Journal of Geophysics
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Analogue-model magnetic field responses of an ocean channel, an island and a seamount in the Hainan Island region
Get PDFThe electromagnetic responses of a shallow channel (Hainan Strait), an island (Hainan Island) in a shallow coastal sea, and a large flat-topped seamount (Zhongsha Islands) in a deep ocean were studied using a scaled laboratory analogue model. To examine the responses, in-phase and quadrature Hz and HY magnetic fields are presented for traverses over the channel, the island and the seamount for simulated geomagnetic variations with periods in the range 5–500 min. The in-phase Hz and HY channel and island responses were found to decrease rapidly with increasing period, reaching negligible values at about 60-min periods, while the in-phase Hz and HY seamount responses had significant values over the entire 5–500-min period range. The quadrature Hz and HY channel and island responses had maxima at a period of approximately 20 min when the 0.25 km depth ocean in the channel and surrounding the island was 0.025 skin depth (δ). The shapes of the quadrature Hz and HY seamount response curves showed a transition from a "channel response" to an "island response" at a period of approximately 20 min, the same period at which maximum in-phase responses occur. At this period the surrounding ocean depth is 0.2–0.4 δ and the ocean depth over the seamount is 0.05 δ. The quadrature Hz and HY seamount responses were each at a maximum at approximately 100 min, when the surrounding ocean depth was 0.1–0.2 δ and the ocean over the seamount is 0.025 δ. The addition of a conducting plate to simulate conducting mantle structure at a depth of 100 km led to much less attenuation for the case of the seamount than for the channel or island due to the deep ocean surrounding the seamount, effectively shielding the conducting mantle from the overhead primary source field.
ARK: https://n2t.net/ark:/88439/y035553
Permalink: https://geophysicsjournal.com/article/302
 
Ground-satellite coordinated study of the April 5, 1979 events: observation of 0+ cyclotron waves
Get PDFIn a period of particularly large magnetospheric disturbance, large amplitude pc2 waves were observed in the late dawn sector on GEOS-2 and in the vicinity of the foot-point of the satellite's field line. The waves have dominantly left-hand polarization and their frequency is closely correlated to the O+ gyrofrequency in the magnetosphere. After the start of the pc2 activity, the GEOS-2 particle detectors measured an enhanced flux of energetic O+ ions in the energy range from 0.9-16 keV. By calculating the dispersion of ion cyclotron waves in a multicomponent plasma, it is shown that the energetic O+ ions can destabilize the observed pc2 waves.
ARK: https://n2t.net/ark:/88439/y043392
Permalink: https://geophysicsjournal.com/article/110
 
Domain state of Ti-rich titanomagnetites deduced from domain structure observations and susceptibility measurements
Get PDFDomain structure observations and measurements of the temperature dependence of susceptibility on synthetic and natural titanomagnetites are reported. At room temperature Ti-rich titanomagnetite particles (x ≈ 0.6) of MD grain size normally develop a very complicated domain structure. The experimental results of our investigations, in addition to simple theoretical calculations, indicate that internal stress is the dominant cause of the observed anomalous domain patterns. Part of the results have already been published by Appel and Soffel (1984). The paper presented here, however, is a far more extended summary of the actual state of our research.
ARK: https://n2t.net/ark:/88439/y026104
Permalink: https://geophysicsjournal.com/article/212
 
Frequency dependence of Q for seismic body waves in the Earth's mantle
Get PDFIn this paper an attempt is made to determine the frequency dependence of Q in the Earth's mantle in the frequency range 0.03–1.5 Hz from the spectral ratio of teleseismic S- and P-waves. Digital broad-band data of 17 earthquakes at 40° < ∆ < 90° recorded at the Central Seismological Observatory of the Federal Republic of Germany at Erlangen were analysed. The method implies the following assumptions: frequency independence of the crustal transfer function, proportionality of Qp (f) and Qs (f), and proportionality of P- and S-source spectra. This last and most critical assumption was carefully investigated by kinematic and dynamic source models. The calculated Q-spectra for the individual events vary considerably but all have in common a general increasing trend with frequency which can best be represented by a power law Q ≈ f α with 0.25 < α < 0.6. A further increase in slope near 1 Hz suggests an absorption band corner with an upper cut-off relaxation time τm = 0.33 ± 0.18 s. The significance of the Q-spectra and their variability is estimated by manipulating semi-synthetic seismograms with different error-producing processes such as length and shape of the time window, superposition of noise, digital filter process and source spectra. It is concluded that none of these processes is able to destroy or to imitate the observed increasing trend of Q with frequency. The results are compared with those from other seismological investigations and from laboratory experiments on mantle rocks at high temperature and in the seismic frequency band.
ARK: https://n2t.net/ark:/88439/y081538
Permalink: https://geophysicsjournal.com/article/270
 
Magnetospheric hydromagnetic waves: their eigenperiods, amplitudes and phase variations; a tutorial introduction
Get PDFThe detection of hydromagnetic waves on spacecraft and at the Earth's surface indicate disturbances of the geomagnetosphere. These disturbances are diagnostic of processes and boundaries occurring within the plasma of the Earth's space environment. In order to delineate the processes and boundaries a dense network of measuring points is desirable. Parameters that have proved useful are the frequency and the relative amplitudes and phases of the waves at different positions in space. This paper summarises some of the characteristics of hydromagnetic waves in the magnetosphere and the modification of the signal brought about by the boundary conditions imposed by the ionosphere-neutral atmosphere-Earth system.
ARK: https://n2t.net/ark:/88439/y039423
Permalink: https://geophysicsjournal.com/article/243
 
Deep electromagnetic studies of the Baltic Shield
Get PDFThe first systematic collection of magnetotelluric, magnetovariational and frequency sounding results for the Baltic Shield is presented. The great variations in data processing and interpretation methodology make it difficult to present any unified summary of the existing results. Conductive regions in the crust occur in all parts of the shield; sometimes in connection with schist zones, variations in bedrock structure and composition, and sometimes as apparent conductive layering in the depth range 15-25 km. The extent of this layer, as well as the conductive layer at asthenospheric depths in the northern parts of the shield, cannot yet be established. The usefulness of combining electromagnetic and other geophysical data is indicated by the example from central Finland, where deep seismic sounding results seem to be able to explain long-period anisotropies of magnetotelluric sounding curves.
ARK: https://n2t.net/ark:/88439/y024024
Permalink: https://geophysicsjournal.com/article/295
 
Rheological properties and velocity dispersion of a medium with power-law dependence of Q on frequency
Get PDFThe Kramers-Kronig relations for magnitude and phase of a linear causal filter are used to derive an exact general expression for the viscoelastic modulus M, corresponding to power laws for the quality factor, Q ≈ ωγ. The exponent γ varies from -1 to + 1, such that the spectrum of rheologies extends from a Kelvin-Voigt to a Maxwell body. High- and low-frequency approximations for M(ω) are derived, and in the special cases γ = ±1, ±1/2, ±1/3, ±1/4,... closed-form solutions are given which apply for arbitrary frequencies. With M(ω) at hand, both high-frequency phenomena such as velocity dispersion and low-frequency phenomena such as creep and stress relaxation can be investigated. Results for phase-velocity dispersion are given as well as short- and long-time-scale approximations of the creep and relaxation functions. Simple dissipation operators are derived which can be convolved with theoretical seismograms in order to correct these for the influence of absorption. Some results on relaxation spectra for the case 0 ≤ γ ≤ 1 are summarized in an appendix. Taken together, the results of this paper suggest that media with 0 < γ < 1 should be considered as generalized Maxwell bodies and media with -1 < γ < 0 as generalized Kelvin-Voigt bodies. Application of the Kramers-Kronig relations to the viscoelastic modulus is better than the use of those relations in conjunction with the wavenumber of a plane wave, which is the procedure that has been employed so far.
ARK: https://n2t.net/ark:/88439/y076559
Permalink: https://geophysicsjournal.com/article/82
 
Seismic velocities of granulites from the Seiland Petrographic Province (N. Norway): Implications for Scandinavian lower continental crust
No full textCompressional and shear wave velocities have been measured in the laboratory at up to 1.0 GPa effective pressure on a suite of granulite facies rocks from the Seiland Petrographic Province. The suite may be of Precambrian age and the measurements were made to test a proposal that the suite represents lower continental crust. Compressional wave velocities at 0.4 GPa effective pressure range from 6.41–6.97 km s-1 with a mean of 6.71 km s-1 . Measurements of the parameter (∂V/∂Tp) suggest a value of a –0.8 x 10-3 km s-1 ºC-1 , and the temperature corrected mean velocity of about 6.5 km s-1 at 20 km depth is comparable to that found in parts of the lower crust in Scandinavia. The mean Poisson's ratio of 0.29 at 0.4 GPa is slightly higher than that found (0.276), but could be reduced if heterogeneity in the suite and the effect of higher pressures are taken into account. The occurrence of lower continental crust as a thrust slice in the Province is consistent with current ideas on the geological evolution of this area.
ARK: https://n2t.net/ark:/88439/y080958
Permalink: https://geophysicsjournal.com/article/260
 
The upper mantle structure under South-East Europe derived from GRF broadband records of Greek earthquakes
Get PDFBroadband recordings from the GRF (Grafenberg) Array of the strongest earthquakes from Greece are examined. A P-wave seismogram section of a number of events in the range between 1,300 and 2,100 km epicentral distance is constructed. The dominant feature in this section is the second arrival, which is the reflection from the 400 km discontinuity. Characteristic amplitude changes of this phase across the array are observed. The apparent velocity across the array of the first arriving P phase is very slow, indicating a slower upper mantle in SE Europe than in other regions. There is also a very weak indication of a third phase. The resulting model of the upper mantle, which was derived with the aid of theoretical seismograms, shows a pronounced discontinuity at a depth of 400 km. The time difference between the observed first two phases can be used for a fast estimation of the epicentral distance.
ARK: https://n2t.net/ark:/88439/y035633
Permalink: https://geophysicsjournal.com/article/19
 
On the influence of ionospheres with non-uniform conductivity distribution on hydromagnetic waves
Get PDFA theoretical model is proposed which describes the influence of non-uniform ionospheric height-integrated conductivity distributions on VLF-pulsations. The assumption is made that the field-aligned currents carried by the wave-field are closed by polarisation currents in the magnetosphere and by the irrotational part of the height-integrated ionospheric currents. Current continuity at the magnetosphere-ionosphere boundary provides for a differential equation governing the reflected electric field for arbitrary non-uniform conductivity distributions. Model calculations for simple, but realistic conductivity and electric field distributions show that local shifts of the ionospheric field maximum against that of the magnetic field below the ionosphere as well as double-peak distributions of the electric field can occur. Strong electric field anomalies i.e. significant deviations of the electric field distribution as compared with the uniform case occur in conductivity gradient zones and fall off rapidly outside. The previously predicted 90° rotation between the magnetic field below and above the ionosphere does not hold generally because the rotation angle depends strongly on the conductivity gradients.
ARK: https://n2t.net/ark:/88439/y072759
Permalink: https://geophysicsjournal.com/article/157