arXiv subjects

S. M. Mahajan

Publications and source records attributed to S. M. Mahajan.

46 records · Page 3Linked to original sources

arXiv2009

Weak and strong regimes of incompressible magnetohydrodynamic turbulence

It is shown that in the framework of the weak turbulence theory, the autocorrelation and cascade timescales are always of the same order of magnitude. This means that, contrary to the general belief, any model of turbulence which implies a large number of collisions among wave packets for an efficient energy cascade (such as the Iroshnikov-Kraichnan model) are not compatible with the weak turbulence theory.

physics.plasm-ph↗

arXiv2009

Spectrum of Global Magnetorotational Instability in a Narrow Transition Layer

The Global Magnetorotational Instability (MRI) is investigated for a configuration in which the rotation frequency changes only in a narrow transition region. If the vertical wavelength of the unstable mode is of the same order or smaller than the width of this region, the growth rates can differ significantly from those given by a local analysis. In addition, the non-axisymmetric spectrum admits overstable modes with a non-trivial dependence on azimuthal wavelength, a feature missed by the local theory. In the limit of vanishing transition region width, the Rayleigh-centrifugal instability is recovered in the axisymmetric case, and the Kelvin-Helmholtz instability in the non-axisymmetric case.

astro-ph.IM↗

arXiv2008

Global axisymmetric Magnetorotational Instability with density gradients

We examine global incompressible axisymmetric perturbations of a differentially rotating MHD plasma with radial density gradients. It is shown that the standard magnetorotational instability, (MRI) criterion drawn from the local dispersion relation is often misleading. If the equilibrium magnetic field is either purely axial or purely toroidal, the problem reduces to finding the global radial eigenvalues of an effective potential. The standard Keplerian profile including the origin is mathematically ill-posed, and thus any solution will depend strongly on the inner boundary. We find a class of unstable modes localized by the form of the rotation and density profiles, with reduced dependence on boundary conditions.

astro-ph↗

arXiv2004

Waves, Coriolis force and the dynamo effect

Dynamo activity caused by waves in a rotating magneto-plasma is investigated. In astrophysical environments such as accretion disks and at sufficiently small spatial scales, the Hall effect is likely to play an important role. It is shown that a combination of the Coriolis force and Hall effect can produce a finite $α$-effect by generating net helicity in the small scales. The shear/ion-cyclotron normal mode of the Hall plasma is the dominant contributor to the dynamo action for short scale motions.

astro-ph↗

arXiv2004

Direct simulations of helical Hall-MHD turbulence and dynamo action

Direct numerical simulations of turbulent Hall dynamos are presented. The evolution of an initially weak and small scale magnetic field in a system maintained in a stationary turbulent regime by a stirring force at a macroscopic scale is studied to explore the conditions for exponential growth of the magnetic energy. Scaling of the dynamo efficiency with the Reynolds numbers is studied, and the resulting total energy spectra are found to be compatible with a Kolmogorov type law. A faster growth of large scale magnetic fields is observed at intermediate intensities of the Hall effect.

astro-ph↗

arXiv2002

Angular momenta creation in relativistic electron-positron plasma

Creation of angular momentum in a relativistic electron-positron plasma is explored. It is shown that a chain of angular momentum carrying vortices is a robust asymptotic state sustained by the generalized nonlinear Schrodinger equation characteristic to the system. The results may suggest a possible electromagnetic origin of angular momenta when it is applied to the MeV epoch of the early Universe.

physics.plasm-ph↗

arXiv2001

Self-trapping of strong electromagnetic beams in relativistic plasmas

Interaction of an intense electromagnetic (EM) beam with hot relativistic plasma is investigated. It is shown that the thermal pressure brings about a fundamental change in the dynamics - localized, high amplitude, EM field structures, not accessible to a cold (but relativisic) plasma, can now be formed under well- defined conditions. Examples of the trapping of EM beams in self-guiding regimes to form stable 2D solitonic structures in a pure e-p plasma are worked out.

physics.plasm-ph↗

arXiv2000

Vortex solitons - Mass, Energy and Angular momentum bunching in relativistic electron-positron plasmas

It is shown that the interaction of large amplitude electromagnetic waves with a hot electron-positron (e-p) plasma (a principal constituent of the universe in the MeV epoch) leads to a bunching of mass, energy, and angular momentum in stable, long-lived structures. Electromagnetism in the MeV epoch, then, could provide a possible route for seeding the observed large-scale structure of the universe.

astro-ph↗

arXiv1996

Acoustic perturbations in special-relativistic parallel flows

Acoustic perturbations in a parallel relativistic flow of an inviscid fluid are considered. The general expression for the frequency of the sound waves in a uniformly (with zero shear) moving medium is derived. It is shown that relativity evokes a difference in the frequencies of the sound-type perturbations propagating along and against the current. Besides, it is shown that the perturbations are not purely irrotational as they are in nonrelativistic case. For a non-uniformly (with nonzero shear) moving fluid a general set of equations, describing the evolution of the acoustic perturbations in relativistic sheared flows, is obtained and analysed when the temperature is nonrelativistic. It is shown that, like the nonrelativistic case, in the new system: (a) the excitation of vortical, transiently growing perturbations, and (b) the excitation of sound-type perturbations, extracting the kinetic energy of the background flow, are possible. It is demonstrated that the relativistic character of the motion significantly intensifies the efficiency of both these processes. Finally, the possible relevance of this study to processes in astrophysical shear flows is pointed out.

astro-ph↗

arXiv1996

Beat-wave generation of plasmons in semiconductor plasmas

It is shown that in semiconductor plasmas, it is possible to generate large amplitude plasma waves by the beating of two laser beams with frequency difference close to the plasma frequency. For narrow gap semiconductors (for example n-type InSb), the system can simulate the physics underlying beat wave generation in relativistic gaseous plasmas.

cond-mat↗