69 research outputs found
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
What can the kinematic complexity of astrophysical shear flows lead to
We develop a method for investigating waves and instabilities in astrophysical shear flows with complex kinematics. Using new tools, we find an unexpected richness in the spectrum of compressible fluctuations sustained by such a flow. The principal characteristic of the revealed exotic phenomena is their asymptotic persistence in the absence of viscosity. "Echoing" as well as strongly unstable (including parametrically driven) solutions are identified. Examples of astrophysical shear flows with nontrivial velocity structure where these method and results can be applied are also discussed
The fourth order of accuracy sequential type rational splitting of inhomogeneous evolution problem
In the present work symmetrized sequential type decomposition scheme of the fourth degree precision for the solution of inhomogeneous evolution problem is constructed. The fourth degree precision is reached by introducing the complex parameter α = 1/2 ± i(1/2√3) and by the approximation of the semigroup through the rational approximation. For the considered scheme the explicit a priori estimation is obtained
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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
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
Radio Emission by Particles due to Pulsar Spin
We present a relativistic model for the motion of charged particles in
rotating magnetic field lines projected on to a plane perpendicular to the
rotation axis. By making an approximation that the projected field lines are
straight, an analytical expression is obtained for the particle trajectory. The
motive behind developing this model is to elucidate some of the effects of
rotation in pulsar profiles. There is a significant contribution to the
curvature of particle trajectory due to the rotation of pulsar, which is in
addition to the inherent curvature of the field lines. The asymmetry in the
observed pulse shapes can be explained by considering the
aberration-retardation effects. The single sign circular polarization that has
been observed in many pulsars, might be due to the relative orientation of
sight line with respect to the particle trajectory plane.Comment: 19 pages, 6 figues. Submitted to Astronomy and Astrophysic
Linear coupling and over-reflection phenomena of magnetohydrodynamic waves in smooth shear flows
Special features of magnetohydrodynamic waves linear dynamics in smooth shear
flows are studied. Quantitative asymptotic and numerical analysis are performed
for wide range of system parameters when basic flow has constant shear of
velocity and uniform magnetic field is parallel to the basic flow. The special
features consist of magnetohydrodynamic wave mutual transformation and
over-reflection phenomena. The transformation takes place for arbitrary shear
rates and involves all magnetohydrodynamic wave modes. While the
over-reflection occurs only for slow magnetosonic and Alfv\'en waves at high
shear rates. Studied phenomena should be decisive in the elaboration of the
self-sustaining model of magnetohydrodynamic turbulence in the shear flows
Dynamics of Charged Particles in the Radio Emission Region of Pulsar Magnetosphere
We consider the classical picture of three dimensional motion of charged
particles in pulsar magnetosphere. We adopt a perturbative method to solve the
equation of motion, and find the trajectory of particles as they move along the
rotating dipolar magnetic field lines. Our aim is to study the influence of
rotation on the pulsar radio emission by considering the constrained motion of
particles along the open dipolar magnetic field lines. We find that the
rotation induces a significant curvature into the particle trajectories. Our
model predicts the intensity on leading side dominates over that of trailing
side. We expect that if there is any curvature induced radio emission from the
region close to the magnetic axis then it must be due to the rotation induced
curvature. Our model predicts the radius--to--frequency mapping (RFM) in the
core emissions.Comment: 16 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysics (2007
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