Model comparison for the density structure across solar coronal waveguides 

Inigo Arregui Submitted: 20150909 02:50
The spatial variation of physical quantities, such as the mass density, across solar atmospheric waveguides governs the timescales and spatial scales for wave damping and energy dissipation. The direct measurement of the spatial distribution of density, however, is difficult and indirect seismology inversion methods have been suggested as an alternative. We applied Bayesian inference, model comparison, and modelaveraging techniques to the inference of the crossfield density structuring in solar magnetic waveguides using information on periods and damping times for resonantly damped magnetohydrodynamic (MHD) transverse kink oscillations. Three commonly employed alternative profiles were used to model the variation of the mass density across the waveguide boundary. Parameter inference enabled us to obtain information on physical quantities such as the Alfvén travel time, the density contrast, and the transverse inhomogeneity length scale. The inference results from alternative density models were compared and their differences quantified. Then, the relative plausibility of the considered models was assessed by performing model comparison. Our results indicate that the evidence in favor of any of the three models is minimal, unless the oscillations are strongly damped. In such a circumstance, the application of modelaveraging techniques enables the computation of an evidenceweighted inference that takes into account the plausibility of each model in the calculation of a combined inversion for the unknown physical parameters.
Authors: I. Arregui, R. Soler, A. Asensio Ramos
Projects: None

Publication Status: ApJ, accepted.
Last Modified: 20150909 09:49



Prominence seismology 

Inigo Arregui Submitted: 20120201 10:38
Given the difficulty in directly determining prominence physicalparametersfrom observations, prominence seismology stands as an alternativemethod toprobe the nature of these structures. We show recent examples of theapplication ofmagnetohydrodynamic (MHD) seismology techniques to infer physicalparameters inprominence plasmas. They are based on the application of inversiontechniques usingobserved periods, damping times, and plasma flow speeds of prominencethread oscillations.The contribution of Hinode to the subject has been of centralimportance.We show an example based on data obtained with Hinode's Solar OpticalTelescope.Observations show an active region limb prominence, composed by amyriad of thinhorizontal threads that flow following a path parallel to thephotosphere and displaysynchronous vertical oscillations. The coexistence of waves and flowscan be firmlyestablished. By making use of an interpretation based on transverseMHD kink oscillations,a seismological analysis of this event is performed. It is shown thatthe combinationof high quality Hinode observations andproper theoretical models allows flowsand waves to become two useful characteristics for our understanding ofthe nature ofsolar prominences.
Authors: I. Arregui, J. L. Ballester, R. Oliver, R. Soler, J. Terradas
Projects: None

Publication Status: 4th Hinode Science Meeting: Unsolved Problems and New Insights ASP Conference Series, Vol. 455, pp. 211218 (2012)
Last Modified: 20120201 12:07



Damping mechanisms for oscillations in solar prominences 

Inigo Arregui Submitted: 20100219 01:15
Small amplitude oscillations are a commonly observed feature in prominences/filaments. These oscillations appear to be of local nature, are associated to the fine structure of prominence plasmas, and simultaneous flows and counterflows are also present. The existing observational evidence reveals that small amplitude oscillations, after excited, are damped in short spatial and temporal scales by some as yet not well determined physical mechanism(s). Commonly, these oscillations have been interpreted in terms of linear magnetohydrodynamic (MHD) waves, and this paper reviews the theoretical damping mechanisms that have been recently put forward in order to explain the observed attenuation scales. These mechanisms include thermal effects, through nonadiabatic processes, mass flows, resonant damping in nonuniform media, and partial ionization effects. The relevance of each mechanism is assessed by comparing the spatial and time scales produced by each of them with those obtained from observations. Also, the application of the latest theoretical results to perform prominence seismology is discussed, aiming to determine physical parameters in prominence plasmas that are difficult to measure by direct means.
Authors: I. Arregui, J.L. Ballester
Projects: None

Publication Status: SSR (submitted)
Last Modified: 20100219 08:55



Subject will be restored when possible 

Inigo Arregui Submitted: 20070828 07:26
The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs in a zeroeta
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 cutoff
frequency. When nonuniformity in the equilibrium is included, surface and body modes are damped due
to resonant absorption. In this paper, first, a normalmode analysis is performed and the period,
the damping rate, and the spatial structure of eigenfunctions are obtained. Then, the timedependent
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

Publication Status: Solar Physics. Accepted
Last Modified: 20070830 21:46



The resonant damping of fast magnetohydrodynamic oscillations in a system of two coronal slabs 

Inigo Arregui Submitted: 20070809 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
surfacelike oscillations, while sausage bodylike 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:

Publication Status: Submitted
Last Modified: 20070810 11:00



The influence of the internal structuring of coronal loops on the properties of their damped transverse oscillations 

Inigo Arregui Submitted: 20070208 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 subresolution
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 subresolution 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:

Publication Status: A&A, Vol. 466, 11451151 (2007)
Last Modified: 20070426 02:46



MHD seismology of coronal loops using the period and damping of quasimode kink oscillations 

Inigo Arregui Submitted: 20061123 01:45
We combine the magnetohydrodynamic (MHD) theory of resonantly damped quasimode 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 quasimode period and damping, in onedimensional fully nonuniform 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 onedimensional solution curve in the twodimensional parameter space (density contrast, transverse inhomogeneity lengthscale). 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 onedimensional solution curve, in the threedimensional parameter space (density contrast, transverse inhomogeneity lengthscale, 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.
Authors: I. Arregui, J. Andries, T. Van Doorsselaere, M. Goossens, S. Poedts
Projects:

Publication Status: Published, A&A, Vol 463, 333338
Last Modified: 20070130 05:20




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