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Author name code: goossens
ADS astronomy entries on 2022-09-14
author:"Goossens, Marcel"
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Title: Non-linear damping of kink waves through uniturbulence and
its role in heating the wind
Authors: van Doorsselaere, Tom; Li, Bo; Goossens, Marcel; Ruderman,
Michael; Magyar, Norbert; Ismayilli, Rajab; Hnat, Bogdan
2022cosp...44.1346V Altcode:
Numerical simulations have revealed a new type of turbulence of
unidirectional waves in a plasma that is perpendicularly structured
(Magyar et al. 2017), named uniturbulence. For this new type of
turbulence, the transverse structuring modifies the upward propagating
wave to have both Elsasser variables, leading to the well-known
perpendicular cascade. In this talk, we discuss our analytical
description of the non-linear evolution of kink waves in a cylindrical
flux tube, which are prone to uniturbulence. We show that they lead
to a non-linear cascade for both propagating and standing waves. We
calculate explicit expressions for the wave pressure and energy cascade
rate. We compare the damping times from our formula with the results
of numerical simulations and observations. In both cases we find a
reasonably good match. The comparison with the simulations show that
the non-linear damping dominates in the high amplitude regime, while
the low amplitude regime shows damping by resonant absorption. In
the comparison with the observations, we find a power law inversely
proportional to the amplitude $\eta^{-1}$ as an outer envelope for our
Monte Carlo data points, which fits with the observed data points. These
numerically tested and observationally verified damping rates may be
used to implement extra heating in coronal models. That in turn allows
for a comparison with in-situ data.
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Title: The properties of magnetoacoustic waves in magnetic flux
tubes with background rotational flows
Authors: Skirvin, Samuel; Fedun, Viktor; van Doorsselaere, Tom;
Goossens, Marcel; Verth, Gary; Claes, Niels
2022cosp...44.2547S Altcode:
Rotational flows are commonly observed features in structures within
the lower solar atmosphere, for example in solar vortices and within
intergranular lanes. It is to be expected that the presence of an
equilibrium rotational flow inside a magnetic flux tube will modify
the behaviour of propagating magnetoacoustic (MA) waves inside the
structure. Using a previously developed numerical eigensolver, we obtain
the eigenvalues for both the sausage and kink modes of the magnetic flux
tube in the presence of a background rotational flow under photospheric
conditions. The effect of both the amplitude of the rotational flow and
the flow's radial structure are investigated. It is found that when
the rotational flow is linear, the modified slow continuum, shifted
due to the background flow, reduces to single-point values. However,
in the case when the flow is non linear, the modified slow continuum now
occupies a band of frequencies. The radial structure of the background
rotational flow has important consequences for determining which
wave modes are absorbed into the continua. Furthermore, we present
for the first time 2D velocity field visualisations showing the
resulting wave perturbations alongside incorporation of the additional
background flow for both the sausage and kink modes. For both cases of
linear and nonlinear background rotational flow, it is shown that the
velocity fields with the additional background flow display different
characteristics when compared to the perturbation alone, which may
be useful for observers when interpreting high resolution data from
e.g. DKIST. Finally, we present initial results investigating the
nature of MA waves in rotating magnetic flux tubes with an additional
vertical flow component, a configuration commonly seen in simulations
of solar vortices.
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Title: Seismology in the presence of flow. Fast spatial damping of
counterpropagating kink waves.
Authors: Goossens, Marcel; Terradas, Jaume; Soler, Roberto
2022cosp...44.2500G Altcode:
This presentation is concerned with the propagation of MHD waves
in the presence of flows. An important theoretical finding is that
counterstreaming MHD waves undergo fast spatial damping due to resonant
absorption even when the flows are rather slow. The analysis deals with
propagating waves in cylindrically symmetric 1-D plasma flux tubes
with an axial magnetic field and an axial flow. The inhomogeneity in
the radial direction causes resonant damping of the propagating kink
waves. Analytic expressions are presented for the phase velocities and
damping lengths of the forward and backward propagating kink waves in
the thin tube and thin boundary approximations. The analytic expressions
are confirmed by numerical simulations that go beyond the TT and TB
approximations. Information on the phase velocities of the backward
and forward propagating waves enables us to determine the internal and
external Alfv\'{e}n velocities. This seismological method is applied to
the observations by Morton et al. 2015. An important result is that the
effect of flow on the damping depends on the propagation direction of
the waves. For forward propagating waves the damping length increases
with respect to that in the static caae and the waves become less
damped. For backward propagating waves flow has the opposite effect. The
damping length decreases and the damping is more efficient and causes
fast damping of counterpropating waves. This phenomenon provides a
natural explanation for the lack of observed backward propagating waves.
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Title: Non-linear damping of surface Alfvén waves due to
Uniturbulence
Authors: Ismayilli, Rajab; Van Doorsselaere, Tom; Goossens, Marcel;
Magyar, Norbert
2022FrASS...8..241I Altcode:
This investigation is concerned with uniturbulence associated with
surface Alfvén waves that exist in a Cartesian equilibrium model
with a constant magnetic field and a piece-wise constant density. The
surface where the equilibrium density changes in a discontinuous
manner are the source of surface Alfvén waves. These surface Alfvén
waves create uniturbulence because of the variation of the density
across the background magnetic field. The damping of the surface
Alfvén waves due to uniturbulence is determined using the Elsässer
formulation. Analytical expressions for the wave energy density,
the energy cascade, and the damping time are derived. The study
of uniturbulence due to surface Alfvén waves is inspired by the
observation that (the fundamental radial mode of) kink waves behave
similarly to surface Alfv\'en waves. The results for this relatively
simple case of surface Alfv\'en waves can help us understand the more
complicated case of kink waves in cylinders. We perform a series of 3D
ideal MHD simulations for a numerical demonstration of the non-linearly
self-cascading model of unidirectional surface Alfv\'en waves using
the code \textrm{MPI-AMRVAC}. We show that surface Alfv\'en waves
damping time in the numerical simulations follows well our analytical
prediction for that quantity. Analytical theory and the simulations show
that the damping time is inversely proportional to the amplitude of the
surface Alfv\'en waves and the density contrast. This unidirectional
cascade may play a role in heating the coronal plasma.
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Title: Acoustic Wave Properties in Footpoints of Coronal Loops in
3D MHD Simulations
Authors: Riedl, Julia M.; Van Doorsselaere, Tom; Reale, Fabio;
Goossens, Marcel; Petralia, Antonino; Pagano, Paolo
2021ApJ...922..225R Altcode: 2021arXiv210902971R
Acoustic waves excited in the photosphere and below might play
an integral part in the heating of the solar chromosphere and
corona. However, it is yet not fully clear how much of the initially
acoustic wave flux reaches the corona and in what form. We investigate
the wave propagation, damping, transmission, and conversion in the lower
layers of the solar atmosphere using 3D numerical MHD simulations. A
model of a gravitationally stratified expanding straight coronal
loop, stretching from photosphere to photosphere, is perturbed at
one footpoint by an acoustic driver with a period of 370 s. For this
period, acoustic cutoff regions are present below the transition region
(TR). About 2% of the initial energy from the driver reaches the
corona. The shape of the cutoff regions and the height of the TR show
a highly dynamic behavior. Taking only the driven waves into account,
the waves have a propagating nature below and above the cutoff region,
but are standing and evanescent within the cutoff region. Studying
the driven waves together with the background motions in the model
reveals standing waves between the cutoff region and the TR. These
standing waves cause an oscillation of the TR height. In addition,
fast or leaky sausage body-like waves might have been excited close
to the base of the loop. These waves then possibly convert to fast or
leaky sausage surface-like waves at the top of the main cutoff region,
followed by a conversion to slow sausage body-like waves around the TR.
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Title: Erratum: "Nonlinear Damping of Standing Kink Waves Computed
With Elsässer Variables" (2021, ApJ, 910, 58)
Authors: Van Doorsselaere, Tom; Goossens, Marcel; Magyar, Norbert;
Ruderman, Michael S.; Ismayilli, Rajab
2021ApJ...920..162V Altcode:
No abstract at ADS
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Title: Chromospheric Heating by Magnetohydrodynamic Waves and
Instabilities
Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson,
M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.;
Murawski, K.; Zaqarashvili, T. V.
2021JGRA..12629097S Altcode: 2021arXiv210402010S
The importance of the chromosphere in the mass and energy transport
within the solar atmosphere is now widely recognized. This review
discusses the physics of magnetohydrodynamic waves and instabilities
in large-scale chromospheric structures as well as in magnetic flux
tubes. We highlight a number of key observational aspects that have
helped our understanding of the role of the solar chromosphere
in various dynamic processes and wave phenomena, and the heating
scenario of the solar chromosphere is also discussed. The review
focuses on the physics of waves and invokes the basics of plasma
instabilities in the context of this important layer of the solar
atmosphere. Potential implications, future trends and outstanding
questions are also delineated.
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Title: Nonlinear Damping of Standing Kink Waves Computed With
Elsässer Variables
Authors: Van Doorsselaere, Tom; Goossens, Marcel; Magyar, Norbert;
Ruderman, Michael S.; Ismayilli, Rajab
2021ApJ...910...58V Altcode: 2021arXiv210414331V
In a previous paper, we computed the energy density and the nonlinear
energy cascade rate for transverse kink waves using Elsässer
variables. In this paper, we focus on the standing kink waves, which
are impulsively excited in coronal loops by external perturbations. We
present an analytical calculation to compute the damping time due to
the nonlinear development of the Kelvin-Helmholtz instability. The
main result is that the damping time is inversely proportional to
the oscillation amplitude. We compare the damping times from our
formula with the results of numerical simulations and observations. In
both cases we find a reasonably good match. The comparison with the
simulations shows that the nonlinear damping dominates in the high
amplitude regime, while the low amplitude regime shows damping by
resonant absorption. In the comparison with the observations, we find
a power law inversely proportional to the amplitude η<SUP>-1</SUP>
as an outer envelope for our Monte Carlo data points.
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Title: Mixed properties of magnetohydrodynamic waves undergoing
resonant absorption in the cusp continuum
Authors: Goossens, M.; Chen, S. -X.; Geeraerts, M.; Li, B.; Van
Doorsselaere, T.
2021A&A...646A..86G Altcode: 2020arXiv201206303G
Context. Observations of magnetohydrodynamic (MHD) waves in the
structured solar atmosphere have shown that these waves are damped
and can thus contribute to atmospheric heating. In this paper,
we focus on the damping mechanism of resonant absorption in the
cusp continuum. This process takes places when waves travel through
an inhomogeneous plasma. <BR /> Aims: Our aim is to determine the
properties of MHD waves undergoing resonant absorption in the cusp
continuum in the transition layer of a cylindrical solar atmospheric
structure, such as a photospheric pore or a coronal loop. Depending on
which quantities dominate, one can assess what type of classical MHD
wave the modes in question resemble most. <BR /> Methods: In order
to study the properties of these waves, we analytically determine
the spatial profiles of compression, displacement, and vorticity for
waves with frequencies in the cusp continuum, which undergo resonant
absorption. We confirm these analytical derivations via numerical
calculations of the profiles in the resistive MHD framework. <BR
/> Results: We show that the dominant quantities for the modes in
the cusp continuum are the displacement parallel to the background
magnetic field and the vorticity component in the azimuthal direction
(i.e. perpendicular to the background magnetic field and along the
loop boundary).
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Title: Damping of Slow Surface Kink Modes in Solar Photospheric
Waveguides Modeled by One-dimensional Inhomogeneities
Authors: Chen, Shao-Xia; Li, Bo; Van Doorsselaere, Tom; Goossens,
Marcel; Yu, Hui; Geeraerts, Michaël
2021ApJ...908..230C Altcode: 2020arXiv201215426C
Given the recent interest in magnetohydrodynamic (MHD) waves in pores
and sunspot umbrae, we examine the damping of slow surface kink modes
(SSKMs) by modeling solar photospheric waveguides with a cylindrical
inhomogeneity comprising a uniform interior, a uniform exterior,
and a continuous transition layer (TL) in between. Performing
an eigenmode analysis in linear, resistive, gravity-free MHD, our
approach is idealized in that, among other things, our equilibrium is
structured only in the radial direction. We can nonetheless address
two damping mechanisms simultaneously, one being the ohmic resistivity
and the other being the resonant absorption of SSKMs in the cusp
and Alfvén continua. We find that the relative importance of the
two mechanisms depends sensitively on the magnetic Reynolds number
(R<SUB>m</SUB>). Resonant absorption is the sole damping mechanism for
realistically large values of R<SUB>m</SUB>, and the cusp resonance in
general dominates the Alfvén one unless the axial wavenumbers are at
the lower end of the observationally relevant range. We also find that
the thin-boundary approximation holds only when the TL-width-to-radius
ratios are much smaller than nominally expected. The ohmic resistivity
is far more important for realistically small R<SUB>m</SUB>. Even in
this case, SSKMs are only marginally damped, with damping-time-to-period
ratios reaching ∼10 in the parameter range we examine.
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Title: Resonant absorption: Transformation of compressive motions
into vortical motions
Authors: Goossens, M.; Arregui, I.; Soler, R.; Van Doorsselaere, T.
2020A&A...641A.106G Altcode: 2020arXiv200908152G
This paper investigates the changes in spatial properties when
magnetohydrodynamic (MHD) waves undergo resonant damping in the
Alfvén continuum. The analysis is carried out for a 1D cylindrical
pressure-less plasma with a straight magnetic field. The effect of the
damping on the spatial wave variables is determined by using complex
frequencies that arise as a result of the resonant damping. Compression
and vorticity are used to characterise the spatial evolution of the
MHD wave. The most striking result is the huge spatial variation in the
vorticity component parallel to the magnetic field. Parallel vorticity
vanishes in the uniform part of the equilibrium. However, when the
MHD wave moves into the non-uniform part, parallel vorticity explodes
to values that are orders of magnitude higher than those attained by
the transverse components in planes normal to the straight magnetic
field. In the non-uniform part of the equilibrium plasma, the MHD wave
is controlled by parallel vorticity and resembles an Alfvén wave,
with the unfamiliar property that it has pressure variations even in
the linear regime.
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Title: Wave Pressure and Energy Cascade Rate of Kink Waves Computed
with Elsässer Variables
Authors: Van Doorsselaere, Tom; Li, Bo; Goossens, Marcel; Hnat,
Bogdan; Magyar, Norbert
2020ApJ...899..100V Altcode: 2020arXiv200715411V
Numerical simulations have revealed a new type of turbulence of
unidirectional waves in a plasma that is perpendicularly structured,
named uniturbulence. For this new type of turbulence, the transverse
structuring modifies the upward propagating wave to have both Elsässer
variables, leading to the well-known perpendicular cascade. In
this paper, we study an analytical description of the nonlinear
evolution of kink waves in a cylindrical flux tube that are prone to
uniturbulence. We show that they lead to a nonlinear cascade for both
propagating and standing waves. We calculate explicit expressions for
the wave pressure and energy cascade rate. The computed damping rate
$\tau /P$ depends on the density contrast of the flux tube and the
background plasma and is inversely proportional to the amplitude of
the kink wave. The dependence on the density contrast shows that it
plays a role especially in the lower solar corona. These expressions
may be added in Alfvén wave driven models of the solar atmosphere
(such as the Alfvén Wave Solar Model (AWSOM)), modifying it to UAWSOM
(Uniturbulence and the Alfvén Wave Solar Model).
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Title: Ultra-long and quite thin coronal loop without significant
expansion
Authors: Li, Dong; Yuan, Ding; Goossens, Marcel; Van Doorsselaere,
Tom; Su, Wei; Wang, Ya; Su, Yang; Ning, Zongjun
2020A&A...639A.114L Altcode: 2020arXiv200602629L
Context. Coronal loops are the basic building blocks of the solar
corona. They are related to the mass supply and heating of solar plasmas
in the corona. However, their fundamental magnetic structures are still
not well understood. Most coronal loops do not expand significantly,
but the diverging magnetic field would have an expansion factor of
about 5-10 over one pressure scale height. <BR /> Aims: We investigate
a unique coronal loop with a roughly constant cross section. The loop
is ultra long and quite thin. A coronal loop model with magnetic
helicity is presented to explain the small expansion of the loop
width. <BR /> Methods: This coronal loop was predominantly detectable
in the 171 Å channel of the Atmospheric Imaging Assembly (AIA). Then,
the local magnetic field line was extrapolated within a model of the
potential field source-surface. Finally, the differential emission
measure analysis made from six AIA bandpasses was applied to obtain the
thermal properties of this loop. <BR /> Results: This coronal loop has a
projected length of roughly 130 Mm, a width of about 1.5 ± 0.5 Mm, and
a lifetime of about 90 min. It follows an open magnetic field line. The
cross section expanded very little (i.e., 1.5-2.0) along the loop length
during its whole lifetime. This loop has a nearly constant temperature
at about 0.7 ± 0.2 MK, but its density exhibits the typical structure
of a stratified atmosphere. <BR /> Conclusions: We use the theory of a
thin twisted flux tube to construct a model for this nonexpanding loop
and find that with sufficient twist, a coronal loop can indeed attain
equilibrium. However, we cannot rule out other possibilities such as
footpoint heating by small-scale reconnection or an elevated scale
height by a steady flow along the loop. <P />Movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038433/olm">https://www.aanda.org</A>
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Title: Understanding Uniturbulence: Self-cascade of MHD Waves in
the Presence of Inhomogeneities
Authors: Magyar, N.; Van Doorsselaere, T.; Goossens, M.
2019ApJ...882...50M Altcode: 2019arXiv190710408M
It is widely accepted in the MHD turbulence community that the
nonlinear cascade of wave energy requires counterpropagating Alfvénic
wave packets, along some mean magnetic field. This fact is an obvious
outcome of the MHD equations under the assumptions of incompressibility
and homogeneity. Despite attempts to relax these assumptions in the
context of MHD turbulence, the central idea of turbulence generation
persists. However, once the assumptions of incompressiblity and
homogeneity break down, the generally accepted picture of turbulent
cascade generation is not universal. In this paper, we show that
perpendicular inhomogeneities (across the mean magnetic field)
lead to propagating wave solutions that are necessarily described
by co-propagating Elsässer fields, already in the incompressible
case. One simple example of these wave solutions is the surface
Alfvén wave on a planar discontinuity across the magnetic field. We
show through numerical simulations how the nonlinear self-deformation
of these unidirectionally propagating waves leads to a cascade of
wave energy across the magnetic field. The existence of this type of
unidirectional cascade might have an additional strong effect on the
turbulent dissipation rate of dominantly outward-propagating Alfvénic
waves in structured plasma, as in the solar corona and solar wind.
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Title: Resonant absorption of the slow sausage wave in the slow
continuum (Corrigendum)
Authors: Yu, D. J.; Van Doorsselaere, T.; Goossens, M.
2019A&A...626C...2Y Altcode:
No abstract at ADS
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Title: Understanding uniturbulence: self-cascade of MHD waves in
the presence of inhomogeneities
Authors: Magyar, Norbert; Van Doorsselaere, Tom; Goossens, Marcel
2019AAS...23412506M Altcode:
It is a generally accepted fact in the MHD turbulence community
that the nonlinear cascade of wave energy requires the existence of
counter-propagating Alfvénic wave-packets, along some mean magnetic
field. This fact is an obvious outcome of the MHD equations when
assuming an incompressible and homogenoeus plasma. There have been
relatively few attempts to relax these assumptions in the context
of MHD turbulence studies. However, it should be clear that once
these assumptions brake down, the generally accepted picture of
turbulent cascade generation is not universal. In the context of
longitudinally stratified plasmas (i.e. gravitationally stratified
coronal holes), it has been known since the 70's that inhomogeneities
along the mean magnetic field lead to the linear coupling of sunward
and anti-sunward propagating waves. This leads to co-propagating
disturbances of Elsasser fields, which can interact coherently to
initiate a nonlinear cascade. The alternative case of perpendicular
inhomogeneity (across the mean magnetic field) was even less studied
in the context of MHD turbulence. In this study we show that these type
of inhomogeneities lead also to co-propagating Elsasser fields, already
in the incompressible case. We show how the nonlinear self-deformation
of these unidirectionally propagating waves leads to a cascade in
k-space across the magnetic field. The existence of this type of
unidirectional cascade might have an additional strong effect on the
turbulent dissipation rate of dominantly outward propagating Alfvénic
waves in structured plasma, as in solar coronal holes.
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Title: XMM-Newton and INTEGRALanalysis of the Supergiant Fast X-ray
Transient IGR J17354-3255
Authors: Goossens, M. E.; Bird, A. J.; Hill, A. B.; Sguera, V.; Drave,
S. P.
2019MNRAS.485..286G Altcode: 2018arXiv181111882G; 2018MNRAS.tmp.3090G
We present the results of combined INTEGRALand XMM-Newton observations
of the supergiant fast X-ray transient (SFXT) IGR J17354-3255. Three
XMM-Newton observations of lengths 33.4, 32.5, and 21.9 ks were
undertaken, the first an initial pointing to identify the correct
source in the field of view and the latter two performed around
periastron. Simultaneous INTEGRALobservations across {∼ }66
{{per cent}} of the orbital cycle were analysed but the source was
neither detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light
curves display a range of moderately bright X-ray activity but there
are no particularly strong flares or outbursts in any of the three
observations. We show that the spectral shape measured by XMM-Newton can
be fitted by a consistent model throughout the observation, suggesting
that the observed flux variations are driven by obscuration from a
wind of varying density rather than changes in accretion mode. The
simultaneous INTEGRALdata rule out simple extrapolation of the simple
power-law model beyond the XMM-Newton energy range.
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Title: Mixed properties of MHD waves in non-uniform plasmas
Authors: Goossens, Marcel L.; Arregui, Inigo; Van Doorsselaere, Tom
2019FrASS...6...20G Altcode:
This paper investigates the mixed properties of MHD waves in a
non-uniform plasma. It starts with a short revision of MHD waves in
a uniform plasma of infinite extent. In that case the MHD waves do
not have mixed properties. They can be separated in Alfvén waves and
magneto-sonic waves. The Alfvén waves propagate parallel vorticity
and are incompressible. In addition they have no parallel displacement
component. The magneto-sonic waves are compressible and in general do
have a parallel component of displacement but do not propagate parallel
vorticity. This clear separation has been the reason why there has
been a strong inclination in the literature to use this classification
in the study of MHD waves in non-uniform plasmas. The main part of
this paper is concerned with MHD waves in a non-uniform plasma. It is
shown that the MHD waves in that situation in general propagate both
vorticity and compression and hence have mixed properties. Finally,
the close connection between resonant absorption and MHD waves with
mixed properties is discussed.
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Title: The Nature of Elsässer Variables in Compressible MHD
Authors: Magyar, N.; Van Doorsselaere, T.; Goossens, M.
2019ApJ...873...56M Altcode: 2019arXiv190201619M
The Elsässer variables are often used in studies of plasma turbulence,
in helping differentiate between MHD waves propagating parallel or
antiparallel to the main magnetic field. While for pure Alfvén waves
in a homogeneous plasma the method is strictly valid, we show that
compressible, magnetoacoustic waves are in general described by both
Elsässer variables. Furthermore, in a compressible and inhomogeneous
plasma, the pure MHD waves (Alfvén, fast and slow) are no longer normal
modes, but waves become linearly coupled or display mixed properties
of Alfvén and magnetoacoustic nature. These waves are necessarily
described by both Elsässer variables, and therefore the Elsässer
formalism cannot be used to strictly separate parallel and antiparallel
propagating waves. Nevertheless, even in an inhomogeneous plasma,
for a highly Alfvénic wave the Elsässer variable corresponding to
the propagation direction appears still dominating. We suggest that
for Alfvénic waves, the relative amplitude of Elsässer variables
depends on the local degree of inhomogeneity and other plasma and wave
properties. This finding has implications for turbulence studies in
inhomogeneous and compressible plasmas, such as the solar corona and
solar wind.
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Title: No unique solution to the seismological problem of standing
kink magnetohydrodynamic waves
Authors: Arregui, I.; Goossens, M.
2019A&A...622A..44A Altcode: 2018arXiv181207266A
The aim of this paper is to point out that the classic seismological
problem using observations and theoretical expressions for the periods
and damping times of transverse standing magnetohydrodynamic waves
in coronal loops is better referred to as a reduced seismological
problem. "Reduced" emphasises the fact that only a small number
of characteristic quantities of the equilibrium profiles can be
determined. Reduced also implies that there is no unique solution to
the full seismological problem. Even the reduced seismological problem
does not allow a unique solution. Bayesian inference results support
our mathematical arguments and offer insight into the relationship
between the algebraic and the probabilistic inversions.
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Title: First Evidence of the Frequency Filtering of Magnetoacoustic
Waves in the Flaring Star EK Dra
Authors: Srivastava, A. K.; Pandey, J. C.; Karmakar, Subhajeet;
Chowdhury, Partha; Moon, Y. -J.; Goossens, Marcel; Jelínek, P.;
Mathioudakis, M.; Doyle, J. G.; Dwivedi, B. N.
2018arXiv180408858S Altcode:
Using the data obtained from XMM-Newton, we show the gradual evolution
of two periodicities of ~4500 s and ~2200 s in the decay phase of the
flare observed in a solar analog EK Dra. The longer period evolves
firstly for first 14 ks, while the shorter period evolves for next
10 ks in the decay phase. We find that these two periodicities are
associated with the magnetoacoustic waves triggered in the flaring
region. The flaring loop system shows cooling and thus it is subjected
to the change in the scale height and the acoustic cut-off period. This
serves to filter the longer period magnetoacoustic waves and enables
the propagation of the shorter period waves in the later phase of
the flare. We provide the first clues of the dynamic behaviour of
EK Dra's corona which affects the propagation of waves and causes
their filtering.
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Title: Generalized Phase Mixing: Turbulence-like Behavior from
Unidirectionally Propagating MHD Waves
Authors: Van Doorsselaere, T.; Magyar, N.; Goossens, M. L.
2017AGUFMSH42B..04V Altcode:
We introduce a new mechanism for the generation of turbulence
in a perpendicularly structured plasma from unidirectional
Alfvenic drivers. We present the results of three-dimensional (3D)
ideal magnetohydrodynamics (MHD) simulations on the dynamics of a
perpendicularly inhomogeneous plasma disturbed by propagating Alfvenic
waves. Simpler versions of this scenario have been extensively studied
as the phenomenon of phase mixing. We show that, by generalizing
the textbook version of phase mixing, interesting phenomena are
obtained, such as turbulence-like behavior and complex current-sheet
structure, a novelty in longitudinally homogeneous plasma excited
by unidirectionally propagating waves. This study is in the setting
of a coronal hole. However, it constitutes an important finding for
turbulence-related phenomena in astrophysics in general, relaxing the
conditions that have to be fulfilled in order to generate turbulent
behavior.
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Title: Driven Transverse Waves Lead to Turbulent Coronal Loops
and Heating
Authors: Van Doorsselaere, T.; Karampelas, K.; Magyar, N.; Antolin,
P.; Goossens, M. L.
2017AGUFMSH41C..05V Altcode:
In this talk, I will show our recent results on 3D simulations of
coronal loops driven with transverse waves at the footpoints. We find
that the transverse waves convert to turbulence via the Kelvin-Helmholtz
instability (for standing waves) or uniturbulence (for propagating
waves). The latter is turbulence generated from the interaction of
the driven propagating waves with the counterpropagating waves which
are generated in-situ because of the plasma structure. Both of these
turbulence generation mechanisms lead to fully turbulent loops, which
allow for efficient energy dissipation and heating.
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Title: Generalized phase mixing: Turbulence-like behaviour from
unidirectionally propagating MHD waves
Authors: Magyar, Norbert; Van Doorsselaere, Tom; Goossens, Marcel
2017NatSR...714820M Altcode: 2017arXiv170202346M
We present the results of three-dimensional (3D) ideal
magnetohydrodynamics (MHD) simulations on the dynamics of a
perpendicularly inhomogeneous plasma disturbed by propagating Alfvénic
waves. Simpler versions of this scenario have been extensively studied
as the phenomenon of phase mixing. We show that, by generalizing
the textbook version of phase mixing, interesting phenomena are
obtained, such as turbulence-like behavior and complex current-sheet
structure, a novelty in longitudinally homogeneous plasma excited
by unidirectionally propagating waves. This study is in the setting
of a coronal hole. However, it constitutes an important finding for
turbulence-related phenomena in astrophysics in general, relaxing the
conditions that have to be fulfilled in order to generate turbulent
behavior.
---------------------------------------------------------
Title: Resonant Absorption of Surface Sausage and Surface Kink Modes
under Photospheric Conditions
Authors: Yu, Dae Jung; Van Doorsselaere, Tom; Goossens, Marcel
2017ApJ...850...44Y Altcode: 2017arXiv171003350Y
We study the effect of resonant absorption of surface sausage and
surface kink modes under photospheric conditions where the slow surface
sausage modes undergo resonant damping in the slow continuum and the
surface kink modes in the slow and Alfvén continua at the transitional
layers. We use recently derived analytical formulas to obtain the
damping rate (time). By considering linear density and linear pressure
profiles for the transitional layers, we show that resonant absorption
in the slow continuum could be an efficient mechanism for the wave
damping of the slow surface sausage and slow surface kink modes while
the damping rate of the slow surface kink mode in the Alfvén continuum
is weak. It is also found that the resonant damping of the fast surface
kink mode is much stronger than that of the slow surface kink mode,
showing a similar efficiency as under coronal conditions. It is worth
noting that the slow body sausage and kink modes can also resonantly
damp in the slow continuum for those linear profiles.
---------------------------------------------------------
Title: Resonant absorption of the slow sausage wave in the slow
continuum
Authors: Yu, D. J.; Van Doorsselaere, T.; Goossens, M.
2017A&A...602A.108Y Altcode:
<BR /> Aims: General analytical formulas for the damping rate by
resonant absorption of slow sausage modes in the slow (cusp) continuum
are derived and the resonant damping of the slow surface mode under
photospheric conditions is investigated. <BR /> Methods: The connection
formula across the resonant layer is used to derive the damping rate
for the slow sausage mode in the slow continuum by assuming a thin
boundary. <BR /> Results: It is shown that the effect of the resonant
damping on the slow surface sausage mode in the slow continuum, which
has been underestimated in previous interpretations, could be efficient
under magnetic pore conditions. A simplified analytical formula for
the damping rate of slow surface mode in the long wavelength limit
is derived. This formula can be useful for a rough estimation of the
damping rate due to resonant absorption for observational wave damping.
---------------------------------------------------------
Title: Resonant Absorption of Axisymmetric Modes in Twisted Magnetic
Flux Tubes
Authors: Giagkiozis, I.; Goossens, M.; Verth, G.; Fedun, V.; Van
Doorsselaere, T.
2016ApJ...823...71G Altcode: 2017arXiv170609665G
It has been shown recently that magnetic twist and axisymmetric MHD
modes are ubiquitous in the solar atmosphere, and therefore the study of
resonant absorption for these modes has become a pressing issue because
it can have important consequences for heating magnetic flux tubes in
the solar atmosphere and the observed damping. In this investigation,
for the first time, we calculate the damping rate for axisymmetric MHD
waves in weakly twisted magnetic flux tubes. Our aim is to investigate
the impact of resonant damping of these modes for solar atmospheric
conditions. This analytical study is based on an idealized configuration
of a straight magnetic flux tube with a weak magnetic twist inside as
well as outside the tube. By implementing the conservation laws derived
by Sakurai et al. and the analytic solutions for weakly twisted flux
tubes obtained recently by Giagkiozis et al. we derive a dispersion
relation for resonantly damped axisymmetric modes in the spectrum of the
Alfvén continuum. We also obtain an insightful analytical expression
for the damping rate in the long wavelength limit. Furthermore, it
is shown that both the longitudinal magnetic field and the density,
which are allowed to vary continuously in the inhomogeneous layer, have
a significant impact on the damping time. Given the conditions in the
solar atmosphere, resonantly damped axisymmetric modes are highly likely
to be ubiquitous and play an important role in energy dissipation. We
also suggest that, given the character of these waves, it is likely
that they have already been observed in the guise of Alfvén waves.
---------------------------------------------------------
Title: Spectral variation in the supergiant fast X-ray transient
SAX J1818.6-1703 observed by XMM-Newton and INTEGRAL
Authors: Boon, C. M.; Bird, A. J.; Hill, A. B.; Sidoli, L.; Sguera,
V.; Goossens, M. E.; Fiocchi, M.; McBride, V. A.; Drave, S. P.
2016MNRAS.456.4111B Altcode: 2016arXiv160101591B
We present the results of a 30 ks XMM-Newton observation of the
supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 - the first
in-depth soft X-ray study of this source around periastron. INTEGRAL
observations shortly before and after the XMM-Newton observation show
the source to be in an atypically active state. Over the course of the
XMM-Newton observation, the source shows a dynamic range of ∼100 with
a luminosity greater than 1 × 10<SUP>35</SUP> erg s<SUP>-1</SUP>
for the majority of the observation. After an ∼6 ks period of
low-luminosity (∼10<SUP>34</SUP> erg s<SUP>-1</SUP>) emission, SAX
J1818.6-1703 enters a phase of fast flaring activity, with flares ∼250
s long, separated by ∼2 ks. The source then enters a larger flare
event of higher luminosity and ∼8 ks duration. Spectral analysis
revealed evidence for a significant change in spectral shape during
the observation with a photon index varying from Γ ∼ 2.5 during the
initial low-luminosity emission phase, to Γ ∼ 1.9 through the fast
flaring activity, and a significant change to Γ ∼ 0.3 during the main
flare. The intrinsic absorbing column density throughout the observation
(n<SUB>H</SUB> ∼ 5 × 10<SUP>23</SUP> cm<SUP>-2</SUP>) is among the
highest measured from an SFXT, and together with the XMM-Newton and
INTEGRAL luminosities, consistent with the neutron star encountering
an unusually dense wind environment around periastron. Although
other mechanisms cannot be ruled out, we note that the onset of the
brighter flares occurs at 3 × 10<SUP>35</SUP>erg s<SUP>-1</SUP>,
a luminosity consistent with the threshold for the switch from a
radiative-dominated to Compton cooling regime in the quasi-spherical
settling accretion model.
---------------------------------------------------------
Title: Dissipationless Damping of Compressive MHD Modes in Twisted
Flux Tubes
Authors: Giagkiozis, I.; Fedun, V.; Verth, G.; Goossens, M. L.;
Van Doorsselaere, T.
2015AGUFMSH53B2488G Altcode:
Axisymmetric modes in straight magentic flux tubes exhibit a cutoff
in the long wavelength limit and no damping is predicted. However,
as soon as weak magnetic twist is introduced inside as well as outside
the magnetic flux tube the cutoff recedes. Furthermore, when density
variations are also incomporated within the modelresonant absorption
appears. In this work we explore analytically the expected damping times
for waves within the Alfven continuum for different solar atmospheric
conditions. Based on the results in this work we offer insight on
recent observations of sausage wave damping in the chromosphere.
---------------------------------------------------------
Title: Apparent Cross-field Superslow Propagation of
Magnetohydrodynamic Waves in Solar Plasmas
Authors: Kaneko, T.; Goossens, M.; Soler, R.; Terradas, J.; Van
Doorsselaere, T.; Yokoyama, T.; Wright, A. N.
2015ApJ...812..121K Altcode: 2015arXiv150903042K
In this paper we show that the phase-mixing of continuum Alfvén
waves and/or continuum slow waves in the magnetic structures of the
solar atmosphere as, e.g., coronal arcades, can create the illusion of
wave propagation across the magnetic field. This phenomenon could be
erroneously interpreted as fast magnetosonic waves. The cross-field
propagation due to the phase-mixing of continuum waves is apparent
because there is no real propagation of energy across the magnetic
surfaces. We investigate the continuous Alfvén and slow spectra
in two-dimensional (2D) Cartesian equilibrium models with a purely
poloidal magnetic field. We show that apparent superslow propagation
across the magnetic surfaces in solar coronal structures is a
consequence of the existence of continuum Alfvén waves and continuum
slow waves that naturally live on those structures and phase-mix as
time evolves. The apparent cross-field phase velocity is related to
the spatial variation of the local Alfvén/slow frequency across the
magnetic surfaces and is slower than the Alfvén/sound velocities for
typical coronal conditions. Understanding the nature of the apparent
cross-field propagation is important for the correct analysis of
numerical simulations and the correct interpretation of observations.
---------------------------------------------------------
Title: MHD Seismology of a loop-like filament tube by observed
kink waves
Authors: Pant, Vaibhav; Srivastava, Abhishek K.; Banerjee, Dipankar;
Goossens, Marcel; Chen, Peng-Fei; Joshi, Navin Chandra; Zhou, Yu-Hao
2015RAA....15.1713P Altcode: 2015arXiv150302281P
We report and analyze observational evidence of global kink oscillations
in a solar filament as observed in Hα by instruments administered
by National Solar Observatory (NSO)/Global Oscillation Network Group
(GONG). An M1.1-class flare in active region (AR) 11692 occurred on
2013 March 15 and induced a global kink mode in the filament lying
towards the southwest of AR 11692. We find periods of about 61-67
minutes and damping times of 92-117 minutes at positions of three
vertical slices chosen in and around the filament apex. We find that
the waves are damped. From the observed period of the global kink
mode and damping timescale using the theory of resonant absorption,
we perform prominence seismology. We estimate a lower cut-off value
for the inhomogeneity length scale to be around 0.34-0.44 times the
radius of the filament cross-section.
---------------------------------------------------------
Title: Energy and energy flux in axisymmetric slow and fast waves
Authors: Moreels, M. G.; Van Doorsselaere, T.; Grant, S. D. T.; Jess,
D. B.; Goossens, M.
2015A&A...578A..60M Altcode:
<BR /> Aims: We aim to calculate the kinetic, magnetic, thermal, and
total energy densities and the flux of energy in axisymmetric sausage
modes. The resulting equations should contain as few parameters as
possible to facilitate applicability for different observations. <BR />
Methods: The background equilibrium is a one-dimensional cylindrical
flux tube model with a piecewise constant radial density profile. This
enables us to use linearised magnetohydrodynamic equations to calculate
the energy densities and the flux of energy for axisymmetric sausage
modes. <BR /> Results: The equations used to calculate the energy
densities and the flux of energy in axisymmetric sausage modes depend
on the radius of the flux tube, the equilibrium sound and Alfvén
speeds, the density of the plasma, the period and phase speed of the
wave, and the radial or longitudinal components of the Lagrangian
displacement at the flux tube boundary. Approximate relations for
limiting cases of propagating slow and fast sausage modes are also
obtained. We also obtained the dispersive first-order correction term
to the phase speed for both the fundamental slow body mode under
coronal conditions and the slow surface mode under photospheric
conditions. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425468/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Prominence seismology using the period ratio of transverse
thread oscillations
Authors: Soler, R.; Goossens, M.; Ballester, J. L.
2015A&A...575A.123S Altcode: 2015arXiv150105238S
The ratio of the period of the fundamental mode to that of the first
overtone of kink oscillations (hereafter 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 because
of 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.
---------------------------------------------------------
Title: The Transverse and Rotational Motions of Magnetohydrodynamic
Kink Waves in the Solar Atmosphere
Authors: Goossens, M.; Soler, R.; Terradas, J.; Van Doorsselaere,
T.; Verth, G.
2014ApJ...788....9G Altcode:
Magnetohydrodynamic (MHD) kink waves have now been observed to be
ubiquitous throughout the solar atmosphere. With modern instruments,
they have now been detected in the chromosphere, interface region,
and corona. The key purpose of this paper is to show that kink waves
do not only involve purely transverse motions of solar magnetic flux
tubes, but the velocity field is a spatially and temporally varying
sum of both transverse and rotational motion. Taking this fact into
account is particularly important for the accurate interpretation of
varying Doppler velocity profiles across oscillating structures such
as spicules. It has now been shown that, as well as bulk transverse
motions, spicules have omnipresent rotational motions. Here we emphasize
that caution should be used before interpreting the particular MHD
wave mode/s responsible for these rotational motions. The rotational
motions are not necessarily signatures of the classic axisymmetric
torsional Alfvén wave alone, because kink motion itself can also
contribute substantially to varying Doppler velocity profiles observed
across these structures. In this paper, the displacement field of the
kink wave is demonstrated to be a sum of its transverse and rotational
components, both for a flux tube with a discontinuous density profile at
its boundary, and one with a more realistic density continuum between
the internal and external plasma. Furthermore, the Doppler velocity
profile of the kink wave is forward modeled to demonstrate that,
depending on the line of sight, it can either be quite distinct or
very similar to that expected from a torsional Alfvén wave.
---------------------------------------------------------
Title: Nonlinear Kink Oscillations of Coronal Magnetic Loops
Authors: Ruderman, M. S.; Goossens, M.
2014SoPh..289.1999R Altcode:
We studied nonlinear kink oscillations of a thin magnetic tube using the
cold-plasma approximation. We assumed that the plasma density varies
along the tube but does not vary in the radial direction. Using the
regular perturbation method, we show that the nonlinearity does not
affect the oscillation amplitude. We also calculated the nonlinear
correction to the oscillation frequency, which is proportional to the
oscillation amplitude squared. As an example, we considered nonlinear
oscillations of a coronal magnetic loop of half-circle shape in an
isothermal atmosphere with equal plasma temperatures inside and outside
the loop.
---------------------------------------------------------
Title: New insights on accretion in supergiant fast X-ray transients
from XMM-Newton and INTEGRAL observations of IGR J17544-2619
Authors: Drave, S. P.; Bird, A. J.; Sidoli, L.; Sguera, V.; Bazzano,
A.; Hill, A. B.; Goossens, M. E.
2014MNRAS.439.2175D Altcode: 2014MNRAS.tmp..299D; 2014arXiv1401.3570D
XMM-Newton observations of the supergiant fast X-ray transient IGR
J17544-2619 are reported and placed in the context of an analysis
of archival INTEGRAL/IBIS data that provide a refined estimate
of the orbital period at 4.9272 ± 0.0004 d. A complete outburst
history across the INTEGRAL mission is reported. Although the new
XMM-Newton observations (each lasting ∼15 ks) targeted the peak
flux in the phase-folded hard X-ray light curve of IGR J17544-2619,
no bright outbursts were observed, the source spending the majority
of the exposure at intermediate luminosities of the order of several
10<SUP>33</SUP> erg s<SUP>-1</SUP> (0.5-10 keV) and displaying only
low level flickering activity. For the final portion of the exposure,
the luminosity of IGR J17544-2619 dropped to ∼4 × 10<SUP>32</SUP>
erg s<SUP>-1</SUP> (0.5-10 keV), comparable with the lowest luminosities
ever detected from this source, despite the observations being taken
near to periastron. We consider the possible orbital geometry of IGR
J17544-2619 and the implications for the nature of the mass transfer and
accretion mechanisms for both IGR J17544-2619 and the supergiant fast
X-ray transients (SFXTs) population. We conclude that accretion under
the `quasi-spherical accretion' model provides a good description of
the behaviour of IGR J17544-2619 and suggests an additional mechanism
for generating outbursts based upon the mass accumulation rate in the
hot shell (atmosphere) that forms around the neutron star under the
quasi-spherical formulation. Hence, we hope to aid in explaining the
varied outburst behaviours observed across the SFXT population with
a consistent underlying physical model.
---------------------------------------------------------
Title: Frequency and Damping Rate of Fast Sausage Waves
Authors: Vasheghani Farahani, S.; Hornsey, C.; Van Doorsselaere, T.;
Goossens, M.
2014ApJ...781...92V Altcode:
We investigate the frequency and damping rate of fast axisymmetric
waves that are subject to wave leakage for a one-dimensional magnetic
cylindrical structure in the solar corona. We consider the ideal
magnetohydrodynamic (MHD) dispersion relation for axisymmetric MHD
waves superimposed on a straight magnetic cylinder in the zero β
limit, similar to a jet or loop in the solar corona. An analytic
study accompanied by numerical calculations has been carried out to
model the frequency, damping rate, and phase speed of the sausage wave
around the cut-off frequency and in the long wavelength limit. Analytic
expressions have been obtained based on equations around the points
of interest. They are linear approximations of the dependence of the
sausage frequency on the wave number around the cut-off wavelength
for both leaky and non-leaky regimes and in the long wavelength
limit. Moreover, an expression for the damping rate of the leaky sausage
wave has been obtained both around the cut-off frequency and in the long
wavelength limit. These analytic results are compared with numerical
computations. The expressions show that the complex frequencies are
mainly dominated by the density ratio. In addition, it is shown that
the damping eventually becomes independent of the wave number in
the long wavelength limit. We conclude that the sausage mode damping
directly depends on the density ratios of the internal and external
media where the damping declines in higher density contrasts. Even
in the long wavelength limit, the sausage mode is weakly damped for
high-density contrasts. As such, sausage modes could be observed for
a significant number of periods in high-density contrast loops or jets.
---------------------------------------------------------
Title: The Behavior of Transverse Waves in Nonuniform Solar Flux
Tubes. II. Implications for Coronal Loop Seismology
Authors: Soler, Roberto; Goossens, Marcel; Terradas, Jaume; Oliver,
Ramón
2014ApJ...781..111S Altcode: 2013arXiv1312.5079S
The seismology of coronal loops using observations of damped transverse
oscillations in combination with results from theoretical models is a
tool to indirectly infer physical parameters in the solar atmospheric
plasma. Existing seismology schemes based on approximations of the
period and damping time of kink oscillations are often used beyond
their theoretical range of applicability. These approximations
assume that the variation of density across the loop is confined
to a nonuniform layer much thinner than the radius of the loop,
but the results of the inversion problem often do not satisfy this
preliminary hypothesis. Here, we determine the accuracy of the analytic
approximations of the period and damping time, and the impact on
seismology estimates when largely nonuniform loops are considered. We
find that the accuracy of the approximations when used beyond their
range of applicability is strongly affected by the form of the density
profile across the loop, that is observationally unknown and so must
be arbitrarily imposed as part of the theoretical model. The error
associated with the analytic approximations can be larger than 50%
even for relatively thin nonuniform layers. This error directly
affects the accuracy of approximate seismology estimates compared to
actual numerical inversions. In addition, assuming different density
profiles can produce noncoincident intervals of the seismic variables
in inversions of the same event. The ignorance about the true shape
of density variation across the loop is an important source of error
that may dispute the reliability of parameters seismically inferred
assuming an ad hoc density profile.
---------------------------------------------------------
Title: Wave Energy Deposition in the Solar Corona
Authors: Van Doorsselaere, Tom; Goossens, Marcel; Verth, Gary; Soler,
Roberto; Gijsen, Stief; Andries, Jesse
2014cosp...40E3464V Altcode:
Recently, a significant amount of transverse wave energy has been
estimated propagating along solar atmospheric magnetic fields. However,
these estimates have been made with the classic bulk Alfven wave
model which assumes a homogeneous plasma. In this talk, the kinetic,
magnetic, and total energy densities and the flux of energy are first
computed for transverse MHD waves in one-dimensional cylindrical flux
tube models with a piecewise constant density profile. There are
fundamental deviations from the properties for classic bulk Alfven
waves. (1) There is no local equipartition between kinetic and magnetic
energy. (2) The flux of energy and the velocity of energy transfer have,
in addition to a component parallel to the magnetic field, components
in the planes normal to the magnetic field. (3) The energy densities
and the flux of energy vary spatially, contrary to the case of classic
bulk Alfven waves. This last property is then used to connect the
energy flux in such a simple model to the energy flux in multiple flux
tube systems. We use the plasma filling factor f to derive an ad-hoc
formula for estimating the energy that is propagated in bundles of
loops. We find that the energy flux in kink waves is lower than the
energy computed from a bulk Alfven wave interpretation, by a factor
that is (approximately) between f and 2f. We consider some geometric
models to quantify this correction factor.
---------------------------------------------------------
Title: Sausage wave oscillations and dampings in the corona
Authors: Vasheghani Farahani, Soheil; Van Doorsselaere, Tom; Goossens,
Marcel; Hornsey, Christopher
2014cosp...40E3475V Altcode:
The frequency and damping rate of fast axisymmetric (Sausage)waves
that experience leakage from a coronal structure e.g. jet or loop is
studied. In this line we consider a 1-D magnetic cylindrical structure
which resembles a jet or loop in the solar corona. We consider the
ideal magnetohydrodynamic (MHD) dispersion relation for axisymmetric
MHD waves superimposed on a straight magnetic cylinder in the zero
β limit. An analytic study accompanied by numerical calculations
has been carried out to model the frequency, damping rate, and phase
speed of the sausage wave around the cut-off frequency and in the
long wavelength limit. Analytic expressions have been obtained for
the damping and frequency of the sausage wave around the cut-off and
in the long wave-length limit. These analytic results are compared
with numerical computations. The expressions show that the complex
frequencies are mainly dominated by the density ratio. In addition,
it is shown that the damping eventually becomes independent of the wave
number in the long wavelength limit. Hence, interestingly when a high
density jet or loop ejects from the solar atmosphere, long wave-length
sausage waves guided by the Jet or loop would be observable for a
significant number of periods.
---------------------------------------------------------
Title: Doppler displacements in kink MHD waves in solar flux tubes
Authors: Goossens, Marcel; Van Doorsselaere, Tom; Terradas, Jaume;
Verth, Gary; Soler, Roberto
2014cosp...40E1045G Altcode:
Doppler displacements in kink MHD waves in solar flux tubes Presenting
author: M. Goossens Co-authors: R. Soler, J. Terradas, T. Van
Doorsselaere, G. Verth The standard interpretation of the transverse MHD
waves observed in the solar atmosphere is that they are non-axisymmetric
kink m=1) waves on magnetic flux tubes. This interpretation is based
on the fact that axisymmetric and non-axisymmetric fluting waves do
not displace the axis of the loop and the loop as a whole while kink
waves indeed do so. A uniform transverse motion produces a Doppler
displacement that is constant across the magnetic flux tube. A recent
development is the observation of Doppler displacements that vary across
the loop. The aim of the present contribution is to show that spatial
variations of the Doppler displacements across the loop can be caused by
kink waves. The motion associated with a kink wave is purely transverse
only when the flux tube is uniform and sufficiently thin. Only in that
case do the radial and azimuthal components of displacement have the
same amplitude and is the azimuthal component a quarter of a period
ahead of the radial component. This results in a unidirectional
or transverse displacement. When the flux tube is non-uniform and
has a non-zero radius the conditions for the generation of a purely
transverse motion are not any longer met. In that case the motion in
a kink wave is the sum of a transverse motion and a non-axisymmetric
rotational motion that depends on the azimuthal angle. It can produce
complicated variations of the Doppler displacement across the loop. I
shall discuss the various cases of possible Doppler displacenents
that can occur depending on the relative sizes of the amplitudes of
the radial and azimuthal components of the displacement in the kink
wave and on the orientation of the line of sight.
---------------------------------------------------------
Title: X6.9-class Flare-induced Vertical Kink Oscillations in
a Large-scale Plasma Curtain as Observed by the Solar Dynamics
Observatory/Atmospheric Imaging Assembly
Authors: Srivastava, A. K.; Goossens, M.
2013ApJ...777...17S Altcode:
We present rare observational evidence of vertical kink oscillations
in a laminar and diffused large-scale plasma curtain as observed
by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory. The X6.9-class flare in active region 11263 on 2011 August
9 induces a global large-scale disturbance that propagates in a narrow
lane above the plasma curtain and creates a low density region that
appears as a dimming in the observational image data. This large-scale
propagating disturbance acts as a non-periodic driver that interacts
asymmetrically and obliquely with the top of the plasma curtain
and triggers the observed oscillations. In the deeper layers of the
curtain, we find evidence of vertical kink oscillations with two periods
(795 s and 530 s). On the magnetic surface of the curtain where the
density is inhomogeneous due to coronal dimming, non-decaying vertical
oscillations are also observed (period ≈ 763-896 s). We infer that the
global large-scale disturbance triggers vertical kink oscillations in
the deeper layers as well as on the surface of the large-scale plasma
curtain. The properties of the excited waves strongly depend on the
local plasma and magnetic field conditions.
---------------------------------------------------------
Title: The Behavior of Transverse Waves in Nonuniform Solar Flux
Tubes. I. Comparison of Ideal and Resistive Results
Authors: Soler, Roberto; Goossens, Marcel; Terradas, Jaume; Oliver,
Ramón
2013ApJ...777..158S Altcode: 2013arXiv1309.3423S
Magnetohydrodynamic (MHD) waves are ubiquitously observed in the
solar atmosphere. Kink waves are a type of transverse MHD waves in
magnetic flux tubes that are damped due to resonant absorption. The
theoretical study of kink MHD waves in solar flux tubes is usually
based on the simplification that the transverse variation of density
is confined to a nonuniform layer much thinner than the radius of
the tube, i.e., the so-called thin boundary approximation. Here, we
develop a general analytic method to compute the dispersion relation
and the eigenfunctions of ideal MHD waves in pressureless flux tubes
with transversely nonuniform layers of arbitrary thickness. Results
for kink waves are produced and compared with fully numerical resistive
MHD eigenvalue computations in the limit of small resistivity. We find
that the frequency and resonant damping rate are the same in both
ideal and resistive cases. The actual results for thick nonuniform
layers deviate from the behavior predicted in the thin boundary
approximation and strongly depend on the shape of the nonuniform
layer. The eigenfunctions in ideal MHD are very different from
those in resistive MHD. The ideal eigenfunctions display a global
character regardless of the thickness of the nonuniform layer, while
the resistive eigenfunctions are localized around the resonance
and are indistinguishable from those of ordinary resistive Alfvén
modes. Consequently, the spatial distribution of wave energy in the
ideal and resistive cases is dramatically different. This poses a
fundamental theoretical problem with clear observational consequences.
---------------------------------------------------------
Title: Discovering a 5.72-d period in the supergiant fast X-ray
transient AX J1845.0-0433
Authors: Goossens, M. E.; Bird, A. J.; Drave, S. P.; Bazzano, A.;
Hill, A. B.; McBride, V. A.; Sguera, V.; Sidoli, L.
2013MNRAS.434.2182G Altcode: 2013MNRAS.tmp.1807G; 2013arXiv1307.0709G
Temporal analysis of INTEGRAL/IBIS data has revealed a 5.7195 ±
0.0007 d periodicity in the supergiant fast X-ray transient source
AX J1845.0-0433, which we interpret as the orbital period of the
system. The new-found knowledge of the orbital period is utilized
to investigate the geometry of the system by means of estimating an
upper limit for the size of the supergiant (<27 R<SUB>⊙</SUB>)
as well as the eccentricity of the orbit (ɛ < 0.37).
---------------------------------------------------------
Title: X6.9-class Flare Induced Vertical Kink Oscillations in a
Large-Scale Plasma Curtain as Observed by SDO/AIA
Authors: Srivastava, A. K.; Goossens, M.
2013arXiv1308.5758S Altcode:
We present rare observational evidence of vertical kink oscillations
in a laminar and diffused large-scale plasma curtain as observed by
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory (SDO). The X6.9 class flare in the Active Region 11263
on 09 August 2011, induces a global large-scale disturbance that
propagates in a narrow lane above the plasma curtain and creates a low
density region that appears as a dimming in the observational image
data. This large-scale propagating disturbance acts as a non-periodic
driver that interacts asymmetrically and obliquely with the top of the
plasma curtain, and triggers the observed oscillations. In the deeper
layers of the curtain, we find evidence of vertical kink oscillations
with two periods (795 s and 530 s). On the magnetic surface of the
curtain where the density is inhomogeneous due to the coronal dimming,
non-decaying vertical oscillations are also observed (period $\approx$
763-896 s). We infer that the global large-scale disturbance triggers
vertical kink oscillations in the deeper layers as well as on the
surface of the large-scale plasma curtain. The properties of the excited
waves strongly depend on the local plasma and magnetic field conditions.
---------------------------------------------------------
Title: INTEGRAL and XMM-Newton observations of IGR J16418-4532:
evidence of accretion regime transitions in a supergiant fast X-ray
transient
Authors: Drave, S. P.; Bird, A. J.; Sidoli, L.; Sguera, V.; McBride,
V. A.; Hill, A. B.; Bazzano, A.; Goossens, M. E.
2013MNRAS.433..528D Altcode: 2013MNRAS.tmp.1439D; 2013arXiv1305.0430D
We report on combined INTEGRAL and XMM-Newton observations of
the supergiant fast X-ray transient (SFXT) IGR J16418-4532. The
observations targeted the X-ray eclipse region of IGR J16418-4532's
orbit with continuous INTEGRAL observations across ∼25 per cent
of orbital phase and two quasi-simultaneous XMM-Newton observations
of length 20 and 14 ks, occurring during and just after the eclipse,
respectively. An enhanced INTEGRAL emission history is provided with
19 previously unreported outbursts identified in the archival 18-60
keV data set. The XMM-Newton eclipse observation showed prominent Fe
emission and a flux of 2.8 × 10<SUP>-13</SUP> erg cm<SUP>-2</SUP>
s<SUP>-1</SUP> (0.5-10 keV). Through the comparison of the detected
eclipse and post-eclipse flux, the supergiant mass-loss rate through
the stellar wind was determined as Ṁ<SUB>w</SUB> = 2.3-3.8 ×
10<SUP>-7</SUP> M<SUB>⊙</SUB> yr<SUP>-1</SUP>. The post-eclipse
XMM-Newton observation showed a dynamic flux evolution with signatures
of the X-ray pulsation, a period of flaring activity, structured
n<SUB>H</SUB> variations and the first ever detection of an X-ray
intensity dip, or `off-state', in a pulsating SFXT. Consideration
is given to the origin of the X-ray dip, and we conclude that the
most applicable of the current theories of X-ray dip generation
is that of a transition between Compton-cooling-dominated and
radiative-cooling-dominated subsonic accretion regimes within the
`quasi-spherical' model of wind accretion. Under this interpretation,
which requires additional confirmation, the neutron star in IGR
J16418-4532 possesses a magnetic field of ∼10<SUP>14</SUP> G,
providing tentative observational evidence of a highly magnetized
neutron star in a SFXT for the first time. The implications of these
results on the nature of IGR J16418-4532 itself and the wider SFXT
class are discussed.
---------------------------------------------------------
Title: Cross-sectional area and intensity variations of sausage modes
Authors: Moreels, M. G.; Goossens, M.; Van Doorsselaere, T.
2013A&A...555A..75M Altcode:
Context. The observations obtained using the Rapid Oscillations
in the Solar Atmosphere instrument (ROSA) show variations in
both cross-sectional area and intensity for magnetic pores in
the photosphere. <BR /> Aims: We study the phase behaviour between
cross-sectional area and intensity variations for sausage modes in a
photospheric context. We aim to determine the wave mode by looking at
the phase difference between the cross-sectional area and intensity
variations. <BR /> Methods: We used a straight cylinder as a model
for the flux tube. The plasma is uniform both inside and outside
the flux tube with a possible jump in the equilibrium values at the
boundary, the magnetic field is directed along the flux tube. We
derived analytic expressions for the cross-sectional area variation
and the total intensity variation. Using these analytic expressions,
we calculated the phase differences between the cross-sectional area
and the intensity variations. These phase differences were then
used to identify the wave mode. <BR /> Results: We found that for
slow sausage modes the cross-sectional area and intensity variations
are always in phase, while for fast sausage modes the variations are
in antiphase. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Erratum: "Energy Content and Propagation in Transverse Solar
Atmospheric Waves" <A href="/abs/2013ApJ...768..191G">(2013, ApJ,
768, 191)</A>
Authors: Goossens, M.; Van Doorsselaere, T.; Soler, R.; Verth, G.
2013ApJ...771...74G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Confirmation of the superorbital modulation of the high
mass X-ray binaries 4U 1909+07, IGR J16479-4514 and IGR J16418-4532
with INTEGRAL/IBIS
Authors: Drave, S. P.; Bird, A. J.; Goossens, M. E.; Sidoli, L.;
Sguera, V.; Fiocchi, M.; Bazzano, A.
2013ATel.5131....1D Altcode:
Following the recent announcement of the detection of superorbital
periods in the wind-fed supergiant X-ray binaries 4U 1909+07,
IGR J16418-4532 and IGR J16479-4514 by Corbet and Krimm 2013 (Atels
#5119 and #5126) we investigated archival INTEGRAL data to search for
additional signatures of these periods.
---------------------------------------------------------
Title: Kelvin-Helmholtz and Rayleigh-Taylor instabilities in partially
ionised prominences
Authors: Díaz, A. J.; Soler, R.; Ballester, J. L.; Goossens, M.
2013hsa7.conf..776D Altcode:
We study the modification of the classical criterion for the linear
onset and growing rate of the Kelvin-Helmholtz Instability (KHI) and the
Rayleigh-Taylor instability (RTI) in a partially ionised plasma in the
two-fluid description. The plasma is composed of a neutral fluid and
an electron-ion fluid, coupled by means of particle collisions. The
governing linear equations and appropriate boundary conditions,
including gravitational terms, are derived and applied to the case
a single interface between two partially ionised plasmas. For high
collision frequencies and low density contrasts the KHI is present for
super-Alfvénic velocity shear only. For high density contrasts the
threshold velocity shear can be reduced to sub-Alfvénic values. For
the particular case of turbulent plumes in prominences, we conclude
that sub-Alfvénic flow velocities can trigger the KHI thanks to the
ion-neutral coupling, but with long time scales. Ion-neutral collisions
have a strong impact on the RTI growth rate, which can be decreased
by an order of magnitude compared to the value in the collisionless
case. The time scale for the development of the instability is much
longer than in the classical incompressible fully ionised case. This
result may explain the existence of prominence fine structures with
life times of the order of 30 minutes.
---------------------------------------------------------
Title: Energy Content and Propagation in Transverse Solar Atmospheric
Waves
Authors: Goossens, M.; Van Doorsselaere, T.; Soler, R.; Verth, G.
2013ApJ...768..191G Altcode:
Recently, a significant amount of transverse wave energy has been
estimated propagating along solar atmospheric magnetic fields. However,
these estimates have been made with the classic bulk Alfvén wave
model which assumes a homogeneous plasma. In this paper, the kinetic,
magnetic, and total energy densities and the flux of energy are computed
for transverse MHD waves in one-dimensional cylindrical flux tube models
with a piecewise constant or continuous radial density profile. There
are fundamental deviations from the properties for classic bulk Alfvén
waves. (1) There is no local equipartition between kinetic and magnetic
energy. (2) The flux of energy and the velocity of energy transfer have,
in addition to a component parallel to the magnetic field, components
in the planes normal to the magnetic field. (3) The energy densities
and the flux of energy vary spatially, contrary to the case of classic
bulk Alfvén waves. This last property has the important consequence
that the energy flux computed with the well known expression for bulk
Alfvén waves could overestimate the real flux by a factor in the
range 10-50, depending on the flux tube equilibrium properties.
---------------------------------------------------------
Title: Effect of partial ionization on wave propagation in solar
magnetic flux tubes
Authors: Soler, R.; Díaz, A. J.; Ballester, J. L.; Goossens, M.
2013A&A...551A..86S Altcode: 2013arXiv1301.5214S
Observations show that waves are ubiquitous in the solar atmosphere and
may play an important role for plasma heating. The study of waves in
the solar corona is usually based on linear ideal magnetohydrodynamics
(MHD) for a fully ionized plasma. However, the plasma in the photosphere
and the chromosphere is only partially ionized. Here we theoretically
investigate the impact of partial ionization on MHD wave propagation
in cylindrical flux tubes in a two-fluid model. We derive the general
dispersion relation that takes into account the effects of neutral-ion
collisions and the neutral gas pressure. We assumed the neutral-ion
collision frequency to be an arbitrary parameter. Specific results for
transverse kink modes and slow magnetoacoustic modes are shown. We find
that the wave frequencies only depend on the properties of the ionized
fluid when the neutral-ion collision frequency is much lower that
the wave frequency. For high collision frequencies that realistically
represent the solar atmosphere, ions and neutrals behave as a single
fluid with an effective density corresponding to the sum of densities
of fluids plus an effective sound velocity computed as the average of
the sound velocities of ions and neutrals. The MHD wave frequencies
are modified accordingly. The neutral gas pressure can be neglected
when studying transverse kink waves but it has to be included for a
consistent description of slow magnetoacoustic waves. The MHD waves are
damped by neutral-ion collisions. The damping is most efficient when
the wave frequency and the collision frequency are on the same order of
magnitude. For high collision frequencies slow magnetoacoustic waves
are more efficiently damped than transverse kink waves. In addition,
we find the presence of cut-offs for certain combinations of parameters
that cause the waves to become non-propagating.
---------------------------------------------------------
Title: Analytic Approximate Seismology of Propagating
Magnetohydrodynamic Waves in the Solar Corona
Authors: Goossens, M.; Soler, R.; Arregui, I.; Terradas, J.
2012ApJ...760...98G Altcode: 2012arXiv1210.2689G
Observations show that propagating magnetohydrodynamic (MHD) waves are
ubiquitous in the solar atmosphere. The technique of MHD seismology
uses the wave observations combined with MHD wave theory to indirectly
infer physical parameters of the solar atmospheric plasma and magnetic
field. Here, we present an analytical seismological inversion scheme for
propagating MHD waves. This scheme uses the observational information
on wavelengths and damping lengths in a consistent manner, along
with observed values of periods or phase velocities, and is based
on approximate asymptotic expressions for the theoretical values of
wavelengths and damping lengths. The applicability of the inversion
scheme is discussed and an example is given.
---------------------------------------------------------
Title: Transverse kink oscillations in the presence of twist
Authors: Terradas, J.; Goossens, M.
2012A&A...548A.112T Altcode: 2012arXiv1210.8093T
Context. 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. <BR /> Aims: The goal of this work is to study the fingerprints
of magnetic twist on stable transverse kink oscillations. <BR />
Methods: 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. <BR />
Results: 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. <BR
/> Conclusions: 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.
---------------------------------------------------------
Title: Damped kink oscillations of flowing prominence threads
Authors: Soler, R.; Ruderman, M. S.; Goossens, M.
2012A&A...546A..82S Altcode: 2012arXiv1209.3382S
Transverse oscillations of thin threads in solar prominences are
frequently reported in high-resolution observations. Two typical
features of the observations are that the oscillations are damped
in time and that simultaneous mass flows along the threads are
detected. Flows cause the dense threads to move along the prominence
magnetic structure while the threads are oscillating. The oscillations
have been interpreted in terms of standing magnetohydrodynamic (MHD)
kink waves of the magnetic flux tubes, which support the threads. The
damping is most likely due to resonant absorption caused by plasma
inhomogeneity. The technique of seismology uses the observations
combined with MHD wave theory to estimate prominence physical
parameters. This paper presents a theoretical study of the joint effect
of flow and resonant absorption on the amplitude of standing kink waves
in prominence threads. We find that flow and resonant absorption can
either be competing effects on the amplitude or both can contribute to
damp the oscillations depending on the instantaneous position of the
thread within the prominence magnetic structure. The amplitude profile
deviates from the classic exponential profile of resonantly damped
kink waves in static flux tubes. Flow also introduces a progressive
shift of the oscillation period compared to the static case, although
this effect is in general of minor importance. We test the robustness
of seismological estimates by using synthetic data aiming to mimic real
observations. The effect of the thread flow can significantly affect the
estimation of the transverse inhomogeneity length scale. The presence
of random background noise adds uncertainty to this estimation. Caution
needs to be paid to the seismological estimates that do not take the
influence of flow into account.
---------------------------------------------------------
Title: Standing and propagating MHD waves in coronal loops
Authors: Goossens, Marcel
2012cosp...39..648G Altcode: 2012cosp.meet..648G
I shall review recent theoretical results on standing and propagating
MHD waves in solar coronal loops. First, I shall focus on periods,
damping times and damping lengths and show how they can be used to
do seismology. Second I shall address the nature of non-axisymmetric
MHD waves. I shall show that the fundamental radial modes of all
non-axisymmetric MHD waves with phase velocities in between the
internal and the external Alfven velocities are essentially surface
waves. Consequences for the transport of energy will be discussed.
---------------------------------------------------------
Title: Surface Alfvén Waves in Solar Flux Tubes
Authors: Goossens, M.; Andries, J.; Soler, R.; Van Doorsselaere, T.;
Arregui, I.; Terradas, J.
2012ApJ...753..111G Altcode: 2012arXiv1205.0935G
Magnetohydrodynamic (MHD) waves are ubiquitous in the solar
atmosphere. Alfvén waves and magneto-sonic waves are particular
classes of MHD waves. These wave modes are clearly different and have
pure properties in uniform plasmas of infinite extent only. Due to
plasma non-uniformity, MHD waves have mixed properties and cannot
be classified as pure Alfvén or magneto-sonic waves. However,
vorticity is a quantity unequivocally related to Alfvén waves as
compression is for magneto-sonic waves. Here, we investigate MHD waves
superimposed on a one-dimensional non-uniform straight cylinder with
constant magnetic field. For a piecewise constant density profile,
we find that the fundamental radial modes of the non-axisymmetric
waves have the same properties as surface Alfvén waves at a true
discontinuity in density. Contrary to the classic Alfvén waves in
a uniform plasma of infinite extent, vorticity is zero everywhere
except at the cylinder boundary. If the discontinuity in density is
replaced with a continuous variation of density, vorticity is spread
out over the whole interval with non-uniform density. The fundamental
radial modes of the non-axisymmetric waves do not need compression
to exist unlike the radial overtones. In thin magnetic cylinders,
the fundamental radial modes of the non-axisymmetric waves with phase
velocities between the internal and the external Alfvén velocities
can be considered as surface Alfvén waves. On the contrary, the radial
overtones can be related to fast-like magneto-sonic modes.
---------------------------------------------------------
Title: Search for counterparts of newly discovered INTEGRAL/IBIS
sources
Authors: Landi, R.; Bassani, L.; Masetti, N.; Bazzano, A.; Ubertini,
P.; Bird, A. J.; Goossens, M.
2012ATel.4166....1L Altcode:
With respect to the recent INTEGRAL/IBIS 9-year Galactic
Hard X-ray Survey (Krivonos et al. 2012, arXiv:1205.3941),
we used archival Swift/XRT data to find likely counterparts
for two hard X-ray sources newly detected by INTEGRAL: SWIFT
J0958.0-4208 and IGR J22534+6243. <P />SWIFT J0958.0-4208 This
source is also reported in the Swift/BAT 58-month catalogue
(http://heasarc.nasa.gov/docs/swift/results/bs58mon/).
---------------------------------------------------------
Title: On the nature of the SWIFT/INTEGRAL source SWIFT J1508.6-4953
(also PMN J1508-4953)
Authors: Landi, R.; Bassani, L.; Masetti, N.; Bazzano, A.; Parisi,
P.; Drave, S.; Goossens, M.
2012ATel.4167....1L Altcode:
This source is listed in the recent INTEGRAL/IBIS
9-year Galactic Hard X-ray Survey (Krivonos et al. 2012,
arXiv:1205.3941) and also appears in the BAT 58-month catalogue
(http://heasarc.nasa.gov/docs/swift/results/bs58mon/). <P />It has
been associated with the radio source PMN J1508-4953, also reported
as a GeV emitter in the 2nd Fermi catalogue (Nolan et al. 2012, ApJS,
199, 31). We use archival Swift/XRT data to investigate its nature.
---------------------------------------------------------
Title: INTEGRAL detects the BeXRB GS 0834-43 returning to an active
state
Authors: Drave, S. P.; Sguera, V.; Bird, A. J.; Goossens, M.; Sidoli,
L.; Bazzano, A.; Fiocchi, M.
2012ATel.4218....1D Altcode:
During INTEGRAL Galactic Plane Scan (GPS) observations between
2012-06-26 06:44:07 and 2012-06-26 15:27:19 (UTC) the BeXRB GS 0834-43
was detected at a high level of significance of 47 sigma by IBIS/ISGRI
(net exposure time of 15.8 ks). The source had a count rate of 20.1
± 0.4 in the 18-60 keV energy band, corresponding to a flux of 109
± 2 mCrab. GS 0834-43 was also in the field of view of the JEM-X
soft X-ray instrument for an exposure time of 1.7 ks during these
observations where it was detected at a significance of 19 sigma.
---------------------------------------------------------
Title: INTEGRAL detection of renewed activity from SAX J2103.5+4545
Authors: Sguera, V.; Drave, S.; Goossens, M.; Bird, A. J.; Sidoli,
L.; Fiocchi, M.; Bazzano, A.; Tarana, A.
2012ATel.4168....1S Altcode:
During recent INTEGRAL Galactic Plane Scanning observation (PI:
A. Bazzano), starting on 2012 June 12 at 01:00 UTC, IBIS/ISGRI
detected renewed activity from the Be HMXB SAX J2103.5+4545. The
source was detected at 29 sigma level (18-60 keV) with a flux of 61
+- 2 mCrab (15 ks exposure time). This detection confirms the X-ray
activity predicted by Konstantinova et al. (ATel #4068). A preliminary
IBIS/ISGRI spectral analysis reveals a 18-60 keV spectrum which can
be described by a power law with photon index 2.6 +- 0.2, the 18-60
keV flux is 7.6 x 10<SUP>-10</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Inversion of Physical Parameters in Solar Coronal Magnetic
Structures
Authors: Arregui, I. .; Ballester, J.; Goossens, M.; Oliver, R.;
Ramos, A.
2012ASPC..456..121A Altcode:
Magnetohydrodynamic seismology aims to determine difficult to measure
physical parameters in the solar corona by a combination of observed and
theoretical properties of waves and oscillations. We describe relevant
examples of the application of seismology techniques to transversely
oscillating coronal loops and prominence fine structures. We also show
how the use of statistical techniques, based on Bayesian inference,
can be of high value in the determination of physical parameters in
these structures, by consistently taking into account the information
from observations.
---------------------------------------------------------
Title: Kelvin-Helmholtz Instability in Partially Ionized Compressible
Plasmas
Authors: Soler, R.; Díaz, A. J.; Ballester, J. L.; Goossens, M.
2012ApJ...749..163S Altcode: 2012arXiv1202.4274S
The Kelvin-Helmholtz instability (KHI) has been observed in the
solar atmosphere. Ion-neutral collisions may play a relevant role for
the growth rate and evolution of the KHI in solar partially ionized
plasmas such as in, e.g., solar prominences. Here, we investigate the
linear phase of the KHI at an interface between two partially ionized
magnetized plasmas in the presence of a shear flow. The effects
of ion-neutral collisions and compressibility are included in the
analysis. We obtain the dispersion relation of the linear modes and
perform parametric studies of the unstable solutions. We find that,
in the incompressible case, the KHI is present for any velocity
shear regardless of the value of the collision frequency. In the
compressible case, the domain of instability depends strongly on the
plasma parameters, especially the collision frequency and the density
contrast. For high collision frequencies and low density contrasts the
KHI is present for super-Alfvénic velocity shear only. For high density
contrasts the threshold velocity shear can be reduced to sub-Alfvénic
values. For the particular case of turbulent plumes in prominences,
we conclude that sub-Alfvénic flow velocities can trigger the KHI
thanks to the ion-neutral coupling.
---------------------------------------------------------
Title: Transverse coronal loop oscillations seen in unprecedented
detail by AIA/SDO
Authors: White, Rebecca.; Verwichte, Erwin.; Soler, Roberto.; Goossens,
Marcel; Van Doorsselaere, Tom.; Arregui, Inigo.
2012decs.confE..18W Altcode:
We present an observational study of transverse oscillations of eleven
coronal loops observed in three separate events using data from the
Solar Dynamics Observatory (SDO) which provides unprecedented temporal
and spatial resolution of the solar corona. We study oscillatory events
using the Atmospheric Imaging Assembly (AIA) instrument on board SDO,
primarily in the 171 Angstrom bandpass to obtain information on loop
lengths, periods and damping times. Where possible, data from SDO/AIA
has been complimented with data from STEREO in order to obtain an
estimation of the 3D loop geometry. Local coronal plasma properties
are often difficult to measure using direct methods, however they
can be probed using the diagnostic power of MHD waves. In particular,
coronal loop oscillations interpreted as the fast MHD kink mode provide
an excellent tool for investigating such properties using the technique
of coronal seismology. By probing the local coronal plasma, important
information on the physical conditions in the vicinity of events such as
solar flares and CMEs can be determined. Further to the observational
study, analytic and Bayesian seismology inversion techniques are
applied to the transverse loop oscillations under the thin tube, thin
boundary approximations and under the assumption that they are damped
via the mechanism of resonant absorption. This technique allows a 3D
parameter space to be constructed that relates the density contrast,
the loop inhomogeneity length scale and the Alfven travel time.
---------------------------------------------------------
Title: Resonant Alfvén waves in partially ionized plasmas of the
solar atmosphere
Authors: Soler, R.; Andries, J.; Goossens, M.
2012A&A...537A..84S Altcode: 2011arXiv1111.4134S
Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar
atmosphere. In magnetic waveguides resonant absorption due to plasma
inhomogeneity naturally transfers wave energy from large-scale motions
to small-scale motions. In the cooler parts of the solar atmosphere
as, e.g., the chromosphere, effects due to partial ionization may
be relevant for wave dynamics and heating. <BR /> Aims: We study
resonant Alfvén waves in partially ionized plasmas. <BR /> Methods:
We use the multifluid equations in the cold plasma approximation. We
investigate propagating resonant MHD waves in partially ionized flux
tubes. We use approximate analytical theory based on normal modes in
the thin tube and thin boundary approximations along with numerical
eigenvalue computations. <BR /> Results: We find that the jumps of
the wave perturbations across the resonant layer are the same as in
fully ionized plasmas. The damping length due to resonant absorption is
inversely proportional to the frequency, while that due to ion-neutral
collisions is inversely proportional to the square of the frequency. For
observed frequencies in the solar atmosphere, the amplitude of MHD
kink waves is more efficiently damped by resonant absorption than by
ion-neutral collisions. <BR /> Conclusions: Most of the energy carried
by chromospheric kink waves is converted into localized azimuthal
Alfvén waves that can deposit energy in the coronal medium. The
dissipation of wave energy in the chromosphere due to ion-neutral
collisions is only effective for high-frequency waves. The chromosphere
acts as a filter for kink waves with periods shorter than 10 s.
---------------------------------------------------------
Title: Spectral Analysis of New X-ray Outbursts from the SFXT AX
J1845.0-0433
Authors: Goossens, M.; Bazzano, A.; Bird, T.; Drave, S.; Hill, A.;
Sguera, V.; Sidoli, L.
2012int..workE..28G Altcode: 2012PoS...176E..28G
No abstract at ADS
---------------------------------------------------------
Title: LYRA Observations of Two Oscillation Modes in a Single Flare
Authors: Van Doorsselaere, T.; De Groof, A.; Zender, J.; Berghmans,
D.; Goossens, M.
2011ApJ...740...90V Altcode:
We analyze light curves from the LYRA irradiance experiment on
board PROBA2 during the flare of 2010 February 8. We see both long-
and short-period oscillations during the flare. The long-period
oscillation is interpreted in terms of standing slow sausage modes;
the short-period oscillation is thought to be a standing fast sausage
mode. The simultaneous presence of two oscillation modes in the same
flaring structure allows for new coronal seismological applications. The
periods are used to find seismological estimates of the plasma-β and
the density contrast of the flaring loop. Also the wave mode number
is estimated from the observed periods.
---------------------------------------------------------
Title: Resonantly Damped Propagating Kink Waves in Longitudinally
Stratified Solar Waveguides
Authors: Soler, R.; Terradas, J.; Verth, G.; Goossens, M.
2011ApJ...736...10S Altcode: 2011arXiv1105.0067S
It has been shown that resonant absorption is a robust physical
mechanism for explaining the observed damping of magnetohydrodynamic
kink waves in the solar atmosphere due to naturally occurring plasma
inhomogeneity in the direction transverse to the direction of the
magnetic field. Theoretical studies of this damping mechanism were
greatly inspired by the first observations of post-flare standing
kink modes in coronal loops using the Transition Region and Coronal
Explorer. More recently, these studies have been extended to explain
the attenuation of propagating coronal kink waves observed by the
Coronal Multi-Channel Polarimeter. In the present study, for the first
time we investigate the properties of propagating kink waves in solar
waveguides including the effects of both longitudinal and transverse
plasma inhomogeneity. Importantly, it is found that the wavelength is
only dependent on the longitudinal stratification and the amplitude
is simply a product of the two effects. In light of these results the
advancement of solar atmospheric magnetoseismology by exploiting high
spatial/temporal resolution observations of propagating kink waves
in magnetic waveguides to determine the length scales of the plasma
inhomogeneity along and transverse to the direction of the magnetic
field is discussed.
---------------------------------------------------------
Title: Alfvénic waves with sufficient energy to power the quiet
solar corona and fast solar wind
Authors: McIntosh, Scott W.; de Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo; Boerner, Paul; Goossens, Marcel
2011Natur.475..477M Altcode:
Energy is required to heat the outer solar atmosphere to millions of
degrees (refs 1, 2) and to accelerate the solar wind to hundreds of
kilometres per second (refs 2-6). Alfvén waves (travelling oscillations
of ions and magnetic field) have been invoked as a possible mechanism
to transport magneto-convective energy upwards along the Sun's magnetic
field lines into the corona. Previous observations of Alfvénic waves
in the corona revealed amplitudes far too small (0.5kms<SUP>-1</SUP>)
to supply the energy flux (100-200Wm<SUP>-2</SUP>) required to
drive the fast solar wind or balance the radiative losses of the
quiet corona. Here we report observations of the transition region
(between the chromosphere and the corona) and of the corona that
reveal how Alfvénic motions permeate the dynamic and finely structured
outer solar atmosphere. The ubiquitous outward-propagating Alfvénic
motions observed have amplitudes of the order of 20kms<SUP>-1</SUP> and
periods of the order of 100-500s throughout the quiescent atmosphere
(compatible with recent investigations), and are energetic enough to
accelerate the fast solar wind and heat the quiet corona.
---------------------------------------------------------
Title: Resonant MHD Waves in the Solar Atmosphere
Authors: Goossens, Marcel; Erdélyi, Robert; Ruderman, Michael S.
2011SSRv..158..289G Altcode: 2010SSRv..tmp..182G
The linear theory of MHD resonant waves in inhomogeneous plasmas
is reviewed. The review starts from discussing the properties of
driven resonant MHD waves. The dissipative solutions in Alfvén
and slow dissipative layers are presented. The important concept of
connection formulae is introduced. Next, we proceed on to non-stationary
resonant MHD waves. The relation between quasi-modes of ideal MHD and
eigenmodes of dissipative MHD are discussed. The solution describing
the wave motion in non-stationary dissipative layers is given. It is
shown that the connection formulae remain valid for non-stationary
resonant MHD waves. The initial-value problem for resonant MHD waves
is considered. The application of theory of resonant MHD waves to
solar physics is discussed.
---------------------------------------------------------
Title: Magnetohydrodynamic Waves and Seismology of the Solar
Atmosphere
Authors: Erdélyi, Robertus; Goossens, Marcel
2011SSRv..158..167E Altcode: 2011SSRv..tmp..229E; 2011SSRv..tmp...84E; 2011SSRv..tmp..153E
No abstract at ADS
---------------------------------------------------------
Title: Kink oscillations of flowing threads in solar prominences
Authors: Soler, R.; Goossens, M.
2011A&A...531A.167S Altcode: 2011arXiv1106.3937S
Context. Recent observations by Hinode/SOT show that MHD waves
and mass flows are simultaneously present in the fine structure
of solar prominences. <BR /> Aims: We investigate standing kink
magnetohydrodynamic (MHD) waves in flowing prominence threads from
a theoretical point of view. We model a prominence fine structure
as a cylindrical magnetic tube embedded in the solar corona with its
ends line-tied in the photosphere. The magnetic cylinder is composed
of a region with dense prominence plasma, which is flowing along
the magnetic tube, whereas the rest of the flux tube is occupied
by coronal plasma. <BR /> Methods: We use the WKB approximation to
obtain analytical expressions for the period and the amplitude of
the fundamental mode as functions of the flow velocity. In addition,
we solve the full problem numerically by means of time-dependent
simulations. <BR /> Results: We find that both the period and the
amplitude of the standing MHD waves vary in time as the prominence
thread flows along the magnetic structure. The fundamental kink
mode is a good description for the time-dependent evolution of the
oscillations, and the analytical expressions in the WKB approximation
are in agreement with the full numerical results. <BR /> Conclusions:
The presence of flow modifies the period of the oscillations with
respect to the static case. However, for realistic flow velocities
this effect might fall within the error bars of the observations. The
variation of the amplitude due to the flow leads to apparent damping
or amplification of the oscillations, which could modify the real rate
of attenuation caused by an additional damping mechanism.
---------------------------------------------------------
Title: Spatial Damping of Propagating Kink Waves Due to Resonant
Absorption: Effect of Background Flow
Authors: Soler, R.; Terradas, J.; Goossens, M.
2011ApJ...734...80S Altcode: 2011arXiv1104.1791S
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 direction perpendicular 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 analog. For slow,
sub-Alfvénic 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.
---------------------------------------------------------
Title: Magnetoseismological Determination of Magnetic Field and
Plasma Density Height Variation in a Solar Spicule
Authors: Verth, G.; Goossens, M.; He, J. -S.
2011ApJ...733L..15V Altcode:
The variation of magnetic field strength and plasma density along a
solar spicule is determined by the use of magnetoseismology. From
Solar Optical Telescope observations of a kink wave propagating
along a spicule, by estimating the spatial change in phase speed and
velocity amplitude, a novel approach is demonstrated to determine
the chromospheric height variation of both magnetic field and plasma
density. Furthermore, the magnetoseismological estimate of the plasma
density gradient is combined with electron density estimates from
spectroscopy to determine the changing degree of ionization of hydrogen
along a spicule.
---------------------------------------------------------
Title: Propagating Kink Waves in Stratified Magnetic Waveguides of
the Solar Corona
Authors: Soler, Roberto; Terradas, J.; Verth, G.; Goossens, M.
2011SPD....42.1805S Altcode: 2011BAAS..43S.1805S
Recent observations using the Coronal Multi-Channel Polarimeter (CoMP)
show ubiquitous propagating waves of low amplitude in magnetic loops
of the solar corona. These observations have been interpreted as
magnetohydrodynamic (MHD) resonant kink waves. It has been shown
that resonant absorption is a robust physical mechanism to explain
the observed damping of MHD kink waves in the solar atmosphere due to
naturally occurring plasma inhomogeneity in the direction transverse
to the magnetic field. In the present study, for the first time we
investigate the properties of propagating kink waves in solar magnetic
waveguides including the effects of both longitudinal and transverse
plasma inhomogeneity. Importantly, it is found that the wavelength is
only dependent on the longitudinal stratification and the amplitude
is simply a product of the two effects. In light of these results the
advancement of solar atmospheric magnetoseismology by exploiting high
spatial/temporal resolution observations of propagating kink waves
in magnetic waveguides to determine the length scales of the plasma
inhomogeneity along and transverse to the direction of the magnetic
field is discussed.
---------------------------------------------------------
Title: The Thermal Instability of Solar Prominence Threads
Authors: Soler, R.; Ballester, J. L.; Goossens, M.
2011ApJ...731...39S Altcode: 2011arXiv1102.2317S
The fine structure of solar prominences and filaments appears as thin
and long threads in high-resolution images. In Hα observations of
filaments, some threads can be observed for only 5-20 minutes before
they seem to fade and eventually disappear, suggesting that these
threads may have very short lifetimes. The presence of an instability
might be the cause of this quick disappearance. Here, we study the
thermal instability of prominence threads as an explanation of their
sudden disappearance from Hα observations. We model a prominence thread
as a magnetic tube with prominence conditions embedded in a coronal
environment. We assume a variation of the physical properties in the
transverse direction so that the temperature and density continuously
change from internal to external values in an inhomogeneous transitional
layer representing the particular prominence-corona transition
region (PCTR) of the thread. We use the nonadiabatic and resistive
magnetohydrodynamic equations, which include terms due to thermal
conduction parallel and perpendicular to the magnetic field, radiative
losses, heating, and magnetic diffusion. We combine both analytical and
numerical methods to study linear perturbations from the equilibrium
state, focusing on unstable thermal solutions. We find that thermal
modes are unstable in the PCTR for temperatures higher than 80,000 K,
approximately. These modes are related to temperature disturbances that
can lead to changes in the equilibrium due to rapid plasma heating or
cooling. For typical prominence parameters, the instability timescale
is of the order of a few minutes and is independent of the form of
the temperature profile within the PCTR of the thread. This result
indicates that thermal instability may play an important role for the
short lifetimes of threads in the observations.
---------------------------------------------------------
Title: Seismology of Transversely Oscillating Coronal Loops with
Siphon Flows
Authors: Terradas, J.; Arregui, I.; Verth, G.; Goossens, M.
2011ApJ...729L..22T Altcode: 2011arXiv1101.5238T
There are ubiquitous flows observed in the solar atmosphere of
sub-Alfvénic speeds; however, after flaring and coronal mass ejection
events flows can become Alfvénic. In this Letter, we derive an
expression for the standing kink mode frequency due to siphon flow in
coronal loops, valid for both low and high speed regimes. It is found
that siphon flow introduces a linear, spatially dependent phase shift
along coronal loops and asymmetric eigenfunctions. We demonstrate
how this theory can be used to determine the kink and flow speed of
oscillating coronal loops with reference to an observational case
study. It is shown that the presence of siphon flow can cause the
underestimation of magnetic field strength in coronal loops using the
traditional seismological methods.
---------------------------------------------------------
Title: Selective spatial damping of propagating kink waves due to
resonant absorption
Authors: Terradas, J.; Goossens, M.; Verth, G.
2010A&A...524A..23T Altcode: 2010arXiv1004.4468T
Context. There is observational evidence of propagating kink waves
driven by photospheric motions. These disturbances, interpreted
as kink magnetohydrodynamic (MHD) waves are attenuated as they
propagate upwards in the solar corona. <BR /> Aims: We show that
resonant absorption provides a simple explanation to the spatial
damping of these waves. <BR /> Methods: Kink MHD waves are studied
using a cylindrical model of solar magnetic flux tubes, which includes
a non-uniform layer at the tube boundary. Assuming that the frequency
is real and the longitudinal wavenumber complex, the damping length and
damping per wavelength produced by resonant absorption are analytically
calculated in the thin tube (TT) approximation, valid for coronal
waves. This assumption is relaxed in the case of chromospheric tube
waves and filament thread waves. <BR /> Results: The damping length of
propagating kink waves due to resonant absorption is a monotonically
decreasing function of frequency. For kink waves with low frequencies,
the damping length is exactly inversely proportional to frequency, and
we denote this as the TGV relation. When moving to high frequencies,
the TGV relation continues to be an exceptionally good approximation
of the actual dependency of the damping length on frequency. This
dependency means that resonant absorption is selective as it favours
low-frequency waves and can efficiently remove high-frequency waves
from a broad band spectrum of kink waves. The efficiency of the
damping due to resonant absorption depends on the properties of the
equilibrium model, in particular on the width of the non-uniform layer
and the steepness of the variation in the local Alfvén speed. <BR />
Conclusions: Resonant absorption is an effective mechanism for the
spatial damping of propagating kink waves. It is selective because
the damping length is inversely proportional to frequency so that the
damping becomes more severe with increasing frequency. This means that
radial inhomogeneity can cause solar waveguides to be a natural low-pass
filter for broadband disturbances. Kink wave trains travelling along,
e.g., coronal loops, will therefore have a greater proportion of the
high-frequency components dissipated lower down in the atmosphere. This
could have important consequences for the spatial distribution of wave
heating in the solar atmosphere.
---------------------------------------------------------
Title: Observational Evidence of Resonantly Damped Propagating Kink
Waves in the Solar Corona
Authors: Verth, G.; Terradas, J.; Goossens, M.
2010ApJ...718L.102V Altcode: 2010arXiv1007.1080V
In this Letter, we establish clear evidence for the resonant
absorption damping mechanism by analyzing observational data from
the novel Coronal Multi-Channel Polarimeter. This instrument has
established that in the solar corona there are ubiquitous propagating
low-amplitude (≈1 km s<SUP>-1</SUP>) Alfvénic waves with a wide
range of frequencies. Realistically interpreting these waves as the
kink mode from magnetohydrodynamic wave theory, they should exhibit a
frequency-dependent damping length due to resonant absorption, governed
by the Terradas-Goossens-Verth relation showing that transverse plasma
inhomogeneity in coronal magnetic flux tubes causes them to act as
natural low-pass filters. It is found that the observed frequency
dependence on damping length (up to about 8 mHz) can be explained by
the kink wave interpretation; and furthermore, the spatially averaged
equilibrium parameter describing the length scale of transverse plasma
density inhomogeneity over a system of coronal loops is consistent
with the range of values estimated from Transition Region and Coronal
Explorer observations of standing kink modes.
---------------------------------------------------------
Title: The effect of longitudinal flow on resonantly damped kink
oscillations
Authors: Terradas, J.; Goossens, M.; Ballai, I.
2010A&A...515A..46T Altcode: 2009arXiv0912.4136T
Context. The most promising mechanism acting towards damping the
kink oscillations of coronal loops is resonant absorption. In
this context most of previous studies neglected the effect of the
obvious equilibrium flow along magnetic field lines. The flows are
in general sub-Alfvénic and hence comparatively slow. <BR /> Aims:
Here we investigate the effect of an equilibrium flow on the resonant
absorption of linear kink MHD waves in a cylindrical magnetic flux
tube with the aim of determining the changes in the frequency of the
forward and backward propagating waves and in the modification of the
damping times due to the flow. <BR /> Methods: A loop model with both
the density and the longitudinal flow changing in the radial direction
is considered. We use the thin tube thin boundary (TTTB) approximation
in order to calculate the damping rates. The full resistive eigenvalue
problem is also solved without assuming the TTTB approximation. <BR />
Results: Using the low ratio of flow and Alfvén speeds we derive simple
analytical expressions to the damping rate. The analytical expressions
are in good agreement with the resistive eigenmode calculations. <BR
/> Conclusions: Under typical coronal conditions the effect of the
flow on the damped kink oscillations is weak when the characteristic
scale of the density layer is similar or lower than the characteristic
width of the velocity layer. However, in the opposite situation the
damping rates can be significantly altered, specially for the backward
propagating wave which is undamped while the forward wave is overdamped.
---------------------------------------------------------
Title: Magnetoseismology: Eigenmodes of Torsional Alfvén Waves in
Stratified Solar Waveguides
Authors: Verth, G.; Erdélyi, R.; Goossens, M.
2010ApJ...714.1637V Altcode:
There have recently been significant claims of Alfvén wave observation
in the solar chromosphere and corona. We investigate how the radial and
longitudinal plasma structuring affects the observational properties
of torsional Alfvén waves in magnetic flux tubes for the purposes of
solar magnetoseismology. The governing magnetohydrodynamic equations
of these waves in axisymmetric flux tubes of arbitrary radial and
axial plasma structuring are derived and we study their observable
properties for various equilibria in both thin and finite-width
magnetic flux tubes. For thin flux tubes, it is demonstrated that
observation of the eigenmodes of torsional Alfvén waves can provide
temperature diagnostics of both the internal and surrounding plasma. In
the finite-width flux tube regime, it is shown that these waves are the
ideal magnetoseismological tool for probing radial plasma inhomogeneity
in solar waveguides.
---------------------------------------------------------
Title: Kelvin-Helmholtz Instability in Coronal Magnetic Flux Tubes
due to Azimuthal Shear Flows
Authors: Soler, R.; Terradas, J.; Oliver, R.; Ballester, J. L.;
Goossens, M.
2010ApJ...712..875S Altcode: 2009arXiv0912.3649S
Transverse oscillations of coronal loops are often observed and
have been theoretically interpreted as kink magnetohydrodynamic
(MHD) modes. Numerical simulations by Terradas et al. suggest that
shear flows generated at the loop boundary during kink oscillations
could give rise to a Kelvin-Helmholtz instability (KHI). Here, we
investigate the linear stage of the KHI in a cylindrical magnetic flux
tube in the presence of azimuthal shear motions. We consider the basic,
linearized MHD equations in the β = 0 approximation and apply them
to a straight and homogeneous cylindrical flux tube model embedded in
a coronal environment. Azimuthal shear flows with a sharp jump of the
velocity at the cylinder boundary are included in the model. We obtain
an analytical expression for the dispersion relation of the unstable
MHD modes supported by the configuration, and compute analytical
approximations of the critical velocity shear and the KHI growth rate
in the thin tube limit. A parametric study of the KHI growth rates is
performed by numerically solving the full dispersion relation. We find
that fluting-like modes can develop a KHI in timescales comparable to
the period of kink oscillations of the flux tube. The KHI growth rates
increase with the value of the azimuthal wavenumber and decrease with
the longitudinal wavenumber. However, the presence of a small azimuthal
component of the magnetic field can suppress the KHI. Azimuthal motions
related to kink oscillations of untwisted coronal loops may trigger
a KHI, but this phenomenon has not been observed to date. We propose
that the azimuthal component of the magnetic field is responsible for
suppressing the KHI in a stable coronal loop. The required twist is
small enough to prevent the development of the pinch instability.
---------------------------------------------------------
Title: Torsional Alfvén waves in small scale current threads of
the solar corona
Authors: Copil, P.; Voitenko, Y.; Goossens, M.
2010A&A...510A..17C Altcode:
Context. The magnetic field structuring in the solar corona occurs
on large scales (loops and funnels), but also on small scales. For
instance, coronal loops are made up of thin strands with different
densities and magnetic fields across the loop. <BR /> Aims: We consider
a thin current thread and model it as a magnetic flux tube with twisted
magnetic field inside the tube and straight field outside. We prove
the existence of trapped Alfvén modes in twisted magnetic flux tubes
(current threads) and we calculate the wave profile in the radial
direction for two different magnetic twist models. <BR /> Methods: We
used the Hall MHD equations that we linearized in order to derive and
solve the eigenmode equation for the torsional Alfvén waves. <BR />
Results: We show that the trapped Alfv én eigenmodes do exist and
are localized in thin current threads where the magnetic field is
twisted. The wave spectrum is discrete in phase velocity, and the
number of modes is finite and depends on the amount of the magnetic
field twist. The phase speeds of the modes are between the minimum of
the Alfvén speed in the interior and the exterior Alfén speed. <BR
/> Conclusions: Torsional Alfvén waves can be guided by thin twisted
magnetic flux-tubes (current threads) in the solar corona. We suggest
that the current threads guiding torsional Alfvén waves, are subject
to enhanced plasma heating due to wave dissipation.
---------------------------------------------------------
Title: Farley-Buneman Instability in the Solar Chromosphere
Authors: Gogoberidze, G.; Voitenko, Y.; Poedts, S.; Goossens, M.
2009ApJ...706L..12G Altcode: 2009arXiv0902.4426G
The Farley-Buneman instability (FBI) is studied in the partially
ionized plasma of the solar chromosphere taking into account the
finite magnetization of the ions and Coulomb collisions. We obtain the
threshold value for the relative velocity between ions and electrons
necessary for the instability to develop. It is shown that Coulomb
collisions play a destabilizing role in the sense that they enable the
instability even in the regions where the ion magnetization is larger
than unity. By applying these results to chromospheric conditions, we
show that the FBI cannot be responsible for the quasi-steady heating
of the solar chromosphere. However, we do not exclude the instability
development locally in the presence of strong cross-field currents
and/or strong small-scale magnetic fields. In such cases, FBI should
produce locally small-scale, ~0.1-3 m, density irregularities in the
solar chromosphere. These irregularities can cause scintillations of
radio waves with similar wave lengths and provide a tool for remote
chromospheric sensing.
---------------------------------------------------------
Title: On the nature of kink MHD waves in magnetic flux tubes
Authors: Goossens, M.; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J. L.
2009A&A...503..213G Altcode: 2009arXiv0905.0425G
Context: Magnetohydrodynamic (MHD) waves are often reported in the
solar atmosphere and usually classified as slow, fast, or Alfvén. The
possibility that these waves have mixed properties is often ignored. <BR
/>Aims: The goal of this work is to study and determine the nature
of MHD kink waves. <BR />Methods: 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 which allows
for MHD radiation. <BR />Results: 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 (k<SUB>z</SUB> R)^2. 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
Alfvén frequency, i.e. in the resonant layer. <BR />Conclusions:
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 Alfvénic. However, it is better to realize that kink waves have
mixed properties and cannot be put in one single box.
---------------------------------------------------------
Title: The Nature of Kink MHD Waves in Magnetic Flux Tubes
Authors: Goossens, Marcel; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J.
2009SPD....40.1306G Altcode:
We examine the nature of MHD kink waves. This is done by determining
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. The overall conclusion
is that kink waves are very robust and do not care about the details
of the MHD wave environment. In all three cases the frequency and the
damping rate are for most practical purposes the same. 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 magnetized flux tube is the magnetic tension force. The
gradient pressure force cannot be neglected except when the frequency
of the kink wave is equal or slightly differs from the local Alfvén
frequency, i.e. in the resonant layer. In a non-magnetic external
plasma the wave is of course acoustic. The adjective fast is not the
correct adjective to characterize kink waves. If an adjective is to
be used it should be Alfvénic. However, it is better to realize that
kink waves have mixed properties and cannot be put in one single box.
---------------------------------------------------------
Title: The influence of longitudinal density variation in coronal
loops on the eigenfunctions of kink-oscillation overtones
Authors: Andries, J.; Arregui, I.; Goossens, M.
2009A&A...497..265A Altcode:
Context: As coronal loops are spatially at least partially resolved
in the longitudinal direction, attempts have been made to use
the longitudinal profiles of the oscillation amplitudes as a
seismological tool. <BR />Aims: We aim to derive simple formulae to
assess which oscillation modes and which quantities of the oscillation
(displacement or compression) are most prone to modifications induced
by stratification of the equilibrium density along the loop. We
furthermore clarify and quantify that the potential of such a method
could be enhanced if observational profiles of the compression in the
oscillations could be determined. <BR />Methods: By means of a linear
expansion in the longitudinal stratification along with the “thin
tube” approximation, the modifications to the eigenfunctions are
calculated analytically. The results are validated by direct numerical
computations. <BR />Results: Higher axial overtones are found to be
more affected by equilibrium stratification and hence would provide a
much better tool if observed. For the k-1th overtone the compression
is found to be around (k + 2)^2/k<SUP>2</SUP> times more sensitive to
longitudinal density variation than the displacement. While the linear
formulae do give a good indication of the strength of the effects
of longitudinal density stratification, the numerical computations
indicate that the corrections to the approximate analytical results
are significant and cannot be neglected under the expected coronal
conditions.
---------------------------------------------------------
Title: Damping of Filament Thread Oscillations: Effect of the Slow
Continuum
Authors: Soler, R.; Oliver, R.; Ballester, J. L.; Goossens, M.
2009ApJ...695L.166S Altcode: 2009arXiv0902.0572S
Transverse oscillations of small amplitude are commonly seen in
high-resolution observations of filament threads, i.e., the fine
structures of solar filaments/prominences, and are typically damped
in a few periods. Kink wave modes supported by the thread body
offer a consistent explanation of these observed oscillations. Among
the proposed mechanisms to explain the kink mode damping, resonant
absorption in the Alfvén continuum seems to be the most efficient
as it produces damping times of about three periods. However, for a
nonzero-β plasma and typical prominence conditions, the kink mode is
also resonantly coupled to slow (or cusp) continuum modes, which could
further reduce the damping time. In this Letter, we explore for the
first time both analytically and numerically the effect of the slow
continuum on the damping of transverse thread oscillations. The thread
model is composed of a homogeneous and straight cylindrical plasma,
an inhomogeneous transitional layer, and the homogeneous coronal
plasma. We find that the damping of the kink mode due to the slow
resonance is much less efficient than that due to the Alfvén resonance.
---------------------------------------------------------
Title: Nonlinear Instability of Kink Oscillations due to Shear Motions
Authors: Terradas, J.; Andries, J.; Goossens, M.; Arregui, I.; Oliver,
R.; Ballester, J. L.
2008ApJ...687L.115T Altcode: 2008arXiv0809.3664T
First results from a high-resolution three-dimensional nonlinear
numerical study of the kink oscillation are presented. We show in
detail the development of a shear instability in an untwisted line-tied
magnetic flux tube. The instability produces significant deformations
of the tube boundary. An extended transition layer may naturally evolve
as a result of the shear instability at a sharp transition between
the flux tube and the external medium. We also discuss the possible
effects of the instability on the process of resonant absorption when an
inhomogeneous layer is included in the model. One of the implications
of these results is that the azimuthal component of the magnetic field
of a stable flux tube in the solar corona, needed to prevent the shear
instability, is probably constrained to be in a very specific range.
---------------------------------------------------------
Title: Three-Dimensional MHD Wave Propagation and Conversion to
Alfvén Waves near the Solar Surface. I. Direct Numerical Solution
Authors: Cally, P. S.; Goossens, M.
2008SoPh..251..251C Altcode: 2007arXiv0711.0498C
The efficacy of fast - slow MHD mode conversion in the surface
layers of sunspots has been demonstrated over recent years
using a number of modelling techniques, including ray theory,
perturbation theory, differential eigensystem analysis, and direct
numerical simulation. These show that significant energy may be
transferred between the fast and slow modes in the neighbourhood
of the equipartition layer where the Alfvén and sound speeds
coincide. However, most of the models so far have been two
dimensional. In three dimensions the Alfvén wave may couple to the
magnetoacoustic waves with important implications for energy loss from
helioseismic modes and for oscillations in the atmosphere above the
spot. In this paper, we carry out a numerical "scattering experiment,"
placing an acoustic driver 4 Mm below the solar surface and monitoring
the acoustic and Alfvénic wave energy flux high in an isothermal
atmosphere placed above it. These calculations indeed show that energy
conversion to upward travelling Alfvén waves can be substantial,
in many cases exceeding loss to slow (acoustic) waves. Typically,
at penumbral magnetic field strengths, the strongest Alfvén fluxes
are produced when the field is inclined 30° - 40° from the vertical,
with the vertical plane of wave propagation offset from the vertical
plane containing field lines by some 60° - 80°.
---------------------------------------------------------
Title: Resonant Absorption in Complicated Plasma Configurations:
Applications to Multistranded Coronal Loop Oscillations
Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L.;
Andries, J.; Goossens, M.
2008ApJ...679.1611T Altcode: 2008arXiv0802.0591T
We study the excitation and damping of transverse oscillations in a
multistranded model of a straight line-tied coronal loop. The transverse
geometry of our equilibrium configuration is quite irregular and
more realistic than the usual cylindrical loop model. By numerically
solving the time-dependent ideal magnetohydrodynamic equations in
two dimensions, we show how the global motion of the whole bundle of
strands, excited by an external disturbance, is converted into localized
Alfvénic motions due to the process of resonant absorption. This
process produces the attenuation of the transverse oscillations. At
any location in the structure, two dominant frequencies are found:
the frequency of the global mode or quasi-mode, and the local Alfvén
frequency. We find that the mechanism of mode conversion, due to the
coupling between fast and Alfvén waves, is not compromised by the
complicated geometry of the model. We also show that it is possible to
have energy conversion not only at the external edge of the composite
loop, but also inside the structure. The implications of these results
and their relationship with the observations are discussed.
---------------------------------------------------------
Title: Analytic approximate seismology of transversely oscillating
coronal loops
Authors: Goossens, M.; Arregui, I.; Ballester, J. L.; Wang, T. J.
2008A&A...484..851G Altcode: 2008arXiv0804.3877G
Aims: We present an analytic approximate seismic inversion scheme for
damped transverse coronal loop oscillations based on the thin tube and
thin boundary approximation for computing the period and the damping
time. <BR />Methods: Asymptotic expressions for the period and damping
rate are used to illustrate the process of seismological inversion in a
simple and easy to follow manner. The inversion procedure is formulated
in terms of two simple functions, which are given by simple closed
expressions. <BR />Results: The analytic seismic inversion shows that
an infinite amount of 1-dimensional equilibrium models can reproduce
the observed periods and damping times. It predicts a specific range of
allowable values for the Alfvén travel time and lower bounds for the
density contrast and the inhomogeneity length scale. When the results
of the present analytic seismic inversion are compared with those of
a previous numerical inversion, excellent agreement is found up to
the point that the analytic seismic inversion emerges as a tool for
validating results of numerical inversions. Actually it helped us to
identify and correct inaccuracies in a previous numerical investigation.
---------------------------------------------------------
Title: Seismology of kink oscillations in coronal loops: Two decades
of resonant damping
Authors: Goossens, Marcel
2008IAUS..247..228G Altcode: 2007IAUS..247..228G
The detection of rapidly damped transverse oscillations in coronal
loops by Aschwanden et al. (1999) and Nakariakov et al. (1999) gave
a strong impetus to the study of MHD waves and their damping. The
common interpretation of the observations of these oscillations is
based on kink modes. This paper reviews how the observed period and
damping time can be reproduced by MHD wave theory when non-uniform
equilibrium models are considered that have a transversal variation
of the local Alfven velocity. The key point here is that resonant
absorption cannot be avoided and occurs as natural damping mechanism
for kink waves in non-uniform equilibrium models. The present paper
starts with work by Hollweg & Yang (1988) and discusses subsequent
developments in theory and their applications to seismology of coronal
loops. It addresses the consistent use of observations of periods and
damping times as seismological tools within the framework of resonant
absorption. It shows that within the framework of resonant absorption
infinitely many equilibrium models can reproduce the observed values
of periods and damping times.
---------------------------------------------------------
Title: On the Scaling of the Damping Time for Resonantly Damped
Oscillations in Coronal Loops
Authors: Arregui, Iñigo; Ballester, José Luis; Goossens, Marcel
2008ApJ...676L..77A Altcode: 2008arXiv0802.1143A
There is not as yet full agreement on the mechanism that causes the
rapid damping of the oscillations observed by TRACE in coronal loops. It
has been suggested that the variation of the observed values of the
damping time as function of the corresponding observed values of the
period contains information on the possible damping mechanism. The
aim of this Letter is to show that, for resonant absorption, this is
definitely not the case unless detailed a priori information on the
individual loops is available.
---------------------------------------------------------
Title: Torsional Alfvén waves in small scale density threads of
the solar corona
Authors: Copil, P.; Voitenko, Y.; Goossens, M.
2008A&A...478..921C Altcode:
The density structuring of the solar corona is observed at large scales
(loops and funnels), but also at small scales (sub-structures of loops
and funnels). Coronal loops consist of thin density threads with sizes
down to (and most probably below) the resolution limit. We study
properties of torsional Alfvén waves propagating in inhomogeneous
cylindrical density threads using the two-fluid magnetohydrodynamic
equations. The eigenmode solutions supported by such a structure
are obtained and analysed. It is shown that the dispersive and
dissipative effects become important for the waves localised in
thin threads. In this case, the Alfvén wave continuum is replaced
with a discrete spectrum of Alfvén waves. This mathematical model is
applied to the waves propagating in coronal structures. In particular,
we consider ~1 Hz Alfvén waves propagating along density threads with
a relatively smooth radial profile, where a density contrast of about
1.1 is attained at radial distances of about 0.1 km. We found that the
dissipation distance of these waves is less than the typical length
of hot coronal loops, 50 Mm. Torsional Alfvén waves are localised in
thin density threads and produce localised heating. Therefore, these
waves can be responsible for coronal heating and for maintenance of
small-scale coronal structuring.
---------------------------------------------------------
Title: On the Excitation of Leaky Modes in Cylindrical Loops
Authors: Terradas, J.; Andries, J.; Goossens, M.
2007SoPh..246..231T Altcode:
The role of leaky waves in the coronal loop oscillations observed by
TRACE is not yet clearly understood. In this work, the excitation of
fast waves in solar coronal loops modelled as dense plasma cylindrical
tubes in a uniform straight magnetic field is investigated. We study
the trapped and especially leaky modes (whose energy escapes from
the tube) that result from an initial disturbance by solving the
time-dependent problem numerically. We find that the stationary state
of the tube motion is given by the trapped normal modes. By contrast,
the transient behaviour between the initial and the stationary phase
is dominated by wave leakage. The so-called trig leaky modes are
clearly identified since the transient behaviour shows periods and
damping times that are in agreement with the values calculated from the
normal-mode analysis. Consequently, these radiating modes have physical
significance. However, we have not found any evidence for the excitation
of other types of modes, such as the principal leaky kink mode.
---------------------------------------------------------
Title: Spatial magneto-seismology: effect of density stratification
on the first harmonic amplitude profile of transversal coronal
loop oscillations
Authors: Verth, G.; Van Doorsselaere, T.; Erdélyi, R.; Goossens, M.
2007A&A...475..341V Altcode:
Context: The new generation of extreme-ultraviolet (EUV) imagers onboard
missions such as the Solar Dynamics Observatory (SDO) and Solar Orbiter
(SO) will provide the most accurate spatial measurements of post-flare
coronal loop oscillations yet. The amplitude profiles of these loop
oscillations contain important information about plasma fine structure
in the corona. <BR />Aims: We show that the position of the anti-nodes
of the amplitude profile of the first harmonic of the standing fast kink
wave of a coronal loop relate to the plasma density stratification of
that loop. <BR />Methods: The MHD kink transversal waves of coronal
loops are modelled both numerically and analytically. The numerical
model implements the implicit finite element code pollux. Dispersion
relations are derived and solved analytically. The results of
the two methods are compared and verified. <BR />Results: Density
stratification causes the anti-nodes of the first harmonic to shift
towards the loop footpoints. The greater the density stratification,
the larger the shift. The anti-node shift of the first harmonic of
a semi-circular coronal loop with a density scale height H=50 Mm and
loop half length L=100 Mm is approximately 5.6 Mm. Shifts in the Mm
range are measureable quantities providing valuable information about
the subresolution structure of coronal loops. <BR />Conclusions:
The measurement of the anti-node shift of the first harmonic of the
standing fast kink wave of coronal loops is potentially a new tool in
the field of solar magneto-seismology, providing a novel complementary
method of probing plasma fine structure in the corona.
---------------------------------------------------------
Title: Coronal loop oscillations: energy considerations and initial
value problem
Authors: Terradas, J.; Andries, J.; Goossens, M.
2007A&A...469.1135T Altcode:
Context: Flares and eruptions in the solar corona generate
oscillations of loops which have been interpreted as eigenmodes
(mainly the fundamental kink mode, although other modes can also be
excited). From the theoretical point of view the excitation of the
tube eigenmodes due to an initial disturbance has not been studied in
much detail. <BR />Aims: The main aim of this work is to calculate for
a given initial disturbance the amount of energy that is deposited
in the trapped fast mode oscillation, how it depends on the initial
perturbation and how it is distributed among the different eigenmodes
(kink and fluting and also the longitudinal harmonics). <BR />Methods:
We calculate, using analytical expressions, the amplitude and the energy
of the oscillation of the magnetic tube for different kinds of initial
excitations. <BR />Results: We find that external excitations deposit
a small amount of energy in the tube. We show that fluting modes have
quite small energies in comparison with the energy of the kink mode
(around three orders of magnitude for the first fluting mode). On
the contrary, the longitudinal fundamental mode and the longitudinal
harmonics have energies of the same order of magnitude. In addition, we
find that the loop length and density contrast can be important factors
that determine the amount of energy that is trapped by the loop. <BR
/>Conclusions: The energy deposited in loops is typically six orders
of magnitude smaller than the energy of the initial disturbance (for
external excitations). However, it strongly depends on the distance
of the initial perturbation and also on the loop properties (length
and density). Fluting modes in coronal loops are very difficult to
excite. Longitudinal harmonics are in principle more easily excited.
---------------------------------------------------------
Title: Global oscillations in a magnetic solar model. II. Oblique
propagation
Authors: Pintér, B.; Erdélyi, R.; Goossens, M.
2007A&A...466..377P Altcode:
The coupling of solar global acoustic oscillations to a magnetised
solar atmosphere is studied here. The solar interior - atmosphere
interface is modelled by a non-magnetic polytrope interior overlayed
by a planar atmosphere embedded in non-uniform horizontal atmospheric
magnetic field. Pintér & Goossens (1999, A&A, 347, 321) showed
that parallel propagating acoustic waves can couple resonantly to
local magnetohydrodynamic (MHD) slow continuum modes only. In general,
global acoustic modes can, however, propagate in arbitrary directions
with respect to local atmospheric fields giving rise to an additional
efficient coupling mechanism that has consequences on mode damping and
atmospheric energetics. In this paper we study obliquely propagating
global modes that can couple also to local MHD Alfvén continuum
modes. The atmospheric magnetic effects on global mode frequencies are
still much of a debate. In particular, the resulting frequency shifts
and damping rates of global modes caused by the resonant interaction
with both local Alfvén and slow waves are investigated. We found the
coupling of global f and p modes and the Lamb mode, that penetrate into
the magnetic solar atmosphere, will strongly depend on the direction of
propagation with respect to the solar atmospheric magnetic field. These
frequency shifts, as a function of the propagation direction, give us
a further elegant tool and refinement method of local helioseismology
techniques. Finally we briefly discuss the importance of studying
obliquely propagating waves and discuss the results in the context of
possible helioseismic observations. <P />Appendix A is only available
in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Damping of Torsional Modes in the Solar Corona
Authors: Copil, Paula; Voitenko, Yuriy; Goossens, Marcel
2007AIPC..895..147C Altcode:
The Alfvén wave is one of the classic waves in magnetoplasma. It is an
electromagnetic-hydrodynamic wave, in which the restoring force comes
from the magnetic tension, while the ions provide the inertia. We have
studied the propagation and dissipation of torsional Alfvén waves in
an inhomogeneous cylindrical plasma taking into account the effects
due to finite Larmor gyroradius.
---------------------------------------------------------
Title: MHD seismology of coronal loops using the period and damping
of quasi-mode kink oscillations
Authors: Arregui, I.; Andries, J.; Van Doorsselaere, T.; Goossens,
M.; Poedts, S.
2007A&A...463..333A Altcode:
Aims:We combine the magnetohydrodynamic (MHD) theory of resonantly
damped quasi-mode kink oscillations with observational estimates of
the period and damping of transverse coronal loop oscillations to
extract information on physical parameters in oscillating loops. <BR
/>Methods: A numerical study of the quasi-mode period and damping,
in one-dimensional fully non-uniform flux tubes, is used to obtain
equilibrium models that reproduce the observed periods and damping
rates. This scheme is applied to 11 loop oscillation events. <BR
/>Results: When only the damping rate is used, the valid equilibrium
models form a one-dimensional solution curve in the two-dimensional
parameter space (density contrast, transverse inhomogeneity
length-scale). Lower limits to the transverse inhomogeneity are
obtained in the limit of high contrast loops. When both the period and
the damping rate are used, the equilibrium Alfvén speed (or Alfvén
travel time) comes into play. The valid equilibrium models then form
a one-dimensional solution curve in the three-dimensional parameter
space (density contrast, transverse inhomogeneity length-scale, Alfvén
speed or Alfvén travel time). The projection of these solutions onto
the Alfvén speed axis is found to be constrained to a rather limited
interval. Upper limits to the internal Alfvén speed are derived for
9 of the 11 analysed events.
---------------------------------------------------------
Title: MHD Waves and Shocks Generated during Magnetic Field
Reconnection
Authors: Bárta, M.; Karlický, M.; Vršnak, B.; Goossens, M.
2007CEAB...31..165B Altcode:
We use a 2D MHD model of magnetic field reconnection to investigate
if and how bursts of reconnection activity, changes of the magnetic
field and shock wave generation are related. We found that major
bursts of power dissipated into Joule heat occur during topological
transitions of the magnetic field structure. These bursts are followed
by shocks and waves. Along the plasma outflow jet not only MHD waves,
but also ion-sound shocks are formed. After the phase of more or
less quiet reconnection (Petschek-type) the tearing mode produces
plasmoids. The interactions of these plasmoids are associated with
further bursts of the reconnection activity and a complex structure
of shock waves. Finally, all these processes are discussed as possible
sources of various radio bursts.
---------------------------------------------------------
Title: Seismology of Coronal Loops Using the Period and Damping of
Quasi-Mode Kink Oscillations
Authors: Arregui, I.; Andries, J.; Van Doorsselaere, T.; Goossens,
M.; Poedts, S.
2006ESASP.617E..81A Altcode: 2006soho...17E..81A
No abstract at ADS
---------------------------------------------------------
Title: Seismology of Transversely Oscillating Loops Using Periods
and Damping Times
Authors: Goossens, Marcel; Arregui, I.; Andries, J.; Van Doorsselaere,
T.
2006SPD....37.1804G Altcode: 2006BAAS...38..247G
Periods and damping times of quasi-mode fundamental kink oscillations
have been computed for non-uniform cylindrical models of coronal
loops. The radial inhomogeneity length-scale, the density contrast
and the internal Alfvén velocity are three equilibrium quantities
that determine the theoretical values of the period and damping
times in 1-D equilibrium models. From a seismological point of view
this means that observed values of period and damping time can be
recovered by an infinite number of equilibrium models. In other words,
observed values of period and damping time of the fundamental kink
oscillation mode do not allow a unique identification of even a 1-D
equilibrium model. Only if there is additional information on one of
the three equilibrium quantities, can we use the observed values of
period and damping time to determine the two remaining equilibrium
quantities. However, it is not all bad news. It turns out that, even
without additional information, we can determine upper limits to the
internal Alfvén velocity. We apply this scheme to the set of 11 loop
oscillation events studied in Goossens et al. 2002 and find constraints
on the equilibrium parameters for these 11 events.
---------------------------------------------------------
Title: Damping of magnetohydrodynamic waves by resonant absorption
in the solar atmosphere
Authors: Goossens, M.; Andries, J.; Arregui, I.
2006RSPTA.364..433G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasi-mode damping in two-dimensional fully non-uniform
coronal loops
Authors: Arregui, I.; Van Doorsselaere, T.; Andries, J.; Goossens,
M.; Poedts, S.
2006RSPTA.364..529A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Energization of Plasma Species by Intermittent Kinetic
Alfvén Waves
Authors: Voitenko, Yuriy; Goossens, Marcel
2006SSRv..122..255V Altcode:
We propose a new phase-mixing sweep model of coronal heating and solar
wind acceleration based on dissipative properties of kinetic Alfvén
waves (KAWs). The energy reservoir is provided by the intermittent
∼1 Hz MHD Alfvén waves excited at the coronal base by magnetic
restructuring. These waves propagate upward along open magnetic
field lines, phase-mix, and gradually develop short wavelengths
across the magnetic field. Eventually, at 1.5-4 solar radii they
are transformed into KAWs. We analyze several basic mechanisms for
anisotropic energization of plasma species by KAWs and find them
compatible with observations. In particular, UVCS (onboard SOHO)
observations of intense cross-field ion energization at 1.5-4 solar
radii can be naturally explained by non-adiabatic ion acceleration in
the vicinity of demagnetizing KAW phases. The ion cyclotron motion is
destroyed there by electric and magnetic fields of KAWs.
---------------------------------------------------------
Title: Oblique aperiodic instability driven by cross-field current
in space plasmas
Authors: Siversky, T.; Voitenko, Yu.; Goossens, M.
2006AdSpR..37..625S Altcode:
Electric currents across the magnetic field can occur in the
solar atmosphere because of quasi-stationary magnetic shear, plasma
non-uniformity, or MHD waves. We propose a new physical mechanism that
can cause the anomalous dissipation of these currents and initiate
energy release in solar plasmas. In the framework of linear kinetic
theory, we show that the cross-field currents are unstable with respect
to low-frequency perturbations. The instability growth rate is quite
high, about 2% of the ion-cyclotron frequency when the differential
electron/ion velocity is 0.2 of the ion thermal speed. We identify the
unstable perturbations as degenerated ion-acoustic modes coupled with
backward kinetic Alfvén modes.
---------------------------------------------------------
Title: Magnetic interfaces in the solar atmosphere: waves,
instabilities and energy release
Authors: Voitenko, Y.; Siversky, T.; Copil, P.; Goossens, M.
2006cosp...36.3364V Altcode: 2006cosp.meet.3364V
Numerous Yohkoh and SOHO observations suggest that the events of
impulsive plasma heating in the solar atmosphere flares nanoflares
blinkers etc are due to the energy released during magnetic
reconnection Magnetic reconnection occurs in magnetic interfaces
between interacting magnetic fluxes Classical transport coefficients
cannot explain the observed rates of energy release As a consequence
several current-driven plasma micro-instabilities have been suggested as
mechanisms causing anomalous resistivity and faster energy release The
common difficulty of models based on the current-driven instabilities
is that the threshold currents for these instabilities are rather high
and require very thin interfaces which are subject to quick disruption
In this situation the fast Petschek regime of magnetic reconnection can
hardly be obtained In our study we take into account that inhomogeneous
shear plasma flows and currents as well as considerable guide magnetic
field components are typical for coronal magnetic interfaces We find
that the shear plasma flows and current inhomogeneity drastically
decrease the threshold currents for kinetic Alfven and ion-acoustic
instabilities As a result these instabilities can develop anomalous
resistivity much earlier in relatively smooth and stable interfaces
which make the standard Petschek model more realistic for the solar
corona Moreover inhomogeneous currents that are typical for the
quasi-steady solar corona can also drive these instabilities which
can therefore contribute to the quasi-steady heating of the corona
---------------------------------------------------------
Title: Non-adiabatic acceleration of ions by kinetic Alfven waves
Authors: Voitenko, Y.; Goossens, M.
2006cosp...36.3372V Altcode: 2006cosp.meet.3372V
Strong energization of ions across the background magnetic field is one
of most interesting observations in the solar corona at 1 5-4 solar
radii and in the auroral zones of the terrestrial magnetosphere at 1
5-4 Earth radii The commonly accepted interpretation of this phenomenon
is based on the ion-cyclotron resonant heating by high-frequency waves
in the solar corona or stochastic heating by small-scale waves in the
auroral zones We propose another mechanism where the cross-field ion
energization is due to non-adiabatic acceleration by low-frequency
kinetic Alfven waves KAWs In the vicinity of even demagnetizing wave
phases all ions undergo a simultaneous increase of their cross-field
velocities similar to particle acceleration in quasi-perpendicular
shocks It is therefore intuitively understandable why the particles
that move against the waves enter the regime of acceleration easier
In-situ measurements of electro-magnetic fields in the auroral
zones and remote spectroscopic coronal observations are compatible
with low-frequency KAW turbulence We demonstrate that the sporadic
appearance of super-critical gradients in KAW turbulence is sufficient
for the cross-field energization of ions observed in these regions
---------------------------------------------------------
Title: Voitenko and Goossens Reply:
Authors: Voitenko, Y.; Goossens, M.
2005PhRvL..95z9502V Altcode:
A Reply to the Comment by P. K. Shukla and L. Stenflo.
---------------------------------------------------------
Title: Seismology of Coronal Loops Using Resonant Absorption
Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens,
M.; Poedts, S.
2005ESASP.600E..21A Altcode: 2005dysu.confE..21A; 2005ESPM...11...21A
No abstract at ADS
---------------------------------------------------------
Title: Anomalous Viscous Dissipation of Slow Magneto-Acoustic Waves
Authors: Siversky, T.; Voitenko, Y.; Goossens, M.
2005ESASP.600E..99S Altcode: 2005ESPM...11...99S; 2005dysu.confE..99S
No abstract at ADS
---------------------------------------------------------
Title: Phase Mixing of MHD ALFVÉN Waves and Origin of Solar Wind
Authors: Voitenko, Y.; Goossens, M.
2005ESASP.600E.103V Altcode: 2005ESPM...11..103V; 2005dysu.confE.103V
No abstract at ADS
---------------------------------------------------------
Title: Linear Sources of Acousticwaves in the Shear Flows of Solar
Convection
Authors: Tevzadze, A. G.; Chagelishvili, G. D.; Goossens, M.
2005ESASP.600E..53T Altcode: 2005ESPM...11...53T; 2005dysu.confE..53T
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of Coronal Loop Oscillations Recent Improvements
and Computational Aspects
Authors: van Doorsselaere, T.; Arregui, I.; Andries, J.; Goossens,
M.; Poedts, S.
2005SSRv..121...79V Altcode:
We will discuss the observed, heavily damped transversal oscillations
of coronal loops. These oscillations are often modeled as transversal
kink oscillations in a cylinder. Several features are added to the
classical cylindrical model. In our models we include loop curvature,
longitudinal density stratification, and highly inhomogeneous radial
density profiles. In this paper, we will first give an overview of
recently obtained results, both analytically and numerically. After
that, we shed a light on the computational aspects of the modeling
process. In particular, we will focus on the parallellization of the
numerical codes.
---------------------------------------------------------
Title: Foreword: Computing in Space and Astrophysical Plasmas
Authors: Goossens, Marcel; Poedts, Stefaan; Voitenko, Yuriy; Chian,
Abraham C. -L.
2005SSRv..121....1G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Shear Flow Instabilities in Low-Beta Space Plasmas
Authors: Siversky, Taras; Voitenko, Yuriy; Goossens, Marcel
2005SSRv..121..343S Altcode:
We study instabilities driven by a sheared plasma flow in the
low-frequency domain. Two unstable branches are found: the ion-sound
mode and the kinetic Alfvén mode. Both instabilities are aperiodic. The
ion-sound instability does not depend on the plasma β (gas/magnetic
pressure ratio) and has a maximum growth rate of about 0.1 of the
velocity gradient dV <SUB>0</SUB>/dx. On the other hand, the kinetic
Alfvén instability is stronger for larger β and dominates the
ion-sound instability for β > 0.05. Possible applications for
space plasmas are shortly discussed.
---------------------------------------------------------
Title: Dynamics of Coronal Loop Oscillations
Authors: van Doorsselaere, T.; Arregui, I.; Andries, J.; Goossens,
M.; Poedts, S.
2005ESASP.596E..44V Altcode: 2005ccmf.confE..44V
No abstract at ADS
---------------------------------------------------------
Title: Resonantly damped fast MHD kink modes in longitudinally
stratified tubes with thick non-uniform transitional layers
Authors: Arregui, I.; Van Doorsselaere, T.; Andries, J.; Goossens,
M.; Kimpe, D.
2005A&A...441..361A Altcode:
Resonantly damped fast kink quasi-modes are computed in fully <P
/>resistive magnetohydrodynamics (MHD) for two-dimensional equilibrium
<P />models. The equilibrium model is a straight cylindrically symmetric
flux <P />tube with a plasma density that is non-uniform both across
and along the <P />loop. The non-uniform layer across the loop is not
<P />restricted to be thin, but its thickness can reach values up to
the <P />loop diameter. <P />Our results indicate that <P />the period
and damping of coronal loop oscillations mainly depend on the density
contrast <P />and the inhomogeneity length-scale and are independent
of the details of <P />longitudinal stratification, depending on the
weighted mean density, <P />weighted with the wave energy. For fully
non-uniform loops, quasi-modes can <P />interact with resistive Alfvén
eigenmodes leading to avoided crossings <P />and gaps in the complex
frequency plane. The present study extends previous <P />studies on
coronal loop oscillations in one-dimensional equilibrium models <P
/>with thick boundary layers and in equilibria with longitudinally
stratified loops under the <P />thin boundary approximation, and allow
for a better comparison between <P />observations and theory raising
the prospect of coronal seismology using <P />the time damping of
coronal loop oscillations.
---------------------------------------------------------
Title: Nonlinear coupling of Alfvén waves with widely different
cross-field wavelengths in space plasmas
Authors: Voitenko, Yuriy M.; Goossens, Marcel
2005JGRA..11010S01V Altcode:
Multiscale activity and dissipation of Alfvén waves play an important
role in a number of space and astrophysical plasmas. A popular
approach to study the evolution and damping of MHD Alfvén waves
assumes a gradual evolution of the wave energy to small dissipative
length scales. This can be done by local nonlinear interactions
among MHD waves with comparable wavelengths resulting in turbulent
cascades or by phase mixing and resonant absorption. We investigate an
alternative nonlocal transport of wave energy from large MHD length
scales directly into the dissipation range formed by the kinetic
Alfvén waves (KAWs). KAWs have very short wavelengths across the
magnetic field irrespectively of their frequency. We focus on the
nonlinear mechanism for the excitation of KAWs by MHD Alfvén waves
via resonant decay AW → KAW<SUB>1</SUB> + KAW<SUB>2</SUB>. The
resonant decay conditions can be satisfied in a rarified plasmas,
where the gas/magnetic pressure ratio is less than the electron/ion
mass ratio. The decay is efficient at low amplitudes of the magnetic
field in the MHD waves, B/B<SUB>0</SUB> ∼ 10<SUP>-2</SUP>. In turn,
the nonlinearly driven KAWs have sufficiently short wavelengths for the
dissipative effects to become significant. Therefore the cross-scale
nonlinear coupling of Alfvén waves can provide a mechanism for the
replenishment of the dissipation range and the consequent energization
in space plasmas. Two relevant examples of this scenario in the solar
corona and auroral zones are discussed.
---------------------------------------------------------
Title: Solar coronal loop oscillations: theory of resonantly damped
oscillations and comparison with observations
Authors: Goossens, M.; Andries, J.; Arregui, I.; Doorsselaere, T. V.;
Poedts, S.
2005AIPC..784..114G Altcode:
One of the proposed damping mechanisms of coronal transverse loop
oscillations in the kink mode is resonant absorption as a result
of the spatial variation of the Alfvén velocity in the equilibrium
configuration. Analytical expressions for the period and the damping
time exist for 1-D cylindrical equilibrium models with thin non-uniform
transitional layers. Comparison with observations indicates that the
assumption of thin non-uniform transitional layers is not a very
accurate approximation of reality. This contributions starts with
a short review of observations on transverse oscillations in solar
coronal loops. Then it presents results on periods and damping times
of resonantly damped kink mode oscillations for (i) fully non-uniform
1-D cylindrical equilibrium models in which the equilibrium quantities
vary in the radial direction across the magnetic field from the centre
of the loop up to its boundary and (ii) non-uniform 2-D cylindrical
equilibrium models in which the equilibrium quantities vary both in the
radial direction across the magnetic field and in the axial direction
along the magnetic field. An important point is that the periods and
damping times obtained for these fully non-uniform models can differ
substantially from those obtained for thin non-uniform transitional
layers. This contribution then reports on a consistency test between
theory and observations showing that there is a very good agreement
within the observational inaccuracies.
---------------------------------------------------------
Title: Damping of phase-mixed slow magneto-acoustic waves: Real
or apparent?
Authors: Voitenko, Y.; Andries, J.; Copil, P. D.; Goossens, M.
2005A&A...437L..47V Altcode:
The propagation of slow magnetoacoustic waves along a multithreaded
coronal loop is modelled analytically by means of a ray tracing
method. It is shown how cross field gradients build up due to phase
mixing. The cross field gradients can enhance shear viscosity so
that it dominates over compressive viscosity. Nevertheless the short
dissipation distances (~10<SUP>7</SUP> m) observed for slow waves in
coronal loops require very small cross field length scales which imply
a filamentary structure on scales at least three orders of magnitude
below the current detection limit of TRACE and close to the limit where
magnetohydrodynamic (MHD) theory breaks down. The observed dissipation
distances can alternatively be explained by phase mixing in its ideal
regime, where the apparent damping is due to the spatial integration
of the phase mixed amplitudes by the observation.
---------------------------------------------------------
Title: Determination of the Coronal Density Stratification from the
Observation of Harmonic Coronal Loop Oscillations
Authors: Andries, Jesse; Arregui, Inigo; Goossens, Marcel
2005ApJ...624L..57A Altcode:
The recent detection of multiple harmonic standing transverse
oscillations in coronal loops by Verwichte et al. is of special
importance, as it allows one to obtain information on the longitudinal
density variation in loops. Verwichte et al. detected the simultaneous
presence of both the fundamental and the first-overtone mode in two
coronal loops. Here we point out that the ratio of the period of the
fundamental mode to the period of the overtone mode differs from 2
in loops with longitudinal density stratification. Conversely, the
difference between this ratio and 2 can be used as a seismological
tool to obtain information about the density scale height in loops.
---------------------------------------------------------
Title: Cross-Scale Nonlinear Coupling and Plasma Energization by
Alfvén Waves
Authors: Voitenko, Y.; Goossens, M.
2005PhRvL..94m5003V Altcode:
We present a new channel for the nonlocal transport of wave energy from
the large (MHD) scales to the small (kinetic) scales generated by the
resonant decay of MHD Alfvén waves into kinetic Alfvén waves. This
process does not impose any restriction on the wave numbers or
frequencies of initial MHD waves, which makes it superior compared
to the mechanisms of spectral transport studied before. Because of
dissipative properties of the nonlinearly driven kinetic Alfvén
waves, the decay leads to plasma heating and particle acceleration,
which is observed in a variety of space and astrophysical plasmas. Two
examples in the solar corona and the terrestrial magnetosphere are
briefly discussed.
---------------------------------------------------------
Title: Coronal loop oscillations. Calculation of resonantly damped
MHD quasi-mode kink oscillations of longitudinally stratified loops
Authors: Andries, J.; Goossens, M.; Hollweg, J. V.; Arregui, I.;
Van Doorsselaere, T.
2005A&A...430.1109A Altcode:
The observed coronal loop oscillations and their damping are often
theoretically described by the use of a very simple coronal loop
model, viz. a straight, longitudinally invariant, axi-symmetric, and
pressureless flux tube with a different density inside and outside
of the loop. In this paper we generalize the model by including
longitudinal density stratification and we examine how the longitudinal
density stratification alters the linear eigenmodes of the system,
their oscillation frequencies, and the damping rates by resonant
absorption. <P />Appendix A is only available in electronic form at
http://www.edpsciences.org
---------------------------------------------------------
Title: Proton versus electron heating in solar flares
Authors: Gordovskyy, M.; Zharkova, V. V.; Voitenko, Yu. M.; Goossens,
M.
2005AdSpR..35.1743G Altcode:
Proton and electron heating of a flaring atmosphere is compared
in a kinetic approach for the particles ejected from a non-neutral
reconnecting current sheet (RCS) located above the top of reconnected
flaring loops in a two-ribbon flare. Two kinds of high-energy particles
are considered: particles accelerated by a super-Dreicer electric field
and those ejected from the reconnection region as neutral outflows, or
separatrix jets. The beam electrons are assumed to deposit their energy
in Coulomb collisions and Ohmic heating of the ambient plasma particles
by the electric field induced by the precipitating beams. The protons
are assumed to deposit their energy in generation of kinetic Alfvén
waves (KAWs), which, in turn, dissipate due to Cherenkov resonant
scattering on the ambient plasma electrons. The beam electrons are
found to provide a fast (within a few tenth of a second) heating of
the atmosphere that is well spread in depth from the corona to the
lower chromosphere. The protons are shown to precipitate to the lower
atmosphere much slower (up to few seconds for beam and up to 10-20 s
for slow jets). Slow jet protons provide heating of the two compact
regions: the first located at the top of a flaring loop just below
the RCS, and the second one appearing at the transition region (TR)
and upper chromosphere; fast beam protons deposit their energy in the
TR and chromosphere only.
---------------------------------------------------------
Title: Numerical Solutions for Resonantly Damped MHD Quasi-Modes in
Two-Dimensional Coronal Loops
Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens, M.
2004ESASP.575...85A Altcode: 2004soho...15...85A
No abstract at ADS
---------------------------------------------------------
Title: Damping of Coronal Loop Oscillations: Calculation of Resonantly
Damped Kink Oscillations of One-dimensional Nonuniform Loops
Authors: Van Doorsselaere, T.; Andries, J.; Poedts, S.; Goossens, M.
2004ApJ...606.1223V Altcode:
The analytic study of coronal loop oscillations in equilibrium states
with thin nonuniform boundary layers is extended by a numerical
investigation for one-dimensional nonuniform equilibrium states. The
frequency and the damping time of the ideal kink quasi mode are
calculated in fully resistive MHD. In this numerical investigation there
is no need to adopt the assumption of a thin nonuniform boundary layer,
which is essential for analytic theory. An important realization is
that analytical expressions for the damping rate that are equivalent
for thin nonuniform layers give results differing by a factor of 2
when they are used for thick nonuniform layers. Analytical theory for
thin nonuniform layers does not allow us to discriminate between these
analytical expressions. The dependence of the complex frequency of the
kink mode on the width of the nonuniform layer, on the length of the
loop, and on the density contrast between the internal and the external
region is studied and is compared with analytical theory, which is valid
only for thin boundaries. Our numerical results enable us to show that
there exists an analytical expression for thin nonuniform layers that
might be used as a qualitative tool for extrapolation into the regime
of thick nonuniform layers. However, when the width of the nonuniform
layer is varied, the differences between our numerical results and the
results obtained with the version of the analytical approximation that
can be extended into the regime of thick nonuniform layers are still
as large as 25%.
---------------------------------------------------------
Title: Cross-Field Heating of Coronal Ions by Low-Frequency Kinetic
Alfvén Waves
Authors: Voitenko, Yuriy; Goossens, Marcel
2004ApJ...605L.149V Altcode:
Low-frequency kinetic Alfvén waves (KAWs) are studied as a possible
source for the strong heating of ions across the magnetic field in the
solar corona. It is shown that test ions moving in the electromagnetic
fields of KAWs undergo an increase in their cross-field energy because
of the superadiabatic acceleration in the vicinity of the demagnetizing
wave phases. In particular, it is found that KAW wave trains, with a
transversal wavelength of the order of 40 proton gyroradii and with
a peak wave/background magnetic field ratio >~0.1, increase the
cross-field energy of O<SUP>5+</SUP> oxygen ions by 1-2 orders. The
required short perpendicular wavelengths can be produced by the phase
mixing of MHD Alfvén waves, propagating upward from the coronal
base. The superadiabatic acceleration provides an alternative to
the ion-cyclotron explanation for the intense transverse heating of
O<SUP>+5</SUP> and Mg<SUP>9+</SUP> ions observed by the Solar and
Heliospheric Observatory at 1.5-3 solar radii.
---------------------------------------------------------
Title: Radio signatures of Langmuir-Alfvén turbulence in the solar
atmosphere
Authors: Chian, A. C. -L.; Goossens, M.; Miranda, R. A.; Rempel,
E. L.; Sirenko, O.; Voitenko, Y.
2004IAUS..223...95C Altcode: 2005IAUS..223...95C
Radio emissions from the solar active regions can be
generated by nonlinear coupling of Langmuir waves with Alfvén
waves. Multi-wavelength observations can be used to provide evidence
for Langmuir-Alfvén turbulence in the solar atmosphere.
---------------------------------------------------------
Title: Damping of Coronal Loop Oscillations
Authors: van Doorsselaere, T.; Andries, J.; Poedt, S.; Goossens, M.
2004ESASP.547..453V Altcode: 2004soho...13..453V
The analytic study of coronal loop oscillations in equilibrium states
with thin nonuniform boundary layers is extended by a numerical
investigation for 1D nonuniform equilibrium states. The frequency and
the damping time of the ideal kink quasi-mode are calculated in fully
resistive MHD. In this numerical investigation there is no need to adopt
the assumption of a thin nonuniform boundary layer which is essential
for analytic theory. The dependence of the complex frequency of the
kink mode on the width of the nonuniform layer, the length of the loop
and the density contrast between the internal and the external region
is studied and is compared with analytical theory which is only valid
for thin boundaries. When the width of the nonuniformlayer is varied,
the differences between our numerical results and the results obtained
with analytical formula, still amount up to 25%.
---------------------------------------------------------
Title: Cross-field ion acceleration by intermittent kinetic Alfven
waves in space plasmas
Authors: Voitenko, Yu.; Goossens, M.
2004cosp...35.4011V Altcode: 2004cosp.meet.4011V
Intense cross-field energization of the ions is often observed in
the solar corona, solar wind and terrestrial magnetosphere. This
energization has been mainly attributed to the ion-cyclotron damping
or stochastic acceleration by high-frequency waves. We investigate
the possibility that this energization is due to low-frequency Alfven
waves that are short-wavelength across the magnetic field - kinetic
Alfven waves (KAWs). The energy reservoir for KAWs is provided by
the large-scale MHD waves that are widespread in space. Phase mixing,
parametric decay instabilities and turbulent cascade work in the same
direction: they reduce the perpendicular length scales of MHD waves and
convert them into KAWs. Short transversal wavelengths of the order of
several proton gyroradii make KAWs accessible for the super-adiabatic
acceleration of the ions in the vicinity of demagnetizing wave
phases. The resulting ion energization is primarily across the
background magnetic field. The main properties of this process are:
(i) it is non-resonant, hence it does not need any wave coherence;
(ii) it has a well-defined threshold character ; (iii) it can be
efficient with rare super-critical waves; (iv) it can be reduced or
enforced by the bulk field-aligned ion motion.
---------------------------------------------------------
Title: Ion Heating across the Magnetic Field in the Solar Corona by
Kinetic Alfvén Waves
Authors: Voitenko, Y.; Goossens, M.
2004ESASP.547..381V Altcode: 2004soho...13..381V
The perpendicular heating of the ions observed by SOHO in the
solar corona at 2-4 solar radii has been mainly attributed to the
ion-cyclotron damping of high-frequency Alfvén waves. We investigate
an alternative mechanism of heating by low-frequency Alfvén waves that
have short wavelengths across the magnetic field - kinetic Alfvén waves
(KAWs). The energy reservoir for these kinetic waves is provided by
low-frequency large-scale MHD waves that are launched in the corona by
the photospheric motions or excited at the coronal base by magnetic
restructuring. The short perpendicular wavelengths, developed by
phase mixing, convert MHD Alfvén waves into KAWs. KAWs can be also
excited in situ by various linear and nonlinear mechanisms. We show that
above a threshold value of the wave amplitude, KAWs can stochastically
accelerate ions across the background magnetic field. In particular,
KAWs with transversal wavelengths of the order of the ion inertial
length and with a wave/background magnetic field ratio of the order
0.1, can contribute to the stochastic heating of oxygen ions O5+ . We
discuss advantages of this mechanism over the ion-cyclotron heating
scheme for the intense transverse heating of ions observed by SOHO at
2-4 solar radii.
---------------------------------------------------------
Title: Nonlinear damping of MHD waves and origin of solar wind
Authors: Voitenko, Yu.; Goossens, M.
2004cosp...35.3973V Altcode: 2004cosp.meet.3973V
We propose a new wave model for the fast solar wind heating and
acceleration. The principal point of our model is that the energy is
deposited in the plasma by the kinetic Alfven waves (KAWs), which
have a sufficiently short wavelength across the magnetic field for
the dissipative effects to become significant. The energy reservoir is
provided by the low-frequency large-scale MHD waves that are far from
the dissipative range. Such waves, polarized in the sense of Alfven wave
(AW) and/or fast wave (FW), are supposed to be launched in the corona
by the photospheric motions or excited at the coronal base by magnetic
restructuring. We show that the presence of finite-amplitude MHD waves
gives rise to the nonlinear excitation of KAWs via parametric decay,
which results in a jump-like transport of MHD wave energy directly
in the dissipation range. The part of wave flux polarized in the
sense of FW decay very fast, as soon as resonant propagation angle is
developed due to wave refraction. The FW-driven KAWs provide heating
at low heights, at the coronal base. A flux of MHD AWs propagates
farther upward up to the height where plasma beta becomes equal to the
electron/ion mass ratio Me/Mi. As soon as plasma beta drops below Me/Mi,
MHD AWs undergo a strong parametric decay into KAWs. Due to their short
perpendicular wavelengths, the nonlinearly excited KAWs dissipate via
collisionless and collisional wave-particle interaction, which, in turn,
gives rise to the plasma heating and particles acceleration. Thus,
the flux of KAWs, that propagates further upward, can easily increase
plasma beta again well above Me/Mi in the high corona and provide the
energy source for the solar wind acceleration.
---------------------------------------------------------
Title: Current driven kinetic Alfven instabilities in the solar
atmosphere
Authors: Voitenko, Yu.; Siverskyy, T.; Goossens, M.
2004cosp...35.4032V Altcode: 2004cosp.meet.4032V
We study two physical mechanisms that can cause anomalous dissipation
of currents in the solar atmosphere. The first one is a resonant
kinetic instability of ion-cyclotron kinetic Alfvén waves (ICKAWs)
driven by currents flowing parallel or perpendicular to the background
magnetic field [1]. These currents can be generated by coronal magnetic
shears or plasma non-uniformity, or by MHD wave phase mixing. Since the
frequencies of the excited ICKAWs are close to the proton cyclotron
frequency, the inverse turbulent cascade transports the wave energy
to lower frequencies where the waves can induce transversal heating of
heavier ion species by the ion-cyclotron resonant interaction. Another,
non-resonant instability of low-frequency KAWs, can be excited by
electrons due to their acceleration in the electric field which
is parallel to the magnetic field. Strong, super-Dreicer electric
fields, which are required for this instability, can be generated
during magnetic reconnection events in the solar atmosphere (flares,
micro- and nanoflares). This non-resonant KAW instability can produce
low-frequency pulsations of plasma emission, as is observed during
solar flares. Both resonant and non-resonant instabilities excite
waves with very short transversal wavelengths of the order of the ion
gyroradius. This makes these waves accessible for the plasma heating
and stochastic ion acceleration across the magnetic field. Both
mechanisms transform the energy of currents into the perpendicular
energy of ions. The scattering of current electrons by waves results
in anomalous transport coefficients and fast energy release. The large
transversal temperatures of ions in the places where the energy is
released could serve as a signature for these processes.
---------------------------------------------------------
Title: Observational Tests of Damping by Resonant Absorption in
Coronal Loop Oscillations
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Andries,
Jesse; Goossens, Marcel; Van Doorsselaere, Tom
2003ApJ...598.1375A Altcode: 2003astro.ph..9470A
One of the proposed damping mechanisms of coronal (transverse)
loop oscillations in the kink mode is resonant absorption as a
result of the Alfvén speed variation at the outer boundary of
coronal loops. Analytical expressions for the period and damping
time exist for loop models with thin nonuniform boundaries. They
predict a linear dependency of the ratio of the damping time to the
period on the thickness of the nonuniform boundary layer. Ruderman and
Roberts used a sinusoidal variation of the density in the nonuniform
boundary layer and obtained the corresponding analytical expression
for the damping time. Here we measure the thickness of the nonuniform
layer in oscillating loops for 11 events, by forward-fitting of the
cross-sectional density profile n<SUB>e</SUB>(r) and line-of-sight
integration to the cross-sectional fluxes F(r) observed with
TRACE 171 Å. This way we model the internal (n<SUB>i</SUB>) and
external electron density (n<SUB>e</SUB>) of the coronal plasma in
oscillating loops. This allows us to test the theoretically predicted
damping rates for thin boundaries as a function of the density ratio
χ=n<SUB>e</SUB>/n<SUB>i</SUB>. Since the observations show that
the loops have nonuniform density profiles, we also use numerical
results for damping rates to determine the value of χ for the
loops. We find that the density ratio predicted by the damping time,
χ<SUB>LEDA</SUB>=0.53+/-0.12, is a factor of ~1.2-3.5 higher than the
density ratio estimated from the background fluxes, χ=0.30+/-0.16. The
lower densities modeled from the background fluxes are likely to be
a consequence of the neglected hotter plasma that is not detected
with the TRACE 171 Å filter. Taking these corrections into account,
resonant absorption predicts damping times of kink-mode oscillations
that are commensurable with the observed ones and provides a new
diagnostic of the density contrast of oscillating loops.
---------------------------------------------------------
Title: Nonlinear excitation of kinetic Alfvén waves and whistler
waves by electron beam-driven Langmuir waves in the solar corona
Authors: Voitenko, Yu.; Goossens, M.; Sirenko, O.; Chian, A. C. -L.
2003A&A...409..331V Altcode:
We study a new nonlinear excitation mechanism of kinetic Alfvén waves
(KAWs) and whistler waves (Ws) by electron beam-driven Langmuir waves
(Ls). The generation conditions for the parametric decay instability
L rightleftarrows W + KAW are determined and the growth rate is
calculated. We show that the resonant pairs of KAWs and whistler
waves are nonlinearly coupled to the pump Langmuir waves and their
amplitudes undergo exponential growth from the thermal level. The
perpendicular dispersion of KAWs strongly increases the coupling due
to the nonlinear current parallel to the ambient magnetic field. Our
study suggests that the nonlinear coupling of Langmuir wave energy
into KAWs and whistlers can provide an efficient sink for weakly
dispersive Langmuir waves excited by fast electron beams in the solar
corona when the electron plasma frequency is lower than the electron
gyrofrequency. This condition can be satisfied in the low-density
magnetic filaments that are rooted in the depleted patches at the
coronal base and extend to the high corona. At the same time, the
Langmuir-driven KAWs and whistlers give rise to scattering and/or thin
structures of radio emission penetrating through, or generated in these
regions. Since the decay into sunward propagating KAWs is strongest,
the nonlinearly driven KAWs can be easily distinguished from the waves
generated at the coronal base and propagating away from the Sun. Our
results may be used in the analysis of solar radio data and for remote
probing of the coronal plasma, magnetic fields, and waves.
---------------------------------------------------------
Title: An introduction to plasma astrophysics and magnetohydrodynamics
Authors: Goossens, Marcel
2003ASSL..294.....G Altcode: 2003ipam.book.....G
No abstract at ADS
---------------------------------------------------------
Title: Kinetic Excitation Mechanisms for ION-Cyclotron Kinetic
Alfvén Waves in Sun-Earth ConnectionI
Authors: Voitenko, Yuriy; Goossens, Marcel
2003SSRv..107..387V Altcode:
We study kinetic excitation mechanisms for high-frequency dispersive
Alfvén waves in the solar corona, solar wind, and Earth's
magnetosphere. The ion-cyclotron and Cherenkov kinetic effects are
important for these waves which we call the ion-cyclotron kinetic
Alfvén waves (ICKAWs). Ion beams, anisotropic particles distributions
and currents provide free energy for the excitation of ICKAWs in space
plasmas. As particular examples we consider ICKAW instabilities in the
coronal magnetic reconnection events, in the fast solar wind, and in
the Earth's magnetopause. Energy conversion and transport initiated by
ICKAW instabilities is significant for the whole dynamics of Sun-Earth
connection chain, and observations of ICKAW activity could provide a
diagnostic/predictive tool in the space environment research.
---------------------------------------------------------
Title: Nonlinear wave dynamics in the dissipation range
Authors: Voitenko, Y.; Goossens, M.
2003PADEU..13..153V Altcode:
There is abundant observational evidence that the ions in the solar
corona (in particular, O(+5) ) are heated anisotropicaly, predominantly
across the background magnetic field. This heating is usually attributed
to the dissipation of ion-cyclotron waves. We study an alternative
possibility with the dissipation range in the solar corona formed by
the kinetic Alfvén waves (KAWs) which are very short- wavelengths
across the magnetic field. Instead of transport of MHD wave energy
towards to the range of ion-cyclotron waves, we study transport into
the dissipation range of KAWs. We show that the nonlinear excitation of
short-wavelength (of the order 10 m) KAWs in the extended solar corona
and solar wind can be provided by upward-propagating fast and Alfvén
MHD waves launched from the coronal base by the convection or magnetic
reconnection. KAWs are very efficient in the energy exchange with plasma
particles, providing plasma heating and particles acceleration. In
particular, these transversal wavelengths make KAWs accessible for the
stochastic perpendicular heating of oxygen ions when the wave/background
magnetic field ratio exceeds 0.005. Both the quasi-steady coronal
heating and the transient heating events observed by Yohkoh and SOHO
may be due to KAWs that are nonlinearly excited by MHD waves.
---------------------------------------------------------
Title: On the quasi-current-free electrodynamics of current-carrying
hot space plasma
Authors: Gubchenko, V. M.; Biernat, H. K.; Goossens, M.
2003AdSpR..31.1277G Altcode:
A reduced self-consistent kinetic approach is developed for the study
of nonlinear plasma electrodynamics near weakly magnetized regions in a
current carrying collisionless plasma. In these regions in addition to
quasineutral we have quasi-current-free dynamics. Quasi-current free
nonlinear plasma dynamics is realized when the diamagnetic spatial
dispersion parameter originating from the diamagnetic current is small
compared to the plasma and field dynamic scales. In this case, the eddy
currents consisting of diamagnetic and resistive parts are mutually
compensated. We illustrate this effect in a liner approach to plasma
electrodynamics. A quasi MHD beam-like approach to the accelerated
electrons forming resistivity and a kinetic approach to the diamagnetic
plasma particles allows us to develop an analytical self-similar
solution to the one dimensional magnetic annihilation problem.
---------------------------------------------------------
Title: Influence of a uniform coronal magnetic field on solar p-modes
Authors: Vanlommel, P.; Debosscher, A.; Andries, J.; Goossens, M.
2002ESASP.506..893V Altcode: 2002svco.conf..893V; 2002ESPM...10..893V
The influence of a constant coronal magnetic field on solar global
oscillations is investigated for a simple planar equilibrium model. The
model consists of an atmosphere with a constant horizontal magnetic
field on top of an unmagnetized solar interior. The focus is on the
possible resonant coupling of global solar oscillation modes to local
slow continuum modes of the atmosphere and the consequent damping of the
global oscillations. The physical process of resonant absorption of the
acoustic modes with frequency in the cusp continuum is mathematically
completely described by the ideal MHD differential equations which
for this particular equilibrium model reduce to the hypergeometric
differential equation.
---------------------------------------------------------
Title: On the nature of umbral oscillations: theory and observation
by CDS/SoHO
Authors: Banerjee, D.; O'Shea, E.; Goossens, M.; Poedts, S.; Doyle,
J. G.
2002ESASP.506..427B Altcode: 2002ESPM...10..427B; 2002svco.conf..427B
We will present solutions for magneto-acoustic-gravity (or MAG)
waves. The possible wave modes in the 3-5 min range will be
discussed. We will then present observations of sunspots performed
in the EUV wavelength range with the Coronal Diagnostic Spectrometer
(CDS) on SoHO. We examine the time series for the line intensities
and relative velocities and calculate their power spectrum using
wavelet transforms. We find oscillations in the chromosphere and
transition region above the sunspots in the temperature range logT =
4.6 -5.4. Most of the spectral power above the umbra is contained in
the 5-7 mHz frequency range. When the CDS slit crosses the sunspot
plume a clear 3 min oscillation is observed. The observations are
interpreted in terms of slow magnetoacoustic waves propagating upwards.
---------------------------------------------------------
Title: Damping of coronal loop oscillations by resonant absorption
of quasi-mode kink oscillations
Authors: Goossens, M.; Andries, J.; Aschwanden, M. J.
2002ESASP.506..629G Altcode: 2002svco.conf..629G; 2002ESPM...10..629G
Damped quasi-mode kink oscillations in cylindrical flux tubes are
capable of explaining the observed rapid damping of the coronal loop
oscillations when the ratio of the inhomogneity length scale to the
radius of the loop is allowed to vary from loop to loop. They do not
need to invoke anomalously low Reynolds numbers. The theoretical
expressions for the decay time by Hollweg & Yang (1988) and
Ruderman & Roberts (2002) are used to estimate the ratio of
the length scale of inhomogneity compared to the loop radius for a
collection of loop oscillations.
---------------------------------------------------------
Title: Coronal loop oscillations. An interpretation in terms of
resonant absorption of quasi-mode kink oscillations
Authors: Goossens, M.; Andries, J.; Aschwanden, M. J.
2002A&A...394L..39G Altcode:
Damped quasi-mode kink oscillations in cylindrical flux tubes are
capable of explaining the observed rapid damping of the coronal
loop oscillations when the ratio of the inhomogeneity length scale
to the radius of the loop is allowed to vary from loop to loop,
without the need to invoke anomalously low Reynolds numbers. The
theoretical expressions for the decay time by Hollweg & Yang
(\cite{hollweg1988}) and Ruderman & Roberts (\cite{ruderman2002})
are used to estimate the ratio of the length scale of inhomogeneity
compared to the loop radius for a collection of loop oscillations.
---------------------------------------------------------
Title: On the theory of MAG waves and a comparison with sunspot
observations from CDS/SoHO
Authors: Banerjee, D.; O'Shea, E.; Goossens, M.; Doyle, J. G.;
Poedts, S.
2002A&A...395..263B Altcode:
We examine the influence of non-adiabatic effects on the modes of an
isothermal stratified magnetic atmosphere. We present new solutions for
magneto-acoustic-gravity (or MAG) waves in the presence of a radiative
heat exchange based on Newton's law of cooling. An analytic expression
for the dispersion relation is derived, which allows the effect of a
weak magnetic field on the modes to be studied. The insight so gained
proves useful in extending the computations to the moderate-high field
case. In the second part we present observations of two sunspots
obtained in the EUV wavelength range with the Coronal Diagnostic
Spectrometer (CDS) on SoHO. We examine the time series for the line
intensities and relative velocities and calculate their power spectra
using wavelet transforms. We find oscillations in the chromosphere
and transition region above the sunspots in the temperature range
log T = 4.6-5.4 K. Most of the spectral power above the umbra is
contained in the 5-7 mHz frequency range. When the CDS slit crosses
the sunspot umbra a clear 3 min oscillation is observed. The observed
oscillation frequencies are compared with the computed frequencies and
the observations are interpreted in terms of the slow magneto-acoustic
waves.
---------------------------------------------------------
Title: Waves and oscillations in magnetic fields
Authors: Goossens, Marcel; de Groof, Anik; Andries, Jesse
2002ESASP.505..137G Altcode: 2002solm.conf..137G; 2002IAUCo.188..137G
This paper gives an overview of the theory of MHD waves in magnetic
plasma configurations in the solar atmosphere. The emphasis is on basic
properties that are independent of specific equilibrium models but
are rather related to the intrinsic structuring and non-uniformity
of the plasma. The discussion is confined to MHD waves in uniform
and 1-d cylindrical equilibrium models of magnetic flux tubes with
a straight magnetic field. These models contain sufficient physics
for understanding basic properties of MHD waves and still allow for
a relatively straightforward and transparent mathematical analysis.
---------------------------------------------------------
Title: Slow MAG waves in the sunspot umbra as observed by CDS/SOHO
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.;
Poedts, S.
2002ESASP.505..187B Altcode: 2002solm.conf..187B; 2002IAUCo.188..187B
We present observations, in the EUV wavelength range, of two
sunspots, carried out by the Coronal Diagostic Spectrometer (CDS)
on SoHO. We examine the time series for the line intensities and
relative velocities and calculate their power spectrum using wavelet
transforms. We find oscillations in the chromosphere and transition
region above the sunspots in the temperature range logT = 4.6 -
5.4. Most of the spectral power above the umbra are contained in the
5 - 7 mHz frequency range. When the CDS slit croses the sunspot plume
a clear 3 in oscillation is observed. We also present new solutions
for magnetic-acoustic-gravity (or MAG) waves in the presence of
radiative heat exchange based on Newton's law of cooling. The observed
oscillation frequencies are compared with the computed frequencies. The
observations are interpreted in terms of slow magnetoacoustic waves
propagating upwards.
---------------------------------------------------------
Title: Frame dependence of the negative energy wave formula!?
Authors: Andries, Jesse; Goossens, Marcel
2002ESASP.505..341A Altcode: 2002solm.conf..341A; 2002IAUCo.188..341A
In this paper we show that the classical negative energy wave
(N.E.W.) formula is frame dependent. By analogy with a simple mechanical
problem, we show that the negligible second order perturbations
become energetically important and function as an energy source for
the linear waves.
---------------------------------------------------------
Title: Fast and Alfvén waves driven by azimuthal footpoint motions
Authors: de Groof, Anik; Goossens, Marcel
2002ESASP.505..389D Altcode: 2002solm.conf..389D; 2002IAUCo.188..389D
The excitation of Alfvén and fast magnetosonic waves in footpoint
driven coronal loops is studied in the framework of resonant
absorption. Previous studies revealed that in case of radial footpoint
motions, quasi-modes are essential for effective wave dissipation
in the loops. We now investigate the role they play in azimuthally
driven loops. For a periodic driver, the efficiency of resonant
absorption strongly depends on the driving frequency ω<SUB>d</SUB>:
only for quasi-mode frequencies coupling has a positive effect on the
growth of the (single) Alfvén resonance. The problem of single-shell
heating can be solved by considering a more realistic, random driver:
a variety of resonant Alfvén waves are excited and multiple resonant
peaks appear, with length scales which are short enough for effective
dissipation. When more realistic loop lengths are considered, the
resonant surfaces are even more numerous resulting in globally heated
loops.
---------------------------------------------------------
Title: Nonlinear excitation of small-scale Alfvén waves by fast
waves and plasma heating in the solar atmosphere
Authors: Voitenko, Yuriy; Goossens, Marcel
2002SoPh..209...37V Altcode:
We study a nonlinear mechanism for the excitation of kinetic Alfvén
waves (KAWs) by fast magneto-acoustic waves (FWs) in the solar
atmosphere. Our focus is on the excitation of KAWs that have very
small wavelengths in the direction perpendicular to the background
magnetic field. Because of their small perpendicular length scales,
these waves are very efficient in the energy exchange with plasmas and
other waves. We show that the nonlinear coupling of the energy of the
finite-amplitude FWs to the small-scale KAWs can be much faster than
other dissipation mechanisms for fast wave, such as electron viscous
damping, Landau damping, and modulational instability. The nonlinear
damping of the FWs due to decay FW = KAW + KAW places a limit on the
amplitude of the magnetic field in the fast waves in the solar corona
and solar-wind at the level B/B<SUB>0</SUB>∼10<SUP>−2</SUP>. In
turn, the nonlinearly excited small-scale KAWs undergo strong
dissipation due to resistive or Landau damping and can provide coronal
and solar-wind heating. The transient coronal heating observed by
Yohkoh and SOHO may be produced by the kinetic Alfvén waves that
are excited by parametric decay of fast waves propagating from the
reconnection sites.
---------------------------------------------------------
Title: Nonlinear interaction of kinetic Alfvén waves and radio
waves in the solar corona
Authors: Sirenko, O.; Voitenko, Yu.; Goossens, M.
2002A&A...390..725S Altcode:
This paper investigates the nonlinear interaction of short wavelength
kinetic Alfvén waves (KAWs) with extraordinary (x-) and ordinary (o-)
mode radio waves using two fluid magnetohydrodynamics. The focus is on
the interaction of a preexistent KAW with an ordinary electromagnetic
wave, giving rise to an extraordinary electromagnetic wave. The equation
governing the time evolution of the amplitude of the excited x-mode
is derived. The growth time of the x-mode wave is determined and the
corresponding interaction distance is computed for active regions in
the low corona and for high corona. The nonlinear coupling appears to
be quite efficient for reasonable amplitudes of KAWs in the corona,
B<SUB>A</SUB>/B<SUB>0</SUB>=0.02-0.2. We suggest that this process
provides a new possibility for the detection and remote sensing of
coronal kinetic Alfvén waves by means of radio observations.
---------------------------------------------------------
Title: Fast and Alfvén waves driven by azimuthal footpoint
motions. I. Periodic driver
Authors: De Groof, A.; Paes, K.; Goossens, M.
2002A&A...386..681D Altcode:
The excitation of Alfvén and fast magneto-acoustic waves in coronal
loops driven by footpoint motions is studied in linear, ideal MHD. The
analysis is restricted to azimuthally polarized footpoint motions
so that only Alfvén waves are directly excited which couple to
fast magneto-acoustic waves at later times. In the present study a
periodic driver is applied at one end of the loop. The effects of a
more realistic random driver are studied in the companion paper De
Groof & Goossens (2002) (hereafter referred to as Paper II). The
first part of the paper is devoted to the study of resonant absorption
and phase-mixing in the absence of coupling (azimuthal wavenumber
k<SUB>y</SUB>=0). Since the density varies across the loop, resonances
occur at the magnetic surfaces where the driving frequency equals the
local Alfvén frequency. In a second part where Alfvén waves with
k<SUB>y</SUB> !=q 0 coupling to fast waves are taken into account, we
find that the behaviour of the MHD waves is strongly dependent on the
driving frequency omega<SUB>d</SUB> . Especially driving frequencies
equal to a quasi-mode frequency seem to make the difference. The fast
waves excited in these cases are global oscillations of the system
and form quasi-modes as they are damped through the resonant coupling
with Alfvén modes. Since these resonances occur at the same location
where the original Alfvén wave peaks, the resonant peak is further
amplified. While in most cases coupling has a negative effect on the
growth of the directly excited Alfvén waves, driving with a quasi-mode
frequency leads to a faster growth of the resonant peaks and a more
efficient decrease in length scales than in the uncoupled case.
---------------------------------------------------------
Title: Fast and Alfvén waves driven by azimuthal footpoint
motions. II. Random driver
Authors: De Groof, A.; Goossens, M.
2002A&A...386..691D Altcode:
The excitation of Alfvén and fast magneto-acoustic waves in coronal
loops driven by footpoint motions is studied in linear, ideal MHD. The
analysis is restricted to azimuthally polarized footpoint motions
so that only Alfvén waves are directly excited to couple to fast
magneto-acoustic waves at later times. In the companion paper De
Groof et al. (\cite{Groof02a}) (hereafter referred to as Paper I),
the behaviour of the MHD waves is studied in case of a monochromatic
driver. In the present study, the effects of a more realistic random
driver are investigated.\ First, we consider loops of equal length and
width in order to limit the number of quasi-modes in the frequency
range of the driver so that the influence of quasi-modes in the
system can easily be detected. In contrast to the single resonant
surface which was found in case of a periodic driver (see Paper I),
a random pulse train excites a variety of resonant Alfvén waves and
consequently the small length scales built up are spread over the
whole width of the loop. The specific effects of the quasi-modes are
not so easily recognized as for radial footpoint motions (De Groof
& Goossens \cite{Groof00a}) since the resonances corresponding to
directly and indirectly excited Alfvén waves are mixed together. In
the second part of the paper, longer loops are considered. Since more
quasi-modes are involved, the resonant surfaces are more numerous and
widely spread throughout the whole loop volume. On the other hand,
it takes more time for the MHD waves to cross the loop and to form
standing waves. Nevertheless this negative effect does not have too much
impact since the simulations show that after a small time interval,
resonant surfaces are created all over the loop, with length scales
which are short enough for effective dissipation.
---------------------------------------------------------
Title: Excitation of high-frequency Alfvén waves by plasma outflows
from coronal reconnection events
Authors: Voitenko, Yuriy; Goossens, Marcel
2002SoPh..206..285V Altcode:
We study a kinetic excitation mechanism for high-frequency dispersive
Alfvén waves in the solar corona by magnetic reconnection events. The
ion-cyclotron and Cerenkov kinetic effects are important for these waves
which we call the ion-cyclotron kinetic Alfvén waves (IC KAWs). The
plasma outflowing from the reconnection site sets up a neutralized
proton beam in the surrounding plasma, providing free energy for
the excitation of waves. The dependence of the phase velocity of
the IC KAW on the parallel wavenumber is different from that on
the perpendicular wavenumber. The phase velocity is an increasing
function of the perpendicular wavenumber and overtakes the Alfvén
velocity for sufficiently large values of k<SUB>⊥</SUB>. However,
the phase velocity is a decreasing function of k<SUB>∥</SUB>, and
sufficiently large values of k<SUB>∥</SUB> result in a phase velocity
below the Alfvén velocity. As a result, the IC KAWs can undergo
the Cerenkov resonance with both super- and sub-Alfvénic particles,
and for the waves to be excited the outflow velocity does not need to
be super-Alfvénic, as for KAWs, but the beam/Alfvén velocity ratio
can span a wide range of values. High growth rates of the order of
γ∼10<SUP>4</SUP> s<SUP>−1</SUP> are found for the values of
the plasma parameters typical for the low solar corona. The waves
excited by (sub-)Alfvénic beams are damped mainly due to kinetic
wave-particle interactions with ions at the cyclotron resonance
(ion-cyclotron damping), and with ions and electrons at the Cerenkov
resonance (Landau damping). Therefore, IC KAWs can heat the plasma
species of the corona in both the parallel and perpendicular direction,
giving rise to an anisotropic heating of the ions. The observational
consequences of the processes under study are discussed.
---------------------------------------------------------
Title: Long-Period Oscillations in Polar Coronal Holes as Observed
by CDS on SOHO
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.
2002mwoc.conf...19B Altcode:
We examine spectral time series of the coronal line Mg ix 368Å,
the transition region line O v 629Å, and the chromospheric line He
i 584Å observed during several periods of 2000, with the Coronal
Diagnostic Spectrometer (CDS) onboard the SoHO spacecraft. We study
different parts of coronal holes, plumes and inter-plumes (off-limb)
as a separate study. We report here on a time series analysis, using
wavelet methods, of small individual regions in the polar coronal
hole. The wavelet analysis allows us to derive the duration as well
as the periods of the oscillations. The statistical significance of
the oscillations was estimated by using a randomisation method. Our
observations indicate the presence of compressional waves with periods
of 20-30 minutes or longer. These slow magneto-acoustic waves may
provide enough energy flux for the acceleration of the fast solar wind.
---------------------------------------------------------
Title: Influence of a uniform coronal magnetic field on solar p modes:
coupling to slow resonant MHD waves
Authors: Vanlommel, P.; Debosscher, A.; Andries, J.; Goossens, M.
2002SoPh..205....1V Altcode:
The influence of a constant coronal magnetic field on solar global
oscillations is investigated for a simple planar equilibrium model. The
model consists of an atmosphere with a constant horizontal magnetic
field and a constant sound speed, on top of an adiabatic interior
having a linear temperature profile. The focus is on the possible
resonant coupling of global solar oscillation modes to local slow
continuum modes of the atmosphere and the consequent damping of
the global oscillations. In order to avoid Alfvén resonances, the
analysis is restricted to propagation parallel to the coronal magnetic
field. Parallel propagating oscillation modes in this equilibrium model
have already been studied by Evans and Roberts (1990). However, they
avoided the resonant coupling to slow continuum modes by a special
choice of the temperature profile. The physical process of resonant
absorption of the acoustic modes with frequency in the cusp continuum
is mathematically completely described by the ideal MHD differential
equations which for this particular equilibrium model reduce to the
hypergeometric differential equation. The resonant layer is correctly
dealt with in ideal MHD by a proper treatment of the logarithmical
branch cut of the hypergeometric function. The result of the resonant
coupling with cusp waves is twofold. The eigenfrequencies become
complex and the real part of the frequency is shifted. The shift of
the real part of the frequency is not negligible and within the limit
of observational accuracy. This indicates that resonant interactions
should definitely be taken into account when calculating the frequencies
of the global solar oscillations.
---------------------------------------------------------
Title: Influence of a Uniform Coronal Magnetic Field on Solar p Modes:
Coupling to Slow Resonant MHD Waves
Authors: Vanlommel, P.; Debosscher, A.; Andries, J.; Goossens, M.
2002ASPC..259..480V Altcode: 2002rnpp.conf..480V; 2002IAUCo.185..480V
No abstract at ADS
---------------------------------------------------------
Title: Signatures of very long period waves in the polar coronal holes
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.
2001A&A...380L..39B Altcode:
We examine long spectral time series of a coronal hole observed on
the 7th March 2000 with the Coronal Diagnostic Spectrometer (CDS)
on-board SoHO. The observations were obtained in the chromospheric He
I, and a series of higher temperature oxygen lines. In this letter we
report on the presence of long period oscillations in a polar coronal
hole region on the disk. Our observations indicate the presence of
compressional waves with periods of 20-30 min or longer.
---------------------------------------------------------
Title: Long period oscillations in the inter-plume regions of the Sun
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.
2001A&A...377..691B Altcode:
We examine long spectral time series of inter-plume lanes observed on
the 14th and 15th March 2000 with the Coronal Diagnostic Spectrometer
(CDS) on-board SoHO. The observations were obtained in lines over a wide
temperature range, from the chromosphere to the corona. The statistical
significance of the oscillations was estimated by using a randomisation
method. Our observations indicate the presence of compressional waves
with periods of 20-50 min or longer, both off-limb and on-disk and up
to 70 min further out to at least 25 arcsec off-limb. To our knowledge
this is the first time that long period oscillations in the inter-plume
regions close to the limb of the Sun have been detected. We interpret
these oscillations as outward propagating slow magneto-acoustic waves
which may contribute significantly to the heating of the lower corona
by compressive dissipation and which may also provide some of the
enough energy flux required for the acceleration of the fast solar
wind. These slow waves may have been produced at the network boundaries
in the coronal hole.
---------------------------------------------------------
Title: Hidden problems of thin-flux-tube approximation
Authors: Zhugzhda, Y. D.; Goossens, M.
2001A&A...377..330Z Altcode:
This paper scrutinizes the validity of the thin-flux-tube approximation
for magnetic flux tubes embedded in a surrounding magnetic plasma. It
is shown that the thin-flux-tube approximation gives an accurate
description of surface waves for C<SUB>T</SUB><SUP>2</SUP>>
C<SUB>Ae</SUB><SUP>2</SUP>, body waves for C_Te<SUP>2</SUP>>
C<SUB>T</SUB><SUP>2</SUP> and surface leaky waves for
C<SUB>Te</SUB><SUP>2</SUP>< C<SUB>T</SUB><SUP>2</SUP> <
C<SUB>Ae</SUB><SUP>2</SUP>. The Leibovich-Roberts equation for
nonlinear surface waves in a flux tube embedded in a field free plasma
is generalized to a flux tube immersed in a magnetic plasma. The
generalized Leibovich-Roberts (GLR) equation describes the propagation
of nonlinear slow surface, body and surface leaky waves in tubes. The
shortcomings of the GLR equation are discussed. The Korteweg-de Vries
equation (KdV) is generalized for surface waves. The advantage of the
second order thin-flux-tube approximation is shown. Two scenarios for
the heating of coronal loops are discussed. It is emphasized, that
the application of the thin-flux-tube approximation to thin tubes of
non-zero diameter has to take into account possible wave emission by
the tube and shock front formation for amplitudes in excess of some
critical value.
---------------------------------------------------------
Title: The influence of resonant MHD wave coupling in the boundary
layer on the reflection and transmission process
Authors: Andries, J.; Goossens, M.
2001A&A...375.1100A Altcode:
When a wave is incident on an inhomogeneous boundary layer separating
two plasmas in relative motion, it is often argued that the resonant
absorption rate can become negative when there is a sufficiently large
velocity shear, thus giving energy back to the wave. However such
treatment fails to distinguish the two energy exchange processes that
are operative: extraction of wave energy by dissipative processes around
the resonance point and energy exchange between the wave and the flow
of the medium. By aid of the proper wave energy definitions recently
discussed by Walker (\cite{walker}), rather than by the conventional
concept of “negative energy" waves we reveal the important dependence
of the resonant amplification process on the precise structure
of the boundary layer profiles. We conclude that stratification in
boundary layers is very important not only because the resulting wave
coupling can cause resonant instabilities for velocity shears below
the Kelvin-Helmholtz threshold, but also because the coupling can
stabilize the Kelvin-Helmholtz instability above this threshold.
---------------------------------------------------------
Title: On the role of ion-cyclotron kinetic Alfvén waves in the
solar wind: results from HELIOS and expectations for Solar Orbiter
Authors: Voitenko, Yu.; Goossens, M.; Marsch, E.
2001ESASP.493..411V Altcode: 2001sefs.work..411V
No abstract at ADS
---------------------------------------------------------
Title: The nature of network oscillations
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.
2001A&A...371.1137B Altcode:
We examine time-series of spectral data obtained from the Coronal
Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements
of Emitted Radiation instrument (SUMER) onboard the Solar Heliospheric
Observatory (SOHO) spacecraft, in the period 30-31 July 1996. The
observations were obtained in lines, ranging in temperature from
12 000 K to 10<SUP>6</SUP> K, covering the low chromosphere to the
corona. We report here on a time series analysis, using wavelet methods,
of small individual network regions in the quiet Sun. The wavelet
analysis allows us to derive the duration as well as the periods of
the oscillations. The statistical significance of the oscillations
was estimated by using a randomisation method. The oscillations are
considered to be due to waves, which are produced in short bursts with
coherence times of about 10-20 min. The low chromospheric and transition
region lines show intensity and velocity power in the 2-4 mHz range. The
coronal line Mg x does not show any statistically significant power
in this range. In general, it is thought likely that the chromosphere
and possibly the transition region oscillates in response to forcing
by the p-modes, but they are also influenced strongly by the presence
of magnetic fields. The observed 2-4 mHz network oscillations can
thus be interpreted in terms of kink and sausage waves propagating
upwards along thin magnetic flux tubes. We perform a linear numerical
computation comparing the results with our observations.
---------------------------------------------------------
Title: Nonlinear resonant absorption of fast magnetoacoustic waves
due to coupling into slow continua in the solar atmosphere
Authors: Erdélyi, R.; Ballai, I.; Goossens, M.
2001A&A...368..662E Altcode:
Nonlinear resonant absorption of fast magnetoacoustic (FMA) waves in
inhomogeneous weakly dissipative, isotropic and anisotropic plasmas
in static and steady equilibria is studied. Both isotropic and
anisotropic plasmas are considered and for the background equilibrium
state 1D planar static and steady models are used. The equilibrium
configuration consists of three layers, where an inhomogeneous
magnetised plasma slab is surrounded by two homogeneous magnetised
semi-infinite plasma regions. The propagating FMA waves are partly
absorbed due to coupling to local nonlinear slow magnetohydrodynamic
(MHD) waves in the inhomogeneous layer, and are partly reflected. The
coefficient of wave energy resonant absorption is derived using two
simplifying assumptions (i) weak nonlinearity and (ii) the thickness
of the inhomogeneous layer is small compared to the wavelength of the
waves, i.e. the so-called long-wavelength approximation is used.
---------------------------------------------------------
Title: Kelvin-Helmholtz instabilities and resonant flow instabilities
for a coronal plume model with plasma pressure
Authors: Andries, J.; Goossens, M.
2001A&A...368.1083A Altcode:
In this paper we continue the study of the effect of the velocity shear
between the coronal plume and the interplume region on the spectrum
of MHD waves trapped in the plume. In Andries et al. (\cite{andries})
we have illustrated the concept of resonant flow instability of the
trapped modes both in a 1-D slab model and a 1-D cylindrical model for
a coronal plume in which plasma-pressure was neglected. The important
result of that paper was that the threshold values of the velocity
shear are significantly smaller for resonant instability than for
Kelvin-Helmholtz instability to occur. The aim of this paper is to
study the effect of plasma pressure on the eigenmodes of the plume. As
expected we find slow waves in addition to the fast waves. Furthermore
there are two different types of Kelvin-Helmholtz instability. Along
with the fact that now not only Alfvén but also slow resonances can
occur this all leads to a wide variety of ranges of the velocity shear
for which instability can be present. Estimates of these ranges for
different equilibrium quantities can be obtained without going through
the elaborate numerical procedures of calculating the eigenmodes. We
show that the instability that will most probably occur in coronal
plumes is due to an Alfvén resonance of slow body modes. These
instabilities could lead to disruption of the coronal plumes and to
the mixing with interplume plasma. However we point out that there
might be a strong dependence of the resonant flow instability upon
the velocity profile that is to be investigated further.
---------------------------------------------------------
Title: On the nature of network oscillations
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.;
Fleck, B.
2001ESASP.464..175B Altcode: 2001soho...10..175B
We examine time-series of spectral data obtained from the Solar
Ultraviolet Measurements of Emitted Radiation instrument (SUMER), on
board SOHO in the period 10-31 July 1996. Observations were obtained
in lines, ranging in temperature from 12,000 K to 10<SUP>6</SUP>K,
covering the low chromosphere to the corona. In this short contribution
we report on the time series analysis on one of these dataset, using
wavelet methods, of small individual network regions in the quiet
Sun. The wavelet analysis allows us to derive the duration as well
as the periods of the oscillations. The statistical significance of
the oscillations was estimated by using a randomisation method. The
oscillations are considered to be due to waves, which are produced
in short bursts with coherence times of about 10-20 minutes. The
low chromospheric and transition region lines show intensity and
velocity power in the 2.4 mHz range. The observed 2-4 mHz network
oscillations can be interpreted in terms of kink and sausage waves
propagating upwards along thin magnetic flux tubes. The kink waves can
be generated by random foot-point motions, e.g. by exploding granules,
at the photospheric level. As they propagate within flux tubes, their
amplitude grows exponentially with height and becomes non-linear. The
waves can thereby undergo a mode transformation and become sausage
type waves, which are more easily detected on the disk.
---------------------------------------------------------
Title: Nonlinear Damping of Fast Waves and Plasma Heating in the
Solar Corona
Authors: Voitenko, Y.; Goossens, M.
2001IAUS..203..517V Altcode:
Fast waves can be excited in the corona by compressional perturbations
of magnetic field lines which are anchored into the dense convective
zone and displaced by the plasma motions there. The consequent linear
dissipation of fast waves in the resonant layers can contribute to
coronal heating. A difficulty of this dissipation mechanism is that
the setup time of the linear resonance (the time required for the
creation of sufficiently short length-scales) is long compared to the
sub-minute variations in the coronal heating process. This suggests a
more effective mechanism for the structurization of waves in the solar
corona. We propose a new, nonlinear mechanism for the dissipation
of fast waves in the corona. In the framework of two-fluid MHD we
show that fast waves are nonlinearly coupled to the kinetic Alfvén
waves - Alfvén waves with short wavelengths across B<SUB>0</SUB>,
background magnetic field. The nonlinear coupling is effective for the
amplitudes of the launched fast waves in the range 0.01 to 0.03 for
B/B<SUB>0</SUB> (B is wave magnetic field), implied by spectroscopic
observations. As the excited AWs have very short wavelengths, they
are damped almost immediatelly by the linear kinetic or collisional
dissipation. Therefore, the resulting plasma heating has the overall
timescale of the order of the characteristic time of nonlinear
interaction, which can easily be in the sub-minute range.
---------------------------------------------------------
Title: Long Period Oscillations in Polar Plumes as observed by CDS
on SoHO
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.
2001IAUS..203..244B Altcode:
We examine spectral time series of coronal line Mg IX 368 Å and
transition region line O V 629 Å, observed with the Coronal Diagnostic
Spectrometer (CDS) onboard the SOHO spacecraft. Primarily we were
looking for intensity and velocity oscillations in polar plumes,
however by chance we detected a giant macro-spicule at the limb and
were able to follow its dynamical structure. Blue and red-shifted
emission in the O V line indicates that it is probably a rotating
twisted magnetic jet. Emission is also detected in the Mg IX 368
Å line, at a temperature of 1 million K. Both Fourier and wavelet
transforms have been applied independently to the analysis of the
oscillations in order to find the most reliable periods. We report
here on the existence of long period oscillations in the polar plumes
as observed in the O V 629 Å line. Our observations indicate the
presence of compressional waves with periods of 20-25 minutes.
---------------------------------------------------------
Title: Influence of equilibrium flows and the atmospheric magnetic
field on solar oscillation modes
Authors: Pintér, B.; Erdélyi, R.; New, R.; Goossens, M.
2001ESASP.464..227P Altcode: 2001soho...10..227P
The competing effects of an atmospheric magnetic field and an
equilibrium flow in the internal regions of the Sun are studied on
the helioseismic f- and p-modes. The Sun is modeled as a multi-layered
plasma, where the upper parts, representing the chromosphere and corona,
are embedded in a unidirectional though inhomogeneous magnetic field,
meanwhile the lower part, representing the sub-photospheric polytropic
region, is in a steady equilibrium state. The steady state sub-surface
region can be considered as a first approximation of dynamic motions
(e.g., convective motion, differential rotation, sub-surface flows,
meridional flows, etc.). The obtained frequency shifts of the different
eigenmodes are associated with flow and magnetic effects. We also found
damping of the eigenfrequencies which apparently can be associated with
the universal mechanism of resonant absorption. Resonant absorption
(already known as a viable heating mechanism in the solar corona)
is present due to inhomogeneities in the atmosphere which give rise
to Alfvén and slow continua. Damping of helioseismic modes occurs
when the modes are coupled into these continua. When both atmospheric
magnetic field and sub-surface flows are present, a complex picture of
competition of these two effects is found. The theoretically predicted
frequency shifts in a steady state are in excellent agreement with the
observed values. For related works see also the papers by Erdélyi &
Taroyan and Varga & Erdélyi in the present Volume.
---------------------------------------------------------
Title: Alfvén waves in space plasmas: dispersive and kinetic effects
Authors: Voitenko, Yu.; Goossens, M.; Yukhimuk, A.; Voytsekhovskaia, A.
2001KosNT...7S..67V Altcode:
New linear and nonlinear properties of Alfvén waves induced by
finite-ion-Larmor radius effects in space plasmas are discussed.
---------------------------------------------------------
Title: Nonlinear coupling of O- and X-mode radio emission and Alfven
waves in the solar corona
Authors: Sirenko, O.; Voitenko, Yu.; Goossens, M.; Yukhimuk, A.
2000AIPC..537..287S Altcode: 2000wdss.conf..287S
The nonlinear coupling of extraordinary and ordinary waves via
kinetic Alfven waves (KAWs) is investigated on the basis of two fluid
magnetohydrodynamics. The equation governing the time dependence
of electric field of excited O-mode is found. We estimate the time
of effective coupling between modes and corresponding interaction
distance in solar corona. Our theoretical results show that the X-
and O-mode couplings via Alfven waves can be efficient depolarization
mechanism for the coronal radioemission. .
---------------------------------------------------------
Title: Resonant absorption in randomly driven coronal loops
Authors: de Groof, Anik; Goossens, Marcel
2000AIPC..537..208D Altcode: 2000wdss.conf..208D
De Groof et al. '98 [1] and '00 [2] studied the time evolution of fast
magnetosonic and Alfvén waves in a coronal loop driven by radially
polarized footpoint motions in linear ideal MHD. Footpoint driving
seems to be an efficient way of generating resonant absorption since the
input energy is mainly stored in body modes which keep the energy in the
loop. The most important feature in this study is the stochastic driving
of the loop. While in earlier models with a periodic driver or a single
pulse, the loop is only heated at one single layer, we now find multiple
resonance layers which results in a more globally heated loop. Moreover,
these resonances (created on a realistic time scale) have length scales
which are small enough to explain energy dissipation. An important
aspect to take into account is the mass transfer between corona and
chromosphere since the density becomes time dependent and consequently,
the resonant surfaces shift throughout the loop [3]. Combined with
the multiple resonances we found in the previous study, this result
can lead to the globally heated coronal loops we observe. .
---------------------------------------------------------
Title: Linear dynamics of the solar convection zone: Excitation of
waves in unstably stratified shear flows
Authors: Chagelishvili, G. D.; Tevzadze, A. G.; Goossens, M.
2000AIPC..537..200C Altcode: 2000astro.ph..6132C; 2000wdss.conf..200C
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. .
---------------------------------------------------------
Title: Long period oscillations in the polar plumes
Authors: Banerjee, D.; O'Shea, E.; Doyle, J. G.; Goossens, M.
2000AIPC..537..160B Altcode: 2000wdss.conf..160B
We examine spectral time series of the transition region line O v
629 Å, observed with the Coronal Diagnostic Spectrometer (CDS) on
the SoHO spacecraft. Both Fourier and wavelet transforms have been
applied independently to the analysis of plume oscillations in order
to find the most reliable periods. The wavelet analysis enables us to
derive the duration as well as the periods of the oscillations. Our
observations indicate the presence of compressional waves with periods
of 10-20 minutes. We have also detected a 10+/-2 minute periodicity
in the network regions of the north polar coronal hole. The waves
are produced in short bursts with coherence times of about 20-30
minutes. We interpret these oscillations as outward propagating slow
magneto-acoustic waves, which may contribute significantly to the
heating of the lower corona by compressive dissipation and which may
also provide enough energy flux for the acceleration of the fast solar
wind. The data support the idea that the same driver is responsible
for the network and plume oscillations with the network providing the
magnetic channel through which the waves propagate upwards from the
lower atmosphere to the plumes. .
---------------------------------------------------------
Title: Note on dispersive Alfven waves in the solar corona
Authors: Voitenko, Yu.; Goossens, M.; Shukla, P.; Stenflo, L.
2000AIPC..537..303V Altcode: 2000wdss.conf..303V
The dispersive (kinetic) Alfvén waves in the ion-cyclotron frequency
regime (ICKAWs), with frequencies close to the ion cyclotron frequency
and with wavelengths close to the ion gyroradius, are studied in
view of their ability to interact strongly with the plasma of solar
corona. The combined action of finite ion cyclotron frequency and
finite ion gyroradius effects has a profound influence on the Alfvén
wave. The phase velocity of ICKAWs along the background magnetic field
can be slower than the Alfvén velocity (due to finite ion cyclotron
frequency effects), but also faster than the Alfvén velocity (due
to finite ion gyroradius effects). The phase velocity depends on
both the perpendicular and parallel wavenumbers, and spans a range of
velocities from sub-Alfvénic to super-Alfvénic. Since these waves have
a field-aligned component of electric field, they undergo Cherenkov
resonant interaction and can be excited by the ion beam component set
up in the low corona by the reconnection outflows from chromospheric
reconnection events. .
---------------------------------------------------------
Title: Linear and nonlinear waves in dilute plasmas
Authors: Ballai, István; Erdélyi, Róbert; Goossens, Marcel
2000AIPC..537..152B Altcode: 2000wdss.conf..152B
Small-amplitude magnetohydrodynamic (MHD) waves are studied in a dilute
collisionless plasma with an anisotropic pressure distribution. The
parallel and perpendicular pressure are defined with the aid of
two polytropic pressure laws. For specific values of the polytropic
indices, previous results obtained with the usual Chew-Goldberger-Low
(CGL) double-adiabatic (i) and double-isothermal (ii) models are
recovered. The double-polytropic model can be considered as the
counterpart of the single-polytropic model. Dispersion relations for
the linear waves are derived and analyzed in the presence of pressure
anisotropy. The weakly nonlinear dynamics is shown to be governed by
the Benjamin-Ono equation. The results are discussed in the CGL and
double-isothermal limits. .
---------------------------------------------------------
Title: Absorption of fast magnetosonic waves in the solar atmosphere
in the limit of weak nonlinearity
Authors: Ballai, István; Erdélyi, Róbert; Goossens, Marcel
2000AIPC..537..144B Altcode: 2000wdss.conf..144B
We study the resonant absorption of fast magnetoacoustic (FMA) waves in
inhomogeneous weakly dissipative, isotropic and anisotropic plasmas. The
equilibrium states on which the waves are superimposed is static
or stationary and are assumed to be in a 1-D planar geometry. The
equilibrium model consists of three layers with an inhomogeneous
magnetized plasma surrounded by two homogeneous magnetized semi-infinite
plasmas. The propagating FMA waves are partly absorbed and partly
reflected by coupling to local nonlinear slow magnetohydrodynamic
(MHD) waves in the inhomogeneous layer. The dissipation acts only in a
narrow layer called the dissipative layer which embraces the resonant
magnetic surface. In linear theory it has been shown that in the
vicinity of the resonant surface the energy density, the amplitudes
of waves and the spatial gradients become large, suggesting that in
this region nonlinearity might be important. The wave motions far from
the dissipative layer are described by the linear, ideal MHD equations,
while inside this layer the wave motions are governed by the full system
of the dissipative, nonlinear MHD equations. The coefficient of wave
energy resonant absorption is derived assuming weak nonlinearity and
long-wavelength approximation. .
---------------------------------------------------------
Title: Alternating current generation in flux tubes by pressure
fluctuations
Authors: Zhugzhda, Yuzef; Goossens, Marcel
2000AIPC..537..317Z Altcode: 2000wdss.conf..317Z
Alternating current in flux tubes is one of the possible contributors
to the heating of the solar corona. The mechanism of the generation of
alternating currents in flux tubes by turbulent pressure fluctuations
in convective zone is proposed. The thin-flux-tube approximation shows,
that the linear and weakly nonlinear torsional waves in twisted flux
tube, are accompanied by tube cross section fluctuations. Consequently,
external pressure fluctuations generate not only slow MHD waves but
torsional waves as well. The torsional waves are nothing else than
alternating current. .
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Title: Kelvin-Helmholtz instability and resonant flow instability
in a 1-dimensional coronal plume model
Authors: Andries, J.; Goossens, M.
2000AIPC..537..136A Altcode: 2000wdss.conf..136A
In a previous paper we have illustrated the concept of resonant flow
instability of the trapped modes both in a 1-D slab model and a 1-D
cylindrical model for a coronal plume in a cold plasma. We found that
much larger values of the velocity shear are needed for Kelvin-Helmholtz
than for resonant instability to occur. The aim of this paper is to
study the effect of a non-zero plasma pressure on the eigenmodes of
the plume structure. We show that the instability most probably to
occur in coronal plumes is due to the resonant coupling of slow body
modes to local resonant Alfvén waves. These instabilities could lead
to disruption of the coronal plumes and to the mixing with interplume
plasma. .
---------------------------------------------------------
Title: Kinetic Alfvén turbulence and non-thermal broadening in
solar flares
Authors: Voitenko, Yu.; Goossens, M.
2000KFNTS...3..437V Altcode:
Recent observations of soft X-ray nonthermal line broadening and hard
X-ray emission in solar flares suggest that the nonthermal broadening
is due to plasma turbulence created in the energy release. We develop
a model of the impulsive excitation of Alfvén wave turbulence and
propose the mechanism for the interruption of spectral energy fluxes in
flaring loops. This interruption determines the nonthermal broadening
in flares and is important for the selective heating of plasma species
and for the acceleration of particles to high energies.
---------------------------------------------------------
Title: Frequency dependence of resonant absorption and over-reflection
of magnetosonic waves in nonuniform structures with shear mass flow
Authors: Csík, Á. T.; Čadež, V. M.; Goossens, M.
2000A&A...358.1090C Altcode:
Processes of resonant absorption and over-reflection of magnetosonic
modes may occur in nonuniform plasmas with a shear mass flow along the
magnetic field. In this work, we study how these two phenomena depend on
the frequency omega of waves propagating in a given direction prescribed
by polar angles theta and phi of wave vector vec k. Plasma is assumed
nonuniform within a transitional layer that separates two semi-infinite
uniform regions. The considered wave propagates only in one of the
two regions and is reflected from the nonuniform transitional layer
either totally or partially depending on the efficiency of resonant
processes. The resonant absorption and over-reflection are strongly
dependent on wave frequency for both the slow and fast magnetosonic
mode as shown in corresponding figures. Under conditions relevant to
the solar atmosphere, we found that typical MHD waves with periods
from few seconds up to much larger values of several minutes and more,
can be resonantly absorbed and also over-reflected. This important fact
has to be taken into account in making models of energy transports in
the corona.
---------------------------------------------------------
Title: Competition of damping mechanisms for the phase-mixed Alfvén
waves in the solar corona
Authors: Voitenko, Y.; Goossens, M.
2000A&A...357.1086V Altcode:
The competition of the linear and nonlinear damping mechanisms for
phase-mixed Alfvén waves in the solar corona is studied. It is shown
that the nonlinear damping of the phase-mixed Alfvén waves due to
their parametric decay is stronger than both collisional and Landau
damping for waves with frequencies below a critical frequency which
depends on the wave amplitude. This critical frequency is close to
the cyclotron frequency ( ~ 10<SUP>5</SUP> s<SUP>-1</SUP> in holes)
even for small wave amplitudes of the order of 1% of the background
value for the magnetic field. This means that the dissipation of the
Alfvén wave flux in the corona can be significantly affected by the
nonlinear wave dynamics. Nonlinear decay of the low-frequency Alfvén
waves transmits a part of the wave energy from the length-scales
created by phase mixing to smaller scales, where the waves damp more
strongly. However, the direction of the effect can be reversed in
the high-frequency domain, 10 s<SUP>-1</SUP><allowbreak omega
<allowbreak 10<SUP>4</SUP> s<SUP>-1</SUP>, where the decay into
counterstreaming waves is strongest, because the wave energy is
quickly transferred to larger scales, where the actual dissipation is
reduced. These effects are introduced by the vector nonlinearity which
involves waves propagating in the different directions across magnetic
field. The effects introduced by the scalar nonlinearity may also become
important in phase mixing (Voitenko & Goossens, in preparation).
---------------------------------------------------------
Title: Nonlinear decay of phase-mixed Alfvén waves in the solar
corona
Authors: Voitenko, Yu.; Goossens, M.
2000A&A...357.1073V Altcode:
This paper presents an analytical investigation of the nonlinear
interaction of phase-mixed Alfvén waves in the framework of two-fluid
magnetohydrodynamics. It focuses on the parametric decay of the
phase-mixed pump Alfvén wave into two daughter Alfvén waves. This
parametric decay is a nonlinear phenomenon which does not occur in ideal
MHD since it is induced by the combined action of finite wave amplitude
and the non-zero gyroradius of the ions. In contrast to intuitive
expectation, the effects of the non-zero gyroradius of the ions are
already important at length scales that are substantially longer than
the ion gyroradius. The parametric decay occurs for relatively small
wave amplitude and is more efficient than collisional damping.
---------------------------------------------------------
Title: Randomly driven fast waves in coronal loops. II. with coupling
to Alfvén waves
Authors: De Groof, A.; Goossens, M.
2000A&A...356..724D Altcode:
We study the time evolution of fast magnetosonic and Alfvén waves in a
coronal loop driven by random footpoint motions. The footpoint motions
are assumed to be polarized normal to the magnetic flux surfaces in
linear ideal MHD. De Groof et al. (1998) (Paper I) showed that the
input energy is mainly stored in the body modes when the fast waves
are decoupled from the Alfvén waves. Hence driving at the loop's feet
forms a good basis for resonant absorption as heating mechanism. In
order to determine the efficiency of resonant absorption, we therefore
study the energy transfer from the body modes to the resonant Alfvén
waves in the case of coupling. We find that the growth of Alfvén mode
energy depends on several parameters. Subsequently we check whether
the necessary small lengthscales are created on a realistic time scale
for the coronal loop. We find that Alfvén resonances are built up
at the magnetic surfaces, where local Alfvén frequencies equal the
quasi-modes frequencies, on time scales comparable to the lifetime
of the loop. Finally we conclude that a random footpoint driving can
produce enough resonances to give rise to a globally heated coronal
loop.
---------------------------------------------------------
Title: Modified Kelvin-Helmholtz Instabilities and Resonant Flow
Instabilities in a One-dimensional Coronal Plume Model: Results for
Plasma β=0
Authors: Andries, J.; Tirry, W. J.; Goossens, M.
2000ApJ...531..561A Altcode:
In this manuscript we study the effect of the velocity shear in the mass
flow between the coronal plume structure and the interplume region on
the spectrum of magnetohydrodynamic (MHD) waves trapped in the plume
structure. To illustrate the concept of resonant flow instability of
the trapped modes, we consider both a one-dimensional slab model and
a one-dimensional cylindrical model for a coronal plume. A nonuniform
intermediate region between the plume and the interplume region is taken
into account so that the waves can be subject to resonant absorption. We
show how the resonance can lead to instability of the trapped modes and
that this resonant instability, which is physically distinct from the
nonresonant Kelvin-Helmholtz instability, occurs for velocity shears
significantly below the Kelvin-Helmholtz threshold. These resonant flow
instabilities could lead to disruption of the coronal plumes and mixing
with interplume plasma. The dependence of the critical velocity shear
for which the resonant instability occurs on the difference between
plume and interplume density is investigated. Our results seem to
suggest that resonant flow instability in a pressureless plasma will
only appear for rather high density contrasts. However, it is clearly
shown that the velocity shear needed for Kelvin-Helmholtz instability
to occur is too high for all density contrasts. Hence, Kelvin-Helmholtz
instability will not be operative in pressureless coronal plumes.
---------------------------------------------------------
Title: Slow surface wave damping in plasmas with anisotropic viscosity
and thermal conductivity
Authors: Ruderman, M. S.; Oliver, R.; Erdélyi, R.; Ballester, J. L.;
Goossens, M.
2000A&A...354..261R Altcode:
This paper studies the damping of slow surface MHD waves propagating
along the equilibrium magnetic field on a finite-thickness magnetic
interface. The plasma is assumed to be strongly magnetised, and the
full Braginskii's expressions for viscosity and the heat flux are
used. The primary focus of the paper is on the competition between
resonant absorption in the thin dissipative layer embracing the ideal
resonant position and the bulk wave damping due to viscosity and thermal
conductivity as damping mechanisms for surface MHD waves. The dependence
of the wave damping decrement on the wave length and the dissipative
coefficients is studied. Application of the obtained results to the
surface MHD wave damping in the solar chromosphere is discussed.
---------------------------------------------------------
Title: Waves in Dusty, Solar, and Space Plasmas
Authors: Verheest, F.; Goossens, M.; Hellberg, M. A.; Bharuthram, R.
2000AIPC..537.....V Altcode: 2000wdss.conf.....V
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear Evolution and Interaction of Phase-Mixed Alfvén
Waves in Solar Corona
Authors: Voitenko, Yu. M.; Goossens, M.
1999ESASP.448..409V Altcode: 1999mfsp.conf..409V; 1999ESPM....9..409V
No abstract at ADS
---------------------------------------------------------
Title: Randomly Driven Fast Waves in Coronal Loops
Authors: de Groof, A.; Goossens, M.
1999ESASP.448..251D Altcode: 1999ESPM....9..251D; 1999mfsp.conf..251D
No abstract at ADS
---------------------------------------------------------
Title: Influence of a Chromospheric Magnetic Field on Solar Acoustic
Modes
Authors: Vanlommel, P.; Goossens, M.
1999ESASP.448..399V Altcode: 1999ESPM....9..399V; 1999mfsp.conf..399V
No abstract at ADS
---------------------------------------------------------
Title: Turbulent Dynamics of Kinetic Alfvén Waves in a Transient
Flare Loop
Authors: Voitenko, Yu. M.; Goossens, M.
1999ESASP.448..735V Altcode: 1999mfsp.conf..735V; 1999ESPM....9..735V
No abstract at ADS
---------------------------------------------------------
Title: Generation of Oscillations in the Solar Convection Zone:
Linear Mechanism of Mode Conversion in Shear Flows
Authors: Chagelishvili, G. D.; Tevzadze, A. G.; Goossens, M.
1999ESASP.448...75C Altcode: 1999ESPM....9...75C; 1999mfsp.conf...75C
No abstract at ADS
---------------------------------------------------------
Title: Oscillations in a magnetic solar model. I. Parallel propagation
in a chromospheric and coronal magnetic field with constant Alfvén
speed
Authors: Pintér, Balázs; Goossens, Marcel
1999A&A...347..321P Altcode:
Oscillation eigenmodes are studied for a planar solar model with a
non-uniform horizontal magnetic field in the atmosphere. The three
layer atmospheric model is the same as in \cite{t2}. The analysis
in that paper is extended to a wide range of parameters. Different
types of oscillation modes are determined for a wide range of the
magnetic field strength and for different degrees of the spherical
harmonic. The emphasis is on the possible coupling of global solar
oscillation modes to localized continuum eigenmodes of the magnetic
atmosphere. For propagation parallel to the magnetic field, the
global oscillation modes can couple only to slow continuum modes and
this is found to occur for a rather large range of parameters. In
addition to the damping of global oscillation modes due to resonant
absorption it was found that the interaction of global eigenmodes
with slow continuum modes leads to unanticipated behaviour of global
eigenmodes. The rather strange behaviour in the slow continuum involves
the disappearance and appearance of global modes and splitting and
merging of global modes. Additionally, frequency shifts of global modes
due to the magnetic field have been examined. The shifts are compared
to observations.
---------------------------------------------------------
Title: Influence of a chromospheric magnetic field on solar acoustic
modes
Authors: Vanlommel, P.; Goossens, M.
1999SoPh..187..357V Altcode:
This paper studies the effect of a magnetic atmosphere on the global
solar acoustic oscillations in a simple Cartesian model. First, the
influence of the ratio of the coronal and the photospheric temperature
τ and the strength of the magnetic field at the base of the corona
B<SUB>c</SUB> on the oscillation modes is studied for a convection
zone-corona model with a true discontinuity. The ratio τ seems to be
an important parameter. Subsequently, the discontinuity is replaced by
an intermediate chromospheric layer of thickness L and the effect of
the thickness on the frequencies of the acoustic waves is studied. In
addition, nonuniformity in the magnetic field, plasma density and
temperature in the transition layer gives rise to continuous Alfvén
and slow spectra. Modes with characteristic frequencies lying within
the range of the continuum may resonantly couple to Alfvén and/or
slow waves.
---------------------------------------------------------
Title: Leaky and resonantly damped flux tube modes reconsidered
Authors: Stenuit, H.; Tirry, W. J.; Keppens, R.; Goossens, M.
1999A&A...342..863S Altcode:
In this research note the results for the eigenfrequencies of the
uniform and non-uniform magnetic flux tubes of Stenuit et al. (1998)
are reconsidered. In that paper it is shown that the eigenfrequencies
may have a damping rate due to two mechanisms causing a loss of
energy. In non-uniform flux tubes the eigenmodes can be damped by
resonant absorption. The other mechanism is leakage of wave energy into
the surroundings, which can occur for both uniform and non-uniform
flux tubes. We point out that the dispersion relations obtained by
Stenuit et al. are correct for leaky and undamped non-leaky modes,
but are not correct for resonantly damped non-leaky modes.
---------------------------------------------------------
Title: Effects of mass flow on resonant absorption and on
over-reflection of magnetosonic waves in low begin {equation}ta
solar plasmas
Authors: Csik, A. T.; Cadez, V. M.; Goossens, M.
1998A&A...339..215C Altcode:
The influence of a stationary mass flow on driven resonant MHD waves is
studied for incoming slow and fast magnetosonic waves with frequencies
in the slow and the Alfven continua. In addition to the classic resonant
absorption already present in a static plasma, driven resonant waves
can also undergo over-reflection. Depending on the strength of the
equilibrium flow a variety of resonant MHD wave phenomena comes into
existence. The resonant absorption and over-reflection are found
for both, slow and fast magnetosonic waves. The main result of this
paper is that even relatively slow flows can have a drastic effect
on the behaviour of MHD waves. This is in particular true under solar
conditions.
---------------------------------------------------------
Title: Resonant Damping of Solar p-Modes by the Chromospheric
Magnetic Field
Authors: Tirry, W. J.; Goossens, M.; Pintér, B.; Čadež, V.;
Vanlommel, P.
1998ApJ...503..422T Altcode:
In this paper, we demonstrate how solar p-mode energy can be
resonantly absorbed by coupling to localized Alfvén waves in the
chromosphere. Nonuniformity in the magnetic field, plasma density, and
temperature in the solar atmosphere give rise to a continuous spectrum
of resonant frequencies. P-modes with characteristic frequencies
within the range of the continuous spectrum may resonantly couple to
localized Alfvén and slow magnetohydrodynamic (MHD) waves, and hence
heat the chromospheric plasma. In dissipative MHD, these p-modes are
recovered as eigenmodes with a damping rate that becomes independent
of the dissipation mechanism in the limit of vanishing dissipation. An
analytical solution of these p-modes is found in the dissipative layer
embracing the resonant magnetic surface. Using the analytical solution
to cross the quasi-singular dissipative layer, the required numerical
effort is limited to the integration of the ideal MHD equations away
from any singularity. This results in a powerful tool for investigating
in a mathematically consistent way the damping by resonant absorption
and the frequency shifts of the solar oscillations arising from the
presence of an overlying magnetic atmosphere in combination with
the resonant absorption process. The outcome is that the mechanism
of resonant absorption might be responsible for the damping of solar
oscillations and should be taken into account in producing a definite
solar model reproducing the observed frequencies of the global solar
oscillations to within the limit of observational uncertainties.
---------------------------------------------------------
Title: Random driven fast waves in coronal loops. I. Without coupling
to Alfven waves
Authors: de Groof, A.; Tirry, W. J.; Goossens, M.
1998A&A...335..329D Altcode:
In this paper we study the time evolution of fast MHD waves in a coronal
loop driven by footpoint motions in linear ideal MHD. We restrict the
analysis to footpoint motions polarized normal to the magnetic flux
surfaces such that the fast waves are driven directly. By supposing
the azimuthal wave number k_y to be zero, the fast waves are decoupled
from the Alfven waves. As a first step to real stochastic driving,
we consider the loop to be driven by a train of identical pulses
with random time intervals in between. The solution is written as a
superposition of eigenmodes whose excitation is determined by the
time dependence of the footpoint motion through a convolution and
by the spatial dependence of the footpoint motion through a scalar
product. An important result from the simulations is that the amount
of kinetic energy associated with the body modes is much larger than
the amount corresponding to the leaky modes. This means that most of
the input energy is stored within the loop. For k_y!=q 0, body modes
can resonantly couple to Alfven waves at certain magnetic surfaces
and hence the energy of the body modes can then be dissipated around
the resonant magnetic surfaces. Using a gamma distribution for the
time intervals between the successive pulses, we analytically derive
a relation between the mean value of the kinetic energy contribution
of each eigenmode, the eigenfrequency, the number of pulses and the
width of the pulses. The larger the variance of the distribution, the
less the power spectrum reveals fine structure, peaks around certain
preferred frequencies. The analytical results confirm the output from
the numerical simulations.
---------------------------------------------------------
Title: MHD eigenmodes in a semi-infinite structured solar atmosphere
Authors: Pinter, B.; Cadez, V. M.; Goossens, M.
1998A&A...332..775P Altcode:
Linear eigenmodes are determined for the solar atmosphere in the
presence of a gravitational field and a magnetic field. The atmosphere
consists of a horizontal nonuniform chromosphere and a semi-infinite
nonuniform corona. It is bounded from below by a heavy and immovable
photosphere. Inhomogeneity is confined to the vertical direction. The
gravitational acceleration is constant and the equilibrium magnetic
field is horizontal. The equilibrium temperature increases linearly
with height in the chromosphere and is constant in the corona. The
equilibrium magnetic field is constant in the chromosphere and it
decreases with height in the corona in such a way that the Alfven
speed is constant there. The results show that there are two sets
of eigenmodes which can be viewed as p- and g-modes modified by the
immobile lower boundary of the atmosphere and by the magnetic field. The
modes can become damped quasi-modes with complex frequencies arising
from the resonant coupling of eigenmodes to the local MHD continua in
the nonuniform chromospheric layer.
---------------------------------------------------------
Title: Resonant flow instability of MHD surface waves
Authors: Tirry, W. J.; Cadez, V. M.; Erdelyi, R.; Goossens, M.
1998A&A...332..786T Altcode:
We study the effect of velocity shear on the spectrum of MHD surface
waves. A nonuniform intermediate region is taken into account,
so that the surface wave can be subject to resonant absorption. In
order to deal in a mathematically and also physically consistent
manner with the resonant wave excitation, we analytically derive
the dissipative solution around the resonant surface in resistive
MHD. Using these analytical solutions in our eigenvalue code, the
effect of the velocity shear on the damping rate of the surface
wave can easily be investigated with limited numerical effort. The
presence of the flow can both increase and decrease the efficiency
of resonant absorption. We also show how the resonance can lead to
instability of the global surface mode for a certain range of values
for the velocity shear. The resonant flow instabilities, which are
physically distinct from the nonresonant Kelvin-Helmholz instabilities
can occur for velocity shears significantly below the Kelvin-Helmholz
threshold. Although resonant absorption as dissipation mechanism is
present, the amplitude of the surface mode grows in time. The resonant
flow instability can be explained in terms of negative energy waves :
to get an unstable negative energy wave, some dissipative process is
required to ensure energy dissipation.
---------------------------------------------------------
Title: Eigenfrequencies and optimal driving frequencies of 1D
non-uniform magnetic flux tubes
Authors: Stenuit, H.; Keppens, R.; Goossens, M.
1998A&A...331..392S Altcode:
The eigenfrequencies and the optimal driving frequencies for flux tubes
embedded in uniform but wave-carrying surroundings are calculated,
based on matching conditions formulated in terms of the normal acoustic
impedances at the flux tube boundary. The requirement of the equality
of the normal acoustic impedance of the transmitted wave field with
the normal acoustic impedance of the outgoing wave field selects the
eigenmodes, while the equality of the ingoing and the transmitted normal
acoustic impedance selects the optimal driving frequencies (Keppens
1996). Even if the flux tube is uniform, the eigenfrequencies can be
complex due to leakage of wave energy into the surroundings. The case
of uniform flux tubes has been considered previously (e.g. Cally 1986),
and serves as a testcase of our formalism. We extend Cally's results
by taking a radial stratification of the flux tube into account. The
non-uniformity of the flux tube can introduce another cause for energy
loss, namely resonant absorption internal to the flux tube. When
resonant absorption occurs, we must incorporate the appropriate jump
conditions over the dissipative layer(s). This can be done using a
simple numerical scheme as introduced by Stenuit et al. (1995).
---------------------------------------------------------
Title: Numerical simulation of twisted solar corona
Authors: Parhi, S.; Goossens, M.; Lakhina, G. S.
1998IAUS..185..467P Altcode:
The solar corona, modelled by a low beta, resistive plasma slab sustains
MHD wave propagations due to footpoint motions in the photosphere. The
profiles for density, magnetic field and driver are considered to be
steep. The numerical simulation presents the evolution of MHD waves. The
resonance layer is clearly observed at the slab edges. Dissipation
takes place around the resonance layer where the perturbation develops
large gradients. The width of the resonance layer is calculated. It is
observed that the thickness of the Alfven resonance layer is more that
that of the slow wave resonance layer. Effort is made to distinguish
between slow and Alfven wave resonance layers. Fast waves develop kink
instability. As plasma evolves the current sheets which provide the
heating at the edges gets distorted and fragment into two current sheets
at each edge which, in turn come closer when the twist is enhanced.
---------------------------------------------------------
Title: Resonant Alfven waves in coronal arcades driven by footpoint
motions
Authors: Ruderman, M. S.; Goossens, M.; Ballester, J. L.; Oliver, R.
1997A&A...328..361R Altcode:
X-ray spectroscopy performed from different astronomical spacecrafts
has shown that the solar corona is structured by magnetic fields
having the shape of loops and arcades. These structures are formed
by stretching and reconnection of magnetic fields, and remain stable
from days to weeks. Also, sporadic or periodic brightenings of such
structures have been detected in UV and soft X-ray observations,
suggesting the existence of propagating waves and plasma heating
within them. In this paper, a mechanism for the deposition of Alfven
wave energy and heating of coronal arcades via resonant absorption is
investigated. An analytical solution to the linear viscous, resistive
MHD equations that describes the steady state of resonant shear Alfven
oscillations in coronal arcades driven by toroidal footpoint motions is
obtained. General expressions for the total amount of dissipated wave
energy and for its spatial distribution within the resonant magnetic
surface is derived.
---------------------------------------------------------
Title: Absorption of magnetosonic waves in presence of resonant slow
waves in the solar atmosphere.
Authors: Cadez, V. M.; Csik, A.; Erdelyi, R.; Goossens, M.
1997A&A...326.1241C Altcode:
The resonant absorption of slow and fast magnetosonic waves in a
nonuniform magnetic plasma is studied for a simple planar equilibrium
model. Propagating slow and fast magnetosonic waves are launched
upwards in a lower uniform layer. They are partially absorbed by
coupling to local resonant waves in an overlying nonuniform plasma
layer at the magnetic surface where the frequency of the incoming wave
equals the local Alfven continuum frequency or the local slow continuum
frequency. The slow magnetosonic waves can only be coupled to resonant
slow continuum waves. For the fast magnetosonic waves there are three
possibilities as they can be coupled to resonant Alfven continuum waves
alone, resonant Alfven continuum waves combined with resonant slow
continuum waves, and resonant slow continuum waves alone. The present
paper focuses on the absorption of magnetosonic waves by coupling
to resonant slow continuum waves either alone or in combination with
resonant Alfven continuum waves. The results show that the resonant
absorption of slow and fast magnetosonic waves at the slow resonance
position strongly depends on the characteristics of the equilibrium
model and of the driving wave. The absorption can produce efficient
local heating of plasma under conditions as in the solar atmosphere.
---------------------------------------------------------
Title: MHD surface type quasi-modes of a current sheet model.
Authors: Tirry, W. J.; Cadez, V. M.; Goossens, M.
1997A&A...324.1170T Altcode:
Resonantly damped surface type quasi-modes are computed as eigenmodes
of the linear dissipative MHD equations for a simple equilibrium model
of a current sheet. The current sheet is modeled by a nonuniform plasma
layer embedded in a uniform plasma environment. The physical equilibrium
variables change in a continuous way in the nonuniform plasma layer. In
particular, this is the case with both the strength and the orientation
of the equilibrium magnetic field resulting from an electric current in
the nonuniform plasma layer. The equilibrium layer can be viewed as a
model for a reconnection site in the solar atmosphere or for current
sheets in the Earth's magnetosphere. Two surface type eigenmodes
(kink and sausage) are numerically found that can propagate along
the nonuniform plasma layer. The phase speeds of these eigenmodes are
smaller than the Alfven speed in the uniform environment. For oblique
propagation to the equilibrium magnetic field, the eigenmodes resonantly
couple to localized Alfven waves leading to damped quasi-modes. It
is shown that the wave damping is strongly anisotropic and that the
dependence of the relative damping rate on the angle of propagation
is different for the sausage and the kink type eigenmodes
---------------------------------------------------------
Title: Temporal evolution of resonant absorption in coronal
loops. Excitation by footpoint motions normal to the magnetic
surfaces.
Authors: Tirry, W. J.; Berghmans, D.; Goossens, M.
1997A&A...322..329T Altcode:
In this paper we study the temporal evolution of linear MHD waves
excited by footpoint motions using an ideal, pressureless slab model
for coronal loops. We choose the footpoint motions to be polarised
normal to the magnetic flux surfaces such that only fast waves
are driven directly, including the so-called quasi-modes. We have
derived a formal analytical solution as a superposition of eigenmodes
describing the system as a function of time. The corresponding
eigenvalue problem is solved numerically. This enables us to study
the influence of the characteristics of the footpoint motion on the
excitation of the quasi-modes. On the magnetic flux surface where the
frequency of these quasi-modes equals the local Alfven frequency,
wave energy is transferred from the quasi-modes towards Alfven
waves. We investigate the time evolution of this process in which
small scale dissipative features are generated which can be relevant
in the context of coronal heating. Special attention is given to the
question whether this generation of small scale dissipative features
takes place on time scales shorter than typical life times of coronal
loops. Expressing the dissipation time scale as function of the length
scale corresponding to the resonances, an estimate for the time when
dissipation becomes important and when our ideal MHD simulation stops
to be valid, can be derived. For typical dissipation coefficients and
length scales, dissipation becomes important in the resonance layer
in a time comparable to the life time of coronal loops.
---------------------------------------------------------
Title: MHD Study of Coronal Waves: A Numerical Approach
Authors: Parhi, S.; Pandey, B. P.; Goossens, M.; Lakhina, G. S.;
de Bruyne, P.
1997Ap&SS.250..147P Altcode:
The solar corona, modelled by a low β, resistive plasma slab sustains
MHD wave propagations due to footpoint motions in the photosphere. The
density, magnetic profile and driver are considered to be neither very
smooth nor very steep. The numerical simulation presents the evolution
of MHD waves and the formation of current sheet. Steep gradients in
slow wave at the slab edges which are signature of resonance layer
where dissipation takes place are observed. Singularity is removed by
the inclusion of finite resistivity. Dissipation takes place around the
resonance layer where the perturbation develops large gradients. The
width of the resonance layer is calculated. The thickness of the
Alfvén resonance layer is more than that of the slow wave resonance
layer. Attempt is made to distinguish between slow and Alfvén wave
resonance layers. Fast waves develop into kink modes. As plasma evolves
the current sheets which provide the heating at the edges gets distorted
and fragment into two current sheets at each edge which in turn come
closer when the twist is enhanced.
---------------------------------------------------------
Title: Direct excitation of resonant torsional Alfven waves by
footpoint motions.
Authors: Ruderman, M. S.; Berghmans, D.; Goossens, M.; Poedts, S.
1997A&A...320..305R Altcode:
The present paper studies the heating of coronal loops by linear
resonant Alfven waves that are excited by the motions of the
photospheric footpoints of the magnetic field lines. The analysis
is restricted to torsionally polarised footpoint motions in an
axially symmetric system so that only torsional Alfven waves are
excited. For this subclass of footpoint motions, the Alfven and cusp
singularities are absent from the analysis which means that resonant
coupling between global modes of the loop and localised oscillations
is avoided. Instead, the focus is on the resonances due to the finite
extent of the loop in the longitudinal direction: at the radii where
Alfven waves travelling back and forth along the length of the loop are
in phase with the footpoint motions, the oscillations grow unbounded
in ideal MHD. Inclusion of electrical resistivity and viscosity as
dissipation mechanisms prevents singular growth and we can look at the
steady state in which the energy injected at the photospheric part
of the loop is balanced by the energy dissipated at the dissipative
layer around the resonance. In this sense, we show that the direct
excitation of Alfven waves by torsionally polarised footpoint motions
leads to a very efficient heating mechanism for coronal loops, even
without resonant coupling to global modes.
---------------------------------------------------------
Title: MHD wave heating of coronal loops
Authors: Poedts, S.; Tirry, W.; Berghmans, D.; Goossens, M.
1997jena.confE..54P Altcode:
The possibility of heating coronal loops by phase-mixing and resonant
absorption of MHD waves is discussed. The focus is on the efficiency and
time scales of the conversion of the wave energy to heat for typical
coronal loop parameter values. Both the sideways excitation of loops
by incident waves and the footpoint driving by convective motions are
discussed. First, the mechanisms of phase-mixing and resonant absorption
are explained in a simple set-up. Next, linear MHD results on solar
coronal loop applications are reviewed. In sideways excited loops
(by incident waves), `quasi-modes' (or `collective modes') play the
crucial role of energy carrier from the external region {through the
flux surfaces} to the resonant layers. The quasi-modes are required
to obtain a reasonable efficiency unless the resonances are located
in the outer region of the loop. In footpoint driven loops, on the
other hand, resonant Alfven can be excited directly and the efficiency
depends of the polarization of the driving source. Recent results take
the variation of plasma density and magnetic field strength {along the
loop} into account. For typical coronal loop parameters, the MHD wave
heating mechanism turns out to be very efficient, i.e. the coupling
of the loop plasma to the external driver is very good and the time
scales for dissipation are much smaller than the typical life time of
a loop. However, the dynamics in the resonant layers is very nonlinear
in the hot (very well conducting) coronal plasma. Computer simulations
show that the shear flow in these layers can become unstable. It will
be shown that the Kelvin-Helmholtz-like instabilities may destroy
the resonant layers and lead to a turbulent state. Finally, some
observational results and consequences are discussed. This brings
us to the problems of the discrepancy between the observed and the
required power spectrum of MHD waves and the distinction between
different candidate heating mechanisms. Scientific visualization of
the observational consequences of the computer simulated results
may lead to different observable features for different candidate
heating mechanisms and, hence, to the identification of the mechanism
responsible for the heating of the coronal loops.
---------------------------------------------------------
Title: Nonlinear development of MHD waves in coronal loops
Authors: Parhi, S.; Pandey, B. P.; Goossens, M.; Lakhina, G. S.;
de Bruyne, P.
1997AdSpR..19.1891P Altcode:
The solar corona, modelled by a low beta, resistive plasma slab sustains
MHD wave propagations due to footpoint motions in the photosphere. The
numerical simulation presents the evolution of MHD waves and the
formation of current sheets. Steep gradients at the slab edges,
which are signatures of resonance layers are observed. Singularities
are removed by the inclusion of finite resistivity. The fast waves
develop kink modes. As the plasma evolves the current sheets which
provide heating at the edges fragment into two current sheets at each
edge which in turn come closer when the twist is enhanced.
---------------------------------------------------------
Title: Quasi-Modes as Dissipative Magnetohydrodynamic Eigenmodes:
Results for One-dimensional Equilibrium States
Authors: Tirry, W. J.; Goossens, M.
1996ApJ...471..501T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Footpoint-driven Coronal Sausage Wave
Authors: Berghmans, David; de Bruyne, Peter; Goossens, Marcel
1996ApJ...472..398B Altcode:
We study the excitation of MHD waves in a coronal loop as its field
line footpoints are forced to follow the photo spheric convective
motions. By focussing on the specific case of cylindrically symmetric
footpoint motions, the original problem is reduced to one in which
fast waves and Alfvén waves are decoupled. This allows for a full
analytical treatment of the photo spheric excitation of both sausage
waves and of torsional Alfvén waves. Previously, Berghmans & De
Bruyne considered the case of torsional Alfvén waves. In the present
paper we extend that analysis to sausage waves that are excited by
radially polarized footpoint motions (e.g., typical for granules). The
time-dependent solution that we obtain is written as a superposition
of body and leaky eigenmodes whose excitation is easily determined
from the imposed footpoint motion. This provides analytical insight
into the dynamics and energetics of both impulsively and periodically
driven sausage waves. In each case, we explain the time evolution
of the generated waves and discuss typical "signatures" that can be
looked for in numerical simulations and possibly in solar observations.
---------------------------------------------------------
Title: Slow Resonant MHD Waves in One-dimensional Magnetic Plasmas
with Anisotropic Viscosity and Thermal Conductivity
Authors: Ruderman, M. S.; Goossens, M.
1996ApJ...471.1015R Altcode:
Slow resonant MHD waves are studied in a compressible plasma with
strongly anisotropic viscosity and thermal conductivity. It is shown
that anisotropic viscosity and/or thermal conductivity removes the slow
singularity which is present in the linear ideal MHD equations. Simple
analytical solutions to the linear dissipative MHD equations are
obtained which are valid in the dissipative layer and in two overlap
regions to the left and the right of the dissipative layer. Asymptotic
analysis of the dissipative solutions enables us to obtain connection
formulae specifying the variations or jumps of the different wave
quantities across the dissipative layer. These connection formulae
coincide with those obtained previously for plasmas with isotropic
viscosity and finite electrical conductivity. The thickness of the
dissipative layer is inversely proportional to the Reynolds number,
in contrast to the case of isotropic dissipative coefficients, where it
is inversely proportional to the cube root of the Reynolds number. The
behavior of the perturbations in the dissipative layer is described
in terms of elementary functions of complex argument.
---------------------------------------------------------
Title: Trapped waves in open stellar atmospheres.
Authors: Pintér, B.; Čadež, V. M.; Goossens, M.
1996POBeo..54...59P Altcode:
Linear MHD waves whose energy remains localized in an unbounded stellar
atmosphere are considered analytically and numerically. The atmosphere
is modelled by a stratified fully ionized plasma with both the gravity
force and the gradient of the unperturbed fluid quantities aligned along
the vertical z-axis. The temperature is taken to have a linear profile
while the magnetic field is assumed horizontal and homogeneous. The
atmosphere is semi-infinite with a solid boundary at its bottom (the
stellar photosphere). In such a system, the authors are looking for
solutions localized in space and that could be treated as eigenmodes.
---------------------------------------------------------
Title: Dissipative instability of the MHD tangential discontinuity in
magnetized plasmas with anisotropic viscosity and thermal conductivity
Authors: Ruderman, M. S.; Verwichte, E.; Erdélyi, R.; Goossens, M.
1996JPlPh..56..285R Altcode:
The stability of the MHD tangential discontinuity is studied in
compressible plasmas in the presence of anisotropic viscosity and
thermal conductivity. The general dispersion equation is derived,
and solutions to this dispersion equation and stability criteria are
obtained for the limiting cases of incompressible and cold plasmas. In
these two limiting cases the effect of thermal conductivity vanishes,
and the solutions are only influenced by viscosity. The stability
criteria for viscous plasmas are compared with those for ideal plasmas,
where stability is determined by the Kelvin—Helmholtz velocity
V<SUB>KH</SUB> as a threshold for the difference in the equilibrium
velocities. Viscosity turns out to have a destabilizing influence when
the viscosity coefficient takes different values at the two sides of
the discontinuity. Viscosity lowers the threshold velocity V below
the ideal Kelvin—Helmholtz velocity VKH, so that there is a range
of velocities between V and V<SUB>KH</SUB> where the overstability is
of a dissipative nature.
---------------------------------------------------------
Title: Effects of flow on resonant absorption of MHD waves in
viscous MHD.
Authors: Erdelyi, R.; Goossens, M.
1996A&A...313..664E Altcode:
The effect of an equilibrium flow on resonant absorption of linear MHD
waves in cylindrical magnetic flux tubes is studied in compressible
viscous MHD. We treat the problem numerically with an application of
the FEM combined with the Galerkin technique. The singularities of
the ideal MHD equations are removed by incorporating a dissipative
effect, namely the classical viscosity. We show that an equilibrium
shear flow can significantly influence the absorption suffered by
the incoming driving waves. The presence of an equilibrium flow may
therefore be very determinant for resonant absorption. A parametric
analysis shows that there are values of the equilibrium velocity
field for which the absorption rate becomes zero, even for rather
small velocity shears. We also found negative absorption of wave
power which apparently can be attributed to the resonant instability
found by Hollweg et al. (1990). For other values of the equilibrium
flow we find that the resonant absorption can be strongly enhanced,
even up to total absorption of the incoming wave.
---------------------------------------------------------
Title: Unified theory of damping of linear surface Alfvén waves in
inhomogeneous incompressible plasmas
Authors: Ruderman, M. S.; Goossens, M.
1996JPlPh..56..107R Altcode:
The viscous damping of surface Alfvén waves in a non-uniform plasma
is studied in the context of linear and incompressible MHD. It is
shown that damping due to resonant absorption and damping on a true
discontinuity are two limiting cases of the continuous variation
of the damping rate with respect to the dimensionless number Rg =
Δλ<SUP>2</SUP>Re, where Δ is the relative variation of the local
Alfvén velocity, λ is the ratio of the thickness of the inhomogeneous
layer to the wavelength, and Re is the viscous Reynolds number. The
analysis is restricted to waves with wavelengths that are long in
comparison with the extent of the non-uniform layer (λ 1), and to
Reynolds numbers that are sufficiently large that the waves are only
slightly damped during one wave period. The dispersion relation is
obtained and first investigated analytically for the limiting cases
of very small (Rg 1) and very large (Rg 1) values of Rg, For very
small values of Rg, the damping rate agrees with that found for a true
discontinuity, while for very large values of Rg, it agrees with the
damping rate due to resonant absorption. The dispersion relation is
subsequently studied numerically over a wide range of values of Rg,
revealing a continuous but nonmonotonic variation of the damping rate
with respect to Rg.
---------------------------------------------------------
Title: Numerical Simulations of Driven MHD Waves in Coronal Loops
Authors: Parhi, S.; De Bruyne, P.; Murawski, K.; Goossens, M.; Devore,
C. R.
1996SoPh..167..181P Altcode:
The solar corona, modeled by a low-β, resistive plasma slab,
sustains MHD wave propagations due to footpoint motions in
the photosphere. Simple test cases are undertaken to verify the
code. Uniform, smooth and steep density, magnetic profile and driver
are considered. The numerical simulations presented here focus on
the evolution and properties of the Alfvén, fast and slow waves in
coronal loops. The plasma responds to the footpoint motion by kink
or sausage waves depending on the amount of shear in the magnetic
field. The larger twist in the magnetic field of the loop introduces
more fast-wave trapping and destroys initially developed sausage-like
wave modes. The transition from sausage to kink waves does not depend
much on the steep or smooth profile. The slow waves develop more
complex fine structures, thus accounting for several local extrema
in the perturbed velocity profiles in the loop. Appearance of the
remnants of the ideal singularities characteristic of ideal plasma is
the prominent feature of this study. The Alfvén wave which produces
remnants of the ideal x<SUP>−1</SUP> singularity, reminiscent of
Alfvén resonance at the loop edges, becomes less pronounced for larger
twist. Larger shear in the magnetic field makes the development of
pseudo-singularity less prominent in case of a steep profile than that
in case of a smooth profile. The twist also causes heating at the edges,
associated with the resonance and the phase mixing of the Alfvén and
slow waves, to slowly shift to layers inside the slab corresponding to
peaks in the magnetic field strength. In addition, increasing the twist
leads to a higher heating rate of the loop. Remnants of the ideal log
¦x¦ singularity are observed for fast waves for larger twist. For
slow waves they are absent when the plasma experiences large twist
in a short time. The steep profiles do not favour the creation of
pseudo-singularities as easily as in the smooth case.
---------------------------------------------------------
Title: Quasi-modes as dissipative MHD eigenmodes : results for
1-dimensional equilibrium states
Authors: Tirry, W. J.; Goossens, M.
1996AAS...188.3609T Altcode: 1996BAAS...28..874T
Quasi-modes which are important for understanding the MHD wave behavior
of solar and astrophysical magnetic plasmas are computed as eigenmodes
of the linear dissipative MHD equations. This eigenmode computation is
carried out with a simple numerical scheme which is based on analytical
solutions to the dissipative MHD equations in the quasi-singular
resonance layer. Non-uniformity in magnetic field and plasma density
gives rise to a continuous spectrum of resonant frequencies. Global
discrete eigenmodes with characteristic frequencies lying within the
range of the continuous spectrum may couple to localised resonant
Alfven waves. In ideal MHD these modes are not eigenmodes of the
Hermitian ideal MHD operator, but are found as a temporal dominant
global exponentially decaying response to an initial perturbation. In
dissipative MHD they are really eigenmodes with damping becoming
independent of the dissipation mechanism in the limit of vanishing
dissipation. An analytical solution of these global modes is found in
the dissipative layer around the resonant Alfvenic position. Using
the analytical solution to cross the quasi-singular resonance layer
the required numerical effort of the eigenvalue scheme is limited
to the integration of the ideal MHD equations in regions away from
any singularity. The presented scheme allows for a straightforward
parametric study. The method is checked with known ideal quasi-mode
frequencies found for a 1-D box model for the Earth's magnetosphere
(Zhu & Kivelson 1988). The agreement is excellent. The dependence
of the oscillation frequency on the wavenumbers for a 1-D slab model
for coronal loops found by Ofman, Davila, & Steinolfson (1995)
is also easily recovered.
---------------------------------------------------------
Title: MHD Surface Waves in a Complex (Longitudinal + Sheared)
Magnetic Field
Authors: Zhelyazkov, I.; Murawski, K.; Goossens, M.
1996SoPh..165...99Z Altcode:
We study the dispersion characteristics of fast MHD surface waves
travelling on a plasma slab immersed in a complex magnetic field
consisting of a large longitudinal B<SUB>0z</SUB> component and a
small sheared B<SUB>0y</SUB> component. The analysis shows that for
typical coronal conditions both the sausage and kink waves are generally
pseudo-surface waves. The tangential magnetic field, B<SUB>0y</SUB>,
modifies the dispersion curves, and for sufficiently large sheared
fields there is a transition from pseudo-surface to pure-surface fast
kink waves.
---------------------------------------------------------
Title: Numerical simulations of MHD wave excitation in bounded
plasma slabs
Authors: Murawski, K.; DeVore, C. R.; Parhi, S.; Goossens, M.
1996P&SS...44..253M Altcode:
Numerical simulations are performed in the framework of nonlinear
resistive 2.5-dimensional magnetohydrodynamics to investigate the
response of a coronal loop to shearing motions of the footpoints. A
simple slab plasma with straight magnetic field lines is used to
model the coronal loop, with the photospheric ends represented by
impermeable walls. The hot, dense loop plasma is approximated by
smoothed step-function profiles. The results show that an initially
excited Alfvén wave reveals a 1/ x-type singularity, which is
characteristic of the linear Alfvén resonances, smoothed by the
resistive dissipation. Both fast and slow magnetosonic waves are driven
by the nonlinear Alfvén wave. The slow magnetosonic waves concentrate
their energies in resonance layers, and their associated flows exhibit
singularities of the 1/ x-type. The Alfvén resonances, which are absent
from the system in the case of linear waves, develop late in time as
the Alfvén-wave amplitude grows into the nonlinear regime, and their
development is accelerated for larger-amplitude driving forces. The
resultant heating associated with the resonances phase mixing of the
waves is concentrated in the region of large Alfvén-speed gradients.
---------------------------------------------------------
Title: Numerical Simulation of Driven MHD Waves in Twisted Coronal
Loops
Authors: de Bruyne, P.; Parhi, S.; Goossens, M.
1996ApL&C..34..163D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: MHD waves in magnetic flux tubes.
Authors: Goossens, M.; Ruderman, M.
1996ASIC..481...61G Altcode:
This review is concerned with a recent development in the analytic
theory of resonant Alfvén waves in nonuniform magnetic flux tubes in
the solar atmosphere. It discusses how an analytic study of simplified
versions of the dissipative MHD equations for plasmas with high Reynolds
numbers leads to the fundamental conservation law and simple analytic
solutions for resonant Alfvén waves. Asymptotic analysis of the
analytic solutions gives jump conditions that connect the solutions
across the dissipative layer.
---------------------------------------------------------
Title: The Linear Spectrum of Twisted Magnetic Flux Tubes in
Viscous MHD
Authors: Pintér, B.; Erdélyi, R.; Goossens, M.
1996ApL&C..34..169P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical Model for the Study of Wave-Mean Flow Interactions
in Solar Magnetohydrodynamics
Authors: Ghizaru, Mihai S.; Goossens, Marcel
1996RoAJ....6...21G Altcode:
A 2D MHD model in cylindrical geometry is developed for the study of
the behaviour of MHD waves incident on critical layers in the presence
of shearing in magnetic and velocity fields in the solar plasma The MHD
code described in this paper is based on the resistive MHD equations
written in (r,z) coordinates. The non-divergence constraint for the
magnetic field is satisfied by using a vector potential. For the time
advancing, an operator splitting procedure is performed when including
the different physical effects, as well as for alternating direction
in the advection terms. The rotating cone test is used for numerical
experiments with different advenction schemes. The model equations are
integrated on an Arakawa b grid, with Bø in the cell center and Aø
in the velocity points. Second order schemes are used for the mass,
momentum and energy advection with a monotonicity maintaining scheme
applied periodically. Implicit methods, treated by gaussian elimination
are used for the toroidal and poloidal magnetic field diffusion.
---------------------------------------------------------
Title: Dissipative MHD solutions for resonant Alfvén waves in
two-dimensional poloidal magnetoplasmas
Authors: Tirry, W. J.; Goossens, M.
1995JGR...10023687T Altcode:
The resonant excitation of Alfvén waves is considered in a resistive
warm plasma embedded in a purely poloidal field. The magnetostatic
equilibrium is invariant in the y direction. The driven problem is
studied in the asymptotic state, so we can assume that all wave fields
vary as exp[i(λy-ωt)]. Resistive solutions are derived in the vicinity
of the resonance with the aid of an asymptotic expansion procedure. We
find that the zeroth-order functions of the flux coordinate have to
satisfy differential equations analogous to those obtained by Goossens
et al. [1995] for the resistive one-dimensional systems. Applied
to the two-dimensional box model for the Earth's magnetosphere, we
recover the results found by Thompson and Wright [1993] in ideal MHD,
but in addition, we obtain the behavior in the dissipative layer in
the asymptotic state.
---------------------------------------------------------
Title: Analytic solutions for resonant Alfvén waves in 1D magnetic
flux tubes in dissipative stationary MHD
Authors: Erdélyi, Róbert; Goossens, Marcel; Ruderman, Michael S.
1995SoPh..161..123E Altcode:
Resonantly driven Alfvén waves are studied in non-uniform stationary
magnetic flux tubes. Analytic dissipative MHD solutions are obtained
for the Lagrangian displacement and the Eulerian perturbation of the
total pressure. These analytic solutions are valid in the dissipative
layer and in the two overlap regions to the left and the right of
the dissipative layer. From these analytic solutions we obtain the
fundamental conservation law and the jump conditions for resonantly
driven Alfvén waves in magnetic flux tubes with an equilibriun
flow. The fundamental conservation law and the jump conditions depend
on the equilibrium flow in a more complicated way than just a Doppler
shift. The effects of an equlibrium flow are not to be predicted easily
in general terms with the exception that the polarization of the driven
Alfvén waves is still in the magnetic surfaces and perpendicular to
the magnetic field lines as it is in a static flux tube.
---------------------------------------------------------
Title: A simple numerical scheme for the computation of resonant
Alfvén waves
Authors: Stenuit, Hilde; Erdélyi, Róbert; Goossens, Marcel
1995SoPh..161..139S Altcode:
The present paper discusses the implementation of the SGHR method
(Sakurai, Goossens, and Hollweg, 1991; Goossens, Ruderman, and Hollweg,
1995) in a numerical scheme for determining resonantly driven Alfvén
waves in nonuniform magnetic flux tubes. This method is based on
jump conditions over the dissipative layer which are obtained from
an asymptotic analysis of analytical solutions to simplified versions
of the linear non-ideal MHD equations in this dissipative layer. The
emphasis is on the computational simplicity and the accuracy of the
method. The method derives its computational simplicity from the fact
that it circumvents the numerical integration of the non-ideal MHD
equations. The implementation only requires the numerical integration
of the ideal MHD equations away from the resonant position. There
is no need for a special integration scheme and a PC suffices as a
hardware tool.
---------------------------------------------------------
Title: Non-stationary resonant Alfvén surface waves in
one-dimensional magnetic plasmas
Authors: Ruderman, M. S.; Tirry, W.; Goossens, M.
1995JPlPh..54..129R Altcode:
This paper uses incompressible visco-resistive MHD to study the
propagation of linear resonant waves in an inhomogeneous plasma. The
background density and magnetic field are assumed to depend only on one
spartial Cartesian coordinate, and the magnetic field is taken to be
unidirectional and perpendicular to the direction of inhomogeneity. The
equation that governs the component of the velocity normal to the plane
formed by the direction of the inhomogeneity and the magnetic field
is derived under the assumption that the coefficients of viscosity
and resistivity are sufficiently small that dissipation of energy
is confined to a narrow dissipative layer. The solutions to this
equation are obtained in the form of decaying normal surface modes
with wavelengths much larger than the characteristic scale of the
inhomogeneity. The effect of non-stationarity inside the dissipative
layer is taken into account, and valid solutions are found even when the
ratio of the thickness of the dissipative layer to the inhomogeneity
length scale is of the order of or smaller than the ratio of the
inhoinogeneity length scale to the wavelength. These solutions are the
generalization of the solutions obtained by Mok and Einaudi, which are
only valid when the first ratio is much larger than the second. The
rate of wave damping is shown to be independent of the values of the
viscosity and the resistivity. However, the behaviour of the solutions
in the dissipative layer depends strongly on the viscosity and the
resistivity. In the case that the effect of dissipation dominates the
effect of non-stationarity, the solutions behave in the dissipative
layer as found by Mok and Einaudi. When the effect of dissipation is
steadily decreased in comparison with the effect of nonstationarity, the
solutions become more and more oscillatory, and their amplitudes grow
very rapidly in the dissipative layer. Eventually, when nonstationarity
dominates dissipation, the amplitudes of the solutions become so
large in the dissipative layer in comparison with those outside the
dissipative layer that practically all the energy of the perturbations
is concentrated in the dissipative layer.
---------------------------------------------------------
Title: Surface Alfvén waves of negative energy
Authors: Ruderman, M. S.; Goossens, M.
1995JPlPh..54..149R Altcode:
The stability of an MHD tangential discontinuity is studied in an
incompressible plasma where viscosity is taken into account at one
side of the discontinuity. When the shear velocity is smaller than the
threshold value for the onset of the Kelvin-Helmholtz (KH) instability,
two surface waves can propagate along the discontinuity. There
is a critical value for the shear velocity, which is smaller than
the threshold value for the onset of the KH instability. When the
shear velocity is smaller than the critical value, the two surface
waves propagate in Opposite directions. When the shear velocity is
larger than the critical velocity, the two waves propagate in the same
direction, and the wave with smaller phase velocity is a negative-energy
wave. Viscosity causes this negative-energy wave to be unstable, and
the instability increment is proportional to the viscosity coefficient.
---------------------------------------------------------
Title: Study of nonlinear MHD equations governing the wave propagation
in twisted coronal loops
Authors: Parhi, S.; Debruyne, P.; Goossens, M.; Zhelyazkov, I.
1995sowi.conf...28P Altcode:
The solar corona, modelled by a low beta, resistive plasma slab,
sustains MHD wave propagations due to shearing footpoint motions in the
photosphere. By using a numerical algorithm the excitation and nonlinear
development of MHD waves in twisted coronal loops are studied. The
plasma responds to the footpoint motion by sausage waves if there is no
twist. The twist in the magnetic field of the loop destroys initially
developed sausage-like wave modes and they become kinks. The transition
from sausage to kink modes is analyzed. The twist brings about mode
degradation producing high harmonics and this generates more complex
fine structures. This can be attributed to several local extrema in
the perturbed velocity profiles. The Alfven wave produces remnants
of the ideal 1/x singularity both for zero and non-zero twist and
this pseudo-singularity becomes less pronounced for larger twist. The
effect of nonlinearity is clearly observed by changing the amplitude
of the driver by one order of magnitude. The magnetosonic waves also
exhibit smoothed remnants of ideal logarithmic singularities when the
frequency of the driver is correctly chosen. This pseudo-singularity
for fast waves is absent when the coronal loop does not undergo any
twist but becomes pronounced when twist is included. On the contrary,
it is observed for slow waves even if there is no twist. Increasing
the twist leads to a higher heating rate of the loop. The larger twist
shifts somewhat uniformly distributed heating to layers inside the
slab corresponding to peaks in the magnetic field strength.
---------------------------------------------------------
Title: Dissipative MHD solutions for resonant AlfvÉn waves in
1-dimensional magnetic flux tubes
Authors: Goossens, Marcel; Ruderman, Michail S.; Hollweg, Joseph V.
1995SoPh..157...75G Altcode:
The present paper extends the analysis by Sakurai, Goossens, and Hollweg
(1991) on resonant Alfvén waves in nonuniform magnetic flux tubes. It
proves that the fundamental conservation law for resonant Alfvén
waves found in ideal MHD by Sakurai, Goossens, and Hollweg remains
valid in dissipative MHD. This guarantees that the jump conditions of
Sakurai, Goossens, and Hollweg, that connect the ideal MHD solutions
forξ<SUB>r</SUB>, andP' across the dissipative layer, are correct. In
addition, the present paper replaces the complicated dissipative MHD
solutions obtained by Sakurai, Goossens, and Hollweg forξ<SUB>r</SUB>,
andP' in terms of double integrals of Hankel functions of complex
argument of order with compact analytical solutions that allow a
straightforward mathematical and physical interpretation. Finally,
it presents an analytical dissipative MHD solution for the component
of the Lagrangian displacement in the magnetic surfaces perpendicular
to the magnetic field linesξ⊥ which enables us to determine the
dominant dynamics of resonant Alfvén waves in dissipative MHD.
---------------------------------------------------------
Title: Resonant absorption of Alfven waves in coronal loops in
visco-resistive MHD.
Authors: Erdelyi, R.; Goossens, M.
1995A&A...294..575E Altcode:
This paper uses numerical simulations in linear visco-resistive MHD
for determining the dominant dissipative mechanism for the stationary
state of resonant absorption of MHD waves in coronal loops. The full
Braginskii viscous stress tensor with five viscosity coefficients and
electrical resistivity are included in the MHD equations. The coronal
loops are approximated by straight, cylindrical, axisymmetric plasma
columns with equilibrium quantities which vary only in the radial
direction. The simulations are carried out with a numerical code that
uses finite elements for the spatial discretization combined with
the Galerkin method. Computations in viscous MHD (in the absence
of electrical resistivity) show that shear viscosity produces the
largest contribution to the absorption and that the contributions due
to the compressive and perpendicular components of the viscous stress
tensor can be neglected for all practical purposes. Computations in
visco-resistive MHD reveal that on the whole it is not possible to
distinguish between shear viscosity and electrical resistivity as
the most efficient dissipative mechanism for resonant absorption. The
relative importance of these two dissipative mechanisms depends on the
equilibrium conditions of density and magnetic field strength so that
there are situations in which either electrical resisitivity or shear
viscosity is the most effective mechanism and in which both mechanisms
are equally efficient.
---------------------------------------------------------
Title: Conservation laws and connection formulae for resonant
MHD waves
Authors: Goossens, Marcel; Ruderman, Michael S.
1995PhST...60..171G Altcode:
This paper reviews a recent development in the theory of resonant MHD
waves in non-uniform plasmas. An asymptotic analysis of the equations
for MHD waves in plasmas with high magnetic Reynolds numbers has shown
that resonant slow and Alfvén waves obey conservation laws and jump
conditions across the dissipative layer. These conservation laws specify
the dominant dynamics of the resonant MHD waves. In combination with
the jump conditions they enable us to understand the basic physics
of resonant MHD waves and also help us with the interpretation of
results of large scale numerical simulations in resistive MHD. They
also can be used to design an accurate and computationally simple
methods for numerical studying resonant MHD waves in non-uniform
plasmas. <P />Conservation laws and jump conditions for resonant
MHD waves are first discussed in linear MHD for 1-dimensional
equilibrium states. Subsequently the generalization of these results
to 2-dimensional equilibrium state in linear MHD and to nonlinear MHD
is reviewed. The interaction of sound waves with an inhomogeneous
plasma is discussed as an application of the theory. Firstly the
results of linear theory are used to consider the interaction of
sound waves with 1-dimensional magnetic tubes. The phenomenon of total
resonant absorption is discussed. Secondly the nonlinear theory of cusp
dissipative layers is used to study the interaction of sound waves with
1-dimensional inhomogeneous plasmas in planar geometry. New effects
that owe their existence to nonlinearity in the cusp dissipative layer
are reviewed.
---------------------------------------------------------
Title: Multiple Scattering and Resonant Absorption of p-Modes by
Fibril Sunspots
Authors: Keppens, R.; Bogdan, T. J.; Goossens, M.
1994ApJ...436..372K Altcode:
We investigate the scattering and absorption of sound waves by
bundles of magnetic flux tubes. The individual flux tubes within the
bundle have thin nonuniform boundary layers where the thermodynamic
and magnetic properties change continuously to their photospheric
levels. In these nonuniform layers, resonant absorption converts some
of the incident acoustic wave energy into heat and thus the flux-tube
bundle appears as a sink of acoustic power. For a fixed amount of
magnetic flux, we find that composite ('spaghetti') sunspots absorb
much more wave energy than their monolithic counterparts, although
both sunspots scatter comparable amounts of the incident acoustic wave
energy. The extra energy drainage results from the interplay of the
wave scattering back and forth between the tubes and the incremental
loss of acoustic power at each interaction with an individual tube due
to the resonant absorption in its boundary layer. The scattering cross
section is not similarly enhanced because the multiply scattered waves
generally interfere destructively in the far field. Another interesting
consequence of the lack of axisymmetry is that composite sunspots may
show acoustic emission for some multipole components, and absorption
for others. The net absorption cross section is however never negative,
and is nonzero only when the projection of the wave phase speed along
the flux-tube bundle is less than the maximal value of the Alfven
speed. Whereas composite sunspots composed of uniformly magnetized
flux tubes posses narrow scattering resonances, the analogous bundle
of nonuniform fibrils instead exhibits corresponding broad absorption
resonances, resulting from the incremental loss of power on successive
scatters. These broad absorption resonances correspond to leaky (MHD
radiating) eigenmodes of the composite structure. When progressively
more flux tubes are clustered, additional oscillation eigenmodes appear
grouped in a complicated band structure characterized by a (nearly)
common speed of propagation along the bundle.
---------------------------------------------------------
Title: Book Review: Solar wind seven / Pergamon Press, 1992
Authors: Goossens, M.
1994SSRv...69..207M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Operator splitting for multidimensional magnetohydrodynamics
Authors: Murawski, K.; Goossens, M.
1994JGR....9911569M Altcode:
Operator splitting and direct time integration of the ideal MHD
equations are compared by using a finite difference flux-corrected
transport method. The comparison is made for impulsively generated
nonlinear waves in a coronal plasma. It suggests that operator splitting
and direct time integration both lead to physically acceptable results
and provide a good agreement with results obtained with the fast
Fourier transform method.
---------------------------------------------------------
Title: Flux corrected transport method for MHD plasma: description
of the numerical algorithm and tests
Authors: Murawski, K.; Goossens, M.
1994A&A...286..943M Altcode:
This paper gives a detailed description of an MHD code for the
simulation of 2D flows and waves in magnetic astrophysical plasmas. The
code uses the flux corrected transport method with Zalesak's
flux limiter for the spatial integration and a predictor-corrector
method for the direct time integration. Results of test problems are
presented in order to assess the numerical accuracy and performance
of the code. The test problems deal with the numerical simulations of
impulsively generated nonlinear MHD waves in coronal density interfaces
and slabs. At present the code uses Cartesian coordinates and the MHD
equations for a (β=0) pressureless ideal plasma. The extension to
more complicated geometries and non-zero β plasmas is straightforward.
---------------------------------------------------------
Title: Excitation of nonlinear MHD waves by foot-point motions
Authors: Murawski, K.; Goossens, M.
1994A&A...286..952M Altcode:
Numerical simulations are performed to investigate the excitation of
magnetohydrodynamic waves in coronal loops by photospheric shearing
motions. In the simulations the solar coronal loop is modelled as
a straight slab of enhanced gas density. The plasma is described by
magnetohydrodynamic equations that include finite gas pressure effects
and magnetic diffusivity. The Alfven wave is artificially suppressed
from the system. MHD waves are excited by imposing oscillations
at one end of the plasma slab. A plasma responds to a foot-point
motion by kink or sausage waves in dependence on the polarization
of a driver. The foot-point motion induces a growth in the parallel
velocity. The growth occurs in a resonant layer on a time scale which
is much less than a typical life time of a loop. The heating associated
with the resonant absorption of the waves is very small and can thus
not be considered as relevant for the coronal heating mechanism. The
nonlinearity of the MHD equations gives rise to a distorted plasma flow,
introducing an asymmetry in the system, and speeds up the occurrence
of the resonant layer.
---------------------------------------------------------
Title: Alfvén wave heating
Authors: Goossens, M.
1994SSRv...68...51G Altcode:
This review discusses Alfvén wave heating in non-uniform plasmas as
a possible means for explaining the heating of the solar corona. It
focusses on recent analytical results that enable us to understand
the basic physics of Alfvén wave heating and help us with the
interpretation of results of numerical simulations. First we
consider the singular wave solutions that are found in linear ideal
MHD at the resonant magnetic surface where the frequency of the wave
equals the local Alfvén frequency. Next, we use linear resistive MHD
for describing the waves in the dissipative region and explain how
dissipation modifies the singular solutions found in linear ideal MHD.
---------------------------------------------------------
Title: Modulations of slow sausage surface waves travelling along
a magnetized slab
Authors: Zhelyazko, I.; Murawski, K.; Goossens, M.; Nenovaki, P.;
Roberts, B.
1994JPlPh..51..291Z Altcode:
In this paper we consider a set of nonlinear MHD equations that admits
in a linear approximation a solution in the form of a slow sausage
surface wave travelling along an isolated magnetic slab. For a wave
of small but finite amplitude, we investigate how a slowly varying
amplitude is modulated by nonlinear self-interactions. A stretching
transformation shows that, at the lowest order of an asymptotic
expansion, the original set of equations with appropriate boundary
conditions (free interfaces) can be reduced to the cubic nonlinear
Schrödinger equation, which determines the amplitude modulation. We
study analytically and numerically the evolution of impulsively
generated waves, showing a transition of the initial states into a
train of solitons and periodic waves. The possibility of the existence
of solitary waves in the solar atmosphere is also briefly discussed.
---------------------------------------------------------
Title: WKB Estimates for the Onset of Ideal Magnetohydrodynamic
Instabilities in Solar Coronal Loops
Authors: Hood, A. W.; de Bruyne, P.; van der Linden, R. A. M.;
Goossens, M.
1994SoPh..150...99H Altcode:
A WKB approach, based on the method of Connor, Hastie, and Taylor
(1979), is used to obtain simple estimates of the critical conditions
for the onset of ideal MHD instabilities in line-tied solar coronal
loops. The method is illustrated for the constant twist, Gold-Hoyle
(1960) field, and the critical conditions are compared with previous and
new numerical results. For the force-free case, the WKB estimate for the
critical loop length reduces to . For the sufficiently non-force-free
case the critical length can be expressed in the forml<SUB>0</SUB>
+l<SUB>1</SUB>/m. The results confirm the findings of De Bruyne and Hood
(1992) that for force-free fields them = 1 mode is the first mode to
become unstable but for the sufficiently strong non-force-free case
this reverses with them → ∞ mode being excited first.
---------------------------------------------------------
Title: Viscous computations of resonant absorption of MHD waves in
flux tubes by fem
Authors: Erdelyi, R.; Goossens, M.
1994Ap&SS.213..273E Altcode:
A numerical code is presented for computing the stationary state of
resonant absorption of MHD waves in cylindrical flux tubes in linear,
compressible, and viscous MHD. The full viscosity stress tensor is
included in the code with the five viscosity coefficients as given
by Braginskii (1965). Also non-zero plasma pressure effects are taken
into account, and the finite elements discretization with the Galerkin
method has been used. The implementation of the stress tensor and the
numerical accuracy of the tensorial viscous MHD code are scrutinized in
test case. The test case involves the absorption of waves in cylindrical
flux tubes considered by Lou (1990) and Goossens and Poedts (1992) in
the context of absorption of acoustic oscillations. The results for
the absorption rates obtained with the tensorial viscous code agree
completely with the results obtained by Lou in a scalar viscous MHD and
by Goossens and Poedts in resistive MHD. This verifies not only the
complicated tensor viscous code but again proves that the absorption
rate is independent of the actual dissipation mechanism.
---------------------------------------------------------
Title: Linear Visco-Resistive Computations of Magnetohydrodynamic
Waves II. Viscous Effects
Authors: Erdelyi, R.; Goossens, M.
1994scs..conf..506E Altcode: 1994IAUCo.144..506E
Resonant absorption of MHD waves in coronal loops is studied in linear,
viscous MHD. Shear viscosity produces absorption and is a viable
candidate for heating coronal loops.
---------------------------------------------------------
Title: Linear Visco-Resistive Computations of Magnetohydrodynamics
Waves I. The Code and Test Cases
Authors: Erdelyi, R.; Goossens, M.; Poedts, S.
1994scs..conf..503E Altcode: 1994IAUCo.144..503E
The stationary state of resonant absorption of linear, MHD waves in
cylindrical magnetic flux tubes is studied in viscous, compressible
MHD with a numerical code using finite element discretization. The
full viscosity tensor with the five viscosity coefficients as given by
Braginskii is included in the analysis. The computations reproduce the
absorption rates obtained by Lou in scalar viscous MHD and Goossens
and Poedts in resistive MHD, which guarantee the numerical accuracy
of the tensorial viscous MHD code.
---------------------------------------------------------
Title: Random velocity field corrections to the f-mode. 3: A
photospheric random flow and chromospheric magnetic field
Authors: Murawski, K.; Goossens, M.
1993A&A...279..225M Altcode:
The combined effect of a photospheric random flow and a chromospheric
magnetic field on the frequencies of the f-mode is studied for a
simple model of the photosphere and chromosphere. The photosphere and
chromosphere are approximated as inviscid fluids with constant mass
densities rho<SUB>1</SUB> and rho<SUB>2</SUB>. The photosphere is taken
to be free of a magnetic field and subject to a random flow and the
chromosphere is in static equilibrium but permeated by a horizontal
magnetic field. Numerical solutions show that the frequencies of the
f-mode depart from the parabola omega<SUP>2</SUP> = gk(rho<SUB>1</SUB>
- rho<SUB>2</SUB>)/rho<SUB>1</SUB> + rho <SUB>2</SUB>) +
k<SUP>2</SUP>(V<SUB>A exp 2</SUB> rho<SUB>2</SUB>/(rho<SUB>1</SUB> +
rho<SUB>2</SUB>) at high wavenumbers k.
---------------------------------------------------------
Title: Total Resonant Absorption of Acoustic Oscillations in Sunspots
Authors: Stenuit, Hilde; Poedts, Stefaan; Goossens, Marcel
1993SoPh..147...13S Altcode:
The question of total resonant absorption of acoustic oscillations in
sunspots is studied for cylindrical 1-D flux tubes that are stratified
only in the radial direction and surrounded by a uniform, non-magnetic
plasma. The numerical investigation of Goossens and Poedts (1992)
in linear resistive MHD is taken further by increasing the strength
of the azimuthal magnetic field in the equilibrium flux tubes. For
relatively strong azimuthal magnetic fields, total absorption is found
over a relatively wide range of spot radii.
---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes - Part Four
Authors: Goossens, Marcel; Hollweg, Joseph V.
1993SoPh..145...19G Altcode:
Resonant absorption of MHD waves on a nonuniform flux tube is
investigated as a driven problem for a 1D cylindrical equilibrium. The
variation of the fractional absorption is studied as a function of
the frequency and its relation to the eigenvalue problem of the MHD
radiating eigenmodes of the nonuniform flux tube is established. The
optimal frequencies producing maximal fractional absorption are
determined and the condition for total absorption is obtained. This
condition defines an impedance matching and is fulfilled for an
equilibrium that is fine tuned with respect to the incoming wave. The
variation of the spatial wave solutions with respect to the frequency
is explained as due to the variation of the real and imaginary parts
of the dispersion relation of the MHD radiating eigenmodes with respect
to the real driving frequency.
---------------------------------------------------------
Title: Non-adiabatic discrete Alfvén waves in coronal loops and
prominences
Authors: Keppens, Rony; van der Linden, Ronald A. M.; Goossens, Marcel
1993SoPh..144..267K Altcode:
Discrete Alfvén waves in coronal loops and prominences are investigated
in non-ideal magnetohydrodynamics. The non-ideal effects included are
anisotropic, thermal conduction, and optically thin radiation. The
classic ideal Alfvén continuum is not altered by these non-ideal
effects, but the discrete Alfvén modes, which exist under certain
conditions above or below the Alfvén continuum in ideal MHD, are
shown to be influenced by non-adiabatic effects.
---------------------------------------------------------
Title: Nonlinearity effects on resonant absorption of surface Alfvén
waves in incompressible plasmas
Authors: Ruderman, M. S.; Goossens, Marcel
1993SoPh..143...69R Altcode:
The resonant absorption of small amplitude surface Alfvén waves is
studied in nonlinear incompressible MHD for a viscous and resistive
plasma. The reductive perturbation method is used to obtain the
equation that governs the spatial and temporal behaviour of small
amplitude nonlinear surface Alfvén waves. Numerical solutions to
this equation are obtained under the initial condition that att =
0 the spatial variation is purely sinusoidal. The numerical results
show that nonlinearity accelerates the wave damping due to resonant
absorption. Resonant absorption is a more efficient wave damping
mechanism than can be anticipated on the basis of linear theory.
---------------------------------------------------------
Title: The Thermal Continuum in Coronal Loops - the Influence of
Finite Resistivity on the Continuous Spectrum
Authors: Ireland, R. C.; van der Linden, R. A. M.; Hood, A. W.;
Goossens, M.
1992SoPh..142..265I Altcode:
The normal mode spectrum for the linearized MHD equations is
investigated for a cylindrical equilibrium. This spectrum is examined
for zero perpendicular thermal conduction, with both zero and non-zero
scalar resistivity. Particular attention is paid to the continuous
branches of this spectrum, or continuous spectra. For zero resistivity
there are three types of continuous spectra present, namely the Alfvén,
slow, and thermal continua. It is shown that when dissipation due to
resistivity is included, the slow and Alfvén continua are removed and
that the thermal continuum is shifted to a different position (where
the shift is independent of the exact value of resistivity). The `old'
location of the thermal continuum is covered by a dense set of nearly
singular discrete modes called a quasi-continuum. The quasi-continuum
is investigated numerically, and the eigenfunctions are shown to have
rapid spatial oscillating behaviour. These oscillations are confined to
the most unstable part of the equilibrium based on the Field criterion,
and may be the cause of fine structure in prominences.
---------------------------------------------------------
Title: The magnetothermal stability of radially stratified line-tied
coronal loops.
Authors: Vanderlinden, Ronald A. M.; Goossens, M.
1992ESASP.348..269V Altcode: 1992cscl.work..269V
The influence of the photospheric anchoring of the magnetic field
lines (line-tying) on the magnetic and thermal (or "magnetothermal")
stability of radially stratified coronal loops is studied. The anchoring
of the magnetic field lines in the photosphere is modelled by line-tying
boundary conditions. Using a simple force-free equilibrium, it is shown
that line-tying has a much stronger influence on a magnetic instability
than on a thermal instability. Thus, line-tying can lead to loops
that are magnetically stable but thermally unstable. In such loops,
condensations can form due to a thermal instability. This may be of
relevance in the context of the formation and eruption of prominences
in the solar corona.
---------------------------------------------------------
Title: The relevance of the ballooning approximation for magnetic,
thermal, and coalesced magnetothermal instabilities
Authors: van der Linden, R. A. M.; Goossens, M.; Hood, A. W.
1992SoPh..140..317V Altcode:
Approximate solutions of the linearized non-adiabatic MHD equations,
obtained using the ballooning method, are compared with `exact'
numerical solutions of the full equations (including the effects
of optically thin plasma radiation). It is shown that the standard
ballooning method, developed within the framework of ideal linear MHD,
can be generalized to non-ideal linear MHD. The localized (ballooning)
spectrum has to be used with caution, but can give valuable (though
limited) information on non-ideal stability.
---------------------------------------------------------
Title: Total Absorption of Sound Waves by Solar Magnetic Flux Tubes
Authors: Hollweg, J. V.; Goossens, M.
1992AAS...180.1702H Altcode: 1992BAAS...24R.753H
No abstract at ADS
---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes - Part Three
Authors: Goossens, Marcel; Hollweg, Joseph V.; Sakurai, Takashi
1992SoPh..138..233G Altcode:
The resonances that appear in the linear compressible MHD formulation
of waves are studied for equilibrium states with flow. The conservation
laws and the jump conditions across the resonance point are determined
for 1D cylindrical plasmas. For equilibrium states with straight
magnetic field lines and flow along the field lines the conserved
quantity is the Eulerian perturbation of total pressure. Curvature
of the magnetic field lines and/or velocity field lines leads to more
complicated conservation laws. Rewritten in terms of the displacement
components in the magnetic surfaces parallel and perpendicular to the
magnetic field lines, the conservation laws simply state that the waves
are dominated by the parallel motions for the modified slow resonance
and by the perpendicular motions for the modified Alfvén resonance.
---------------------------------------------------------
Title: Linear Resistive Magnetohydrodynamic Computations of Resonant
Absorption of Acoustic Oscillations in Sunspots
Authors: Goossens, Marcel; Poedts, Stefaan
1992ApJ...384..348G Altcode:
A numerical study of the resonant absorption of p-modes by sunspots is
performed in linear resistive MHD. A parametric evaluation shows that
the efficiency of the absorption mechanism depends significantly on both
the equilibrium model and the characteristics of the p-modes. Results
from this numerical study of the relevant parameter domain indicate
that the resonant absorption of p-modes is more efficient in larger
sun spots with twisted magnetic fields. This is particularly true for
p-modes with higher azimuthal wave numbers.
---------------------------------------------------------
Title: Modelling stellar surface magnetic fields. I. Search strategies
and uniqueness.
Authors: Stift, M. J.; Goossens, M.
1991A&A...251..139S Altcode:
Within the framework of the Rigid Rotator hypothesis, an elaborate
procedure for the modeling of stellar magnetic surface fields aimed at
ensuring the uniqueness of the final models by means of appropriate
search strategies is proposed. Two different field geometries are
considered for this purpose. Using a sufficiently dense grid, the
entire 6D or 4D parameter space for those models that best predict
the observed integrated longitudinal field and integrated scalar field
strength is explored. In a second step, detailed calculations of Stokes
profiles for a few selected transitions make it possible to pick out
the most promising starting models for the final iterative line profile
modeling procedure, often leading simultaneously to the resolution of
the ambiguity between obliquity angle and inclination in axisymmetric
models. Illustrating the method with the analysis of the magnetic
variations of four well-observed Ap stars it is shown that the starting
models derived are in reasonable agreement with the published spectra.
---------------------------------------------------------
Title: The Thermal Continuum in Coronal Loops - Instability Criteria
and the Influence of Perpendicular Thermal Conduction
Authors: van der Linden, R. A. M.; Goossens, M.
1991SoPh..134..247V Altcode:
The linear MHD spectrum is investigated for cylindrical equilibrium
models under typical coronal conditions. Non-ideal effects are
included and attention is focussed on the thermal instability and
the influence of perpendicular thermal conduction. It is shown that,
when thermal conduction across magnetic field lines is neglected, the
`classic' Alfvén and slow continua are supplemented by a new `thermal'
continuum. Surprisingly, the existence of this non-ideal continuous
spectrum appears to have been overlooked for a very long time. Unlike
the (still purely oscillatory) Alfvén continuum modes and the slow
continuum modes (overstable or damped), the thermal continuum modes
are exponentially growing or decaying in time. As with the Alfvén and
slow continua, discrete modes may be present above or below the thermal
continuum, depending upon the choice of equilibrium parameters. These
modes are localized using a simple WKB approach. The knowledge of the
thermal subspectrum is then exploited to find necessary and sufficient
conditions for instability.
---------------------------------------------------------
Title: On Poloidal Mode Coupling in the Continuous Spectrum of
2d Equilibria
Authors: Poedts, Stefaan; Goossens, Marcel
1991SoPh..133..281P Altcode:
The continuous spectrum of linear ideal MHD is determined analytically
in 2D magnetostatic models for coronal loops and arcades by means
of a perturbation expansion. Poloidal mode coupling, induced by
non-circularity of the cross-sections of the magnetic surfaces and/or
variation of the plasma density along the magnetic field lines, is
shown to occur in first order. The coupling is most pronounced on
and near rational surfaces for particular poloidal and toroidal mode
numbers and produces gaps in the continuous spectrum of ideal MHD.
---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes - Part One
Authors: Sakurai, Takashi; Goossens, Marcel; Hollweg, Joseph V.
1991SoPh..133..227S Altcode:
A basic procedure is presented for dealing with the resonance problems
that appear in MHD of which resonant absorption of waves at the
Alfvén resonance point is the best known example in solar physics. The
procedure avoids solving the full fourth-order differential equation
of dissipative MHD by using connection formulae across the dissipation
layer.
---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
Flux Tubes II. Absorption of Sound Waves by Sunspots
Authors: Sakurai, Takashi; Goossens, Marcel; Hollweg, Joseph V.
1991SoPh..133..247S Altcode:
The absorption of solar five-min oscillations by sunspots is interpreted
as the resonant absorption of sound waves by a magnetic cylinder. The
absorption coefficient is calculated both analytically under
certain simplifying assumptions, and numerically under more general
conditions. The observed magnitude of the absorption coefficient,
which is up to 0.5 or even more, can be explained for suitable ranges
of parameters. Limitations in the present model are also discussed.
---------------------------------------------------------
Title: The Visco Resistive Stability of Arcades in the Corona of
the Sun
Authors: Bogaert, E.; Goossens, M.
1991SoPh..132..109B Altcode:
The stability of ballooning modes in coronal arcades is studied using
linear visco-resistive MHD. Rigid wall conditions are adopted for
modelling the photospheric line-tying of the magnetic field. The full
Braginskii viscosity stress tensor is used and particular attention
is given to the effect of the viscosity coefficient η<SUB>3</SUB>
which was left out of an earlier investigation by Van der Linden,
Goossens, and Hood (1987, 1988). The numerical results for shearless
arcades show that the coefficient η<SUB>3</SUB> has a stabilizing
effect. However, for realistic values of the equilibrium quantities
the stabilizing effect by η<SUB>3</SUB> can be neglected in comparison
with the strong stabilizing effect of the perpendicular viscosity. The
effect of magnetic field strength and mode number on stability are
determined. In particular it is found that there exists a critical
field strength for every mode number such that the mode is stable for
weaker fields and unstable for stronger fields.
---------------------------------------------------------
Title: Analytical study of plasma heating by resonant absorption of
the modified external kink mode
Authors: van Eester, D.; Goossens, M.; Poedts, S.
1991JPlPh..45....3V Altcode:
A simplified analytic description is used to understand recent
results of large-scale numerical simulations of resonant absorption
and to disentangle the basic physics. It is shown that very efficient
absorption takes place at frequencies where a discrete external kink
and an Alfvén continuum mode merge into a modified external kink
mode. The relation between this ‘hybrid’ mode and ‘pure’
continuum or discrete spectrum modes is discussed.
---------------------------------------------------------
Title: Thermal instability in slab geometry in the presence of
anisotropical thermal conduction
Authors: van der Linden, R. A. M.; Goossens, M.
1991SoPh..131...79V Altcode:
Prominences and filaments are thought to arise as a consequence of
a magnetized plasma undergoing thermal instability. Therefore, the
thermal stability of a magnetized plasma is investigated under coronal
conditions. The equilibrium structure of the plasma is approximated by
a 1-D slab configuration. This is investigated in thermal instability
taking into account optically thin plasma radiation and anisotropic
thermal conduction. The thermal conduction perpendicular to the magnetic
field is taken to be small but non-zero.
---------------------------------------------------------
Title: Resonant Absorption of MHD Waves in Magnetic Loops in the
Solar Corona
Authors: Goossens, M.
1991mcch.conf..480G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetohydrodynamic waves and wave heating in non-uniform
plasmas.
Authors: Goossens, M.
1991gamp.conf..137G Altcode:
The present review will not try to give a complete description of the
present status of MHD waves in the solar atmosphere. Instead it will
attempt to outline basic properties of MHD waves in nonuniform plasmas
and will focus on those aspects of MHD waves that are important for
understanding wave heating theories of the solar corona.
---------------------------------------------------------
Title: Magnetohydrodynamic Waves and Wave Heating in Nonuniform
Plasmas
Authors: Goossens, M.
1991assm.conf..137G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Efficiency of Coronal Loop Heating by Resonant
Absorption
Authors: Poedts, Stefaan; Goossens, Marcel; Kerner, Wolfgang
1990ApJ...360..279P Altcode:
The heating of solar coronal loops by resonant absorption of Alfven
waves is investigated in the framework of linearized compressible
resistive MHD. The resonant absorption of the waves incident on
the coronal loops is numerically simulated in straight cylindrical,
axisymmetric loop models externally excited by a periodic source. The
stationary state of this driven system and the ohmic dissipation rate in
this state are determined by a very accurate code based on the finite
element technique. The efficiency of the heating mechanism and the
energy deposition profile in this stationary state strongly depend on
the characteristics of both the external driver and the equilibrium. It
is shown that resonant absorption is very efficient for typical coronal
loops as a considerable part of the energy supplied by the external
source is actually dissipated ohmically and converted into heat. The
heating rate is proportional to the square of the magnitude of the
background magnetic field.
---------------------------------------------------------
Title: Temporal evolution of resonant absorption in solar coronal
loops
Authors: Poedts, Stefaan; Goossens, Marcel; Kerner, Wolfgang
1990CoPhC..59...95P Altcode:
A numerical code is presented for the computation of the
temporal evolution of an externally driven cylindrical plasma
column in the framework of linearized compressible and resistive
magnetohydrodynamics. The partial differential equations are solved with
a semi-discretization method using cubic and quadratic finite elements
for the spatial discretization and a fully implicit time advance. This
numerical technique yields very accurate results even for small values
of the resistivity. With this code it is, amongst others, possible to
simulate the heating of solar coronal loops by the resonant absorption
of waves that inpitch on them in order to determine the role of this
dissipation mechanism in coronal heating. In particular, it is necessary
to find out how the time scales of this heating mechanism compare to the
life of the coronal loops. <P />Present address: JET Joint Undertaking,
Theory Division, Abingdon, Oxfordshire OX14 3EA, England.
---------------------------------------------------------
Title: A combined finite element/Fourier series method for the
numerical study of the stability of line-tied magnetic plasmas
Authors: van der Linden, R. A. M.; Goossens, M.; Kerner, W.
1990CoPhC..59...61V Altcode:
An efficient method is presented for the numerical study of the
stability of line-tied coronal plasmas. This method uses a finite
element discretization in the direction normal to the magnetic flux
surfaces and a Fourier series analysis in the axial direction for
solving the differential equations that govern linear perturbations of
a static equilibrium. It has a wide range of possible applications both
in ideal and non-ideal magnetohydrodynamic stability investigations. <P
/>As an example, the numerical method is used to investigate the ideal
MHD stability of a straight line-tied cylindrical plasma, modelling a
coronal loop. It is found that line-tying has a notable stabilizing
influence on both internal and external modes in the sense that it
reduces the growthrate of the unstable modes as compared to the case
of an infinite or periodic cylinder. For a fixed equilibrium profile,
complete stabilization is obtained as soon as the cylinder length is
reduced below a critical value.
---------------------------------------------------------
Title: Linearly overstable magnetic convection in 1D compressible
and non-uniform plasmas
Authors: Hermans, D.; Kerner, W.; Goossens, M.
1990CoPhC..59..127H Altcode:
The linear overstable magnetic convection in a non-uniform and
compressible plasma slab is computed using a finite element eigenvalue
code. Overstable modes are found to be part of the slow-gravity
subspectrum and can be stabilized by increasing the magnetic field
strength. The periods of overstable oscillations are determined by
the isothermal frequency. Results for two different sequences of
equilibrium configurations are presented.
---------------------------------------------------------
Title: Numerical simulation of the stationary state of periodically
driven coronal loops
Authors: Poedts, Stefaan; Goossens, Marcel; Kerner, Wolfgang
1990CoPhC..59...75P Altcode:
The heating of solar coronal loops by resonant absorption of Alfvén
waves is studied in the framework of linearized, compressible,
resistive MHD by means of numerical simulations in which the loops
are approximated by straight cylindrical, axisymmetric plasma columns
with equilibrium quantities varying only in the radial direction. The
incident waves that excite the loops are modelled by a periodic external
source. The stationary state of this driven system is determined
numerically with a finite element code. The finite element technique is
extremely suitable to compute the nearly-singular solutions and yields
very accurate results. The efficiency of the heating mechanism and the
energy deposition profile in this stationary state strongly depend on
the characteristics of both the external driver and the equilibrium. A
numerical survey of the relevant parameter space shows that resonant
absorption is very efficient for typical coronal parameter values and
appears to be a viable candidate heating mechanism for solar loops. <P
/>Present address: JET Joint Undertaking, Theory Division, Abingdon,
Oxfordshire OX14 3EA, England.
---------------------------------------------------------
Title: Problems in the numerical modelling of plasmas. Contributed
papers. 4. European Workshop on Problems in the Numerical Modelling
of Plasmas (NUMOP 89), Spitzingsee (Germany, F.R.), 1 - 5 Oct 1989.
Authors: Goossens, M.; Kerner, W.
1990CoPhC..59.....G Altcode:
For the invited review papers of this workshop see the main entry
012.063.
---------------------------------------------------------
Title: Problems in the numerical modelling of plasmas. Invited review
papers. 4. European Workshop on Problems in the Numerical Modelling
of Plasmas (NUMOP 89), Spitzingsee (Germany, F.R.), 1 - 5 Oct 1989.
Authors: Goossens, M.; Kerner, W.
1990CoPhR..12.....G Altcode:
For the contributed papers of this workshop see the main entry 012.093.
---------------------------------------------------------
Title: Overstable slow-gravity modes in a thermally conducting
MHD plasma-slab
Authors: Hermans, D.; Goossens, M.; Kerner, W.
1990A&A...231..259H Altcode:
A study of the spectrum of linear motions of a compressible and
thermally conducting plasma slab with a horizontal magnetic field and
a constant vertical gravity field is conducted in order to obtain a
clear picture of overstable linear motions. A numerical scheme is used
based on a finite element discretization which allows for the rigorous
inclusion of the compressibility of the plasma and the nonuniformity of
the equilibrium configuration. It is found that the overstable linear
modes are part of the slow-gravity subspectrum that also contains the
isothermal continuum and two groups of purely exponentially growing
modes, determined by thermal and gravitational effects. The oscillatory
frequencies of the overstable modes are always smaller than the maximum
value of the isothermal frequency, so that the characteristic periods
of the overstable modes are larger than those obtained in the Boussinesq
approximation which are linked to the Alfven frequency.
---------------------------------------------------------
Title: Thermal Instability in Planar Solar Coronal Structures
Authors: van der Linden, R. A. M.; Goossens, M.
1990LNP...363..276V Altcode: 1990doqp.coll..276V; 1990IAUCo.117..276V
Prominentes and filaments are thought to arise as a consequence of
a magnetized plasma undergoing thermal instability. Therefore the
thermal stability of a magnetized plasma is investigated under coronal
conditions. The equilibrium structure of the plasma is approximated by
a 1-D slab configuration. This is investigated on thermal instability
taking into account optically thin plasma radiation and anisotropical
thermal conduction. The thermal conduction perpendicular to the
magnetic field is taken to be small but non-zero. The classical rigid
wall boundary conditions which are often applied in the litterature,
either directly on the plasma or indirectly through some other
medium, are replaced by a more physical situation in which the plasma
column is placed in a low-density background stretching towards
infinity. Results for a uniform equilibrium structure indicate the
major effect of this change is on the eigenfunctions rather than on the
growthrate. Essentially, perpendicular thermal conduction introduces
field-aligned fine-structure. It is also shown that in the presence
of perpendicular thermal conduction, thermal instability in a slab
model is only possible if the inner plasma has the shortest thermal
instability time-scale.
---------------------------------------------------------
Title: Dynamic stabilization of unstable gravity modes by magnetic
fields in non-uniform and compressible plasma
Authors: Hermans, D.; Goossens, M.
1989A&A...225..569H Altcode:
The dynamic stabilization of unstable gravity modes by magnetic
fields is investigated for nonuniform and compressible plasmas. This
is done by studying the spectrum of linear motions of 1D horizontal
plasmas in the presence of a horizontal sheared magnetic field and an
external vertical gravity field. In case of a compressible plasma the
slow subspectrum is the relevant part of the spectrum for the dynamic
stabilization of unstable gravity modes. Because of the nonuniformity
of the basic state this slow subspectrum consists of a discrete part
and a continuous part, with the latter defined by the range of the cusp
frequency. The slow subspectrum is computed for three sequences of basic
states which differ by the profiles of the frequency of Brunt-Vaisala,
the frequency of Alfven, and the cusp frequency. The magnetic field
has a stabilizing effect and complete stability can be obtained under
certain conditions for strong enough fields. The stabilizing effect of
the magnetic field is determined by the variation of the cusp frequency,
and, thus by the slow continuous part of the spectrum. The discrete
slow subspectrum hangs on the slow continuous spectrum, so to speak,
and stability can be obtained by transition of unstable regular normal
modes either to stable regular modes below the slow continuum or into
the slow continuum.
---------------------------------------------------------
Title: A formulation of non-ideal localized (or ballooning) modes
in the solar corona
Authors: Hood, A. W.; van der Linden, R.; Goossens, M.
1989SoPh..120..261H Altcode:
The stability equations for localized (or ballooning) modes in
the solar atmosphere are formulated. Dissipation due to viscosity,
resistivity, and thermal conduction are included using the general
forms due to Braginskii (1965). In addition, the effect of gravity,
plasma radiation, and coronal heating are included. The resulting
equations are one-dimensional and only involve derivatives along
the equilibrium magnetic field. Thus, the stabilising influence of
photospheric line-tying, which is normally neglected in most numerical
simulations, can be studied in a simple manner. Two applications to
sound wave propagation and thermal instabilities in a low-beta plasma
are considered with a view to determining realistic coronal boundary
conditions that model the lower, denser levels of the solar atmosphere
in a simple manner.
---------------------------------------------------------
Title: Alfvén-wave heating in resistive MHD
Authors: Poedts, Stefaan; Kerner, Wolfgang; Goossens, Marcel
1989JPlPh..42...27P Altcode:
Resonant absorption of Alfvén waves in tokamak plasmas is
studied numerically using the linearized equations of resistive
magnetohydrodynamics. A numerical code based on a finite-element
discretization is used for determining the stationary state of a
cylindrical plasma column that is excited by an external periodic
driver. The energy dissipation rate in the stationary state is
calculated and the dependence of the plasma heating on electrical
resistivity, the equilibrium profiles, and the wavenumbers and frequency
of the external driver is investigated. Resonant absorption is extremely
efficient when the plasma is excited with a frequency near that of a
so-called ‘collective mode’. The heating of a plasma by driving
it at the frequencies of discrete Alfvén waves is also investigated.
---------------------------------------------------------
Title: Numerical simulation of coronal heating by resonant absorption
of Alfvén waves
Authors: Poedts, Stefaan; Goossens, Marcel; Kerner, Wolfgang
1989SoPh..123...83P Altcode:
The heating of coronal loops by resonant absorption of Alfvén waves
is studied in compressible, resistive magnetohydrodynamics. The
loops are approximated by straight cylindrical, axisymmetric plasma
columns and the incident waves which excite the coronal loops are
modelled by a periodic external driver. The stationary state of
this system is determined with a numerical code based on the finite
element method. Since the power spectrum of the incident waves is
not well known, the intrinsic dissipation is computed. The intrinsic
dissipation spectrum is independent of the external driver and reflects
the intrinsic ability of the coronal loops to extract energy from
incident waves by the mechanism of resonant absorption.
---------------------------------------------------------
Title: Thermal Instability in Planar Solar Coronal Structures
Authors: van der Linden, R. A. M.; Goossens, M.
1989HvaOB..13..289V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Kink modes in coronal loops.
Authors: Goedbloed, J. P.; Goossens, M.; Poedts, S.
1989plap.work..103G Altcode:
Spectral theory of magnetohydrodynamic waves and instabilities has been
extensively developed. With proper modifications results obtained for
tokamaks can be transferred to the study of stability of coronal flux
loops and heating of the corona by means of Alfvén waves. In tokamaks
external kink modes are stabilized by the geometric constraint that
the modes should fit into the torus. In current-carrying coronal
loops the opposite problem arises, viz. the apparent absence of
external kink modes, as evidenced by their long life-time, spanning
many orders of magnitude of the characteristic growth-time of these
instabilities. Anchoring of the foot points of the field lines in the
photosphere is generally considered to be the responsible agent for
stabilization. Given the overall MHD stability of a coronal magnetic
loop structure, the subtle analysis of Alfvén wave heating by means
of the continuous spectrum may be undertaken. Here, an additional
complication is encountered which turns out to be quite beneficial
though from the point of view of heating efficiency. This gives rise to
improper modes which have both a global character and a non-integrable
part which admits quasi-dissipation.
---------------------------------------------------------
Title: Coronal heating by resonant absorption in resistive MHD.
Authors: Poedts, S.; Goossens, M.; Kerner, W.
1989plap.work..107P Altcode:
The heating of coronal loops by the process of resonant
absorption of Alfvén waves is studied in compressible, resistive
magnetohydrodynamics. The authors consider a one-dimensional,
cylindrical-symmetric plasma column which is excited periodically by
means of an external driver. They determine the intrinsic dissipation
spectrum which is independent of the external driver (whose power
spectrum is not known) and yet reveals some interesting features of
heating by resonant absorption. Resonant absorption is very efficient
for typical coronal loop parameter values. A considerable part of
the energy supplied by the external driver, is actually dissipated
Ohmically and converted into heat. The energy dissipation rate is
almost independent of the resistivity for the relevant values of this
parameter. The efficiency of the heating mechanism strongly depends
on the equilibrium profiles, the wave numbers and the frequency of
the external driver.
---------------------------------------------------------
Title: Deposit of observations of beta Cephei stars in the IUA
archives of unpublished photoelectric observations of variable stars.
Authors: Cuypers, J.; Lampens, P.; Goossens, M.
1988BICDS..35..155C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effects of Parallel and Perpendicular Viscosity on
Resistive Ballooning Modes in Line-Tied Coronal Magnetic Fields
Authors: van der Linden, R.; Goossens, M.; Hood, A. W.
1988SoPh..115..235V Altcode:
The study of resistive ballooning instabilities in line-tied
coronal magnetic fields is extended by including viscosity in the
stability analysis. The equations that govern the resistive ballooning
instabilities are derived and the effects of parallel and perpendicular
viscosity are included using Braginskii's stress tensor. Numerical
solutions to these equations are obtained under the rigid wall
boundary conditions for arcades with cylindrically-symmetric magnetic
fields. It is found that viscosity has a stabilizing effect on the
resistive ballooning instabilities with perpendicular viscosity being
more important by far than parallel viscosity. The strong stabilizing
effect of perpendicular viscosity can lead to complete stabilization
for realistic values of the equilibrium quantities.
---------------------------------------------------------
Title: The continuous spectrum of MHD waves in 2-D solar loops and
arcades - Parametric study of poloidal mode coupling for poloidal
magnetic fields
Authors: Poedts, S.; Goossens, M.
1988A&A...198..331P Altcode:
This parametric study investigates how poloidal mode coupling of ideal
MHD continuum modes of two-dimensional models for coronal loops and
arcades depends on equilibrium quantities. The two physical causes
for poloidal mode coupling considered are non-circularity of the
cross-sections of the flux surfaces and variation of the equilibrium
density along the magnetic field lines. This phenomenon of wave number
coupling is typical for the continuous spectrum of MHD waves of 2D
plasmas. It has two important consequences: the eigenfrequencies
are modified, and the eigenfunctions tend to localize. The present
parametric study is concerned with the dependence of poloidal mode
coupling on the value of the plasma beta (i.e. the ratio of the
plasma pressure to the magnetic pressure), the ellipticity of the
cross-sections, and the variation of the equilibrium density normal to
the magnetic surfaces and along the magnetic field lines. For realistic
values of the parameters, it is found that the continuous spectrum is
modified, the ranges of the continuum frequencies are considerably
enlarged, and the derivatives of the continuum frequencies normal
to the magnetic surfaces are considerably increased. Consequently,
the phasemixing time is reduced, and the efficiency of phase-mixing
as a heating mechanism of solar loops and arcades is increased. The
dissipation of wave energy depends on two spatial coordinates and is
found to be larger at the tops of the coronal loops.
---------------------------------------------------------
Title: Linear spectrum of magnetostatic and thermally conducting
planar plasmas
Authors: Hermans, D.; Goossens, M.; Kerner, W.; Lerbinger, K.
1988PhFl...31..547H Altcode:
The linear spectrum of one-dimensional magnetostatic and thermally
conducting plasmas is analyzed. The model and mathematical expressions
for the eigenvalue problem of linear motions of a one-dimensional
plasma in a gravitational field are presented. The effects of thermal
conduction on the linear spectrum, the ideal slow continuous spectrum,
and the isothermal slow continuous spectrum are investigated. Particular
attention is given to changes in the slow and fast magnetoacoustic modes
and the thermal modes of the isothermal slow continuous spectrum. It
is observed that thermal conduction affects various portions of the
linear spectrum of ideal MHD differently; the ideal Alfven continuous
spectrum is not influenced by thermal conduction; and the isothermal
slow continuous spectrum replaces the ideal slow continuous spectrum.
---------------------------------------------------------
Title: Overstable Convection in a Non-Uniform Magnetic Field
Authors: Hermans, D.; Goossens, M.; Kerner, W.; Lerbinger, K.
1988IAUS..123..395H Altcode:
The linear spectrum of a non-homogeneous, compressible and thermally
conducting planar plasma with a vertical gravitational field and a
sheared horizontal magnetic field is studied. It is shown that the
spectrum has two continuous parts. The Alfvén continuum of linear
ideal MHD is unaffected by radiative conduction, but the slow continuum
is removed and replaced by a new continuous part called isothermic
continuum. Purely exponentially growing and overstable normal modes
have been determined numerically and the distribution of their
eigenfrequencies in the complex plane has been studied.
---------------------------------------------------------
Title: The Continuous Spectrum of Magnetohydrodynamic Waves in 2d
Solar Loops and Arcades - First Results on Poloidal Mode Coupling
for Poloidal Magnetic Fields
Authors: Poedts, S.; Goossens, M.
1987SoPh..109..265P Altcode:
A first attempt is made to study the continuous spectrum of linear
ideal MHD for 2D solar loops and to understand how 2D effects change
the continuum eigenfrequencies and continuum eigenfunctions. The
continuous spectrum is computed for 2D solar loops with purely poloidal
magnetic fields and it is investigated how non-circularity of the
cross-sections of the poloidal magnetic surfaces and variations
of density along the poloidal magnetic field lines change the
continuous spectrum and induce poloidal wave number coupling in the
eigenfunctions. Approximate analytical results and numerical results
are obtained for the eigenfrequencies and the eigenfunctions and the
poloidal wave number coupling is clearly illustrated. It is found
that the continuum frequencies are substantially increased, that the
ranges of the continuum frequencies are considerably enlarged and that
the derivatives of the continuum frequencies normal to the magnetic
surfaces are substantially increased. The eigenfunctions are strongly
influenced by poloidal wave number coupling. Implications of these
findings for the heating mechanisms of resonant absorption and phase
mixing are briefly considered.
---------------------------------------------------------
Title: The effects of viscosity on resistive ballooning modes in
line-tied coronal magnetic fields.
Authors: Vanderlinden, R.; Goossens, M.; Hood, A. W.
1987ESASP.275..109V Altcode: 1987sspp.symp..109V
The study of resistive ballooning instabilities in line-tied
coronal magnetic fields is extended by including viscosity in the
stability analysis. The equations that govern the resistive ballooning
instabilities are derived and the effects of viscosity are included
using Braginskii's stress tensor. Numerical solutions to the equations
are obtained under the rigid wall boundary conditions for arcades with
cylindrically symmetric magnetic fields. The stabilizing influences of
parallel and perpendicular viscosity are investigated. Perpendicular
viscosity is more important and can lead to complete stabilization.
---------------------------------------------------------
Title: Poloidal Mode Coupling of Alfvén Continuum Modes in 2D
Coronal Loops
Authors: Poedts, S.; Goossens, M.
1987rfsm.conf..272P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Poloidal mode coupling of Alfvén continuum modes in 2D
coronal loops.
Authors: Poedts, S.; Goossens, M.
1987rfsm.conf..277P Altcode:
Study of the continuous spectrum of two-dimensional (2D) solar loops is
obviously required to see how 2D effects change the continuous spectrum
and influence resonant absorption and phase mixing. The results show
that the Alfvén continuum of static 2D solar loops can be changed
substantially compared with the 1D plasma case. This can have important
consequences for resonant absorption and phase mixing. In particular
the present results indicate that density variation along magnetic
field lines increases the efficiency of phase mixing of Alfvén
continuum waves.
---------------------------------------------------------
Title: The Alfven-gravity spectrum of an incompressible slab
Authors: Hermans, D.; Goossens, M.
1987A&A...172...85H Altcode:
The linear spectrum of a horizontal incompressible layer in the presence
of a nonuniform magnetic field is studied in order to understand the
dynamic stabilization of the unstable gravity modes. Such a study is
necessary to understand overstable magnetic convection. Overstable
magnetoconvection has been invoked as a possible mechanism for waves
and oscillations in sunspots and for rapid oscillations observed in
Ap-stars. The Alfven-gravity for waves and oscillations in sunspots and
for rapid oscillations observed in Ap-stars. The Alfven-gravity spectrum
is computed for four sequences of equilibrium configurations that
differ by the profiles of the Brunt-Vaisala frequency and the Alfven
frequency, and hence by the Alfven continuum. The evolution of the
spectrum is determined for increasing strength of the magnetic field.
---------------------------------------------------------
Title: Viscous Normal Modes on Coronal Inhomogeneities and Their
Role as a Heating Mechanism
Authors: Steinolfson, R. S.; Priest, E. R.; Poedts, S.; Nocera, L.;
Goossens, M.
1986ApJ...304..526S Altcode:
Viscous damping of Alfven surface waves is examined both analytically
and numerically using incompressible MHD. Normal modes are shown to
exist on discontinuous as well as continuously varying interfaces in
Alfven speed. The waves experience negligible decay below the transition
zone. High-frequency waves damp just above the transition region,
while those of lower frequency lose energy further out. A comparison of
dissipative decay rates shows that wave damping by viscosity proceeds
approximately two orders of magnitude faster than by resistivity.
---------------------------------------------------------
Title: On the existence of the continuous spectrum of ideal MHD in
a 2D magnetostatic equilibrium.
Authors: Goossens, M.; Poedts, S.; Hermans, D.
1985SoPh..102...51G Altcode:
The continuous spectrum of a 2D magnetostatic equilibrium with
y-invariance is derived. It is shown that the continuous spectrum is
given by an eigenvalue problem on each magnetic surface and is related
to the different behaviour of the equilibrium quantities in different
magnetic surfaces. The special case of a uniform poloidal magnetic field
in a 1D equilibrium that is stratified with height, has been considered
in detail and it is found that there is no continuous spectrum. It is
shown that this result relies completely on the artificial property
that the behaviour of the equilibrium quantities along a magnetic field
line is independent of the field line considered. As a consequence
the non-existence of a continuous spectrum in a 1D equilibrium with
a uniform magnetic field cannot be used to argue that the continuous
spectrum has no physical relevance.
---------------------------------------------------------
Title: The continuous spectrum of an axisymmetric self-gravitating and
static equilibrium with a mixed poloidal and toroidal magnetic field
Authors: Poedts, S.; Hermans, D.; Goossens, M.
1985A&A...151...16P Altcode:
The continuous spectrum of the linearized equations of ideal MHD
is investigated for an axisymmetric, self-gravitating equilibrium
with a mixed poloidal and toroidal magnetic field. The continuous
spectrum for a purely poloidal magnetic field is treated as a special
case. The continuous spectrum of a purely poloidal magnetic field
is given by an eigenvalue problem of two uncoupled ordinary second
order differential equations along the magnetic field lines, so
that there are two uncoupled continuous parts of the spectrum: an
Alfvén continuum and a cusp continuum. Variational expressions for
the continuum frequencies are derived for each of the two continua,
and it is found that the Alfvén continuum is always on the stable side
of the spectrum but the cusp continuum can be unstable. The continuous
spectrum in the presence of a mixed poloidal and toroidal magnetic field
is given by an eigenvalue problem of a fourth order system of coupled
ordinary differential equations. A variational expression is derived
for the continuum frequencies and it is found that the equilibrium
gravitational field can lead to an unstable continuous spectrum.
---------------------------------------------------------
Title: Axisymmetric force-free stellar magnetospheres
Authors: Goossens, M.; Hereygers, G.
1985A&A...149..253G Altcode:
Self-consistent axisymmetric force-free magnetospheres are computed on
the assumption that the toroidal component is related to the poloidal
flux function through a power law and that the emergent field has both
a dipolar and a quadrupolar contribution. It is found that there is a
maximum value of the toroidal magnetic field which cannot be exceeded
if the magnetosphere is to be force-free, and this maximum value
decreases when the quadrupolar contribution to the poloidal magnetic
field becomes more important. Simple polynomial expressions are derived
for the quantities that describe the variations of the magnetic field
components over the stellar surface and these polynomial expressions
allow magnetic observations to be modelled with axisymmetric force-free
magnetic fields.
---------------------------------------------------------
Title: The Continuous Spectrum of AN Axisymmetric, Self-Gravitating
Equilibrium in the Presence of a Poloidal Magnetic Field
Authors: Hermans, D.; Goossens, M.; Poedts, S.
1984ESASP.207..297H Altcode:
The continuous spectrum of oscillation frequencies is examined for an
axisymmetric, self-gravitating equilibrium in the presence of a purely
poloidal magnetic field. It is shown that the continuous spectrum is
given by an eigenvalue problem of two uncoupled ordinary second orders
differential equations along the magnetic field lines. The two decoupled
continuous spectra have modes that are polarized either perpendicular or
parallel to the magnetic field lines and may be called Alfven continuum
and cusp continuum in analogy to the linear diffuse pinch. Curvature
and toroidicity influence the two continua, but only the cusp continuum
is affected by gravity and compressibility. Variational expressions
for the continuum frequencies are derived and it is found that only
the cusp continuum can attain negative values. The stability depends
on the distribution of density, pressure, gravity, and magnetic field
along the magnetic field lines.
---------------------------------------------------------
Title: The continuous spectrum of an axisymmetric equilibrium with a
mixed poloidal and toroidal magnetic field and with gravity included.
Authors: Poedts, S.; Goossens, M.; Hermans, D.
1984ESASP.220..201P Altcode: 1984ESPM....4..201P
The continuous spectrum of a static, axisymmetric self-gravitating
equilibrium with a mixed poloidal and toroidal magnetic field is
given by an eigenvalue problem of two coupled ordinary second-order
differential equations. The solutions have motions in the magnetic
surfaces that are not polarized purely perpendicular and purely parallel
to the magnetic field lines and show mixed properties. This coupling of
the classical Alfven and cusp continuum is due to the toroidal magnetic
field component and even persists in the incompressible limit. A
variational expression was derived for the continuum frequencies
and it is shown that the continuum frequencies can be negative. The
stability depends on the distributions of density and pressure in the
magnetic surfaces.
---------------------------------------------------------
Title: Light variability of sigma Scorpii.
Authors: Goossens, M.; Lampens, P.; de Maerschalck, D.; Schrooten, M.
1984A&A...140..223G Altcode:
The light variability of the spectroscopic binary and Beta Cephei star
Sigma Scorpii is studied in view of Van Hoof's (1966) observations,
which when subjected to period analyses in individual years reveal
two oscillations whose periods and mean light curves do not exhibit
any detectable year-to-year variation. Period analyses of the
light observations grouped in orbital phase intervals show that the
largest amplitude oscillation found in the individual years is the
only oscillation in observations grouped along the binary orbit. The
light variability of Sigma Sco can be modelled by two mathematically
equivalent, but physically different, models: two intrinsic pulsations
with constant periods and amplitudes, or one intrinsic pulsation in
which mean light curve amplitude and shape are modulated by tidal
action.
---------------------------------------------------------
Title: Linear incompressible magnetoconvection in a planar layer
with a non-uniform horizontal magnetic field.
Authors: Hermans, D.; Goossens, M.; Polfliet, R.
1984ESASP.220..199H Altcode: 1984ESPM....4..199H
The linear stability of an incompressible planar layer in the presence
of a non-uniform horizontal magnetic field is studied for three
sequences of equilibrium configurations. It is found that the Alfvén
continuum is fundamental for the understanding of the linear stability.
---------------------------------------------------------
Title: Linear Stability of an Incompressible Horizontal Layer with
a Non-uniform Horizontal Magnetic Field
Authors: Hermans, D.; Goossens, M.; Polfliet, R.
1984LIACo..25..387H Altcode: 1984trss.conf..387H; 1984tpss.conf..387H
No abstract at ADS
---------------------------------------------------------
Title: Continuous Spectra of Oscillation Frequencies of an
Axisymmetric Incompressible Equilibrium with a Poloidal Magnetic field
Authors: Goossens, M.; Hermans, D.; Poedts, S.
1984LIACo..25..382G Altcode: 1984trss.conf..382G; 1984tpss.conf..382G
No abstract at ADS
---------------------------------------------------------
Title: Light variations of Sigma Sco (conference paper)
Authors: Goossens, M.; Lampens, P.; de Maerschalck, D.; Schrooten, M.
1983HvaOB...7..215G Altcode:
The light variability of σ Sco can be represented by two mathematically
equivalent but physically different models: (1) two intrinsic pulsations
with constant periods and amplitudes; (2) one intrinsic pulsation of
which the amplitude and the shape of the mean light curve are modulated
by tidal action.
---------------------------------------------------------
Title: A singular perturbation approach to the effect of a weak
magnetic field on stellar oscillations
Authors: Biront, D.; Goossens, M.; Cousens, A.; Mestel, L.
1982MNRAS.201..619B Altcode:
The effect of a weak, axisymmetric magnetic field on linear, adiabatic
oscillations of a star is studied. Over the bulk of the star a regular
perturbation treatment is adequate, but this breaks down near the
surface if the field does not vanish there but extends into the low
density exterior. Singular perturbation methods are applied to the
case of a star with a polytropic atmosphere and a curl-free, dipolar
field in the surface region, oscillating in a mode that is radial in
the absence of the field. Deep down the motions remain nearly radial,
but as the surface is approached the field ensures that the horizontal
and vertical motions are of the same order, possibly affecting the
interpretation of observed light and velocity curves.
---------------------------------------------------------
Title: Frequency analyses of light and radialvelocity observations
of alf Lup.
Authors: Lampens, P.; Goossens, M.
1982A&A...115..413L Altcode:
Photoelectric photometric observations by Van Hoof are used to
study the light variability of α Lupi. Frequency analyses of Van
Hoof's observations in Y and UV light reveal the existence of only
one oscillation with an amplitude larger than the noise level of the
observations. The frequency of this oscillation is determined to be f
= 3.848424 ± O.OOOO4Oc/d and shows no variation that can be detected
with Van Hoof's observations. We show that both the light variability
and the radial velocity variations can be represented by an oscillation
with a constant frequency.
---------------------------------------------------------
Title: Unstable poloidal magnetic fields in stars
Authors: van Assche, W.; Goossens, M.; Tayler, R. J.
1982A&A...109..166V Altcode:
The dynamic stability of an axisymmetric star with a purely poloidal
magnetic field is studied by means of the energy method. It is shown
that a poloidal magnetic field is always unstable at a simple neutral
point, which is encircled by the magnetic field lines. The results
obtained by Wright and by Markey and Tayler for particular field
geometries are thus generalized. The sigma-energy method is used to
obtain the growth rates of instabilities in the neighbourhood of a
simple neutral point. Numerical applications to weak poloidal fields
in polytropes show that these fields are unstable with very short
e-folding times.
---------------------------------------------------------
Title: The magnetic field surface structure of HD 215441
Authors: Goossens, M.; van Assche, W.; Demoitie, R.; Gadeyne, L.
1982Ap&SS..83..213G Altcode:
The oblique rotator model with axisymmetric magnetic fields containing
a toroidal component is adopted for the description of the magnetic
variations observed in Ap stars. The toroidal component is taken as a
non-linear function of the stream function of the poloidal component. A
scheme is presented for the determination fromH <SUB>e</SUB>
andH <SUB>s</SUB> observations of the parameters that describe the
surface distribution of the magnetic field components. This schema is
succesfully applied to the magnetic observations of HD 215441, and the
existence of different magnetic field surface structures that reproduce
equally well the observations has been found. The main characteristics
of the different solutions are discussed.
---------------------------------------------------------
Title: Composite Boundary Layer Approximation for Adiabatic Pulsation
in Stars with a Magnetic Field
Authors: Biront, D.; Cousens, A.; Goossens, M.; Mestel, L.
1982pccv.conf..257B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Frequency Analyses of Light and Radial Observations -
Observations of - LUPI and - Crucis
Authors: Lampens, P.; Goossens, M.; Cuypers, J.
1982pccv.conf..317L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A contribution to the study of the stability and oscillations
of stars with a magnetic field.
Authors: Goossens, M.
1982MeBel..44...23G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Frequency Analysis of Photometric Observations of the
Beta-Cephei Star Nu-Eridani
Authors: Cuypers, J.; Goossens, M.
1981A&AS...45..487C Altcode:
A phase dispersion minimization method is used to perform a frequency
analysis of Van Hoof's photometric observations of the β Cephei
star, ν Eridani. The frequency analysis reveals the existence of four
oscillations in the light variability of p Eridani. In addition to the
primary largest amplitude oscillation with frequency f<SUB>0</SUB>
= 5.76349 c/d, three oscillations have been detected with closely
spaced frequencies : f<SUB>1</SUB> = 5.6373 c/d, f<SUB>2</SUB> =
5.6535 c/d, f<SUB>3</SUB> = 5.6201 c/d. The present numerical values
of the frequencies are in good agreement with the values obtained by
Saito and Kubiak from a frequency analysis of radial velocity data.
---------------------------------------------------------
Title: Additional Results for Unstable Stratified Toroidal Magnetic
Fields in Stars
Authors: Goossens, M.; Biront, D.; Tayler, R. J.
1981Ap&SS..75..521G Altcode:
General conditions for adiabatic instability of weak axisymmetric
toroidal magnetic fields are obtained. In particular all fields which
haveH <SUB>ϕ</SUB>=0 and ∂H {<SUB>ϕ</SUB>/<SUP>2</SUP>}/∂δ>0
simultaneously are unstable. This includes all fields which exist near
to the axis of symmetry and, indeed, any without an infinite current
density anywhere. A detailed discussion is given of fields with an
angular dependenceH {<SUB>ϕ</SUB>/<SUP>2</SUP>}∼|P <SUB>ℓ</SUB>
(cos θ)|. These fields are dynamically unstable with respect to both
axisymmetric and non-axisymmetric perturbations for all values of the
azimuthal wave number |m|. The maximum growth rates are independent
ofm and they are largest for the fields that are defined in the regions
closest to the polar axis.
---------------------------------------------------------
Title: On the effective magnetic field of an oblique rotating star
with an axisymmetric magnetic field
Authors: Goossens, M.; Martens, L.; Gadeyne, L.
1981A&A....95..240G Altcode:
This study considers first the theoretical determination of
the effective magnetic field in an oblique rotating and magnetic
star. An analytical expression for the variation of the effective
field is derived for a surface distribution of a magnetic field that
is axisymmetric about an axis through the stellar centre, but that
is not necessarily irrotational. Subsequently, it is shown that the
effective-field observations only contain partial information on
the surface distribution of a magnetic field if it is assumed to be
axisymmetric but not irrotational.
---------------------------------------------------------
Title: Axisymmetric and mixed poloidal-toroidal magnetic fields for
HD 215441 and 53 Cam
Authors: Goossens, M.; van Assche, W.
1981LIACo..23..277G Altcode: 1981cpsu.conf..277G
A method for the construction of magnetic field geometries from
the observed periodic magnetic variations in Ap stars based on
the oblique rotator model is presented and applied to the stars HD
215441 and 53 Cam. The method involves the fitting of observations
of the surface and effective magnetic fields with the expressions
for the surface distribution of the magnetic field components for an
axisymmetric magnetic field in order to determine the distribution of
the toroidal and poloidal field components. The solutions obtained
for various values of the angles i and beta for HD 215441 are found
to be characterized by a predominant dipole-like surface structure of
the poloidal component with values of the polar strength in the range
40 to 50 kGs and differences in magnetic field at the two poles in the
range -40 kGs to -5 kGs. Solutions for 53 Cam are all characterized by
poloidal components that have a strong quadrupole-like contribution
and relatively strong toroidal components, with a smaller range in
polar differences.
---------------------------------------------------------
Title: The influence of a magnetic field on the oscillations of
early-type stars
Authors: Biront, D.; Goossens, M.; Cousens, A.; Mestel, L.
1981LIACo..23..337B Altcode: 1981cpsu.conf..337B
Reference is made to the contention of Weiss et al. (1981) that
some early-type pulsationally unstable stars almost certainly have
an intrinsic magnetic field. The intent here is to discover what
effects the coupling of the magnetic field and the pulsation have. It
is thought that subsequent work in this field could explain the long
period variability of the light curves of some stars (Blazhko effect)
as a modulation caused by the coupling between the pulsation under
the influence of the magnetic field and the oblique rotation of the
star. It is noted that deep down in the star the thermal pressure
is much greater than the magnetic pressure; over most of the star,
therefore, the effects of the field are negligible. The period, too,
is little affected. An oscillation mode of the star which deep down is
purely radial is considered. It is found that near the surface of the
star the forces exerted by the distorted field give an angle-dependent
horizontal motion comparable to the radial one. A time dependence of
exp(i omega t) is assumed throughout for all perturbation quantities.
---------------------------------------------------------
Title: The Effect of a Magnetic Field on Stellar Pulsations as a
Singular Perturbation Problem
Authors: Goossens, M.; Biront, D.
1980SSRv...27..661G Altcode: 1980IAUCo..58..661G
The perturbation problem that describes the effect of a weak magnetic
field on stellar adiabatic oscillation is considered. This perturbation
problem is singular when the magnetic field does not vanish at the
stellar surface, and a regular perturbation scheme fails where the
magnetic pressure is comparable to the thermodynamic pressure. The
application of the Method of Matched Asymptotic Expansion is used to
obtain expressions for the eigenfunctions and the eigenfrequencies.
---------------------------------------------------------
Title: On unstably stratified toroidal magnetic fields in stars
Authors: Goossens, M.; Tayler, R. J.
1980MNRAS.193..833G Altcode:
A previous discussion of the stability of stars containing an
axisymmetric toroidal magnetic field is extended and it is shown that
there exist continuous perturbations which give a negative value of
(delta) (W to the sigma power) when one of the conditions for stability
is violated; this contradicts a conclusion reached by Dicke. The main
characteristics of the displacement fields that are associated with
the instabilities in the special, but astrophysically relevant, case
of a weak toroidal field are indicated. In particular, axisymmetric
toroidal magnetic fields with a strength of 100 million G in the cores
of solar-type stars produce instabilities with e-folding times of the
order of a few hours.
---------------------------------------------------------
Title: Omicron-Stability Analysis of Hydromagnetic Instabilities in
Stars with Toroidal Magnetic Fields
Authors: Goossens, M.; Biront, D.
1980SSRv...27..667G Altcode: 1980IAUCo..58..667G
σ-stability analysis is used to investigate the adiabatic stability of
a star containing an axisymmetric toroidal magnetic field. Necessary
and sufficient conditions for σ-stability are derived. Special
attention is devoted to the typical hydromagnetic instabilities that
can be introduced by a weak toroidal magnetic field in a star that is
stably stratified in the absence of any magnetic field. An expression
for the maximum growth rate of instability is derived and the basic
properties of the displacement fields associated with the instabilities
are indicated.
---------------------------------------------------------
Title: Periods and Photographic Mean Light Curves of 17 Long Period
Variables in a Field around alpha= 17h, delta= -70d
Authors: Goossens, M.; Waelkens, C.
1980IBVS.1828....1G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Periods and Mean Light Curves of 22 Long Period Variables in
a Field Centered at alpha= 13h, delta= -70d
Authors: Goossens, M.; Stoop, C.; Waelkens, C.
1980IBVS.1760....1G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: -stability analysis of toroidal magnetic fields in stars
Authors: Goossens, M.
1980GApFD..15..123G Altcode:
This study uses -stability analysis to explore the hydromagnetic
instabilities of a star with an axisymmetric toroidal magnetic
field. Necessary and sufficient conditions for -stability are derived
and are used to obtain the maximum growth rate (MGR) of instability. The
concepts determining the point of MGR and the -stability line are
introduced as rigorous tools to investigate the instabilities. The
-stability analysis of a star with a weak toroidal magnetic field is
considered in detail. It is shown that the hydromagnetic stability
of a star with a weak toroidal magnetic field depends solely on the
shape of the impressed toroidal magnetic field. Relevant hydromagnetic
instabilities are in almost all cases due to an unstable stratification
of the toroidal magnetic field with respect to the colatitude. The MGR
depends on the strength of the impressed magnetic field, but important
properties of the hydromagnetic instabilities, such as the extent of
the instability region, the position of the point of MGR, and the shape
of the -stability line, depend only on the topology of the magnetic
field. The results of the -stability analysis of a star with a weak
magnetic field are used to investigate the instabilities caused by
admissible weak toroidal magnetic fields H = Kβ (r sin θ)2β - 1
(with β a parameter) in a polytrope n = 3. These toroidal magnetic
fields are always most unstable under non-axisymmetric perturbations
with |m| = 1 (for β < 2) or under axisymmetric perturbations
(β ≥ 2). The growth rates of the most unstable perturbations lead
to very short e-folding times when applied to real stars with weak
magnetic fields. Instabilities with growth rates comparable to the
MGR are primarily internal phenomena, but interesting instabilities
with e-folding times still relatively short compared to the nuclear
time scale may occur further out.
---------------------------------------------------------
Title: The effective magnetic field of an oblique rotating star
with an irrotational and axisymmetric surface distribution of
magnetic field.
Authors: Goossens, M.
1979A&A....79..210G Altcode:
Summary. This study deals with the theoretical determination of the
effective field, H,, in a rigidly rotating and magnetic star. An
analytical expression is derived for H, when the surface distribution
of the magnetic field is irrotational and axisymmetric about an
axis through the stellar center. This expression is applied to the
axisymmetric and irrotational surface fields that have been considered
in the literature to explain observed magnetic variations. It is
shown that the expressions for these particular geometries, which were
sometimes obtained by elaborate mathematical methods, can be recovered
in an elegant way. Key words: magnetic stars - oblique rotator model -
effective magnetic field
---------------------------------------------------------
Title: On the orientation of magnetic and rotation axes in Ap
stars. II. Results.
Authors: Hensberge, H.; van Rensbergen, W.; Goossens, M.; Deridder, G.
1979A&A....75...83H Altcode:
On the assumption of a decentered magnetic dipole, models are
constructed for 17 Ap stars for which the phase diagram of the effective
magnetic field is known. This model involves the determination of the
displacement of the dipole, the angle between the rotation axis and
the line of sight, the angle between the rotation and magnetic axes,
and a parameter determining the strength of the magnetic field. It
is possible to determine these parameters quite accurately if the
phase diagram of the mean surface magnetic field is also known. It
is shown that, for a sample of 17 stars with theoretically predicted
distribution functions, there is no evidence against randomly oriented
magnetic axes in Ap stars.
---------------------------------------------------------
Title: Unrestricted Second-Order Tensor Virial Equations for Linear
Oscillations of Magnetic Configurations
Authors: Goossens, M.
1979Ap&SS..60..401G Altcode:
This paper deals with the second-order tensor virial equations for the
linear oscillations of a gaseous mass in the presence of a magnetic
field. It is shown that the commonly used linearized versions of
the tensor virial equations are restricted integral equations that
incorporate the linearized equation of motion but not the boundary
condition. These restricted equations only allow trial functions that
fulfil the boundary condition and are of limited practical value. The
unrestricted variational principle for the linear oscillations of a
magnetic configuration is used to derive a more general formulation
of the second-order tensor virial equations so that the linear trial
function ξ<SUB>i</SUB> =X <SUB>ij</SUB> x <SUB>j</SUB> can be used to
study the oscillations of a configuration with a magnetic field that
extends in the exterior vacuum. The unrestricted virial equations
have been applied to Ferraro's model and approximate results for
the eigenfrequencies and eigenfunctions have been obtained for nine
oscillation modes.
---------------------------------------------------------
Title: On the Lamb Frequencies of Spherical Gaseous Star
Authors: Smeyers, P.; Goossens, M.
1979GApFD..11..237S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hydromagnetic Instabilities in Polytropic Stars Containing
Toroidal Mag- netic Fields
Authors: Goossens, M.; Veugelen, R.
1978A&A....70..277G Altcode:
Summary. This study concerns the typically hydromagnetic instabilities
that occur in a polytrope n =3 that is stably stratified in the absence
of any magnetic field and that contains a weak toroidal magnetic
field = Kpr sin 0. It is shown that the non-axisymmetric perturbations
associated with Im 1=1 are the only possible unstable perturbations and
that the most unstable perturbations are localized in the immediate
vicinity of the center of the polytrope. A trial function method
is used to study the characteristic features of the hydromaguetic
instabilities. It is inferred that the motions that are driven by the
most violent hydromagnetic instabilities are characterized by small
vertical wavelenths for perturbations confined close to the polar axis
and by small horizontal wavelengths for other perturbations. Key words:
toroidal magnetic fields - hydromagnetic instabilities
---------------------------------------------------------
Title: Application of the Method of Matched Asymptotic Expansions
to the Radial Oscillations of a Magnetic Cylinder
Authors: Goossens, M.; Cornette, J.
1978Ap&SS..58..227G Altcode:
This study deals with the singular character of the perturbation
introduced into the eigenvalue problem of the linear and adiabatic
oscillations of a gaseous configuration by a magnetic field that is
non-zero on the boundary surface of the configuration. This singular
character implies that a regular perturbation scheme cannot yield
uniformly valid expansion for the eigenfunctions. This investigation
considers the application of the Method of Matched Asymptotic Expansions
(M.M.A.E.) to the latter singular perturbation problem in order to
obtain uniformly valid expansions for the eigenfunctions and first-order
expressions for the eigenfrequencies. As an illustrative example, the
M.M.A.E. is applied to the eigenvalue problem of the linear, radial,
and adiabatic oscillations of a homogeneous cylindrical plasma with
a constant longitudinal magnetic field.
---------------------------------------------------------
Title: On the Lamb frequencies of a spherical gaseous star.
Authors: Smeyers, P.; Goossens, M.
1978GApFD..11..237S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effect of a Poloidal Magnetic Field on the Linear and
Adiabatic Oscillations of a Polytrope n=2
Authors: Goossens, M.
1977Ap&SS..52....3G Altcode:
The frequencies of the linear and adiabatic oscillations of a gaseous
polytrope with a poloidal magnetic field are determined with the aid of
a perturbation method. The influence of the poloidal magnetic field on
the different types of spheroidal oscillation modes is discussed. The
poloidal magnetic field generally strengthens the stability of the
oscillation modes and this effect is the largest in the case of the
non-radialp-modes.
---------------------------------------------------------
Title: On the orientation of magnetic and rotation axes in Ap
stars. Theory of the decentred magnetic dipole.
Authors: Hensberge, H.; van Rensbergen, W.; Goossens, M.; Deridder, G.
1977A&A....61..235H Altcode:
A formalism is developed for determining the angle between the magnetic
and rotational axes of a magnetic star, the angle between its rotational
axis and the line of sight, and the displacement of the dipole along
the magnetic axis. The variation with time of the effective magnetic
field and the mean surface field is analyzed for a centered oblique
rotating magnetic dipole as well as for a decentered dipole. The
effective field is calculated as a function of the cosine of the angle
between the magnetic axis and the line of sight for positive values
of the dipole displacement and various values of the limb darkening
coefficient; the surface field is computed as a function of the cosine
of the same angle for positive values of the dipole displacement. The
results are used to evaluate the ratios between the minimum and maximum
effective and surface magnetic fields. It is noted that these ratios
may be employed to select possible magnetic fields and geometries for
Ap stars if the variations of the two fields with phase are known.
---------------------------------------------------------
Title: Non-Radial Oscillations and Stability of a Polytrope n=3 with
a Toroidal Magnetic Field
Authors: Goossens, M.
1976Ap&SS..44..397G Altcode:
A perturbation method has been applied for the determination of the
frequencies of the linear and adiabatic oscillations of a gaseous
polytropic configuration pervaded by a purely toroidal magnetic
field. The influence of a toroidal magnetic field on the frequencies
of the different types of spheroidal oscillation modes is discussed.
---------------------------------------------------------
Title: Non-Radial Oscillations and Stability of Prendergast's Model
Authors: Goossens, M.
1976Ap&SS..43....9G Altcode:
The frequencies of the linear and adiabatic oscillations of
Prendergast's model are determined with the aid of a perturbation
method. The influence of the magnetic field on the frequencies of the
different types of spheroidal oscillation modes is discussed.
---------------------------------------------------------
Title: Stellar Oscillations and Magnetic Field Perturbations of the
Boundary Conditions
Authors: Goossens, M.; Smeyers, P.; Denis, J.
1976Ap&SS..39..257G Altcode:
The effect of a weak magnetic field on the adiabatic radial and
non-radial oscillations of a stellar configuration is studied by
means of a perturbation method. Special attention is devoted to the
perturbation of the oscillation frequencies resulting from the change
of the boundary conditions caused by the magnetic field. This change is
related to the fact that the introduction of a magnetic field removes
the singularity at the surface of the equilibrium configuration. The
perturbation method is applied to Ferraro's model and the influence
of a magnetic field on the frequencies of the different types of
oscillation modes is discussed.
---------------------------------------------------------
Title: Gravity Modes in Composite Polytropic Stars
Authors: Goossens, M.; Smeyers, P.
1974Ap&SS..26..137G Altcode:
We study gravity modes of composite polytropic stars which consist of
two convectively stable zones separated by a convectively unstable
zone. In addition to the unstable gravity modes associated with the
intermediate zone, we distinguish two types of stable gravity modes,
one type being mainly associated with the core, the other one being
mainly associated with the envelope. We find also some accidental
‘resonances’ between the core and the envelope.
---------------------------------------------------------
Title: First Ephemerides of Five Variable Stars in Eridanus and Fornax
Authors: Deurinck, R.; Goossens, M.
1973IBVS..792....1D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Perturbation of the Radial and Non-Radial Oscillations of a
Star by a Magnetic Field
Authors: Goossens, M.
1972Ap&SS..16..386G Altcode:
A generalization of the perturbation method is applied to the problem
of the radial and the non-radial oscillations of a gaseous star which
is distorted by a magnetic field. An expression is derived for the
perturbation of the oscillation frequencies due to the presence
of a weak magnetic field when the equilibrium configuration is a
spheroid. The particular application to the homogeneous model with a
purely poloidal field inside, due to a current distribution proportional
to the distance from the axis of symmetry, and a dipole type field
outside is considered in detail. The main result is that the magnetic
field has a large and almost stabilizing effect on unstableg-modes,
particularly on higher order modes. With the considered magnetic field
the surface layers appear to have a large weight.