595
Permalink: https://geophysicsjournal.com/article/181
 
Gaussian beams and paraxial ray approximation in three-dimensional elastic inhomogeneous media
Get PDFThe elastodynamic Gaussian beams in 3D elastic inhomogeneous media are derived as asymptotic high-frequency one-way solutions of the elastodynamic equation concentrated close to rays of P and S waves. In this case, the elastodynamic equation is reduced to a parabolic (Schroedinger) equation which further leads to a matrix Riccati equation and the transport equation. Both these equations can be simply solved along the ray, the first numerically and the other analytically. The amplitude profile of the principal displacement component of the elastodynamic Gaussian beams is Gaussian in the plane perpendicular to the ray, with its maximum at the ray. The Gaussian beams are regular along the whole ray, even at caustics. As a limiting case of infinitely broad Gaussian beams, the paraxial ray approximation is obtained. The properties and possible applications of Gaussian beams and paraxial ray approximations in the numerical modelling of seismic wave fields in 3D inhomogeneous media are discussed.
ARK: https://n2t.net/ark:/88439/y030593
Permalink: https://geophysicsjournal.com/article/142
 
Observation of gravity changes during the passage of cold fronts
Get PDFClear observations of small but abrupt changes in gravity during the passage of cold fronts are reported. Instrumental effects can be ruled out by special experiments. Simple models for the atmosphere show that the direction and the order of magnitude of the observed effects can be explained by changes in gravitational attraction of the sensor mass by the atmosphere or by downward acceleration of the ground due to the increasing air pressure. This is additional evidence for the high importance of meteorological causes for long-period seismic noise.
ARK: https://n2t.net/ark:/88439/y040712
Permalink: https://geophysicsjournal.com/article/143
 
Investigations of the internal geomagnetic field by means of a global model of the Earth's crust
Get PDFA global model of the Earth's crust has been constructed of two-layer blocks of 2° x 2° dimension, with thickness and susceptibility selected from a ten-step classification of the various crustal types. Considering induced magnetization only, the magnetic effect of each block has been approximated by a single dipole in the middle of the block, directed parallel to the field for a given main field model (32,400 dipoles altogether). The magnetic field of this global model of the crust has been calculated for an altitude of 450 km, appropriate for a comparison with anomaly fields from satellite surveys. From field values at 1° x 1° grid points, model anomaly charts for the X, Y, Z-components and for the total intensity can be plotted. The underlying crustal parameters have been prepared for modification towards adjusting the model field to the final Magsat anomaly field. The aim is to construct a truly realistic model of the whole Earth's crust. Based on a global distribution of the Z-component of the model field a spherical harmonic analysis has been made by a direct integral method, up to degree and order 35. The energy density spectrum of the magnetic field, apart from the lowest degree terms, resembles a "white" spectrum in which the level nearly meets that obtained for the crustal part of an actual field model (from n = 15 to 29), except for a factor of less than 3. A supplemental evaluation of the core part of the observed spectrum indicates a source depth of some 100 km below the surface of the Earth's core, supported by a similar result for the secular variation of the core field.
ARK: https://n2t.net/ark:/88439/y060920
Permalink: https://geophysicsjournal.com/article/166
 
On the dependence of radar aurora amplitude on ionospheric electron density
Get PDFRadar aurora amplitudes have been correlated with mean electron density measurements by means of ionosondes during two geophysically widely different events. A similar, roughly linear relationship was found between the two quantities in both cases. On the other hand, the amplitude seems to be almost independent of the ambient electric field, once it is well above the instability threshold.
ARK: https://n2t.net/ark:/88439/y093977
Permalink: https://geophysicsjournal.com/article/189
 
Effect of parallel electric fields on whistler mode waves in Jupiter's magnetosphere
Get PDFObservation of auroral hiss at Jupiter by Voyager I has been suggested as being directly related to regions of parallel electric field and auroral particle precipitation. The effect of a parallel electric field on whistler mode propagation in Jupiter's magnetosphere has been studied. The dispersion relation for whistler mode waves in an isothermal one-component electron plasma in the presence of a parallel electric field has been used to study the growth of whistler mode waves propagating in the Jovian magnetosphere. The growth rates have been computed by using the observed plasma parameters at 5.6 Rj . The growth rate, which is found to be a maximum in the equatorial magnetosphere, is reduced to zero in the absence of the electrostatic field. This has lead us to conclude that, in the case of isothermal magnetosplasma, the growth rate is induced by the electrostatic field.
ARK: https://n2t.net/ark:/88439/y007466
Permalink: https://geophysicsjournal.com/article/16