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1.
The Shillong plateau and Mikir Hills form a conspicuous feature in the geologically complex region of Northeastern India. From observations of geomagnetic variations at Shillong a consistent suppression of the vertical (Z) component of variation is noted. From previous analyses of a selected geomagnetic storm observed at Shillong, Ujjain and Jaipur, stations at nearly the same latitude, the complex demodulates ofZ variation at Shillong demonstrate this suppression. Results from a comparative study of bays and storm sudden commencements are also cited. An attempt to calculate induction vectors at Shillong was made but they were found to be indeterminate. However, a case for the presence of induced current concentrations near Shillong is strong, when these characteristics of geomagnetic variations are considered in the context of regional geology. Shillong and the Mikir Hills are the intruded northeastern remnants of the Indian Shield, that have been transported into the Eastern Himalayan syntaxis. But the present data are insufficient to delineate the path of induced currents in the Shillong region or to estimate their intensity. 相似文献
2.
A.K. Agarwal B.P. Singh N. Nityananda 《Physics of the Earth and Planetary Interiors》1979,18(3):232-237
Variations recorded simultaneously at eight Indian stations during a magnetic storm are analysed to obtain the power spectra for each component using the fast Fourier transform. For two selected peaks, observed for H at all stations, at 49.5- and 70-min periods, a complex demodulation is performed for each component. From the variations thus obtained, horizontal polarization ellipses are drawn for four events of 49.5-min period. The variation of the demodulates and polarization ellipses at the stations shows a regional character which coincides with the physiographic grouping of the stations. Anomalously large Z variations are observed at the equatorial stations, which are near the sea. A distinct feature is the suppression of H at Trivandrum where the polarization ellipses confirm the E-W trend of the induced currents in this region. The overall similarity of the horizontal variations is demonstrated by these ellipses. Horizontal disturbance vectors at Annamalainagar, Trivandrum and Sabhawala confirm channelled currents located previously. At Shillong, the induced effects are evident and the currents are deduced to flow eastward, directly beneath the station.The overall regional similarities in variations are clearly manifested. 相似文献
3.
The collisionless Boltzmann equation governing self-gravitating systems such as galaxies has recently been shown to admit
exact oscillating solutions with planar and spherical symmetry. The relation of the spherically symmetric solutions to the
Virial theorem, as well as generalizations to non-uniform spheres, uniform spheroids and discs form the subject of this paper.
These models generalize known families of static solutions. The case of the spheroid is worked out in some detail. Quasiperiodic
as well as chaotic time variation of the two axes is demonstrated by studying the surface of section for the associated Hamiltonian
system with two degrees of freedom. The relation to earlier work and possible implications for the general problem of collisionless
relaxation in self gravitating systems are also discussed 相似文献
4.
Anomalous geomagnetic variations have been observed at all geomagnetic stations in South India and they have earlier been attributed to ocean and crustal conductors. The spectral character of induced variations indicate that, in addition to coastal effects and channelling of currents through the Palk Straits, there is a concentration of currents in a conducting body at deeper levels in the crust or mantle.Current thinking on crustal evolution and the influence of Precambrian lineaments on tectonism and mineralisation up to recent times, provides support for the conjecture that zones of high electrical conductivity exist in the crust, which are related to tectonic and mineral influences. The Eastern Ghats and granulite terrain of South India and Sri Lanka are probably associated with such a zone of fundamental weakness in the crust. Extended graphite mineralisation along the Eastern Ghats, occurrence of economic graphite deposits near Trivandrum and in Sri Lanka and the association of alkaline and ultrabasic metallic minerals with deep faults in South India are indications of the possible chemical causes of high electrical conductivity in some regions of the crust.Similar relations between induced anomalous variations and tectonism and mineralisation in the Precambrian crystalline crust have been found in North America and Scotland. However, the regions of high electrical conductivity have not yet been mapped in detail in South India to confirm such a relationship. 相似文献
5.
The maximum entropy method (MEM) of image reconstructtion is discussed in the context of incomplete Fourier information (as
in aperture synthesis). Several current viewpoints on the conceptual foundation of the method are analysed and found to be
unsatisfactory. It is concluded that the MEM is a form of model-fitting, the model being a non-linear transform of a band-limited
function. A whole family of ’entropies’ can be constructed to give reconstructions which (a) are individually unique, (b)
have sharpened peaks and (c) have flattened baselines. The widely discussed 1nB and - B1nB forms of the entropy are particular
cases and lead to Lorentzian and Gaussian shaped peaks respectively. However, they hardly exhaust the possibilities-for example,
B1/2 is equally good.
The two essential features of peak sharpening and baseline flattening are shown to depend on a parameter which can be controlled
by adding a suitable constant to the zero spacing correlation ρ00. This process, called FLOATing, effectively tames much of the unphysical behaviour noted in earlier studies of the MEM. A
numerical scheme for obtaining the MEM reconstruction is described. This incorporates the FLOAT feature and uses the fast
Fourier transform (FFT), requiring about a hundred FFTs for convergence. Using a model brightness distribution, the MEM reconstructions
obtained for different entropies and different values of the resolution parameter are compared. The results substantiate the
theoretically deduced properties of the MEM.
To allow for noise in the data, the least-squares approach has been widely used. It is shown that this method is biased since
it leads to deterministic residuals which do not have a Gaussian distribution. It is suggested that fitting the noisy data
exactly has the advantage of being unbiased even though the noise appears in the final map. A comparison of the strengths
and weaknesses of the MEM and CLEAN suggests that the MEM already has a useful role to play in image reconstruction. 相似文献
6.
Based on a simple picture of speckle phenomena in optical interferometry it is shown that the recent signal-to-noise ratio
estimate for the so called bispectrum, due to Wirnitzer (1985), does not possess the right limit when photon statistics is
unimportant. In this wave-limit, which is true for bright sources, his calculations over-estimate the signal-to-noise ratio
for the bispectrum by a factor of the order of the square root of the number of speckles. 相似文献
7.
The theory of gravitational lensing of background quasars by stars in the halo of a galaxy is considered. In the limiting
case of small ‘optical depth’, only one star is close enough to the beam to cause strong scattering, and the effect of all
the other stars is treated as a perturbation with both systematic and random components. The perturbation coming from weak
scattering can increase the number of images and the amplification in those cases where the amplification is already high;
such events are preferentially selected in flux limited observations. The theory is applicable to the apparent association
of background quasars with foreground galaxies. A comparison with earlier work on the same problem is given. The relevance
of these results to gravitational lensing by galaxies as perturbed by random inhomogeneities surrounding the ray path is also
briefly discussed. 相似文献
8.
Mita Rajaram B. P. Singh N. Nityananda A. K. Agrawal 《Geophysical Journal International》1979,56(1):127-138
Summary. Transient geomagnetic variations like SSCs, Bays, Sq and storm-time variations show anomalously large Z amplitudes at the three permanent magnetic observatories in India under the equatorial electrojet. Our earlier studies have shown that these anomalies cannot be explained in terms of the usual coastal effect. Another unique feature of this area is the small equatorial enhancement of all magnetic fluctuations, which still remains to be completely understood. It is also noticed that Z/H ratio at Annamalainagar is very large for night-time variations but becomes insignificant for day-time fluctuations, whereas no such large difference is seen at Trivandrum, the other coastal station. All the above features have been explained here by introducing the presence of a conducting channel in the lower crust or upper mantle between India and Sri Lanka Island. The anomalies of the equatorial electrojet in the Indian region thus do not necessitate the conducting surface of the mantle to be deeper in this area as has been suggested by earlier workers. A less conducting mantle in the Indian region otherwise could be difficult to incorporate in the present theories of mantle convection when this area is known to be tectonically active. The presence of the conducting channel is further confirmed by analogue model experiments of Papamastorakis. We further observe that the equatorial electrojet does have an associated internal part in the Indian region. 相似文献
9.
N. Nityananda A.K. Agarwal B.P. Singh 《Physics of the Earth and Planetary Interiors》1981,25(3):226-231
Induction vectors calculated for short-period events such as “bays” and “storm sudden commencements” (SSCs) indicate that the conductor causing the anomaly lies to the south of Sabhawala. The nature of vertical transfer functions obtained from spectral analysis of storm-time variations further confirms the presence of such a conductor. The nature of maximum (Gp) and minimum (G1) response functions suggests that a two-dimensional conductor south of Sabhawala is responsible for anomalies in Z. Invoking a structural model of the Earth's crust beneath the Himalayas, in keeping with accepted orogenic theories, we propose that one structure capable of producing the desired conducting path is a rise of the asthenosphere south of the Himalayan range. Another important factor contributing to the induced variations at Sabhawala is due to a sedimentary trough running parallel to the Himalayas, again to the south. These views are confirmed by a two-dimensional model calculation. It is found that the undulation of the asthenosphere alone does not reproduce the nature of the anomalies in H and Z. 相似文献
10.
N. Nityananda A.K. Agarwal B.P. Singh 《Physics of the Earth and Planetary Interiors》1977,15(1):P5-P9
A study of the horizontal components of certain short-period magnetic-fluctuation events, viz., storm sudden commencements and bays, shows appreciable anomalies at two of the five magnetic observatories in peninsular India. The electric current patterns deduced from the magnetic horizontal induced variations imply channelling of induced currents through the Palk Straits and around the southern tip of the Indian peninsula. An interesting feature of these anomalies is that while the induced magnetic vertical variations are strongly correlated with the horizontal northward magnetic field, the electric current concentrations flow parallel to the coast; southwest, near Annamalainagar, and north near Trivandrum. The observations are interpreted in terms of a regional, east-west, induced electric current pattern which is perturbed in the vicinity of the two observatories by the Indian peninsula. 相似文献