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91.
We present numerical simulations of kinetic Alfvén waves (KAWs) and inertial Alfvén waves (IAWs) applicable to the solar wind, the solar corona, and the auroral regions, respectively, leading to the formation of coherent magnetic structures when the nonlinearity arises from ponderomotive effects and Joule heating. The nonlinear dynamical equation satisfies the modified nonlinear Schrödinger equation. The effect of nonlinear coupling between the main KAW/IAW and the perturbation, producing filamentary structures of the magnetic field, has been studied. Scalings in the spectral index of the power spectrum at different times have been calculated. These filamentary structures can act as a source for particle acceleration by wave?–?particle interaction because the KAWs/IAWs are mixed modes and Landau damping is possible. 相似文献
92.
93.
Suman?MajumdarEmail authorView authors OrcID profile Kanan?K.?Datta Raghunath?Ghara Rajesh?Mondal T.?Roy?Choudhury Somnath?Bharadwaj Sk.?Saiyad?Ali Abhirup?Datta 《Journal of Astrophysics and Astronomy》2016,37(4):32
The line-of-sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reionization is quite unique in many ways compared to any other cosmological signal. Different unique effects, such as the evolution history of the signal, non-linear peculiar velocities of the matter etc. will imprint their signature along the line-of-sight axis of the observed signal. One of the major goals of the future SKA-LOW radio interferometer is to observe the cosmic dawn and the epoch of reionization through this 21-cm signal. It is thus important to understand how these various effects affect the signal for its actual detection and proper interpretation. For more than one and half decades, various groups in India have been actively trying to understand and quantify the different line-of-sight effects that are present in this signal through analytical models and simulations. In many ways the importance of this sub-field under 21-cm cosmology have been identified, highlighted and pushed forward by the Indian community. In this article, we briefly describe their contribution and implication of these effects in the context of the future surveys of the cosmic dawn and the epoch of reionization that will be conducted by the SKA-LOW. 相似文献
94.
T. Roy Choudhury Kanan Datta Suman Majumdar Raghunath Ghara Aseem Paranjape Rajesh Mondal Somnath Bharadwaj Saumyadip Samui 《Journal of Astrophysics and Astronomy》2016,37(4):29
Studying the cosmic dawn and the epoch of reionization through the redshifted 21-cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the Universe. Interpreting the upcoming data would require detailed modelling of the relevant physical processes. In this article, we focus on the theoretical models of reionization that have been worked out by various groups working in India with the upcoming SKA in mind. These models include purely analytical and semi-numerical calculations as well as fully numerical radiative transfer simulations. The predictions of the 21-cm signal from these models would be useful in constraining the properties of the early galaxies using the SKA data. 相似文献
95.
Kanan K. Datta Raghunath Ghara Suman Majumdar T. Roy Choudhury Somnath Bharadwaj Himadri Roy Abhirup Datta 《Journal of Astrophysics and Astronomy》2016,37(4):27
Detection of individual luminous sources during the reionization epoch and cosmic dawn through their signatures in the HI 21-cm signal is one of the direct approaches to probe the epoch. Here, we summarize our previous works on this and present preliminary results on the prospects of detecting such sources using the SKA1-low experiment. We first discuss the expected HI 21-cm signal around luminous sources at different stages of reionization and cosmic dawn. We then introduce two visibility based estimators for detecting such signals: one based on the matched filtering technique and the other relies on simply combing the visibility signal from different baselines and frequency channels. We find that the SKA1-low should be able to detect ionized bubbles of radius \(R_{\mathrm {b}} \gtrsim 10\) Mpc with ~100 h of observations at redshift z~8 provided that the mean outside neutral hydrogen fraction \(\mathrm {x}_{\text {HI}} \gtrsim 0.5\). We also investigate the possibility of detecting HII regions around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock et al. (Nature 474, 7353 (2011)). We find that a 5σ detection is possible with 600 h of SKA1-low observations if the QSO age and the outside xHI are at least ~2×107 Myr and ~0.2 respectively. Finally, we investigate the possibility of detecting the very first X-ray and Ly- α sources during the cosmic dawn. We consider mini-QSOs like sources which emits in X-ray frequency band. We find that with a total ~ 1000 h of observations, SKA1-low should be able to detect those sources individually with a ~ 9σ significance at redshift z=15. We summarize how the SNR changes with various parameters related to the source properties. 相似文献
96.
The self-gravitating instability of an infinitely extending axisymmetric cylinder of viscoelastic medium permeated with non uniform magnetic field and rotation is studied for both the strongly coupled plasma (SCP) and weakly coupled plasma (WCP). The non uniform magnetic field and rotation are considered to act along the axial direction of the cylinder. The normal mode method of perturbations is applied to obtain the dispersion relation. The condition for the onset of gravitational instability has been derived from the dispersion relation under both strongly and weakly coupling limits. It is found that the Jeans criterion for gravitational collapse gets modified due to the presence of shear and bulk viscosities for the SCP, however, the magnetic field and rotation whether uniform or non uniform has no effect on the Jeans criterion of an infinitely extending axisymmetric cylinder of a self-gravitating viscoelastic medium. 相似文献
97.
The transition 111 ? 110 at 4.829 GHz of formaldehyde (H2CO) was the first one showing the anomalous absorption, i.e., the absorption against the cosmic microwave background. Anomalous absorption is an unusual phenomena. Structure of H2CC is very similar to that of H2CO and H2CS. Both H2CO and H2CS have already been identified in a number of cosmic objects. Though H2CC is not yet identified in the cosmic objects, we propose that H2CC may be identified in cool cosmic objects through its transition 111 ? 110 at 4.85 GHz in anomalous absorption. 相似文献
98.
Rajmal Jain Malini Aggarwal Raghunandan Sharma 《Journal of Astrophysics and Astronomy》2010,31(3):155-163
We report observational evidence of the decay of the flux ratio of Fe to Fe-Ni line features as a function of plasma electron
temperature in solar flares in comparison to that theoretically predicted by Phillips (2004). We present the study of spectral
analysis of 14 flares observed by the Solar X-ray Spectrometer (SOXS) — Low Energy Detector (SLD) payload. The SLD payload
employs the state-of-the-art solid state detectors, viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices. The sub-keV energy
resolution of Si PIN detector allows us to study the Fe-line and Fe-Ni line features appearing at 6.7 and 8 keV, respectively,
in greater detail. In order to best-fit the whole spectrum at one time in the desired energy range between 4 and 25 keV we
considered Gaussian-line, the multi-thermal power-law and broken power-law functions. We found that the flux ratio of Fe to
Fe-Ni line features decays with flare electron temperature by the asymptotic form of polynomial of inverse third order. The
relative flux ratio is ∼30 at temperature 12 MK which drops to half, ∼15 at 20 MK, and at further higher temperatures it decreases
smoothly reaching to ∼8 at ∼50 MK. The flux ratio, however, at a given flare plasma temperature, and its decrease with temperature
is significantly lower than that predicted theoretically. We propose that the difference may be due to the consideration of
higher densities of Fe and Fe-Ni lines in the theoretical model of Phillips (2004). We suggest revising the Fe and Fe-Ni line
densities in the corona. The decay of flux ratio explains the variation of equivalent width and peak energy of these line
features with temperature. 相似文献
99.
Saibal Gupta Aditi Das Sudipta Goswami Ananda Modak Suman Mondal 《Journal of the Geological Society of India》2010,75(1):313-322
Inverted metamorphism in the Himalayas is closely associated with the Main Central Thrust (MCT). In the western Himalayas,
the Main Central Thrust conventionally separates high grade metamorphic rocks of the Higher Himalayan Crystalline Sequence
(HHCS) from unmetamorphosed rocks of the Inner sedimentary Belt. In the eastern Himalayas, the Inner sedimentary Belt is absent,
and the HHCS and meta-sedimentary Lesser Himalayan Sequence (LHS) apparently form a continuous Barrovian metamorphic sequence,
leading to confusion about the precise location of the MCT. In this study, it is demonstrated that migmatitic gneisses of
the sillimanite zone in the higher structural levels of the HHCS are multiply deformed, with two phases of penetrative fabric
formation (S1HHCS and S2HHCS) followed by third folding event associated with a spaced, NW-SE trending, north-east dipping foliation (S3HHCS). The underlying LHS schists (kyanite zone and lower) are also multiply deformed, with the bedding S0 being isoclinally folded (F1LHS), and subsequently refolded (F2LHS and F3LHS). The contact zone between the HHCS and LHS is characterized by ductile, top-to-the southwest shearing and stabilization
of a pervasive foliation that is consistently oriented NW-SE and dips northeast. This foliation is parallel to the S3HHCS foliation in the HHCS, and the S2LHS in the LHS. Early lineations in the HHCS and LHS also show different dispersions across the contact shear zone, implying
that pre-thrusting orientations of the two units were distinct. The contact shear zone is therefore interpreted to be a plane
of structural discordance, shows a shear sense consistent with thrust movement and is associated with mineral growth during
Barrovian metamorphism. It may well be considered to represent the MCT in this region. 相似文献
100.
Koushik Sen Ruchika Sharma B. R. Arora Vikram Gupta 《Journal of the Geological Society of India》2010,76(5):322-330
Anisotropy of Magnetic Susceptibility (AMS) and seismic wave velocity studies of some paramagnetic Himalayan granitoids show
good correlation between magnetic fabric anisotropy and P wave velocity (Vp). Vp shows strong positive correlation with magnetic
lineation (L) and degree of magnetic anisotropy (P′) having correlation coefficient (r) values of 0.93 and 0.89 respectively.
Both Vp and Vs show positive correlation with the SiO2 content of Proterozoic and Paleozoic granitoids. Velocity of S wave (Vs) shows negative correlation with mean magnetic susceptibility
(Km) having ‘r’ value of 0.86. The correlation between Vs-Km, Vp-P′, Vp-L also shows >95% probability in Spearman’s rank correlation. Based on the results from the present sample size it is suggested
that, in paramagnetic granites, Vp is proportional to intensity of deformation and preferred orientation of minerals as well as the mineralogy. On the other
hand, Vs is more dependent on the mineralogy alone. 相似文献