Journal of Geophysics
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Bursts of irregular magnetic pulsations during the substorm
Get PDFNew characteristics of PiB magnetic pulsations which are associated with the substorm in the local midnight sector are revealed from the recordings of a meridional network of ground stations. PiB's are observed during distortions of the auroral arc which enclose a local region of upward directed field-aligned current. An enhancement at about 0.3–0.4 Hz is shown to be a permanent feature of PiB's. In addition, a secondary amplitude maximum at about 1 Hz sometimes appears in the recordings far from the auroral zone. The generation of PiB pulsations in the Pi1 period range is located at auroral latitudes at a low altitude. Low-altitude electric fields and acceleration of auroral particles, low-altitude resonance cavities, and isotropic propagation of waves is discussed in the interpretation of PiB pulsations.
ARK: https://n2t.net/ark:/88439/y043502
Permalink: https://geophysicsjournal.com/article/287
 
Ray theoretical seismograms for laterally inhomogeneous structures
No full textApplications of the ray method to the construction of theoretical seismograms for laterally varying layered structures are discussed. Numerical examples are presented. It is shown that the refracted waves are very sensitive to the curvature of interfaces. Certain modifications and improvements of ray theoretical seismograms to increase their accuracy are suggested.
ARK: https://n2t.net/ark:/88439/y076019
Permalink: https://geophysicsjournal.com/article/217
 
Palaeomagnetism of Upper Cretaceous limestones from the Munster Basin, Germany
Get PDFMarine, mainly flat-lying sediments of Late Cretaceous age are exposed throughout a wide area of the Munster Basin (NW-Germany). During the Cenomanian, Turonian, and Campanian fine-grained, grey, marly to pure limestones were deposited. The Campanian limestones carry magnetization components of unknown age due to the presence of secondary goethite and haematite. However, in the Cenomanian and Turonian rocks the natural remanent magnetization (NRM) is due to detrital magnetite and can be associated with the time of deposition. Fold tests confirm a Late Cretaceous age of magnetization in the magnetite-bearing limestones, since the NRM pre-dates latest Cretaceous deformation along the northern margin of the basin. The Munster Basin limestones provide one of the first reliable Cretaceous pole positions (Lat.: 76° N, Long.: 181° E) from stable Europe.
ARK: https://n2t.net/ark:/88439/y023954
Permalink: https://geophysicsjournal.com/article/285
 
Finite element convection models: comparison of shallow and deep mantle convection, and temperatures in the mantle
Get PDFA 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
 
Empirical model of global electron temperature distribution between 300 and 700 km based on data from Aeros-A
Get PDFAn empirical model function of the global electron temperature distribution has been determined based on the measurements of the planar Retarding Potential Analyzer on-board the Aeros-A satellite. The model represents the mean temperature between 300 and 700 km altitude at 0300 LT and 1500 LT depending on latitude, longitude, and height. The model values are compared with all the measured data to show the accuracy achieved and the mean spread of the data for different latitudes. A clear correlation was not found between the electron temperature and geophysical indices such as Kp or sunspot number for the period of low solar activity between January and August, 1973. Seasonal and annual effects could not be detected. The mathematical background and method used to generate the model function is described in the appendix.
ARK: https://n2t.net/ark:/88439/y033123
Permalink: https://geophysicsjournal.com/article/286
 
Palaeomagnetism and the early magmatic history of Fuerteventura (Canary Islands)
Get PDFThermal and alternating field demagnetization combined with studies of the convergence points of remagnetization circles have been carried out on a variety of rocks from Fuerteventura. The older (post-Albian) rocks, including the basement plutonics, the sheeted dike complex and the earliest subaerial lava sequence (lava Series I), have a multicomponent remanence while the younger lava Series II of Miocene/Pliocene age has dominantly a one-component magnetization. Comparison of the results with recent palaeomagnetic data from Gran Canaria/Tenerife and with results from continental Africa and Europe suggest a late Cretaceous origin of the basal intrusive rocks while the subaerial volcanism (lava Series I) most likely initiated at around the Cretaceous/Tertiary boundary. This implies that there is a nearly 50-m.y.-long period of volcanic quiescence and erosion between lava Series I and lava Series II. The apparent contradiction between these conclusions and the few K/Ar dates available is discussed.
ARK: https://n2t.net/ark:/88439/y082688
Permalink: https://geophysicsjournal.com/article/281
 
Q of Mode 0S0
Get PDFEstimates of Q of mode oSo as measured after the Chilean (1960) and Alaskan (1964) earthquakes showed large scatter. The Indonesian quake of August 19, 1977, has provided a new opportunity to determine the attenuation factor for oSo. Time lapse spectra of a priori selected high quality data were analyzed using a maximum-likelihood method and Ricean statistics. Data from South Pole and Los Angeles gave Q values of 6324 (1 ±21 %) and 6859 (1 ± 17%) respectively. Taken together the result is 6687 (1 ± 13%).
ARK: https://n2t.net/ark:/88439/y084078
Permalink: https://geophysicsjournal.com/article/39
 
A two-dimensional magnetometer array for ground-based observations of auroral zone electric currents during the International Magnetospheric Study (IMS)
No full textA two-dimensional magnetometer array was progressively installed in Scandinavia, from 1974 onwards, for operation during the IMS period (1977-1979). The 36 instruments, which are buried in the ground, are of the Gough-Reitzel type, i.e., classical magnetometers, with wire-suspended magnets and optical recording. The time-variation period range observable is from about 50 s to several days. In northern Scandinavia, the spacing between the stations is about 100-150 km in both the north-south and east-west directions. For presentation and analysis of the data a special Cartesian coordinate system has been introduced (the 'Kiruna system'). It is derived by mapping the local Earth's surface onto a tangential plane, with its origin close to Kiruna (geographic coordinates 67.8 N, 20.5 E). First results from some stations show that internal contributions to the recorded horizontal geomagnetic variations are small and possibly negligible at lower frequencies. However, at frequencies above about 2 mHz the variations of the vertical component display a strong amplification near the coast, and indicate the existence of conductivity anomalies at some inland locations. In order to demonstrate the observational capabilities of the array, equivalent overhead current configurations are presented which characterize a substorm recorded on October 7, 1976.
ARK: https://n2t.net/ark:/88439/y001456
Permalink: https://geophysicsjournal.com/article/160
 
Long-wavelength magnetic anomalies as a source of information about deep crustal structure
Get PDFThe nature of long-wavelength magnetic anomalies (λ = 60–300 km) computed for the Ukrainian Shield from the original field by a continuation upward to a height of 10 km is studied. The correlation betweeen the regional anomalies, the crustal thickness and the topography of the Curie isotherm of magnetite is examined. The strongest correlation is established between the regional anomalies and the crustal thickness. Similar results have been obtained by us for the Baltic Shield and by D.H. Hall for the Canadian Shield. It is concluded that the entire lower crust is magnetized, the average magnetization being almost the same for all the ancient shields. These results have been used for the construction of a magnetic model of the Earth's crust. With some exceptions, the magnetization of the lower crust is found to be inhomogeneous and 5-10 times higher than that of the upper crust. Theoretical modelling and experimental results show a high magnetization in the entire sequence of blocks for the case of a thickened crust and, vice versa, weakly magnetized rocks correspond to a smaller thickness of the crust. The present approach may be of potential use for distinguishing and studying crust-upper mantle interaction areas as well as for predicting the topography of the Moho discontinuity.
ARK: https://n2t.net/ark:/88439/y092937
Permalink: https://geophysicsjournal.com/article/282
 
Palaeomagnetic and rockmagnetic properties of the Permian volcanics in the western Southern Alps
Get PDFA palaeomagnetic and rockmagnetic investigation has been carried out in the western Southern Alps. At 31 sites in the Permian volcanics from four regions 349 samples were drilled: 1. the region of Arona (SW of Lago Maggiore), 2. the porphyry district of Lugano and Ganna, 3. the Valle Brembana (N of Bergamo) and 4. the Auccia volcanics (N of Brescia). AF-cleaning as well as continuous and progressive thermal demagnetization reveal, in most of the igneous rocks studied, the presence of a stable characteristic remanent magnetization (ChRM) representing the magnetization acquired at the time of formation of these rocks. Microscopic observations, IRM-acquisition curves and Curie point measurements indicate the ChRM to be carried by Ti-poor magnetite and titanohematite. Stable secondary magnetizations due to oxidation may be present. Their directions, however, are very similar to the primary thermoremanent magnetization (TRM). Therefore it is inferred that the oxidation probably took place shortly after acquisition of the primary TRM. The magnetization directions within individual sites are well grouped (α95 usually < 10°), but the site mean directions are dispersed, due to regional and local tectonic complications. At Arona, Ganna, and Auccia a suitable tilt correction can be made. Since the consistent directions from these sites are very similar to the well defined results from the Bolzano porphyries (Zijderveld et al., 1970), it is suggested that the western and eastern Southern Alps have, on a large scale, behaved as a single tectonic unit. The Southern Alpine block has been rotated anticlockwise by about 50° relative to extra-alpine Europe since the Early Mesozoic. The Permian Southern Alpine palaepoles are situated close to the Permian part of the African polar wander path. Therefore the palaeomagnetic data support geological and sedimentological arguments which consider the Southern Alps as originating on the southern margin of Tethys and forming a parautochthonous extension of the African plate since the Early Mesozoic.
ARK: https://n2t.net/ark:/88439/y013985
Permalink: https://geophysicsjournal.com/article/284