A Global Model of MHD Wave Support of a Stratified Molecular Cloud     

Takahiro Kudoh  Shantanu Basu 《Astrophysics and Space Science》2004,292(1-4):161-166

We perform numerical simulations of nonlinear MHD waves in a gravitationally stratified molecular cloud that is bounded by a hot and tenuous external medium, within a 1.5-dimensional approximation. Under the influence of a driving source of Alfvénic disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a steady state characterized by oscillations about a new time-averaged equilibrium state. The nonlinear effect results in the generation of longitudinal motions and many shock waves. Models of an ensemble of clouds show that, for various strengths of the input energy, the velocity dispersion in the cloud σ ∝ Z ^0.5, where Z is a characteristic size of the cloud. Furthermore, σ is always comparable to the mean Alfvén velocity of the cloud, consistent with observational results.  相似文献   

Variability of extragalactic objects in relation to redshift, color, radio spectral index and absorption lines     

D. Basu 《Journal of Astrophysics and Astronomy》2001,22(4):263-282

Optical variability of extragalactic objects, viz., QSOs, BL Lacs and Seyfert galaxies has been monitored systematically over an appreciable period of time and a large amount of data have accumulated. The present work reports results of investigations involving statistical analysis of updated data on relationships between variability and various observed properties of the objects, viz., redshift, color indices, radio spectral index and absorption lines. It is found that at high frequencies (rest frame) radio spectral index does not change significantly with the degree of variability. However, the degree of variability depends on redshifts. On the other hand, presence or absence of absorption lines is significantly associated with variability for QSOs with larger redshifts (z > 1.0), while no such relationship exists for QSOs at smaller redshifts (z < 1.0) or other objects. Correlation between color indices and redshifts depends on the degree of variability and the sample chosen for the color index.  相似文献   

Some trends in the red-shift distribution of quasi-stellar objects and related peculiar galaxies     

D. Basu  M. A. Abdu 《Astrophysics and Space Science》1972,19(2):303-308

Gaps in the red-shift distribution of quasi-stellar objects and related peculiar galaxies have been studied using 205 sources. The result indicates certain definite trends in the distribution of the gaps but does not suggest any periodicity when the entire sample is considered.  相似文献   

An index of impulsiveness for 2800 MHz impulsive solar noise bursts     

D. Basu  A. E. Covington 《Solar physics》1968,5(1):102-110

The initial growth of intensity, I, of the impulsive solar noise burst observed at 2800 MHz has been fitted with a parabolic curve of the form I=ct ²and the quantity c taken as the index of impulsiveness. Two groups of bursts comprising 85% of all impulsive bursts observed in 1962–63 and 1966 were selected for study. A good fit has been obtained for bursts having peak flux density up to 20 flux units, while for more intense bursts, the average observed growth is more rapid than the parabolic rate. The distribution of the index in the range 0.1 to more than 100 shows two peaks, one for c values 1–10 and another apparent peak for those with c greater than 100. The index is independent of the peak intensity of the burst and its position on the solar disc, while there is a small trend, indicating that shorter bursts are more impulsive than longer duration events. The more easily derived linear rate of rise, b = Peak Flux/Interval from start to peak is related to the parabolic impulsive index by b = 1.86 c ^0.57. The non-linear rate of expansion of a flaring volume suggested by Pneuman when applied to explain the parabolic rise of microwave bursts indicates that the impulsiveness of bursts is inversely related to the contained magnetic field.  相似文献   

Solar Cycle Variations of Large-Scale Flows in the sun     

Basu  Sarbani  Antia  H.M. 《Solar physics》2000,192(1-2):469-480

Using data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO), we study the large-scale velocity fields in the outer part of the solar convection zone using the ring diagram technique. We use observations from four different times to study possible temporal variations in flow velocity. We find definite changes in both the zonal and meridional components of the flows. The amplitude of the zonal flow appears to increase with solar activity and the flow pattern also shifts towards lower latitude with time.  相似文献   

1E 1207.4–5209: Are the optical and X‐ray spectra blueshifted?     

D. Basu 《Astronomische Nachrichten》2006,327(7):724-728

1E 1207.4–5209 is an X‐ray source located at the centre of the supernova remnant (SNR) PKS 1209–52 (G296.5+10.0) and is thought to be an isolated neutron star (INS) associated with the SNR. Its optical spectrum shows several absorption lines and the X‐ray spectrum exhibits three variable absorption features at what appears to be harmonically related wavelengths, the latter being interpreted as due to cyclotron resonance. However, there are several serious problems, uncertainties and difficulties in the above association (SNR/INS) and in the interpretation of the spectra. In view of these, we suggest an alternative explanation of the observed spectra in terms of blueshifts. This implies that the optical and X‐ray absorption spectra are due to the central object of the SNR in association with two separate absorption clouds ejected at successively increasing speeds. The clouds have their origins in jets resulting from the merger of two very massive compact stars. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Homogenization of spatial patterns of hydrologic response in artificially drained agricultural catchments          下载免费PDF全文

Samuel J. Boland‐Brien  Nandita B. Basu  Keith E. Schilling 《水文研究》2014,28(19):5010-5020

Anthropogenic modifications to the landscape, with agricultural activities being a primary driver, have resulted in significant alterations to the hydrologic cycle. Artificial drainage, including surface and subsurface drainage (tile drains), is one of the most extensive manipulations in agricultural landscapes and thus is expected to provide a distinct signature of anthropogenic modification. This study adopts a data synthesis approach in an effort to characterize the signature of artificial subsurface drainage. Daily discharge data from 24 basins across the state of Iowa, which encapsulate a range of anthropogenic modifications, are assessed using a variety of flow metrics. Results indicate that the presence of artificial subsurface drainage leads to a homogenization of landscape hydrologic response. Non‐tiled watersheds exhibit a decrease in the area‐normalized peak discharge and an increase in the baseflow ratio (baseflow/streamflow) with increases in the spatial scale, while scale invariance is apparent in tiled basins. Within‐basin variability in hydrograph recession coefficients also appears to decrease with increases in the proportion of the catchment that is artificially drained. Finally, the differences between tiled and non‐tiled landscapes disappear at scales greater than approximately 2200 km², indicating that this may be a threshold scale for studying the effects of tile drainage. This decrease in within‐basin variability and the scale invariance of hydrologic metrics in artificially drained watersheds are attributed to the creation of a bypass flow hydrologic pathway that bypasses the complexity of the catchment travel paths. Spatial homogeneity in responses implies that it may be possible to develop more parsimonious hydrologic models for these regions. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Influence of heterogeneous refractivity on optical wave propagation in coastal environments     

Ping He  Christopher G. Nunalee  Sukanta Basu  Jean Minet  Mikhail A. Vorontsov  Steven T. Fiorino 《Meteorology and Atmospheric Physics》2015,127(6):685-699

  相似文献   

Observational Support for the Stability Dependence of the Bulk Richardson Number Across the Stable Boundary Layer     

S. Basu  A. A. M. Holtslag  L. Caporaso  A. Riccio  G.-J. Steeneveld 《Boundary-Layer Meteorology》2014,150(3):515-523

The bulk Richardson number ( $Ri_{Bh}$ ; defined over the entire stable boundary layer) is commonly utilized in observational and modelling studies for the estimation of the boundary-layer height. Traditionally, $Ri_{Bh}$ is assumed to be a quasi-universal constant. Recently, based on large-eddy simulation and wind-tunnel data, a stability-dependent relationship has been proposed for $Ri_{Bh}$ . In this study, we analyze extensive observational data from several field campaigns and provide further support for this newly proposed relationship.  相似文献   

Mass accretion by the nuclei of disk galaxies,II     

Tara Bhattacharyya  Anuradha Saha  B. Basu 《Astrophysics and Space Science》1985,113(1):33-51

Repeated explosions in the nuclei of galaxies are now accepted as observationally established phenomena. Each explosion leads to the ejection of gas from the central region of a galaxy with velocities depending on the strength of the explosive event. In the process the nucleus temporarily becomes gas-deficient. It is suggested that the mass los is replenished by the accretion of the mass which is shed by those evolved stars in the galactic bulge that possess relatively low rotational velocities. The gas to be accreted is assumed to be magnetized. In the present model, the accretion rate has been assumed to be a function of both radial distance and time. The cross-radial equation of motion has been solved to derive the expression for the rotational velocity which is found to bealmost linear with the radial distance from the centre. The radial equation has been solved to calculate the time-scale over which the nucleus accumulates sufficient mass to undergo instability and suffer explosion. The calculated time-scale range from few multiples of 10⁷ to a few multiples of 10⁸ yr. This range agrees very well with that as has been suggested on the basis of observation in the case of our own Galaxy.  相似文献