338 research outputs found
A Landau fluid model for warm collisionless plasmas
A Landau fluid model for a collisionless electron-proton magnetized plasma,
that accurately reproduces the dispersion relation and the Landau damping rate
of all the magnetohydrodynamic waves, is presented. It is obtained by an
accurate closure of the hydrodynamic hierarchy at the level of the fourth order
moments, based on linear kinetic theory. It retains non-gyrotropic corrections
to the pressure and heat flux tensors up to the second order in the ratio
between the considered frequencies and the ion cyclotron frequency.Comment: to appear in Phys. Plasma
Instabilities for a relativistic electron beam interacting with a laser irradiated plasma
The effects of a radiation field (RF) on the unstable modes developed in
relativistic electron beam--plasma interaction are investigated assuming that
, where is the frequency of the RF and
is the plasma frequency. These unstable modes are parametrically
coupled to each other due to the RF and are a mix between two--stream and
parametric instabilities. The dispersion equations are derived by the
linearization of the kinetic equations for a beam--plasma system as well as the
Maxwell equations. In order to highlight the effect of the radiation field we
present a comparison of our analytical and numerical results obtained for
nonzero RF with those for vanishing RF. Assuming that the drift velocity
of the beam is parallel to the wave vector of the
excitations two particular transversal and parallel configurations of the
polarization vector of the RF with respect to are
considered in detail. It is shown that in both geometries resonant and
nonresonant couplings between different modes are possible. The largest growth
rates are expected at the transversal configuration when is
perpendicular to . In this case it is demonstrated that in general
the spectrum of the unstable modes in -- plane is split into two
distinct domains with long and short wavelengths, where the unstable modes are
mainly sensitive to the beam or the RF parameters, respectively. In parallel
configuration, , and at short wavelengths
the growth rates of the unstable modes are sensitive to both beam and RF
parameters remaining insensitive to the RF at long wavelengths.Comment: 23 pages, 5 figure
Magnetospheric eclipses in the double pulsar system J0737-3039
We argue that eclipses of radio emission from the millisecond pulsar A in the
double pulsar system J0737-3039 are due to synchrotron absorption by plasma in
the closed field line region of the magnetosphere of its normal pulsar
companion B. A's radio beam only illuminates B's magnetosphere for about 10
minutes surrounding the time of eclipse. During this time it heats particles at
r\gtrsim 10^9 cm to relativistic energies and enables extra plasma to be
trapped by magnetic mirroring. An enhancement of the plasma density by a factor
\sim 10^2 is required to match the duration and optical depth of the observed
eclipses. The extra plasma might be supplied by a source near B through B\gamma
pair creation by energetic photons produced in B's outer gap. Excitation of
pairs' gyrational motions by cyclotron absorption of A's radio beam can result
in their becoming trapped between conjugate mirror points in B's magnetosphere.
Because the trapping efficiency decreases with increasing optical depth, the
plasma density enhancement saturates even under steady state illumination. The
result is an eclipse with finite, frequency dependent, optical depth. After
illumination by A's radio beam ceases, the trapped particles cool and are lost.
The entire cycle repeats every orbital period. We speculate that the
asymmetries between eclipse ingress and egress result in part from the
magnetosphere's evolution toward a steady state when illuminated by A's radio
beam. We predict that A's linear polarization will vary with both eclipse phase
and B's rotational phase.Comment: 8 pages, 1 figure, submitted to ApJ, references corrected,
detectability of reprocessed emission revised, major conclusions unchange
2D continuous spectrum of shear Alfven waves in the presence of a magnetic island
The radial structure of the continuous spectrum of shear Alfven modes is
calculated in the presence of a magnetic island in tokamak plasmas. Modes with
the same helicity of the magnetic island are considered in a slab model
approximation. In this framework, with an appropriate rotation of the
coordinates the problem reduces to 2 dimensions. Geometrical effects due to the
shape of the flux surface's cross section are retained to all orders. On the
other hand, we keep only curvature effects responsible of the beta induced gap
in the low-frequency part of the continuous spectrum. New continuum
accumulation points are found at the O-point of the magnetic island. The
beta-induced Alfven Eigenmodes (BAE) continuum accumulation point is found to
be positioned at the separatrix flux surface. The most remarkable result is the
nonlinear modification of the BAE continuum accumulation point frequency
Radiative Efficiency of Collisionless Accretion
Radiative efficiency of a slowly accreting black hole is estimated using a
two-temperature model of accretion. The radiative efficiency depends on the
magnetic field strength near the Schwarzschild radius. For weak magnetic fields
(magnetic energy=equipartition/1000), the low efficiency 0.0001 assumed in some
theoretical models might be achieved. For stronger fields, a significant
fraction of viscous heat is dissipated by electrons and radiated away resulting
in a larger efficiency. At equipartition magnetic fields, we estimate
efficiency = of order 10%.Comment: 12 pages, Latex, Submitted to Ap
Stable periodic waves in coupled Kuramoto-Sivashinsky - Korteweg-de Vries equations
Periodic waves are investigated in a system composed of a
Kuramoto-Sivashinsky - Korteweg-de Vries (KS-KdV) equation, which is linearly
coupled to an extra linear dissipative equation. The model describes, e.g., a
two-layer liquid film flowing down an inclined plane. It has been recently
shown that the system supports stable solitary pulses. We demonstrate that a
perturbation analysis, based on the balance equation for the field momentum,
predicts the existence of stable cnoidal waves (CnWs) in the same system. It is
found that the mean value U of the wave field u in the main subsystem, but not
the mean value of the extra field, affects the stability of the periodic waves.
Three different areas can be distinguished inside the stability region in the
parameter plane (L,U), where L is the wave's period. In these areas, stable
are, respectively, CnWs with positive velocity, constant solutions, and CnWs
with negative velocity. Multistability, i.e., the coexistence of several
attractors, including the waves with several maxima per period, appears at
large value of L. The analytical predictions are completely confirmed by direct
simulations. Stable waves are also found numerically in the limit of vanishing
dispersion, when the KS-KdV equation goes over into the KS one.Comment: a latex text file and 16 eps files with figures. Journal of the
Physical Society of Japan, in pres
- …