arXiv subjects
Publications and source records attributed to S. M. Chitre.
A simple model of uniformly expanding, homogeneous Universe with a bulk viscosity is studied wherein the inflationary density decays due to viscous dissipation during the expansion phase of the Universe. The model is shown to generate the Cosmic Microwave Background radiation (CMB). We also demonstrate that, at late times, the inflationary density asymptotically approaches a small finite constant value.
Consideration of the entropy production in the creation of the CMB leads to a simple model of the evolution of the universe during this period which suggests a connection between the small observed acceleration term and the early inflation of a closed universe. From this we find an unexpected relationship between the Omega's of cosmology and calculate the total volume of the universe.
With the ultimate aim of using the fundamental or $f$-mode to study helioseismic aspects of turbulence-generated magnetic flux concentrations, we use randomly forced hydromagnetic simulations of a piecewise isothermal layer in two dimensions with reflecting boundaries at top and bottom. We compute numerically diagnostic wavenumber-frequency diagrams of the vertical velocity at the interface between the denser gas below and the less dense gas above. For an Alfvén-to-sound speed ratio of about 0.1, a 5% frequency increase of the $f$-mode can be measured when $k_xH_{\rm p}=3$-$4$, where $k_x$ is the horizontal wavenumber and $H_{\rm p}$ is the pressure scale height at the surface. Since the solar radius is about 2000 times larger than $H_{\rm p}$, the corresponding spherical harmonic degree would be 6000-8000. For weaker fields, a $k_x$-dependent frequency decrease by the turbulent motions becomes dominant. For vertical magnetic fields, the frequency is enhanced for $k_xH_{\rm p}\approx4$, but decreased relative to its nonmagnetic value for $k_xH_{\rm p}\approx9$.
The helioseismic observations of the internal rotation profile of the Sun raise questions about the two-dimensional (2D) nature of the transport of angular momentum in stars. Here we derive a convective prescription for axisymmetric (2D) stellar evolution models. We describe the small scale motions by a spectrum of unstable linear modes in a Boussinesq fluid. Our saturation prescription makes use of the angular dependence of the linear dispersion relation to estimate the anisotropy of convective velocities. We are then able to provide closed form expressions for the thermal and angular momentum fluxes with only one free parameter, the mixing length. We illustrate our prescription for slow rotation, to first order in the rotation rate. In this limit, the thermodynamical variables are spherically symetric, while the angular momentum depends both on radius and latitude. We obtain a closed set of equations for stellar evolution, with a self-consistent description for the transport of angular momentum in convective regions. We derive the linear coefficients which link the angular momentum flux to the rotation rate ($Λ$- effect) and its gradient ($α$-effect). We compare our results to former relevant numerical work.
A putative temporally varying circulation-free magnetic-field configuration is inferred in an equatorial segment of the solar convection zone from the helioseismologically inferred angular-velocity variation, assuming that the predominant dynamics is angular acceleration produced by the azimuthal Maxwell stress exerted by a field whose surface values are consistent with photospheric line-of-sight measurements.
Available helioseismic data now span almost the entire solar activity cycle 23 making it possible to study solar-cycle related changes of the solar rotation rate in detail. In this paper we study how the solar rotation rate, in particular, the zonal flows change with time. In addition to the zonal flows that show a well known pattern in the solar convection zone, we also study changes in the radial and latitudinal gradients of the rotation rate, particularly in the shear layer that is present in the immediate sub-surface layers of the Sun. In the case of the zonal-flow pattern, we find that the band indicating fast rotating region close to the equator seems to have bifurcated around 2005. Our investigation of the rotation-rate gradients show that the relative variation in the rotation-rate gradients is about 20% or more of their average values, which is much larger than the relative variation in the rotation rate itself. These results can be used to test predictions of various solar dynamo models.
The Faraday rotation measurements of multiply-imaged gravitational lens systems can be effectively used to probe the existence of large-scale ordered magnetic fields in lensing galaxies and galaxy clusters. The available sample of lens systems appears to suggest the presence of a coherent large-scale magnetic field in giant elliptical galaxies somewhat similar to the spiral galaxies.
We compare the zonal flow pattern in subsurface layers of the Sun with the distribution of surface magnetic features like sunspots and polar faculae. We demonstrate that in the activity belt, the butterfly pattern of sunspots coincides with the fast stream of zonal flows, although part of the sunspot distribution does spill over to the slow stream. At high latitudes, the polar faculae and zonal flow bands have similar distributions in the spatial and temporal domains.
AIM: To study the variation of the angular momentum and the rotational kinetic energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. METHOD: Inverting helioseismic rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. RESULTS: The temporal variation in angular momentum and kinetic energy at high latitudes (>π/4) through the convection zone is positively correlated with solar activity, whereas at low latitudes it is anticorrelated, except for the top 10% by radius where both are correlated positively. CONCLUSION: The helioseismic data imply significant temporal variation in the angular momentum and the rotational kinetic energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
The latitudinal sound-speed structure of the Sun's convection zone gives insight into the physical processes occurring there, specifically the cellular convection and possibly the presence of magnetic fields. Using helioseismic data from the GONG network and MDI instrument on SOHO, we map the latitudinal acoustic structure of the convection zone from 1995 to 2002. The temporally averaged structure confirms previous findings of an excess in sound speed at the $10^{-4}$ level at 60 degrees latitude. There also appear to be some variations with time, with the peak in sound-speed asphericity at 60 degrees growing towards the maximum of solar activity according to the MDI data. However, we present some evidence that such variation may be associated with instrumental variation between the epochs before and after SOHO was temporarily lost in 1998. Nonetheless, some genuine temporal variation may be present, and we discuss the possible physical causes of that.
Primary inversions of accurately measured solar oscillation frequencies coupled with the equations of thermal equilibrium and other input physics, enable us to infer the temperature and hydrogen abundance profiles inside the Sun. These profiles also help in setting constraints on the input physics as well as on heavy element abundance in the solar core. Using different treatments of plasma screening for nuclear reaction rates, limits on the cross-section of proton-proton nuclear reaction as a function of heavy element abundance in the solar core are obtained and an upper limit on heavy element abundance in the solar core is also derived from these results.
We have computed accurate 1-D solar models including both a macroscopic mixing process in the solar tachocline as well as up-to-date microscopic physical ingredients. Using sound speed and density profiles inferred through primary inversion of the solar oscillation frequencies coupled with the equation of thermal equilibrium, we have extracted the temperature and hydrogen abundance profiles. These inferred quantities place strong constraints on our theoretical models in terms of the extent and strength of our macroscopic mixing, on the photospheric heavy elements abundance, on the nuclear reaction rates such as $S_{11}$ and $S_{34}$ and on the efficiency of the microscopic diffusion. We find a good overall agreement between the seismic Sun and our models if we introduce a macroscopic mixing in the tachocline and allow for variation within their uncertainties of the main physical ingredients. From our study we deduce that the solar hydrogen abundance at the solar age is $X_{\rm inv}=0.732\pm 0.001$ and that based on the $^9$Be photospheric depletion, the maximum extent of mixing in the tachocline is 5% of the solar radius. The nuclear reaction rate for the fundamental $pp$ reaction is found to be $S_{11}(0)=4.06\pm 0.07$ $10^{-25}$ MeV barns, i.e., 1.5% higher than the present theoretical determination. The predicted solar neutrino fluxes are discussed in the light of the new SNO/SuperKamiokande results.
A seismic model for the Sun calculated using the accurate helioseismic data predicts a lower $^{8}{B}$ neutrino flux as compared to the standard solar model (SSM). However, there persists a discrepancy between the predicted and measured neutrino fluxes and it seems necessary to invoke neutrino oscillations to explain the measurements. In this work, we have performed a global, unified oscillation analysis of the latest solar neutrino data (including the results of SNO charged current rate) using the seismic model fluxes as theoretical predictions. We determine the best-fit values of the neutrino oscillation parameters and the $χ^2_{\mathrm min}$ for both $ν_e-ν_{\mathrm active}$ and $ν_e -ν_{\mathrm sterile}$ cases and present the allowed parameter regions in the $Δm^2 - \tan^2 θ$ plane for $ν_e-ν_{\mathrm active}$ transition. The results are compared with those obtained using the latest SSM by Bahcall and his collaborators.
The observed splittings of solar oscillation frequencies can be employed to separate the effects of internal solar rotation and to estimate the contribution from a large-scale magnetic field or any latitude-dependent thermal perturbation inside the Sun. The surface distortion estimated from the rotation rate in the solar interior is found to be in good agreement with the observed oblateness at solar surface. After subtracting out the estimated contribution from rotation, there is some residual signal in the even splitting coefficients, which may be explained by a magnetic field of approximately 20 kG strength located at a depth of 30000 km below the surface or an equivalent aspherical thermal perturbation. An upper limit of 300 kG is derived for a toroidal field near the base of the convection zone.
We propose a mechanism for the fast dissipation of magnetic fields which is effective in a stratified medium where ion motions can be neglected. In such a medium, the field is frozen into the electrons and Hall currents prevail. Although Hall currents conserve magnetic energy, in the presence of density gradients, they are able to create current sheets which can be the sites for efficient dissipation of magnetic fields. We recover the frequency, $ω_{MH}$, for Hall oscillations modified by the presence of density gradients. We show that these oscillations can lead to the exchange of energy between different components of the field. We calculate the time evolution and show that magnetic fields can dissipate on a timescale of order $1/ω_{MH}$. This mechanism can play an important role for magnetic dissipation in systems with very steep density gradients where the ions are static such as those found in the solid crust of neutron stars.
Primary inversions of solar oscillation frequencies coupled with the equations of thermal equilibrium and other input physics, enable us to infer the temperature and hydrogen abundance profiles inside the Sun. These profiles also help in setting constraints on the input physics that is consistent with the accurately measured oscillation frequencies data. Helioseismic limits on the cross-section of proton-proton nuclear reaction as a function of heavy element abundance in the solar core are derived. We demonstrate that it is not possible to infer the heavy element abundance profile, in addition to temperature and hydrogen abundance profiles, with the helioseismic constraints.
A static and circularly symmetric lens characterized by mass and scalar charge parameters is constructed. For the small values of the scalar charge to the mass ratio, the gravitational lensing is qualitatively similar to the case of the Schwarzschild lens; however, for large values of this ratio the lensing characteristics are significantly different. The main features are the existence of two or nil Einstein ring(s) and a radial critical curve, formation of two or four images and possibility of detecting three images near the lens for sources located at relatively large angular positions. Such a novel lens may also be treated as a naked singularity lens.
The primary inversion of the accurately measured frequencies of solar oscillations determines the mechanical properties of the Sun, i.e., the sound speed and density as a function of solar radius. In order to infer the temperature and hydrogen abundance profiles inside the Sun, it becomes necessary to use, in addition, the equations of thermal equilibrium along with the input physics, such as opacities, equation of state and nuclear reaction rates. It then becomes possible to estimate the effects of uncertainties in opacity and nuclear reaction rates on the inferred thermal and composition profiles. The seismically determined temperature and hydrogen abundance profiles turn out to be close to those in a standard evolutionary solar model that includes the diffusion of helium and heavy elements below the convection zone. The most significant departure occurs just below the base of the convection zone where the inferred hydrogen abundance profile is smoother than that in a standard diffusion model. The region just beneath the solar convection zone appears to be mixed, a process which could account for the observed low lithium abundance in the solar envelope. With a reasonable allowance for errors in opacities, the helioseismically estimated cross-section for pp nuclear reaction rate turns out to be (4.15+/-0.25)*1.E-25 MeV barns.