14 research outputs found
On linear coupling of acoustic and cyclotron waves in plasma flows
It is found that in magnetized electrostatic plasma flows the velocity shear
couples ion-acoustic waves with ion-cyclotron waves and leads, under favorable
conditions, to their efficient reciprocal transformations. It is shown that in
a two-dimensional setup this coupling has a remarkable feature: it is governed
by equations that are exactly similar to the ones describing coupling of sound
waves with internal gravity waves [Rogava & Mahajan: Phys. Rev. E vol.55, 1185
(1997)] in neutral fluid flows. Using another noteworthy quantum mechanical
analogy we calculate transformation coefficients and give fully analytic,
quantitative description of the coupling efficiency for flows with low shearing
rates.Comment: 5 pages, no figures. Submitted to "Physics of Plasmas
On over-reflection and generation of Gravito-Alfven waves in solar-type stars
The dynamics of linear perturbations is studied in magnetized plasma shear
flows with a constant shearing rate and with gravity-induced stratification.
The general set of linearized equations is derived and the two-dimensional case
is considered in detail. The Boussinesq approximation is used in order to
examine relatively small-scale perturbations of low-frequency modes:
Gravito-Alfven waves (GAW) and Entropy Mode (EM) perturbations. It is shown
that for flows with arbitrary shearing rate there exists a finite time interval
of non-adiabatic evolution of the perturbations. The non-adiabatic behavior
manifests itself in a twofold way, viz. by the over-reflection of the GAWs and
by the generation of GAWs from EM perturbations. It is shown that these
phenomena act as efficient transformers of the equilibrium flow energy into the
energy of the perturbations for moderate and high shearing rate solar plasma
flows. Efficient generation of GAW by EM takes place for shearing rates about
an order of magnitude smaller than necessary for development of a shear
instability. The latter fact could have important consequences for the problem
of angular momentum redistribution within the Sun and solar-type stars.Comment: 20 pages (preprint format), 4 figures; to appear in The Astrophysical
Journal (August 1, 2007, v664, N2 issue
Quantifying shear-induced wave transformations in the solar wind
The possibility of velocity shear-induced linear transformations of different
magnetohydrodynamic waves in the solar wind is studied both analytically and
numerically. A quantitative analysis of the wave transformation processes for
all possible plasma- regimes is performed. By applying the obtained
criteria for effective wave coupling to the solar wind parameters, we show that
velocity shear-induced linear transformations of Alfv\'en waves into
magneto-acoustic waves could effectively take place for the relatively
low-frequency Alfv\'en waves in the energy containing interval. The obtained
results are in a good qualitative agreement with the observed features of
density perturbations in the solar wind.Comment: Astrophysical Journal (accepted
General-relativistic Model of Magnetically Driven Jet
The general scheme for the construction of the general-relativistic model of
the magnetically driven jet is suggested. The method is based on the usage of
the 3+1 MHD formalism. It is shown that the critical points of the flow and the
explicit radial behavior of the physical variables may be derived through the
jet ``profile function."Comment: 12 pages, LaTex, no figure
Centrifugally driven relativistic dynamics on curved trajectories
Motion of test particles along rotating curved trajectories is considered.
The problem is studied both in the laboratory and the rotating frames of
reference. It is assumed that the system rotates with the constant angular
velocity . The solutions are found and analyzed for the case
when the form of the trajectory is given by an Archimedes spiral. It is found
that particles can reach infinity while they move along these trajectories and
the physical interpretation of their behaviour is given. The analogy of this
idealized study with the motion of particles along the curved rotating magnetic
field lines in the pulsar magnetosphere is pointed out. We discuss further
physical development (the conserved total energy case, when )
and astrophysical applications (the acceleration of particles in active
galactic nuclei) of this theory.Comment: 13 pages, 3 figures, to appear in the vol.35 (July issue) of the
"General Relativity and Gravitation
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Asymptotic persistence of collective modes in shear flows
A new nonasymptotic method is presented that reveals an unexpected richness in the spectrum of fluctuations sustained by a shear flow with nontrivial arbitrary mean kinematics. The vigor of the method is illustrated by analyzing a two-dimensional, compressible hydrodynamic shear flow. The temporal evolution of perturbations spans a wide range of nonexponential behavior from growth-cum oscillations to monotonic growth. The principal characteristic of the revealed exotic collective modes in their asymptotic persistence. {open_quotes}Echoing{close_quotes} as well as unstable (including parametrically-driven) solutions are displayed. Further areas of application, for both the method and the new physics, are outlined
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Shear flow induced wave couplings in the solar wind
A sheared background flow in a plasma induces coupling between different MHD wave modes, resulting in their mutual transformations with corresponding energy redistributing between the modes. In this way, the energy can be transfered from one wave mode to the other, but energy can also be added to or extracted from the background flow. In the present paper it is investigated whether the wave coupling and energy transfer mechanisms can operate under solar wind conditions. It is shown that this is indeed the case. Hence, the long-period waves observed in the solar wind at r > 0.3 AU might be generated by much faster periodic oscillations in the photosphere of the Sun. Other possible consequences for observable beat phenomena in the wind and the acceleration of the solar wind particles are also discussed