Journal of Geophysics
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Pi2 magnetic pulsations, auroral break-ups, and the substorm current wedge: A case study
No full textThe two-dimensional distribution of the characteristics of Pi2 magnetic pulsations observed by the Scandinavian Magnetometer Array (SMA) during the passage of a westward travelling surge on 11 November 1976 and during three sucessive auroral break-ups around magnetic midnight on 15 February 1977 have been studied in relation to the position of active aurora and the break-up current system. On both days the greatest Pi2 amplitudes were collocated with the region where the brightest auroras were observed. The sense of polarization of the horizontal disturbance vectors changed along longitudinal and meridional lines. The two-dimensional equivalent current system of the Pi2 pulsations resembled a circular current vortex around the location of the localized upward field-aligned currents and changed its direction from counterclockwise to clockwise and back to counterclockwise again within one Pi2 cycle. Our observations indicate that the generation of Pi2 pulsations is not directly connected to periodic fluctuations of the complete current system at substorm onset, but that the upward directed field-aligned currents at the western edge of this system play the most important role for the Pi2 generation.
ARK: https://n2t.net/ark:/88439/y071359
Permalink: https://geophysicsjournal.com/article/167
 
The shape of the geomagnetic field through the last 8,500 years over part of the northern hemisphere
Get PDFRegional type curves depicting secular variations of declination and inclination through the last 10,000 calendar years constructed for north-western Europe (356° E, 55° N) and east-central North America (270° E, 46° N) by stacking palaeomagnetic data derived from lake sediment cores are described and analysed. The spectral content and phase relationships of the two pairs of curves show that they have a complex origin with both drifting and standing geomagnetic sources contributing to them. The strongest evidence of drifting sources is provided by the inclination type-curves which exhibit maximum correlation for a phase shift of ~650 years suggestive of westward drift at a rate of about 0.13 degrees a year. At the same time, comparison of the declination type-curves strongly suggests that waxing and waning standing sources were dominant. We show that the difference in relative importance of drifting as compared to standing geomagnetic sources implied by the patterns of correlation deduced respectively for declination and inclination can, at least in principle, be attributed to observation point/geomagnetic source geometry by modelling the secular variation that would be produced by standing but oscillating equatorial dipoles and radial dipoles located deep within the outer core, by a pair of drifting deep-seated radial dipoles of constant intensity, and by drifting sheets of radial dipoles (taken to represent current-loops) located at shallow depth within the outer core. Each of these model sources produces secular variation curves with distinctive shapes and phase relationships. Hence, an attempt is made to identify qualitatively the types and locations of the sources which dominated the secular variations as recorded by our type-curves. One of our most important conclusions is that there appears to be a' turning-point' at ~ 4 750 years before present when the relative amplitudes of the active' standing' sources changed but the characteristics of the drifting sources appear to have remained relatively unchanged.
ARK: https://n2t.net/ark:/88439/y011535
Permalink: https://geophysicsjournal.com/article/263
 
Linearized solutions of kinematic problems of seismic body waves in inhomogeneous slightly anisotropic media
Get PDFThe linearization approach to the evaluation of travel-times of seismic body waves propagating in inhomogeneous, slightly anisotropic media is discussed. General linearization equations are specified both for quasi-compressional and quasi-shear waves. Various situations of seismological interest are investigated in detail. This applies, e.g., to the situation where the unperturbed medium is isotropic and to the case where the unperturbed ray is a plane curve. The numerical examples presented suggest that the linearization approach gives travel-times of seismic body waves with an accuracy sufficient to solve direct and inverse kinematic problems for inhomogeneous anisotropic models of the Earth's crust and the uppermost mantle.
ARK: https://n2t.net/ark:/88439/y051821
Permalink: https://geophysicsjournal.com/article/175
 
Tectonic inferences of paleomagnetic data from some Mesozoic formations in Central Iran
Get PDFMaterial for paleomagnetic research has been collected from sedimentary rocks of Mesozoic age from four localities in Central Iran. Detailed paleomagnetic analyses have been carried out using partial progressive demagnetization procedures both with alternating magnetic fields and with heating in order to isolate the characteristic remanence. The following results were obtained: Early Triassic Sorkh Shales with D = 289°, I = 21°, α95= 14°; Late Jurassic Garedu Red Beds with D = 4°, I = 42°, α95 = 14°; Late Jurassic - Early Cretaceous Bidou Beds with D = 48°, I = 32.5°, α95 = 19°; Middle Cretaceous Dehuk Sandstones with D = 326°, I = 38.5°, α95 = 21°. The large α95 values are due to the rather small collections of specimens, roughly 50 each. From the paleomagnetic data we conclude that in Early Triassic times Central Iran, which forms part of the Iranian-Afghan micro-continent, belonged to Gondwana; since Early Jurassic times the area has been positioned close to the Eurasian continent. The scatter in the declination of the remanence directions can be explained in terms of rotations of individual blocks along the main Central-Iranian fault systems.
ARK: https://n2t.net/ark:/88439/y021734
Permalink: https://geophysicsjournal.com/article/264
 
Ar-40-Ar-39 ages of rocks and glasses from the Noerdlinger Ries Crater and the temperature history of impact breccias
No full textWe report on a 40Ar–39Ar study of hornblende, biotite, and glass samples from the Nordlinger Ries impact crater. The samples are derived from various depths (377–1,200 m) of the Forschungsbohrung 1973 drill core, from the ejecta blanket, and the crystalline crater rim. All mineral separates display 40Ar–39Ar plateau ages of 320 ± 3 m.y. The data represent the first direct age determination of the Ries bedrock. The plateau ages of suevite and moldavite, both generated in the Ries impact, date the cratering event which occurred 15 m.y. ago. The results from the mineral separates imply that shock pressure alone, even as high as 450 kbar, cannot reset K–Ar ages of hornblende and biotite. This result is significant for the interpretation of ages of impact breccias in general, and for the interpretation of lunar highland rock ages in particular. 40Ar–39Ar studies yield, as a side result, data on the natural loss of radiogenic 40Ar. In this report we emphasize on this aspect to estimate the cooling history of the suevite layer. We compare the measured 40Ar-loss to the loss calculated from a simple cooling model of the layer and from the diffusion properties of the minerals as obtained in the stepwise heating experiment. The resulting upper limit for the post-shock equilibrium temperature within the suevite layer is 450°C.
ARK: https://n2t.net/ark:/88439/y028544
Permalink: https://geophysicsjournal.com/article/18
 
Amplitude study of the Pg phase
Get PDFThe amplitude of the Pg phase, as recorded in explosion seismology studies, is analyzed with the aid of synthetic seismograms. Parameters such as source frequency, low-velocity cover above the crust (sediments or weathered layer), low-velocity layers within the upper crust, velocity gradients, thickness of the gradient zone, attenuation and Poisson's ratio strongly influence the amplitude-distance pattern of the Pg phase. A systematic study clearly shows that different models of the continental upper crust display distinct amplitude-distance characteristics. These models could not be distinguished by travel-time interpretation alone. In the presence of gradient zones the amplitude-distance curve shows different patterns depending on the source frequency. The higher the frequency, the more pronounced are the relative maxima in the amplitudes. The presence of a low-velocity cover at the surface accentuates the character of the amplitude-distance curves even if the cover is thin (a few hundred meters). Moreover, a low-velocity cover produces P to S conversions and multiples following the Pg which obscure possible secondary crustal phases. The thickness of the velocity gradient zone influences the amplitude decay and the width of the relative maxima. Low-velocity layers within the upper crust cause a faster drop-off of the amplitudes than would be expected from ray theory. Detailed Pg amplitude studies are thus useful in improving the knowledge of the physical properties of the upper continental crust. The application of the derived criteria to two sets of real data allow us to determine fine details of the velocity-depth function which are of great importance for the understanding of the Earth's crust.
ARK: https://n2t.net/ark:/88439/y083748
Permalink: https://geophysicsjournal.com/article/291
 
Joint two-dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents: 3. Auroral zone currents during the passage of a westward travelling surge
Get PDFGround magnetic perturbations and ionospheric electric fields have been observed by the Scandinavian Magnetometer Array and the STARE radars, respectively, during the passage of a westward travelling surge in the late evening sector. On the basis of these measurements, the three-dimensional current system in the vicinity of the westward travelling surge is modelled. The results support a current model recently proposed by Rostoker and co-workers on the basis of purely magnetic ground-based measurements. In particular, we can identify an upward field-aligned current of about 5 x 104 A in the head of the surge. Ahead of the surge, we find a south-eastward directed electric field. Additionally, we have some evidence for the existence of a southwestward directed electric field east of the surge. The latter may be explained by the generation of polarisation charges at the northern and southern boundaries of the higher conducting region east of the surge's head.
ARK: https://n2t.net/ark:/88439/y068400
Permalink: https://geophysicsjournal.com/article/156
 
On a type classification of lower crustal layers under Precambrian regions
Get PDFVarious parameters pertinent to the lower crustal layer under Precambrian regions are listed for locations where seismic, and geomagnetic or geoelectric, studies have been undertaken. The parameters define three distinct types of lower crustal layer with certain dominant characteristics: Type I – "Normal" – typical continental seismic parameters and a high electrical resistivity (103–104 Ωm); Type II – "Intermediate" – high compressional wave velocity (either fixed Vp = 7.0 km s-1 or transitional Vp = 6.7 → 7.3 km s-1) and a moderate resistivity (100–300 Ωm); Type III – "Low" – a low shear wave velocity layer (LVsL), high Poisson's ratio (> 0.30) and low electrical resistivity (10–50 Ωm). Possible conditions and rock types, existing at the P–T environment of the lower crust and which could account for the observations, are suggested. The zoning of Canada into types implies that Type II layers are shield "edge" effects, and that inability to observe what is regarded as the final stage of development of a shield region under certain shields may be due to their being too small.
ARK: https://n2t.net/ark:/88439/y052001
Permalink: https://geophysicsjournal.com/article/185
 
Inverse problems = Quest for information
Get PDFWe examine the general non-linear inverse problem with a finite number of parameters. In order to permit the incorporation of any a priori information about parameters and any distribution of data (not only of gaussian type) we propose to formulate the problem not using single quantities (such as bounds, means, etc.) but using probability density functions for data and parameters. We also want our formulation to allow for the incorporation of theoretical errors, i.e. non-exact theoretical relationships between data and parameters (due to discretization, or incomplete theoretical knowledge); to do that in a natural way we propose to define general theoretical relationships also as probability density functions. We show then that the inverse problem may be formulated as a problem of combination of information: the experimental information about data, the a priori information about parameters, and the theoretical information. With this approach, the general solution of the non-linear inverse problem is unique and consistent (solving the same problem, with the same data, but with a different system of parameters does not change the solution).
ARK: https://n2t.net/ark:/88439/y048722
Permalink: https://geophysicsjournal.com/article/28
 
Characteristics of westward travelling surges during magnetospheric substorms
Get PDFData from arrays of magnetometers along lines of constant magnetic latitude and longitude supplemented by all-sky camera and riometer data are used to infer the characteristics of the temporal development and the typical scale size of westward travelling surges which occur during magnetospheric substorms. It is found that the motion of the head of the surge can be quite irregular, and that in extreme cases the surge form may grow and decay in a confined longitudinal sector without suffering any significant westward displacement. The positive D-component perturbation, known to be the characteristic signature of a surge, is generally confined within a longitude range of ~6-10° at ~70° N and is thought to be generated by a filamentary southward ionospheric current flowing at the head of the surge. A comprehensive model three-dimensional current system involving this equatorward current and northwestward current flow in the region to the east of the head of the surge is presented through a detailed comparison of model and observed latitude and longitude profiles of the magnetic disturbance. It is found that best agreement is obtained when the entire electrojet system flows from southeast to northwest relative to the lines of constant magnetic latitude.
ARK: https://n2t.net/ark:/88439/y029714
Permalink: https://geophysicsjournal.com/article/128