48 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
Swirling astrophysical flows - efficient amplifiers of Alfven waves
We show that a helical shear flow of a magnetized plasma may serve as an
efficient amplifier of Alfven waves. We find that even when the flow is purely
ejectional (i.e., when no rotation is present) Alfven waves are amplified
through the transient, shear-induced, algebraic amplification process. Series
of transient amplifications, taking place sequentially along the flow, may
result in a cascade amplification of these waves. However, when a flow is
swirling or helical (i.e., some rotation is imposed on the plasma motion),
Alfven waves become subject to new, much more powerful shear instabilities. In
this case, depending on the type of differential rotation, both usual and
parametric instabilities may appear. We claim that these phenomena may lead to
the generation of large amplitude Alfven waves and the mechanism may account
for the appearance of such waves in the solar atmosphere, in accretion-ejecion
flows and in accretion columns. These processes may also serve as an important
initial (linear and nonmodal) phase in the ultimate subcritical transition to
MHD Alfvenic turbulence in various kinds of astrophysical shear flows.Comment: 12 pages, 11 figures, accepted for publication (25-11-02) in
Astronomy and Astrophysic
Amplification of MHD waves in swirling astrophysical flows
Recently it was found that helical magnetized flows efficiently amplify
Alfv\'en waves (Rogava et al. 2003, A&A, v.399, p.421). This robust and
manifold nonmodal effect was found to involve regimes of transient algebraic
growth (for purely ejectional flows), and exponential instabilities of both
usual and parametric nature. However the study was made in the incompressible
limit and an important question remained open - whether this amplification is
inherent to swirling MHD flows per se and what is the degree of its dependence
on the incompressibility condition. In this paper, in order to clear up this
important question, we consider full compressible spectrum of MHD modes:
Alfv\'en waves (AW), slow magnetosonic waves (SMW) and fast magnetosonic waves
(FMW). We find that helical flows inseparably blend these waves with each other
and make them unstable, creating the efficient energy transfer from the mean
flow to the waves. The possible role of these instabilities for the onset of
the MHD turbulence, self-heating of the flow and the overall dynamics of
astrophysical flows are discussed.Comment: 8 pages, 9 figures, accepted for publication (18.03.2003) in the
"Astronomy and Astrophysics
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Escaping radio emission from pulsars: Possible role of velocity shear
It is demonstrated that the velocity shear, intrinsic to the e{sup +}e{sup {minus}} plasma present in the pulsar magnetosphere, can efficiently convert the nonescaping longitudinal Langmuir waves (produced by some kind of a beam or stream instability) into propagating (escaping) electromagnetic waves. It is suggested that this shear induced transformation may be the basic mechanism needed for the eventual generation of the observed pulsar radio emission
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