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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.

---------------------------------------------------------
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.

---------------------------------------------------------
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 (&lt;27 R<SUB>⊙</SUB>)
  as well as the eccentricity of the orbit (ɛ &lt; 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 &amp; 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 &amp; Goossens (1999, A&amp;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 &gt; 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 &gt;~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 &amp; Yang (1988) and
  Ruderman &amp; 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 &amp; Yang
  (\cite{hollweg1988}) and Ruderman &amp; 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 &amp; 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
  &amp; 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>&gt;
  C<SUB>Ae</SUB><SUP>2</SUP>, body waves for C_Te<SUP>2</SUP>&gt;
  C<SUB>T</SUB><SUP>2</SUP> and surface leaky waves for
  C<SUB>Te</SUB><SUP>2</SUP>&lt; C<SUB>T</SUB><SUP>2</SUP> &lt;
  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 &amp;
  Taroyan and Varga &amp; 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. .

---------------------------------------------------------
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>&lt;allowbreak omega
  &lt;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 &amp; 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 &amp; 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 &amp; 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, &amp; 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&gt;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 &lt; 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.

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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.

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Title: On the Lamb Frequencies of Spherical Gaseous Star
Authors: Smeyers, P.; Goossens, M.
1979GApFD..11..237S    Altcode:
  No abstract at ADS

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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

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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.

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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.