The Lohawat howardite: Mineralogy,chemistry and cosmogenic effects     

M. S. Sisodia  A. D. Shukla  K. M. Suthar  R. R. Mahajan  S. V. S. Murty  P. N. Shukla  N. Bhandari  R. Natarajan 《Meteoritics & planetary science》2001,36(11):1457-1466

Abstract— The Lohawat meteorite is a texturally heterogeneous breccia having a variety of mineral and lithic fragments. Among mineral fragments, pyroxenes show a wide range of composition (Wo_0.011–0.17En_0.37–0.78Fs_0.21–0.60) whereas plagioclase is anorthitic (An_0.92Ab_0.07Or_0.007). Abundant rounded “chondrule‐like” objects ranging in size up to ～7 mm, some with concentric layering, have been observed. Petrographic features, trace element composition and rare earth element patterns show the presence of eucritic and diogenitic components confirming that it is a typical howardite. Cosmogenic tracks, rare gases (He, Ne, and Ar) and radionuclides (²²Na and ²⁶Al) were measured. Track density in olivine and plagioclase varies between 0.7 to 6 times 10⁶/cm². ³⁸Ar exposure age is estimated to be ～110 Ma, being the highest among howardites. The track production rates correspond to ablation of 9 to 15 cm, implying a radius for its preatmospheric size of ～27 cm. ²²Na/²⁶Al ～ 1, as expected from the production models and solar modulation of galactic cosmic‐ray fluxes before its fall, suggesting that the meteoroid did not undergo any fragmentation during the past ～2 Ma in interplanetary space. The radiogenic age based on K‐Ar method is 4.3 Ga while the U‐Th‐⁴He age is 3.3 Ga indicating partial loss of He.  相似文献   

Self-modulation of plasma waves in equal mass plasmas     

M. Salahuddin  H. Saleem  P. K. Shukla  G. Murtaza 《Astrophysics and Space Science》1995,234(2):253-257

The self-modulation of weakly relativistic plasma waves in equal mass (electron-positron) plasmas has been investigated by employing the Krylov-Bogoliubov-Mitropolsky perturbation technique. It is found that one-dimensional electron plasma waves are modulationally stable and that they can propagate in the form of dark envelope solitons.  相似文献   

Excitation rate of sodium D line in the nightglow     

A. N. Srivastava  R. V. Shukla 《Astrophysics and Space Science》1970,8(1):136-139

The processes responsible for the emission of Na-D line in the Earth's atmosphere and laboratory are briefly reviewed. From the laboratory results of Ghoshet al. (1970), the rate coefficient of reactions exciting sodium D line is estimated to be 4.73×10^–25 cm⁶/sec², and its intensity in the nightglow is found to be about 114R in summer and 302R in winter.  相似文献   

Self-modulation of pulsar radiation in strongly magnetized electron-positron plasmas     

P. K. Shukla 《Astrophysics and Space Science》1985,114(2):381-385

A nonlinear Schrödinger equation is obtained for linearly polarized electromagnetic waves propagating across the ambient magnetic field in an electron-positron plasma. The nonlinearities arising from wave intensity induced particle mass modulation, as well as harmonic generation are incorporated. Modulational instability and localization of pulsar radiation are investigated.  相似文献   

Excitation of O2 Herzberg Bands in the nightglow     

R. D. Srivastava  R. V. Shukla  A. N. Srivastava 《Astrophysics and Space Science》1971,14(2):317-322

The excitation mechanism for O₂ Herzberg Bands as given by Young and Black (1966) is examined. It is found that O₂ Herzberg Bands are heavily quenched by N atoms, while (0,0) and (0,1) Atmospheric Bands are quenched mainly by CO, NO, O₂ and N₂, NO, O₂ respectively. The emission of Herzberg Bands is found to arise from two layers centred at 80 and 100 km. The rate coefficients of a number of quenching reactions involving atmospheric gases are obtained theoretically.  相似文献   

Bremsstrahlung gamma radiation and interstellar electron spectrum in the local interstellar medium     

C. J. Cesarsky  J. A. Paul  P. G. Shukla 《Astrophysics and Space Science》1978,59(1):73-83

We present a detailed study of the bremsstrahlung gamma-ray emissivity of the galactic disk. We show that there are large uncertainties in the production spectrum of photons in the medium energy range (10–100 MeV) due to our lack of knowledge of the interstellar electron spectrum below a few hundred MeV. In fact, gamma-ray observations can be of great help in determining this spectrum. At present, the spectral shape of the local gamma-ray emissivity above 30 MeV is available, thanks to the SAS-II and the COS-B satellites. Comparing it to our calculations, we determine the local interstellar electron flux in the 50–500 MeV range; the corresponding integrated gamma-ray emissivity above 100 MeV is equal to 2.4×10^–25 photons s^–1 (H-atom)^–1, 60% higher than previously accepted values.  相似文献   

Mineralogy and trace element chemistry of the siliceous earth of barmer basin, Rajasthan: Evidence for a volcanic origin     

M. S. Sisodia  U. K. Singh  G. Lashkari  P. N. Shukla  A. D. Shukla  N. Bhandari 《Journal of Earth System Science》2005,114(2):111-124

We report the presence of a 3–5 cm thick loose fragmental layer in the Siliceous Earth at Matti ka Gol in the Barmer basin of Rajasthan. Petrographic, chemical and mineralogical study reveals the presence of abundant volcanic debris such as glass shards, agglutinates, hollow spheroids, kinked biotites, feldspars showing oscillatory zoning, olivines, ilmenite and native iron. The presence of similar particles in the whole section suggests that the Siliceous Earth is a volcanic ash. Stratigraphic correlation, palynological and microvertebrate data suggest that the Siliceous Earth may have deposited over a short span of time during the Upper Cretaceous to Lower Palaeocene. In view of the possibility that this section may contain K/T impact debris, we looked for grains having impact signatures. Some patches of the Siliceous Earth of Bariyara show the presence of Ni-rich (> 0.5%) vesicular glasses, sanidine spherules, magnesioferrite crystals, soot, etc., but because of their low abundance, it is not possible to establish if they are volcanic, micrometeorite ablation products or a part of the K/T impact ejecta.  相似文献   

Performance of Multi Model Canonical Correlation Analysis (MMCCA) for prediction of Indian summer monsoon rainfall using GCMs output     

Ankita Singh  Nachiketa Acharya  Uma Charan Mohanty  Gopbandhu Mishra 《Comptes Rendus Geoscience》2013,345(2):62-72

The emerging advances in the field of dynamical prediction of monsoon using state-of-the-art General Circulation Models (GCMs) have led to the development of various multi model ensemble techniques (MMEs). In the present study, the concept of Canonical Correlation Analysis is used for making MME (referred as Multi Model Canonical Correlation Analysis or MMCCA) for the prediction of Indian summer monsoon rainfall (ISMR) during June-July-August-September (JJAS). This method has been employed on the rainfall outputs of six different GCMs for the period 1982 to 2008. The prediction skill of ISMR by MMCCA is compared with the simple composite method (SCM) (i.e. arithmetic mean of all GCMs), which is taken as a benchmark. After a rigorous analysis through different skill metrics such as correlation coefficient and index of agreement, the superiority of MMCCA over SCM is illustrated. Performance of both models is also evaluated during six typical monsoon years and the results indicate the potential of MMCCA over SCM in capturing the spatial pattern during extreme years.  相似文献   

Relaxed magnetic field structures in multi-ion plasmas     

M. Iqbal  P. K. Shukla 《Astrophysics and Space Science》2012,339(1):19-23

The steady state solution of a three species magnetoplasma is presented. It is shown that relaxed magnetic field configuration results in a triple curl Beltrami equation which permits the existence of three structures. It is the consequence of inertial effects of the plasma constituents. One of the three vortices is of large scale while the remaining two relaxed structures are of small size of the order of electron skin depth. The magnetic field profiles are given for different Beltrami parameters. The study could be helpful to understand large magnetic field structures in three species plasmas found in space and laboratory.  相似文献   

Development of a modified bare soil and urban index for Landsat 8 satellite data     

Atul Kant Piyoosh  Sanjay Kumar Ghosh 《国际地球制图》2018,33(4):423-442

Mapping of urban area has always been a challenging task due to its similar spectral characteristics with bare soil. The spectral characteristics of urban and bare soil being similar, causes confusion and misclassification among themselves. A new modified normalized difference soil index (MNDSI) has been proposed using PAN and Band 7 of Landsat 8. PAN band of Landsat 8 provides increased contrast between vegetation and land areas without vegetation. Subsequently, MNDSI was used to develop a new normalized ratio urban index (NRUI) by enhancing the capability of biophysical composition index (BCI) in two stages. First, a ratio urban index (RUI) was developed which discriminates urban and soil better than BCI. Second, RUI was further enhanced, subsequently known as NRUI, which is able to discriminate urban area from soil even better than RUI. MNDSI and NRUI show a good discrimination between soil and urban and may be useful for such purposes.  相似文献