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We have investigated the radial dependence of the radial and azimuthal components and the magnitude of the interplanetary magnetic field obtained by the Technical University of Braunschweig magnetometer experiment on-board of Helios-1 from December 10, 1974 to first perihelion on March 15, 1975. Absolute values of daily averages of each quantity have been employed. The regression analysis based on power laws leads to 2.55 γ x r-2.0 , 2.26 γ x r-1.0 and F = 5.53 γ x r-1.6 with standard deviations of 2.5 γ, 2.0 γ and 3.2 γ for the radial and azimuthal components and magnitude, respectively. Here r is the radial distance from the Sun in astronomical units. The results are compared with results obtained for Mariners 4, 5 and 10 and Pioneers 6 and 10. The differences are probably due to different epochs in the solar cycle and the different statistical techniques used.
ARK: https://n2t.net/ark:/88439/y077849
Permalink: https://geophysicsjournal.com/article/113
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The polarization angle of the HELIOS-1 downlink signal has been monitored during two solar occultations in 1975 at two widely separated ground stations. Significant Faraday rotation of the signal occurs whenever the signal ray path passes through the solar corona near superior conjunction. Large-scale variations in the data arise both from rotation of the solar corona and from the slowly changing solar offset (point of smallest heliocentric distance along ray path). A simplified model of the solar corona has been developed to simulate the results of the Faraday rotation measurements. In this model the known polarity of the large-scale interplanetary magnetic field is employed as an aid in determination of the product N · B (electron density x magnetic field) as a function of heliographic longitude and heliocentric distance r within 2-10 R☉ . In this distance range N · B is proportional to r-5.5. If the magnetic field can be assumed to follow an inverse square law over this range, the electron density is found to be decreasing as r-3.5, in good agreement with previous results. The derived longitudinal structure for the corona during both occultations is consistent with synoptic coronal white light observations.
ARK: https://n2t.net/ark:/88439/y065590
Permalink: https://geophysicsjournal.com/article/112
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Qualitative techniques are described by which the domain nature and magnetite-maghemite oxidation state of the iron oxides in basalts may be rapidly identified through measurements of hysteresis and susceptibility over a wide temperature range. Detailed studies made on different suites of basalts have revealed that their magnetic properties in most cases can be explained only on the basis of single-domain behaviour. Also it has been found that the observed variations in these properties between basalts are usually best explained by differences in the position the iron oxide minerals in the different samples occupy along the magnetite-maghemite oxidation chain. These observations suggest that the role of titanium usually found in association with iron oxides in basalts is to subdivide the grains physically rather than to form solid solutions of titanomagnetites or titanomaghemites. Some implications of these results to basalt formation and the magnetic anomalies such rocks could cause are discussed.
ARK: https://n2t.net/ark:/88439/y055011
Permalink: https://geophysicsjournal.com/article/111
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In 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
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Angular variations of seismic velocities have been observed in the Earth and attributed to some form of anisotropy caused by aligned crystals, orientated cracks and inclusions, and laminated strata. The exact analytical expressions for the velocities in each particular symmetry-system, derived from the Kelvin-Christoffel equations, are complicated functions of the elastic constants and cannot be easily manipulated. This paper examines the form of the velocity variations for the several systems of elastic symmetry; five of these seven symmetry-systems have been suggested for possible Earth structures. We shall demonstrate that the approximate equations of Backus (1965) and Crampin (1977 a) are good estimates for the velocity variations in symmetry planes of all symmetry systems, but not in general for off-symmetry planes. These equations are linear in the elastic constants, and provide a convenient link between velocity variations and elastic constants, if used judiciously. The behaviour of shear waves in off-symmetry directions is complicated by pinches, caused by the proximity of shear-wave singularities, where the two shear-waves exchange polarizations. Despite the restrictions to their use, the equations are the fundamental relationship for a number of modelling studies.
ARK: https://n2t.net/ark:/88439/y036503
Permalink: https://geophysicsjournal.com/article/109
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A fundamental problem in the numerical data preprocessing of digital broad-band seismograms is the simulation of arbitrary analog seismograph systems, especially seismometer-galvanometer combinations. A special case of this simulation problem is the deconvolution or restitution problem as the realization of a wide-band seismograph system with a transfer function proportional to ground displacement, velocity or acceleration. The simulation problem can be solved by a digital cascade recursive filter using the bilinear z-transformation. Applications of the simulation filter are: a combined interpretation of digital broad-band and analog narrow-band recordings, a routine analysis of broad-band seismograms consistent with ordinary analog stations, the determination of the local magnitude from simulated Wood-Anderson seismograms and the restitution of broad-band recordings. The relationship between bandwidth, fine structure and information content of seismograms can be demonstrated in an obvious way by comparing broad-band recordings of the Graefenberg-array with simulated seismograms for different standardized seismometer-galvanometer systems.
ARK: https://n2t.net/ark:/88439/y024704
Permalink: https://geophysicsjournal.com/article/108
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Two-dimensional seismic models are used to reproduce the geological sequence cheek-seam-cheek in deep coal mines. The known reflection method based on Rayleigh channel waves is investigated with respect to discontinuities being composed of a termination of the seam and a fractured zone outside the seam. By means of fractured zones having different reflection coefficients discontinuities are designed representing clay hogs or microtectonic faults. Results obtained by investigating amplitudes, frequencies and phase velocities show a strong increase of the rate of amplitude decay with decreasing phase velocity. Furthermore it is demonstrated that the part of the Rayleigh channel wave propagating outside the seam is responsible for the detectability of a discontinuity. Expressing the detectability numerically the ratio ER of the channel wave's energy inside the seam to its total energy has been used. The value 0.5 for ER is derived to be a standard value for distinguishing the phase velocity range for which reliable reflection surveys are possible (ER > 0.5) or not (ER < 0.5).
ARK: https://n2t.net/ark:/88439/y015755
Permalink: https://geophysicsjournal.com/article/107
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The average median valley of the Mid-Atlantic Ridge between 12° and 18° N is described as a smooth depression flanked on both sides by a high. This applies both to the bathymetry and to the gravity anomalies. This picture of the median valley and its walls was obtained by stacking profiles across the valley in 50- to 70-km-wide bands. The reduced median valley can then be interpreted as the result of the parting of the lithosphere and the response of the asthenosphere as a viscous layer to repeated unloading. Fluid dynamic equations show that the response is in general broader than the original load disturbance. We describe this as a viscous lag of the shorter wavelength components. A steady-state solution was reached by numerical methods, showing a depression accompanied by a high on both sides. For the asthenosphere under the Mid-Atlantic Ridge at these latitudes a value followed for the kinematic viscosity of 1.5 x 1019 stokes. The model can be extended to other parts of the mid-ocean ridge system by adapting the time-dependent constants (viscosity and spreading rate). If the viscosity is a factor 5 lower, no median valley results. Rising to isostatic equilibrium of a light body under the floor of the median valley then accounts for the existence of a median ridge like found at Reykjanes Ridge and at the East Pacific Rise. The coefficient of viscosity under the East Pacific Rise would be about 0.4 x 1018 stokes. The concept of a viscous lag of the short-wavelength components replaces Sleep's (1969) original notion of a 'loss of head'. The secondary valleys and ridges found in the median valley and on the flanks of the Mid-Atlantic Ridge crest cannot be explained by the model. They represent essentially a non-continuum process, in which presumably an episodic jumping of the inner valley plays an important role. Additional faulting occurs at the hinge line between the floor and the walls of the median valley.
ARK: https://n2t.net/ark:/88439/y006206
Permalink: https://geophysicsjournal.com/article/106
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In this paper an event of very favourable structure for induction purposes, which was observed by the Scandinavian magnetometer array, is discussed and analysed in detail. The responses derived, in both the time and frequency domains, display a large coast effect at all coastal stations, both on the sea coasts and on the Gulf of Bothnia. Two relatively large inland anomalies are also delineated. The more significant of the two, the Storavan anomaly, is apparent using all the analysis techniques employed, and may be associated with a remnant of the Svionian island arc system. The less dominant anomaly, in the vicinity of Mieron, has no obvious correlation with geology or tectonic formations, and appears to be polarisation sensitive - it is only energised by east-west magnetic fields. First approximation modelling of the coast effect observed by the northwestern stations illustrated that the responses are well satisfied by the conductivity contrast between sea and land. Finally, the validity of the derived induction vectors, and the possible effects of induction for source field studies, are discussed.
ARK: https://n2t.net/ark:/88439/y095007
Permalink: https://geophysicsjournal.com/article/105
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Paleomagnetic measurements made on Eocene-Oligocene intrusives of Chalkidiki (northern Greece) have given normal and reversed directions. The mean direction is D = 37°, I = 30°, α95 = 9° and the pole is 50°N, 140°E. This direction indicates a clockwise rotation of about 25° for Chalkidiki, similar to the rotations observed in western Greece but different from Bulgarian results.
ARK: https://n2t.net/ark:/88439/y081908
Permalink: https://geophysicsjournal.com/article/104
