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21.
The small granite plutons occurring at the contact of the Singhbhum-Orissa Iron Ore craton (IOC) to the north and the Eastern Ghat Granulite Belt (EGGB) to the south in eastern Indian shield are characterised by the presence of enclaves of the granulites of EGGB and the greenschist facies rocks of IOC. These granites also bear the imprints of later cataclastic deformation which is present at the contact of the IOC and the EGGB. In situ Pb-Pb zircon dating of these granites gives minimum age of their formation 2.80 Ga. A whole-rock three point Rb-Sr isochron age of this rock is found to be 2.90 Ga. Therefore, the true age of formation of these granites will be around 2.90–2.80 Ga. These granitic rocks also contain xenocrystic zircon components of 3.50 Ga and show a later metasomatic or metamorphic effect 2.48 Ga obtained from the analyses on overgrowths developed on 2.80 Ga old zircon cores. The presence of granulitic enclaves within these contact zone granite indicates that the granulite facies metamorphism of the EGGB is 2.80 Ga or still older in age. The cataclastic deformations observed at the contact zone of the two adjacent cratons is definitely younger than 2.80 Ga and possibly related to 2.48 Ga event observed from the overgrowths. As 2.80 Ga granite plutons of small dimensions are also observed at the western margin of the IOC; it can be concluded that a geologic event occurred 2.80 Ga over the IOC when small granite bodies evolved at the marginal part of this craton after its stabilisation at 3.09 Ga. 相似文献
22.
Rajat Acharya Bijoy Roy M.R. Sivaraman Ashish Dasgupta 《Journal of Atmospheric and Solar》2010,72(9-10):774-780
The variability of Total Electron Content (TEC) at Trivandrum, located within equatorial anomaly region at the dip equator, with respect to a reference level derived from the TEC measurements at Shimla, located outside the region has been studied during low solar activity period. Chapman function is assumed to hold good for regions outside the anomaly extent. It shows that the difference of total measured TEC at the equator from the derived reference is highly correlated with equatorial electrojet. The observations conform to the previous investigations and are interpreted in light of established relations. A stochastic relationship with electrojet is derived and validated. 相似文献
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24.
We derive the poleward migration trajectory diagram of the filament bands for the years 1915–1982 from the H-alpha synoptic charts. We find that the global solar activity commences soon after the polar field reversal in the form of two components in each hemisphere. The first component we identify with the polar faculae that appear at latitudes 40–70° and migrate polewards. The second and the more powerful component representing the sunspots shows up at 40° latitudes 5–6 years later and drifts equatorward giving rise to the butterfly diagram. Thus the global solar activity is described by the faculae and the sunspots that occur at different latitude belts and displaced in time by 5–6 years. This gives rise to the prolonged duration for the global solar activity lasting for 16–18 years as against the 11 years which has come about based only on the spots. The two components match with the pattern of the coronal emission in 5303 Å line. Finally, we show that the two components of activity also match with the pattern of excess shear associated with the torsional oscillations on the Sun and this provides a link between the torsional oscillations and the magnetic activity. 相似文献
25.
MR Lipiński JSF van der Vyver P Shaw WHH Sauer 《African Journal of Marine Science》2016,38(4):589-593
This short note summarises past and current knowledge of the life cycle of chokka-squid Loligo reynaudii d’Orbigny, 1845, in South African waters. Prior to approximately 2010, the chokka-squid stock was considered simple and uniform, with one paralarval pool, the drift of paralarvae westwards, one main nursery area and one main, long spawning migration of adult squid eastwards, back to the main spawning grounds. Based on new information, this life-history scheme has been revised. Genetically, the stock is uniform; however, morphologically, it comprises three main geographic groups. It is proposed that the differences between the groups originate from many different paralarval events and that short (not long) migrations dominate the life cycle. 相似文献
26.
We have derived the velocities of meridional flows by measuring the latitudinal motions (or drifts) of umbrae of spot groups
classified into three categories of area: 0 – 5 μ, 5 – 10 μ, and >10 μ (μ area in millionths of the solar hemisphere). The latitudinal drifts (or the meridional flows) in all three categories are
directed equatorward in both the northern and southern hemispheres. By sorting the spot groups into three area classes, we
are able to relate the respective latitudinal drifts with the three depths in the convection zone where the footpoints of
the flux loops of the spot groups of each area class are anchored. We obtain estimates of the anchor depths through a comparison
of the rotation rates of the spot groups of each area class with the rotation-rate profiles from helioseismic inversions.
The equatorward drifts obtained provide estimates of the meridional flows at the three depths in the convection zone and thereby
suggest the presence of return meridional flows as envisaged in the flux-transport dynamo models, which have remained undetected
so far. The data sources for this study are measurements of positions and areas of umbrae of sunspots from the photographic
white-light images of the Sun of the Kodaikanal Observatory archives for the period 1906 – 1987 and a very similar, but independent,
data set from the Mt. Wilson Observatory archives for the period 1917 – 1985. 相似文献
27.
From a large sample of the Kodaikanal spectroheliograms in the Call K line we have studied the variations in the intensity of the network elements over two solar cycles and have estimated their contribution to the overall variability seen in the disc-averaged K line profiles. The relative contribution of the network elements and the bright points to the K-emission are of the order of 25% and 15% respectively. We have shown that the area of the network elements is anti-correlated with the solar activity, and it increases by about 24% during the solar minimum compared to the maximum period. 相似文献
28.
Although the Ca ii K232 network is known to be cospatial with magnetic elements there has been doubt as to the magnetic origin of the fainter K2V points. We demonstrate that weak magnetic elements also lie at the roots of the K2V points, and because the latter are numerous they may contribute sensibly to the integrated light profile of Ca ii K.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
29.
K. R. Sivaraman 《Solar physics》1973,29(2):333-340
An improved method is described for the measurement of both the solar radius and the height of the chromosphere in any desired wavelength. Possible sources of uncertainty are discussed and a comparison with other methods is made. The first results from the 1972 observing period are given: R = (960.24 ±0.16) for the continuum at 5011.5 Å and R = (966.9 ±0.4) for H ± 0.5 Å. This yields a mean height of Ha emission of (4900 ± 400) km. 相似文献
30.
Direct photographs of Comet Ikeya-Seki obtained on four consecutive days from October 29 to November 1, 1965, are used for an analysis of the multiple helical structures in the ionised tail. The formation of these structures is explained on the basis of plasma instabilities excited in the tail containing twisted magnetic fields. The growth rate of the modes excited at the mode rational surface agrees well with the observed results. This model also accounts for the presence of harmonic structures seen in the tail of the comet. 相似文献