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Laboratory experiments, each lasting several weeks, have been conducted to establish the characteristics of viscous remanent magnetization (VRM) in oceanic basalts from many sites of the Deep Sea Drilling Program (DSDP). VRM is most pronounced in low-coercivity basalts whose natural remanences (NRM) have low median destructive fields, less than 100 Oe. A simple logarithmic acquisition law is rarely obeyed, but two or three distinct stages are instead observed, in each of which a logarithmic dependence of VRM intensity on acquisition time may be assumed. This observation leads to a simple interpretational model for the nature of VRM in DSDP basalts, but also implies that extrapolation of laboratory observations to geological times is not meaningful. Instead, the ratio of laboratory VRM (acquired in a 1 Oe field during 1000 h) to NRM is used as a minimum indicator of the potential seriousness of VRM. Experiments show that VRM acquired in the presence of NRM is more serious than VRM acquired in alternating field (AF) demagnetized samples. As most published VRM data in DSDP basalts were obtained after AF demagnetization, these are regarded also as minimum estimates of the significance of VRM acquired by oceanic basalts in situ. The consequences of the common occurrence of such an unstable component of magnetization in the oceanic basalt layer are considered in relation to the nature and distribution of oceanic magnetic quiet zones. The Cretaceous, and possibly the Jurassic, magnetic quiet zones are considered adequately explained by constant paleomagnetic field polarity. However, if VRM is a substantial and widespread magnetization component in the oceanic crust, it may not always be appropriate to interpret oceanic magnetic anomalies (or their absence) as an exact record of paleomagnetic field behavior. Remagnetization of the oceanic crust by VRM acquisition may be a viable alternative explanation of the origin of the marginal magnetic quiet zones.
ARK: https://n2t.net/ark:/88439/y064370
Permalink: https://geophysicsjournal.com/article/206
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During the interval 0400-1600 UT on August 31, 1978 strong pc5 activity was observed in the morning and afternoon sector of the magnetosphere. Using data from a world-wide network of ground-magnetometer stations and from the geostationary satellites GEOS 2, GOES 2 and 3 as well as the satellite pair ISEE 1 and 2 a case study was performed with special respect to the question of possible source mechanisms responsible for the observed pulsations. Like earlier workers we came up with the result that the Kelvin-Helmholtz instability at the magnetopause or in the low latitude boundary layer is a likely candidate. In particular we found a change in the azimuthal phase propagation direction from westwards on the morning side and eastward on the afternoon side. Also the sense of polarization and the azimuth of the ground magnetic disturbance in the horizontal plane changed across the meridian of the stagnation point as predicted by a Kelvin-Helmholtz instability source mechanism. A more detailed analysis was carried out for an isolated, large amplitude pc5 event at 1040 UT. At the same time magnetic field observations from the ISEE satellite pair indicate a flux transfer event (FTE) like disturbance at the magnetopause. Also indications of magnetopause boundary oscillations were found, and we feel that this FTE-like event constitutes a possible source for the observed impulsive pc5 event recorded on the ground and on GEOS 2. Furthermore, as the observed pc5 pulsations exhibit a wave-packet structure, we studied whether these wave packets coincided with substorms or substorm intensifications observed simultaneously in the nighttime magnetosphere. However, only a partial one-to-one correlation was found.
ARK: https://n2t.net/ark:/88439/y054611
Permalink: https://geophysicsjournal.com/article/205
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Flat-bedded, shallow water, marine limestones interlayered with marls are well exposed in Franconia and Swabia, Southern Germany. The Upper Jurassic sediments sampled have been dated in detail palaeontologically, and cover the interval from middle Lower Oxfordian to middle Lower Kimmeridgian. Bed by bed sampling of sections at 11 localities throughout a thickness of more than 130 m yielded about 400 samples (1100 specimens) for palaeomagnetic analysis. The NRM intensities appear to correlate with the clay content and averaged 2 x 10–7 Gauss. Progressive alternating field demagnetization of each specimen was used to isolate the direction of the characteristic remanence, whose mean intensity averaged 5 x 10–8 Gauss. Only normally magnetized rocks are found in the lower part of the section, but distinct zones of normal and reversed polarity are found in the upper part (middle Lower Kimmeridgian) in both regions of investigation. The polarity sequence is tentatively correlated with the ocean floor Mesozoic magnetic anomaly sequence. The data yield a reliable estimate of a Late Jurassic palaeomagnetic pole position for stable Europe.
ARK: https://n2t.net/ark:/88439/y044102
Permalink: https://geophysicsjournal.com/article/204
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In the laterally heterogeneous and anisotropic Earth the particle motions of seismic waves are three-dimensionally polarized. This paper presents a method for determining a local wave coordinate system using the zeros of some component operators in rotated systems. It is applicable to broad-band composite waveforms and offers higher precision and resolution for the determination of azimuth and incidence angle than the usual least-squares techniques. The algorithms have been implemented in an interactive program for preprocessing three-component digital recordings. In addition, the program allows the use of particle motion diagrams and component products for the determination of wave types, onset times and pulse durations. Its performance is demonstrated on regional and teleseismic events recorded at the European broad-band stations GRF (Grafenberg, West Germany), KRC (Kasperske Hory, Czechoslovakia) and KSP (Ksiaz, Poland).
ARK: https://n2t.net/ark:/88439/y034913
Permalink: https://geophysicsjournal.com/article/203
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A Finite Element Method for solving the convection problem in a fluid with position-dependent Newtonian viscosity is developed, using bicubic and biquadratic spline functions on a rectangular grid. Introducing weak (less viscous) zones at the active margins of the lithosphere, dynamical mantle convection models are established which have a nearly uniform surface (plate) velocity and a satisfactory heat flux profile. A comparison of upper and deep mantle convection shows:— a moderate increase of viscosity with depth cannot confine the flow to the upper mantle;— in shallow depth convection models the temperature is too low inside the cell, but deep mantle convection models yield satisfactory temperatures for the upper mantle.For that reason deep (or whole) mantle convection should be the favored hypothesis.
ARK: https://n2t.net/ark:/88439/y025744
Permalink: https://geophysicsjournal.com/article/202
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Magnetograms from the Scandinavian Magnetometer Array (SMA) and ionospheric electric field measurements from the Scandinavian Twin Auroral Radar Experiment (STARE) have been evaluated during the passage of several omega bands (or eastward travelling surges) over Northern Scandinavia around 0400 MLT on 16 February 1977. The eastward motion of the omega bands was a pure E × B drift and was associated with Ps6 disturbances of the east-west component particularly of magnetic and electric fields. The two-dimensional distributions of electric and magnetic disturbance fields strongly support the hypothesis of a three-dimensional current system which is embedded in a rather homogeneous westward Hall current and where north-south aligned localized regions of field-aligned currents, flowing upward close to the omega band's wave-crest and downward east and west of it, are travelling eastward with the omega band. These field-aligned currents are associated with eastward and westward electric field disturbances and southward and northward Hall currents between them. It remains unclear whether these Hall currents close within the ionosphere, i.e. forming counterclockwise and clockwise ellipsoidal Hall current vortices, or if they diverge from the ionosphere as field-aligned currents at the southern and northern boundary of the region that is disturbed by the omega band.
ARK: https://n2t.net/ark:/88439/y015335
Permalink: https://geophysicsjournal.com/article/201
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It is generally agreed (Gwinner, 1971) that the Northern Calcareous Alps (together with the Northern Grauwackenzone) rest allochthonously on top of a Penninic basement and have been transported into their present position from the South after Upper Eocene. Their complicated structure is characterized by a series of nappes of various lateral extensions and thicknesses. A sketch map of the Western part of the Northern Calcareous Alps (Fig. 1) shows the main nappes (after Gwinner, 1971).
ARK: https://n2t.net/ark:/88439/y004586
Permalink: https://geophysicsjournal.com/article/200
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New information on the age of the volcanics from the Colli Euganei and Monti Lessini (Northern Italy) collected by Channell et al. (1978) show that the divisions made by Soffel (1972, 1974, 1975a) of palaeomagnetic directions from the two areas into Eocene and Oligocene groups had to be modified. From the data of both sampling areas alone an anticlockwise rotation of the area between Eocene and Oligocene can no longer be clearly demonstrated. However, a combination of the predominantly Oligocene data from the Colli Euganei with the predominently Eocene data from the Monti Lessini as well as their comparison with corresponding palaeomagnetic data from Stable Europe brings enough statistical evidence for an anticlockwise rotation of the area of at least 20-30° between Upper Eocene and Lower to Middle Oligocene.
ARK: https://n2t.net/ark:/88439/y094107
Permalink: https://geophysicsjournal.com/article/199
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The extended range 7 elements (ER7) seismic array, ILPA, has been installed in Iran. Construction was started in the winter of 1973 as a joint undertaking of the governments of Iran and the United States of America. This array began its operation in January 1976.
ARK: https://n2t.net/ark:/88439/y083518
Permalink: https://geophysicsjournal.com/article/198
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The global geomagnetic secular variation in the l960s and l970s was characterized by a rather rapid intensification which was shown to be of internal origin (Nevanlinna, 1980). An external 11-year wave could also be identified (Nevanlinna, 1980; Yukutake and Cain, 1979). In studying the global secular variation for the last few years, an even more rapid global change can be identified, namely a sudden enhancement followed by a deficit, lasting roughly two years from the end of 1977 to 1979. Here we demonstrate that the pulse is of external origin and that it resembles an intensification of the magnetospheric ring-current system lasting for about two years.
ARK: https://n2t.net/ark:/88439/y074919
Permalink: https://geophysicsjournal.com/article/197
