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Inigo Arregui Submitted: 2007-08-28 07:26
The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs in a zero-$eta$
configuration and for parallel propagation of waves does not allow the existence of surface waves.
When oblique propagation of perturbations is considered both surface and body waves are
able to propagate. When the perpendicular wave number is larger than a certain value, the body kink
mode becomes a surface wave. In addition, a sausage surface mode is found below the internal cut-off
frequency. When non-uniformity in the equilibrium is included, surface and body modes are damped due
to resonant absorption. In this paper, first, a normal-mode analysis is performed and the period,
the damping rate, and the spatial structure of eigenfunctions are obtained. Then, the time-dependent
problem is solved, and the conditions under which one or the other type of mode is excited are
investigated.
Authors: I. Arregui, J. Terradas, R. Oliver, J.L. Ballester
Projects: None
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Publication Status: Solar Physics. Accepted
Last Modified: 2007-08-30 21:46
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The resonant damping of fast magnetohydrodynamic oscillations in a system of two coronal slabs
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Inigo Arregui Submitted: 2007-08-09 04:38
Observations of transversal coronal loop oscillations very often show the excitation and damping
of oscillations in groups of coronal loops rather than in individual and isolated structures.
We present results on the oscillatory properties (periods, damping rates, and spatial distribution of
perturbations) for resonantly damped oscillations in a system of two inhomogeneous coronal slabs and compare
them to the properties found in single slab loop models.
A system of two identical coronal loops is modeled, in Cartesian geometry, as being composed by
two density enhancements. The linear magnetohydrodynamic (MHD) wave equations for oblique propagation of waves are
solved and the damping of the different solutions, due to the transversal inhomogeneity of the density profile, is computed.
The physics of the obtained results is analyzed by an examination of the perturbed physical variables.
We find that, due to the interaction between the loops, the normal modes of oscillation present in a single slab split
into symmetric and antisymmetric oscillations when a system of two identical slabs is considered.
The frequencies of these solutions may differ from the single slab results when the distance between the loops is of the
order of a few slab widths. Oblique propagation of waves weakens this interaction, since
solutions become more confined to the edges of the slabs. The damping is strong for
surface-like oscillations, while sausage body-like solutions are unaffected. For some solutions, and small
slab separations, the damping in a system of two loops differs substantially from the damping of a single loop.
Authors: Inigo Arregui, Jaume Terradas, Ramon Oliver, Jose Luis Ballester
Projects:
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Publication Status: Submitted
Last Modified: 2007-08-10 11:00
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The influence of the internal structuring of coronal loops on the properties of their damped transverse oscillations
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Inigo Arregui Submitted: 2007-02-08 07:37
The geometry and physical conditions in solar
coronal loops are complicated and still not well
understood. Recent high resolution
observations, obtained with TRACE, indicate the existence of sub-resolution
transverse structuring, not accessible to direct
observation. This ingredient has not yet been
taken into account in previous theoretical
models used for the study of transversal coronal
loop oscillations and their damping due to
resonant conversion of energy.
This study aims to assess the effect of the
possibly unresolved internal structure of a coronal loop on
the properties of its transverse oscillations and on the
efficiency of resonant absorption as a damping mechanism of these
oscillations. The equilibrium configuration of a single
coronal loop with internal density structuring is
modelled by considering the loop as being
composed by two very close parallel identical coronal slabs in Cartesian geometry.
The period of the oscillation and the damping
time, for the resonantly damped fundamental
kink mode, are computed. These quantities are then compared to those obtained
for two models for a single equivalent slab without internal density structuring.
We find that the period and the damping time of
a coronal loop with internal density structuring
change by less than 15\%, when compared to the
same oscillatory properties of a single coronal
loop with either the same density contrast or a single coronal
loop with the same total mass.
Therefore the internal density structuring of a coronal loop does not
affect very much its oscillatory properties. However, the sub-resolution structuring of a
coronal loop, with different densities in its
components or with different widths could vary these results.
Authors: I. Arregui, J. Terradas, R. Oliver, & J.L. Ballester
Projects:
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Publication Status: A&A, Vol. 466, 1145-1151 (2007)
Last Modified: 2007-04-26 02:46
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MHD seismology of coronal loops using the period and damping of quasi-mode kink oscillations
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Inigo Arregui Submitted: 2006-11-23 01:45
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.
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. 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 Alfven speed (or Alfven 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, Alfven speed or Alfven travel time). The projection of these solutions onto the Alfven speed axis is found to be constrained to a rather limited interval. Upper limits to the internal Alfven speed are derived for 9 of the 11 analysed events.
Authors: I. Arregui, J. Andries, T. Van Doorsselaere, M. Goossens, S. Poedts
Projects:
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Publication Status: Published, A&A, Vol 463, 333-338
Last Modified: 2007-01-30 05:20
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