8,415 research outputs found
Transverse kink oscillations in the presence of twist
Magnetic twist is thought to play an important role in coronal loops. The
effects of magnetic twist on stable magnetohydrodynamic (MHD) waves is poorly
understood because they are seldom studied for relevant cases. The goal of this
work is to study the fingerprints of magnetic twist on stable transverse kink
oscillations. We numerically calculated the eigenmodes of propagating and
standing MHD waves for a model of a loop with magnetic twist. The azimuthal
component of the magnetic field was assumed to be small in comparison to the
longitudinal component. We did not consider resonantly damped modes or kink
instabilities in our analysis. For a nonconstant twist the frequencies of the
MHD wave modes are split, which has important consequences for standing waves.
This is different from the degenerated situation for equilibrium models with
constant twist, which are characterised by an azimuthal component of the
magnetic field that linearly increases with the radial coordinate. In the
presence of twist standing kink solutions are characterised by a change in
polarisation of the transverse displacement along the tube. For weak twist, and
in the thin tube approximation, the frequency of standing modes is unaltered
and the tube oscillates at the kink speed of the corresponding straight tube.
The change in polarisation is linearly proportional to the degree of twist.
This has implications with regard to observations of kink modes, since the
detection of this variation in polarisation can be used as an indirect method
to estimate the twist in oscillating loops
On the nature of kink MHD waves in magnetic flux tubes
Magnetohydrodynamic (MHD) waves are often reported in the solar atmosphere
and usually classified as slow, fast, or Alfv\'en. The possibility that these
waves have mixed properties is often ignored. The goal of this work is to study
and determine the nature of MHD kink waves. This is done by calculating the
frequency, the damping rate and the eigenfunctions of MHD kink waves for three
widely different MHD waves cases: a compressible pressure-less plasma, an
incompressible plasma and a compressible plasma with non-zero plasma pressure
which allows for MHD radiation. In all three cases the frequency and the
damping rate are for practical purposes the same as they differ at most by
terms proportional to . In the magnetic flux tube the kink waves are
in all three cases, to a high degree of accuracy incompressible waves with
negligible pressure perturbations and with mainly horizontal motions. The main
restoring force of kink waves in the magnetised flux tube is the magnetic
tension force. The total pressure gradient force cannot be neglected except
when the frequency of the kink wave is equal or slightly differs from the local
Alfv\'{e}n frequency, i.e. in the resonant layer. Kink waves are very robust
and do not care about the details of the MHD wave environment. The adjective
fast is not the correct adjective to characterise kink waves. If an adjective
is to be used it should be Alfv\'{e}nic. However, it is better to realize that
kink waves have mixed properties and cannot be put in one single box
Linear and nonlinear resonant interaction of sound waves in dissipative layers
The theory of resonant nonlinear magnetohydrodynamic (MHD) waves in dissipative steady plasmas developed by Ballai and Erdélyi is used to study the effect of steady flows on nonlinear resonant heating of MHD waves in (a) linear, (b) weakly and (c) strongly nonlinear approximations. Nonlinear connection formulae for slow MHD waves are derived. This nonlinear theory of driven MHD waves is then used to study the interaction of sound waves with one-dimensional isotropic steady plasmas.
We find that a steady equilibrium flow can significantly influence the efficiency of resonant absorption in the considered limits. In the case of strong nonlinearity, the efficiency of the resonant coupling is found to be proportional to the counterpart obtained in linear theory. The factor of proportion is approximately of the order of unity, justifying the commonly applied linear approximations
Nonlinear theory of non-axisymmetric resonant slow waves in straight magnetic flux tubes
Nonlinear resonant slow magnetohydrodynamic (MHD) waves are studied in weakly dissipative isotropic plasmas for a cylindrical equilibrium model. The equilibrium magnetic field lines are unidirectional and parallel with the z axis. The nonlinear governing equations for resonant slow magnetoacoustic (SMA) waves are derived. Using the method of matched asymptotic expansions inside and outside the narrow dissipative layer, we generalize the connection formulae for the Eulerian perturbation of the total pressure and for the normal component of the velocity.
These nonlinear connection formulae in dissipative cylindrical MHD are an important extention of the connection formulae obtained in linear ideal MHD [Sakurai et al., Solar Phys. 133, 227 (1991)], linear dissipative MHD [Goossens et al., Solar Phys. 175, 75 (1995); Erdélyi, Solar Phys. 171, 49 (1997)] and in nonlinear dissipative MHD derived in slab geometry [Ruderman et al., Phys. Plasmas4, 75 (1997)]. These generalized connection formulae enable us to connect the solutions at both sides of the dissipative layer without solving the MHD equations in the dissipative layer. We also show that the nonlinear interaction of harmonics in the dissipative layer is responsible for generating a parallel mean flow outside the dissipative layer
Prospective five-year subsidence analysis of a cementless fully hydroxyapatite-coated femoral hip arthroplasty component
Early subsidence >1.5 mm is considered to be a predictive factor for later aseptic loosening of the femoral component following total hip arthroplasty (THA). The aim of this study was to assess five-year subsidence rates of the cementless hydroxyapatite-coated twinSys stem (Mathys Ltd., Bettlach, Switzerland).This prospective single-surgeon series examined consecutive patients receiving a twinSys stem at Maria Middelares Hospital, Belgium. Patients aged >85 years or unable to come to follow-up were excluded. Subsidence was assessed using Ein Bild Roentgen Analyse--Femoral Component Analysis (EBRA-FCA). Additional clinical and radiographic assessments were performed. Follow-ups were prospectively scheduled at two, five, 12, 24, and 60 months.In total, 218 THA (211 patients) were included. At five years, mean subsidence was 0.66 mm (95% CI: 0.43-0.90). Of the 211 patients, 95.2% had an excellent or good Harris Hip Score. There were few radiological changes. Kaplan-Meier analysis indicated five-year stem survival to be 98.4% (95% CI: 97.6-100%).Subsidence levels of the twinSys femoral stem throughout the five years of follow-up were substantially lower than the 1.5 mm level predictive of aseptic loosening. This was reflected in the high five-year survival rate
Prominence seismology using the period ratio of transverse thread oscillations
The ratio of the period of the fundamental mode to that of the first overtone
of kink oscillations, from here on the "period ratio", is a seismology tool
that can be used to infer information about the spatial variation of density
along solar magnetic flux tubes. The period ratio is 2 in longitudinally
homogeneous thin tubes, but it differs from 2 due to longitudinal
inhomogeneity. In this paper we investigate the period ratio in longitudinally
inhomogeneous prominence threads and explore its implications for prominence
seismology. We numerically solve the two-dimensional eigenvalue problem of kink
oscillations in a model of a prominence thread. We take into account three
nonuniform density profiles along the thread. In agreement with previous works
that used simple piecewise constant density profiles, we find that the period
ratio is larger than 2 in prominence threads. When the ratio of the central
density to that at the footpoints is fixed, the period ratio depends strongly
on the form of the density profile along the thread. The more concentrated the
dense prominence plasma near the center of the tube, the larger the period
ratio. However, the period ratio is found to be independent of the specific
density profile when the spatially averaged density in the thread is the same
for all the profiles. An empirical fit of the dependence of the period ratio on
the average density is given and its use for prominence seismology is
discussed.Comment: Accepted for publication in A&
Linear dynamics of the solar convection zone: excitation of waves in unstably stratified shear flows
In this paper we report on the nonresonant conversion of convectively
unstable linear gravity modes into acoustic oscillation modes in shear flows.
The convectively unstable linear gravity modes can excite acoustic modes with
similar wave-numbers. The frequencies of the excited oscillations may be
qualitatively higher than the temporal variation scales of the source flow,
while the frequency spectra of the generated oscillations should be
intrinsically correlated to the velocity field of the source flow. We
anticipate that this nonresonant phenomenon can significantly contribute to the
production of sound waves in the solar convection zone.Comment: 8 pages. To appear in the proceedings of the conference "Waves in
Dusty, Solar and Space Plasmas", Leuven, Belgium 21-26 May 200
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