Browsing by Author "Sridhar, S."

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Non-perturbative quasilinear approach to the shear dynamo problem
(2009-10-01) Sridhar, S.; Subramanian, Kandaswamy
We study large–scale dynamo action due to turbulence in the presence of a linear shear ﬂow. Our treatment is quasilinear and equivalent to the standard ‘ﬁrst order smoothing approximation’. However it is non perturbative in the shear strength. We ﬁrst derive an integro–differential equation for the evolution of the mean magnetic ﬁeld, by systematic use of the shearing coordinate transformation and the Galilean invariance of the linear shear ﬂow. We show that, for non helical turbulence, the time evolution of the cross–shear components of the mean ﬁeld do not depend on any other components excepting themselves; this is valid for any Galilean–invariant velocity ﬁeld, independent of its dynamics. Hence, to all orders in the shear parameter, there is no shear–current type effect for non helical turbulence in a linear shear ﬂow, in quasilinear theory in the limit of zero resistivity. We then develop a systematic approximation of the integro–differential equation for the case when the mean magnetic ﬁeld varies slowly compared to the turbulence correlation time. For non-helical turbulence, the resulting partial differential equations can again be solved by making a shearing coordinate transformation in Fourier space. The resulting solutions are in the form of shearing waves, labeled by the wavenumber in the sheared coordinates. These shearing waves can grow at early and intermediate times but are expected to decay in the long time limit. small regions of strong ﬁeld gradients. We investigate the kinetic energy release into heat, mediated by the dynamo action, both in our model and by solving the induction equation with the same ﬂow. We ﬁnd that a ﬂux rope dynamo is an order of magnitude more eﬃcient at converting mechanical energy into heat. The probability density of the magnetic energy release in reconnections has a power-law form with the slope −3, consistent with the Solar corona heating by nanoﬂares.
Pattern speed of the nuclear disk of M31 using a variant of the Tremaine-Weinberg method
(2000-08-25) Sambhus, Niranjan; Sridhar, S.
The twin peaks in the nucleus of M31 have been interpreted by Tremaine as a thick, eccentric, disk of stars orbiting a massive dark object; the required align- ment of the apoapsides of the stellar orbits could be maintained by self–gravity, and the whole structure might be a discrete, nonlinear eigenmode. The pattern speed of this mode could, in principle, be determined by the Tremaine–Weinberg (TW) method, which requires measurements of the surface brightness, and ra- dial velocity along a strip parallel to the line of nodes. However, spectroscopic observations along the line of nodes are not available. We propose a variant of the TW method, which exploits a basic feature of the eccentric disk model, to extract estimates of the pattern speed from Hubble Space Telescope spectroscopic data, taken along the line joining the two peaks. Within limitations imposed by the data, we estimate that the pattern rotates in a prograde manner and, for an assumed disk inclination of 77◦ , the pattern speed |Ωp| < 30 kms−1 pc−1, or period more than 200, 000 years.
Shear dynamo problem: Quasilinear kinematic theory
(2009-04-01) Sridhar, S.; Subramanian, Kandaswamy
Large–scale dynamo action due to turbulence in the presence of a linear shear ﬂow is studied. Our treatment is quasilinear and kinematic but is non perturbative in the shear strength. We derive the integro–differential equation for the evolution of the mean magnetic ﬁeld, by systematic use of the shearing coordinate transformation and the Galilean invariance of the linear shear ﬂow. For non helical turbulence the time evolution of the cross–shear components of the mean ﬁeld do not depend on any other components excepting themselves. This is valid for any Galilean–invariant velocity ﬁeld, independent of its dynamics. Hence the shear–current assisted dynamo is essentially absent, although large–scale non helical dynamo action is not ruled out.
Stellar dynamics around black holes in galactic nuclei
(2015-03-11) Sridhar, S.; Touma, J.
Stellar orbits in triaxial clusters around black holes in galactic nuclei
(2015-03-01) Sambhus, Niranjan; Sridhar, S.
We investigate the orbital structure of a model triaxial star cluster, centered around a supermassive black hole (BH), appropriate to galactic nuclei. Sridhar and Touma {1999) proved that the presence of the BH enforces some regularity in the dynamics within the radius of influence of the BH. We employ their averaging method to reduce the degrees of freedom from three to two. Numerical orbit integrations, together with Poincare surfaces of section allow us to draw a global portrait of the orbital structure; in our calculations we employ a model cluster potential that is triaxial and harmonic. The averaged dynamics of the axisymmetric case is integrable, and we present a detailed comparison of orbits in oblate and prolate axisymmetric potentials. Both cases support resonant orbits with fixed values of eccentricity, inclination, and periapse, whose lines of nodes rotates steadily. These occur for all values of oblateness, but only for axis ratio greater than two, in the prolate case; we identify this phenomenon with the (in)stability of the long axis orbit. We then systematically explore significantly triaxial potentials, possessing small oblateness, or prolateness. Resonant orbits and their families are studied both numerically, and through secular perturbation theory. Chaos is highly suppressed for all the cases we studied, and we obtain effective third integrals. Some of the orbits appear to reinforce the shape of the potential; we provide phase space, as well as real space portraits of these orbits. A particularly promising resonant orbit exists in highly prolate, triaxial potentials.
Turbulent transport of a tracer: An electromagnetic formulation
(2015-03-11) Sridhar, S.