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61.
Anirban Mukhopadhyay Arun Mondal Sandip Mukherjee Dipam Khatua Subhajit Ghosh Debasish Mitra Tuhin Ghosh 《Journal of Earth System Science》2014,123(6):1349-1360
In the Himalayan states of India, with increasing population and activities, large areas of forested land are being converted into other land-use features. There is a definite cause and effect relationship between changing practice for development and changes in land use. So, an estimation of land use dynamics and a futuristic trend pattern is essential. A combination of geospatial and statistical techniques were applied to assess the present and future land use/land cover scenario of Gangtok, the subHimalayan capital of Sikkim. Multi-temporal satellite imageries of the Landsat series were used to map the changes in land use of Gangtok from 1990 to 2010. Only three major land use classes (built-up area and bare land, step cultivated area, and forest) were considered as the most dynamic land use practices of Gangtok. The conventional supervised classification, and spectral indices-based thresholding using NDVI (Normalized Difference Vegetation Index) and SAVI (Soil Adjusted Vegetation Index) were applied along with the accuracy assessments. Markov modelling was applied for prediction of land use/land cover change and was validated. SAVI provides the most accurate estimate, i.e., the difference between predicted and actual data is minimal. Finally, a combination of Markov modelling and SAVI was used to predict the probable land-use scenario in Gangtok in 2020 AD, which indicted that more forest areas will be converted for step cultivation by the year 2020. 相似文献
62.
We study the relationship between full-disk solar radiative flux at different wavelengths and average solar photospheric magnetic-flux
density, using daily measurements from the Kitt Peak magnetograph and other instruments extending over one or more solar cycles.
We use two different statistical methods to determine the underlying nature of these flux – flux relationships. First, we
use statistical correlation and regression analysis and show that the relationships are not monotonic for total solar irradiance
and for continuum radiation from the photosphere, but are approximately linear for chromospheric and coronal radiation. Second,
we use signal theory to examine the flux – flux relationships for a temporal component. We find that a well-defined temporal
component exists and accounts for some of the variance in the data. This temporal component arises because active regions
with high magnetic-field strength evolve, breaking up into small-scale magnetic elements with low field strength, and radiative
and magnetic fluxes are sensitive to different active-region components. We generate empirical models that relate radiative
flux to magnetic flux, allowing us to predict spectral-irradiance variations from observations of disk-averaged magnetic-flux
density. In most cases, the model reconstructions can account for 85 – 90% of the variability of the radiative flux from the
chromosphere and corona. Our results are important for understanding the relationship between magnetic and radiative measures
of solar and stellar variability. 相似文献
63.
Sandip Kumar Chakrabarti 《Astrophysics and Space Science》2005,297(1-4):131-142
Black holes are the most compact objects in the universe. Therefore, matter accreting onto them is likely to radiate photons
of energy comparable to very high gravitational potential energy. We discuss the nature of the emitted radiation in X-rays
and gamma-rays from black hole candidates. We present theoretical solutions, which comprise both Keplerian and sub-Keplerian
components and suggest that shocks in accretion and outflows may play a major role in producing these spectra. 相似文献
64.
65.
Dibyendu Nandy 《Astrophysics and Space Science》2002,282(1):209-219
We report here results from a dynamo model developed on the lines of the Babcock-Leighton idea that the poloidal field is
generated at the surface of the Sun from the decay of active regions. In this model magnetic buoyancy is handled with a realistic
recipe – wherein toroidal flux is made to erupt from the overshoot layer wherever it exceeds a specified critical field B
c (105 G). The erupted toroidal field is then acted upon by the α-effect near the surface to give rise to the poloidal field. In
this paper we study the effect of buoyancy on the dynamo generated magnetic fields. Specifically, we show that the mechanism
of buoyant eruption and the subsequent depletion of the toroidal field inside the overshoot layer, is capable of constraining
the magnitude and distribution of the magnetic field there. We also believe that a critical study of this mechanism may give
us new information regarding the solar interior and end with an example, where we propose a method for estimating an upper
limit of the difusivity within the overshoot layer.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
66.
67.
Sujit Dasgupta Basab Mukhopadhyay Manoj Mukhopadhyay D. R. Nandy 《Journal of the Geological Society of India》2013,81(2):241-247
Two contemporary earthquakes originating in the central Himalayan arc and its foredeep (Sikkim earthquake of 18.09.2011, Mw 6.9, h: 10–60 (?) km and Bihar-Nepal earthquake of 20.08.1988, Mw 6.8, h: 57 km) are commonly associated with transverse lineaments/faults traversing the region. Such lineaments/faults form active seismic blocks defining promontories for the advancing Indian Craton. These actually produce conjugate shear faulting pattern suggestive of pervasive crustal interplay deep inside the mountains. Focal mechanism solutions allow inferring that large part of the current convergence across the central Himalayan arc is accommodated by lateral slip. Similar slip also continues unabated in the densely populated foredeep for distances up to several tens of kilometers south of the Main Boundary Thrust (MBT). 相似文献
68.
Detection of ionospheric perturbation due to a soft gamma ray repeater SGR J1550-5418 by very low frequency radio waves 总被引:1,自引:0,他引:1
Sushanta K. Mondal Sandip K. Chakrabarti Sudipta Sasmal 《Astrophysics and Space Science》2012,338(2):259-264
The properties of cylindrical and spherical dust acoustic (DA) solitary and shock waves in an unmagnetized electron depleted
dusty plasma consisting of inertial dust fluid and ions featuring Tsallis statistics are investigated by employing the reductive
perturbation technique. A Korteweg-de Vries Burgers (KdVB) equation is derived and its numerical solution is obtained. The
effects of ion nonextensivity and dust kinematic viscosity on the basic features of DA solitary and shock waves are discussed
in nonplanar geometry. It is found that nonextensive nonplanar DA waves behave quite differently from their one-dimensional
planar counterpart. 相似文献
69.
Surya K. Maji Sandip K. Chakrabarti Sushanta K. Mondal 《Earth, Moon, and Planets》2012,108(3-4):243-251
Very Low Frequency (VLF) radio waves propagate through the Earth-ionosphere waveguide. Irregularities caused by excess or deficient extreme ultra-violet and X-rays, which otherwise sustain the ionosphere, change the waveguide properties and hence the signals are modified. We report the results of monitoring of the NWC transmitter (19.8?kHz) by a receiver placed at Khukurdaha (22°27′N, 87°45′E) during the partial solar eclipse (75?%) of 15th January, 2010. The propagation path from the transmitter to the receiver crosses the annular eclipse belt. We got a clear depression in the data during the period of the eclipse. Most interestingly, there was also a X-ray flaring activity in the sun on that day which reached its peak (C-type) right after the time when the eclipse reached its maximum. We saw the effects of the occultation of this flare in our VLF signal since a part of the X-ray active region was clearly blocked by the moon. We quantitatively compared by using analogies with previous observations and found best fitting parameters for the time when the flare was occulted. We then reconstructed the VLF signal in the absence of the occulted flare. To our knowledge, this is the first such incident where the solar flare was observed through lunar occultation and that too during a partial eclipse. 相似文献
70.
Sandip Kumar Saha Vasant A. Matsagar Arvind K. Jain 《Bulletin of Earthquake Engineering》2016,14(4):1153-1175
Seismic fragility curves for fixed-base and base-isolated liquid storage tanks are developed under non-stationary earthquakes, and their seismic performance are compared. The correlation between different earthquake intensity measure (IM) parameters and peak response quantities of the base-isolated liquid storage tanks are investigated. The failure criteria are chosen based on (1) the elastic buckling strength of the tank wall, which is defined in terms of critical base shear and critical overturning moment, and (2) in terms of the critical isolation displacement. The uncertainty involved is considered in the earthquake characteristics. Non-stationary earthquake ground motions are generated using Monte Carlo (MC) simulation. Influence of the isolator characteristic parameters and modeling approaches on the seismic fragility of the base-isolated liquid storage tanks is also investigated. Peak ground acceleration is found to be the well correlated IM parameter with the peak response quantities of the base-isolated liquid storage tanks. Substantial decrease in the seismic fragility of the base-isolated liquid storage tanks is observed as compared to the fixed-base tanks. Significant influence of the isolator characteristic parameters on the seismic fragility of the base-isolated liquid storage tanks are reported in the present study. 相似文献