1,296 research outputs found
Transverse waves in coronal flux tubes with thick boundaries: The effect of longitudinal flows
Observations show that transverse magnetohydrodynamic (MHD) waves and flows
are often simultaneously present in magnetic loops of the solar corona. The
waves are resonantly damped in the Alfv\'en continuum because of plasma and/or
magnetic field nonuniformity across the loop. The resonant damping is relevant
in the context of coronal heating, since it provides a mechanism to cascade
energy down to the dissipative scales. It has been theoretically shown that the
presence of flow affects the waves propagation and damping, but most of the
studies rely on the unjustified assumption that the transverse nonuniformity is
confined to a boundary layer much thinner than the radius of the loop. Here we
present a semi-analytic technique to explore the effect of flow on resonant MHD
waves in coronal flux tubes with thick nonuniform boundaries. We extend a
published method, which was originally developed for a static plasma, in order
to incorporate the effect of flow. We allowed the flow velocity to continuously
vary within the nonuniform boundary from the internal velocity to the external
velocity. The analytic part of the method is based on expressing the wave
perturbations in the thick nonuniform boundary of the loop as a Frobenius
series that contains a singular term accounting for the Alfv\'en resonance,
while the numerical part of the method consists of solving iteratively the
transcendental dispersion relation together with the equation for the Alfv\'en
resonance position. As an application of this method, we investigated the
impact of flow on the phase velocity and resonant damping length of MHD kink
waves. We consistently recover results in the thin boundary approximation
obtained in previous studies. We have extended those results to the case of
thick boundaries. We also explored the error associated with the use of the
thin boundary approximation beyond its regime of applicability.Comment: Accepted in A&
Damped transverse oscillations of interacting coronal loops
Damped transverse oscillations of magnetic loops are routinely observed in
the solar corona. This phenomenon is interpreted as standing kink
magnetohydrodynamic waves, which are damped by resonant absorption owing to
plasma inhomogeneity across the magnetic field. The periods and damping times
of these oscillations can be used to probe the physical conditions of the
coronal medium. Some observations suggest that interaction between neighboring
oscillating loops in an active region may be important and can modify the
properties of the oscillations compared to those of an isolated loop. Here we
theoretically investigate resonantly damped transverse oscillations of
interacting non-uniform coronal loops. We provide a semi-analytic method, based
on the T-matrix theory of scattering, to compute the frequencies and damping
rates of collective oscillations of an arbitrary configuration of parallel
cylindrical loops. The effect of resonant damping is included in the T-matrix
scheme in the thin boundary approximation. Analytic and numerical results in
the specific case of two interacting loops are given as an application.Comment: Accepted for publication in A&
Spatial Damping of Propagating Kink Waves Due to Resonant Absorption: Effect of Background Flow
Observations show the ubiquitous presence of propagating magnetohydrodynamic
(MHD) kink waves in the solar atmosphere. Waves and flows are often observed
simultaneously. Due to plasma inhomogeneity in the perpendicular direction to
the magnetic field, kink waves are spatially damped by resonant absorption. The
presence of flow may affect the wave spatial damping. Here, we investigate the
effect of longitudinal background flow on the propagation and spatial damping
of resonant kink waves in transversely nonuniform magnetic flux tubes. We
combine approximate analytical theory with numerical investigation. The
analytical theory uses the thin tube (TT) and thin boundary (TB) approximations
to obtain expressions for the wavelength and the damping length. Numerically,
we verify the previously obtained analytical expressions by means of the full
solution of the resistive MHD eigenvalue problem beyond the TT and TB
approximations. We find that the backward and forward propagating waves have
different wavelengths and are damped on length scales that are inversely
proportional to the frequency as in the static case. However, the factor of
proportionality depends on the characteristics of the flow, so that the damping
length differs from its static analogue. For slow, sub-Alfvenic flows the
backward propagating wave gets damped on a shorter length scale than in the
absence of flow, while for the forward propagating wave the damping length is
longer. The different properties of the waves depending on their direction of
propagation with respect to the background flow may be detected by the
observations and may be relevant for seismological applications.Comment: Accepted for publication in Ap
UMTS radio-over-fiber pico-cell interconnection employing uncooled DFB lasers for multi-mode fibre modulation bandwidth enhancement
This paper analyzes experimentally the use of distributed feedback lasers (DFB) in order to increase modulation bandwidth in multimode fibres, enabling 3 km bidirectional radio-over-multimode fibre UMTS transmission in a frequency-division duplexing (FDD) configuration
Onset of the Kelvin-Helmholtz instability in partially ionized magnetic flux tubes
Context. Recent observations of solar prominences show the presence of
turbulent flows that may be caused by Kelvin-Helmholtz instabilites (KHI).
However, the observed flow velocities are below the classical threshold for the
onset of KHI in fully ionized plasmas.
Aims. We investigate the effect of partial ionization on the onset of KHI in
dense and cool cylindrical magnetic flux tubes surrounded by a hotter and
lighter environment.
Methods. The linearized governing equations of a partially ionized two-fluid
plasma are used to describe the behavior of small-amplitude perturbations
superimposed on a magnetic tube with longitudinal mass flow. A normal mode
analysis is performed to obtain the dispersion relation for linear
incompressible waves. We focus on the appearance of unstable solutions and
study the dependence of their growth rates on various physical parameters. An
analytical approximation of the KHI linear growth rate for slow flows and
strong ion-neutral coupling is obtained. An application to solar prominence
threads is given.
Results. The presence of a neutral component in a plasma may contribute to
the onset of the KHI even for sub-Alfv\'enic longitudinal shear flows.
Collisions between ions and neutrals reduce the growth rates of the unstable
perturbations but cannot completely suppress the instability.
Conclusions. Turbulent flows in solar prominences with sub-Alfv\'enic flow
velocities may be interpreted as consequences of KHI in partially ionized
plasmas.Comment: 8 pages, 4 figures. Accepted for publication in A&
- …