Author name code: arregui ADS astronomy entries on 2022-09-14 author:"Arregui, Inigo" ------------------------------------------------------------------------ Title: Construction of coronal hole and active region magnetohydrostatic solutions in two dimensions: Force and energy balance Authors: Terradas, J.; Soler, R.; Oliver, R.; Antolin, P.; Arregui, I.; Luna, M.; Piantschitsch, I.; Soubrié, E.; Ballester, J. L. Bibcode: 2022A&A...660A.136T Altcode: 2022arXiv220206800T; 2022arXiv220206800J Coronal holes and active regions are typical magnetic structures found in the solar atmosphere. We propose several magnetohydrostatic equilibrium solutions that are representative of these structures in two dimensions. Our models include the effect of a finite plasma-β and gravity, but the distinctive feature is that we incorporate a thermal structure with properties similar to those reported by observations. We developed a semi-analytical method to compute the equilibrium configuration. Using this method, we obtain cold and under-dense plasma structures in open magnetic fields representing coronal holes, while in closed magnetic configurations, we achieve the characteristic hot and over-dense plasma arrangements of active regions. Although coronal holes and active regions seem to be antagonistic structures, we find that they can be described using a common thermal structure that depends on the flux function. In addition to the force balance, the energy balance is included in the constructed models using an a posteriori approach. From the two-dimensional computation of thermal conduction and radiative losses in our models, we infer the required heating function to achieve energy equilibrium. We find that the temperature dependence on height is an important parameter that may prevent the system from accomplishing thermal balance at certain spatial locations. The implications of these results are discussed in detail. Title: Recent Applications of Bayesian Methods to the Solar Corona Authors: Arregui, Iñigo Bibcode: 2022FrASS...926947A Altcode: Solar coronal seismology is based on the remote diagnostics of physical conditions in the corona of the Sun by comparison between model predictions and observations of magnetohydrodynamic wave activity. Our lack of direct access to the physical systems of interest makes information incomplete and uncertain so our conclusions are at best probabilities. Bayesian inference is increasingly being employed in the area, following a general trend in the space sciences. In this paper, we first justify the use of a Bayesian probabilistic approach to seismology diagnostics of solar coronal plasmas. Then, we report on recent results that demonstrate its feasibility and advantage in applications to coronal loops, prominences and extended regions of the corona. Title: Methodology for Predicting the Probability Distribution of the Amplitude of Solar Cycle 25 Authors: Arregui, Iñigo Bibcode: 2022SoPh..297...21A Altcode: A number of precursor-type methods for solar-cycle prediction are based on the use of regression models and confidence-level estimates. A drawback of these methods is that they do not permit one to make probability statements, nor do they offer straightforward ways to propagate the uncertainty from observations to the quantities of interest. We suggest a method for calculating the probability of the maximum amplitude of Solar Cycle 25 using Bayesian inference. We illustrate this approach with the predictions made by one particular phenomenological model that relates the time interval between the termination events of preceding cycles to the amplitude of the next cycle. Our results show well-constrained posterior-predictive distributions for the maximum sunspot number. The impact of uncertainty on the sunspot number and the time interval between terminators is quantified. A comparison between the maximum sunspot numbers of the past and the posterior-predictive distributions computed using the method enables us to quantify the quality of the inference and the prediction. Title: Editorial Appreciation Authors: Arregui, Iñigo; Leibacher, John; Mandrini, Cristina H.; van Driel-Gesztelyi, Lidia; Wheatland, Michael S. Bibcode: 2022SoPh..297...11A Altcode: No abstract at ADS Title: Erratum: "Bayesian Evidence for a Nonlinear Damping Model for Coronal Loop Oscillations" (2021, ApJL, 915, L25) Authors: Arregui, Iñigo Bibcode: 2021ApJ...922L..22A Altcode: No abstract at ADS Title: Bayesian Evidence for a Nonlinear Damping Model for Coronal Loop Oscillations Authors: Arregui, Iñigo Bibcode: 2021ApJ...915L..25A Altcode: 2021arXiv210612243A Recent observational and theoretical studies indicate that the damping of solar coronal loop oscillations depends on the oscillation amplitude. We consider two mechanisms: linear resonant absorption and a nonlinear damping model. We confront theoretical predictions from these models with observed data in the plane of observables defined by the damping ratio and the oscillation amplitude. The structure of the Bayesian evidence in this plane displays a clear separation between the regions where each model is more plausible relative to the other. There is qualitative agreement between the regions of high marginal likelihood and Bayes factor for the nonlinear damping model and the arrangement of observed data. A quantitative application to 101 loop oscillation cases observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) results in the marginal likelihood for the nonlinear model being larger in the majority of them. The cases with conclusive evidence for the nonlinear damping model outnumber considerably those in favor of linear resonant absorption. Title: Coronal Heating by MHD Waves Authors: Van Doorsselaere, Tom; Srivastava, Abhishek K.; Antolin, Patrick; Magyar, Norbert; Vasheghani Farahani, Soheil; Tian, Hui; Kolotkov, Dmitrii; Ofman, Leon; Guo, Mingzhe; Arregui, Iñigo; De Moortel, Ineke; Pascoe, David Bibcode: 2020SSRv..216..140V Altcode: 2020arXiv201201371V The heating of the solar chromosphere and corona to the observed high temperatures, imply the presence of ongoing heating that balances the strong radiative and thermal conduction losses expected in the solar atmosphere. It has been theorized for decades that the required heating mechanisms of the chromospheric and coronal parts of the active regions, quiet-Sun, and coronal holes are associated with the solar magnetic fields. However, the exact physical process that transport and dissipate the magnetic energy which ultimately leads to the solar plasma heating are not yet fully understood. The current understanding of coronal heating relies on two main mechanism: reconnection and MHD waves that may have various degrees of importance in different coronal regions. In this review we focus on recent advances in our understanding of MHD wave heating mechanisms. First, we focus on giving an overview of observational results, where we show that different wave modes have been discovered in the corona in the last decade, many of which are associated with a significant energy flux, either generated in situ or pumped from the lower solar atmosphere. Afterwards, we summarise the recent findings of numerical modelling of waves, motivated by the observational results. Despite the advances, only 3D MHD models with Alfvén wave heating in an unstructured corona can explain the observed coronal temperatures compatible with the quiet Sun, while 3D MHD wave heating models including cross-field density structuring are not yet able to account for the heating of coronal loops in active regions to their observed temperature. Title: Resonant absorption: Transformation of compressive motions into vortical motions Authors: Goossens, M.; Arregui, I.; Soler, R.; Van Doorsselaere, T. Bibcode: 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. Title: Quantifying the evidence for resonant damping of coronal waves with foot-point wave power asymmetry Authors: Montes-Solís, M.; Arregui, I. Bibcode: 2020A&A...640L..17M Altcode: 2020arXiv200803004M We use Coronal Multi-channel Polarimeter (CoMP) observations of propagating waves in the solar corona together with Bayesian analysis to assess the evidence of models with resonant damping and foot-point wave power asymmetries. We considered two nested models: a reduced and a larger model. The reduced model considers resonant damping as the sole cause of the measured discrepancy between outward and inward wave power. The larger model contemplates an extra source of asymmetry with its origin in the foot-points. We first computed the probability distributions of parameters conditional on the models and the observed data. The obtained constraints were then used to calculate the evidence for each model in view of the data. We find that we need to consider the larger model to explain CoMP data and to accurately infer the damping ratio, hence, to better assess the possible contribution of the waves to coronal heating. Title: Coronal Cooling as a Result of Mixing by the Nonlinear Kelvin-Helmholtz Instability Authors: Hillier, Andrew; Arregui, Iñigo Bibcode: 2019ApJ...885..101H Altcode: 2019arXiv190911351H Recent observations show cool, oscillating prominence threads fading when observed in cool spectral lines and appearing in warm spectral lines. A proposed mechanism to explain the observed temperature evolution is that the threads were heated by turbulence driven by the Kelvin-Helmholtz instability that developed as a result of wave-driven shear flows on the surface of the thread. As the Kelvin-Helmholtz instability is an instability that works to mix the two fluids on either side of the velocity shear layer, in the solar corona it can be expected to work by mixing the cool prominence material with that of the hot corona to form a warm boundary layer. In this paper, we develop a simple phenomenological model of nonlinear Kelvin-Helmholtz mixing, using it to determine the characteristic density and temperature of the mixing layer. For the case under study, with constant pressure across the two fluids, these quantities are {ρ }mixed}=\sqrt{{ρ }1{ρ }2} and {T}mixed}=\sqrt{{T}1{T}2}. One result from the model is that it provides an accurate—as determined by comparison with simulation results—determination of the kinetic energy in the mean velocity field. A consequence of this is that the magnitude of turbulence—and with it, the energy that can be dissipated on fast timescales—as driven by this instability can be determined. For the prominence-corona system, the mean temperature rise possible from turbulent heating is estimated to be less than 1% of the characteristic temperature (which is found to be T mixed = 105 K). These results highlight that mixing, and not heating, is likely to be the cause of the observed transition between cool to warm material. One consequence of this result is that the mixing creates a region with higher radiative loss rates on average than either of the original fluids, meaning that this instability could contribute a net loss of thermal energy from the corona, i.e., coronal cooling. Title: Fundamental transverse vibrations of the active region solar corona Authors: Luna, M.; Oliver, R.; Antolin, P.; Arregui, I. Bibcode: 2019A&A...629A..20L Altcode: 2019arXiv190705212L Context. Some high-resolution observations have revealed that the active region solar corona is filled with a myriad of thin strands even in apparently uniform regions with no resolved loops. This fine structure can host collective oscillations involving a large portion of the corona due to the coupling of the motions of the neighbouring strands.
Aims: We study these vibrations and the possible observational effects.
Methods: We theoretically investigated the collective oscillations inherent to the fine structure of the corona. We have called them fundamental vibrations because they cannot exist in a uniform medium. We used the T-matrix technique to find the normal modes of random arrangements of parallel strands. We considered an increasing number of tubes to understand the vibrations of a huge number of tubes of a large portion of the corona. We additionally generated synthetic time-distance Doppler and line-broadening diagrams of the vibrations of a coronal region to compare with observations.
Results: We have found that the fundamental vibrations are in the form of clusters of tubes where not all the tubes participate in the collective mode. The periods are distributed over a wide band of values. The width of the band increases with the number of strands but rapidly reaches an approximately constant value. We have found an analytic approximate expression for the minimum and maximum periods of the band. The frequency band associated with the fine structure of the corona depends on the minimum separation between strands. We have found that the coupling between the strands is on a large extent and the motion of one strand is influenced by the motions of distant tubes. The synthetic Dopplergrams and line-broadening maps show signatures of collective vibrations, not present in the case of purely random individual kink vibrations.
Conclusions: We conclude that the fundamental vibrations of the corona can contribute to the energy budget of the corona and they may have an observational signature.

A movie associated to Fig. 10 is available at http:// https://www.aanda.org Title: Normal Modes of Transverse Coronal Loop Oscillations from Numerical Simulations. I. Method and Test Case Authors: Rial, S.; Arregui, I.; Oliver, R.; Terradas, J. Bibcode: 2019ApJ...876...86R Altcode: 2019arXiv190200211R The purpose of this work is to develop a procedure to obtain the normal modes of a coronal loop from time-dependent numerical simulations with the aim of better understanding observed transverse loop oscillations. To achieve this goal, in this paper we present a new method and test its performance with a problem for which the normal modes can be computed analytically. In a follow-up paper, the application to the simulations of Rial et al. is tackled. The method proceeds iteratively and at each step consists of (i) a time-dependent numerical simulation followed by (ii) the Complex Empirical Orthogonal Function (CEOF) analysis of the simulation results. The CEOF analysis provides an approximation to the normal mode eigenfunctions that can be used to set up the initial conditions for the numerical simulation of the following iteration, in which an improved normal mode approximation is obtained. The iterative process is stopped once the global difference between successive approximate eigenfunctions is below a prescribed threshold. The equilibrium used in this paper contains material discontinuities that result in one eigenfunction with a jump across these discontinuities and two eigenfunctions whose normal derivatives are discontinuous there. After six iterations, the approximations to the frequency and eigenfunctions are accurate to ≲0.7% except for the eigenfunction with discontinuities, which displays a much larger error at these positions. Title: Inference of magnetic field strength and density from damped transverse coronal waves Authors: Arregui, I.; Montes-Solís, M.; Asensio Ramos, A. Bibcode: 2019A&A...625A..35A Altcode: 2019arXiv190305437A A classic application of coronal seismology uses transverse oscillations of waveguides to obtain estimates of the magnetic field strength. The procedure requires information on the density of the structures. Often it ignores the damping of the oscillations. We computed marginal posteriors for parameters such as the waveguide density, the density contrast, the transverse inhomogeneity length scale, and the magnetic field strength under the assumption that the oscillations can be modelled as standing magnetohydrodynamic (MHD) kink modes damped by resonant absorption. Our results show that the magnetic field strength can be properly inferred, even if the densities inside and outside the structure are largely unknown. Incorporating observational estimates of plasma density further constrains the obtained posteriors. The amount of information that is included a priori for the density and the density contrast influences their corresponding posteriors, but very little the inferred magnetic field strength. The decision to include or leave out the information on the damping and the damping timescales has a minimal impact on the obtained magnetic field strength. In contrast to the classic method, which provides numerical estimates with error bars or possible ranges of variation for the magnetic field strength, Bayesian methods offer the full distribution of plausibility over the considered range of possible values. The methods applied to available datasets of observed transverse loop oscillations can be extended to prominence fine structures or chromospheric spicules, and implemented to propagating waves in addition to standing oscillations. Title: Mixed properties of MHD waves in non-uniform plasmas Authors: Goossens, Marcel L.; Arregui, Inigo; Van Doorsselaere, Tom Bibcode: 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. Title: Inference of physical parameters in solar prominence threads Authors: Montes-Solís, M.; Arregui, I. Bibcode: 2019hsax.conf..514M Altcode: We consider magnetohydrodynamics models and observations of transverse oscillations in prominence threads to obtain information on their physical properties such as the magnetic field strength, the plasma density, or the length. We further compare between short and long thread limits in period ratio models and compute the relative plausibility of alternative mechanisms in explaining the observed damping of transverse oscillations. Bayesian techniques are used for both analyses. The results show that the physical parameters of interest can be inferred. Values of period ratio around 1 are more likely in the long thread limit while shorter and larger values are more likely in the short thread limit. The mechanism known as resonant absorption in the Alfvén continuum is the most plausible damping mechanism. Title: Inferring physical parameters in solar prominence threads Authors: Montes-Solís, M.; Arregui, I. Bibcode: 2019A&A...622A..88M Altcode: 2018arXiv181207262M Context. High resolution observations have permitted the resolution of solar prominences/filaments into sets of threads/fibrils. However, the values of the physical parameters of these threads and their structuring remain poorly constrained.
Aims: We use prominence seismology techniques to analyse transverse oscillations in threads by comparing magnetohydrodynamic (MHD) models and observations.
Methods: We applied Bayesian methods to obtain two different types of information. We first inferred the marginal posterior distribution of physical parameters such as the magnetic field strength or length of the thread, when a totally filled tube, partially filled tube, and three damping models are considered as certain; the three damping models are resonant absorption in the Alfvén continuum, resonant absorption in the slow continuum, and Cowling's diffusion. Then, we compared the relative plausibility between alternative MHD models by computing the Bayes factors.
Results: Well-constrained probability density distributions can be obtained for the magnetic field strength, length of the thread, density contrast, and parameters associated with the damping models. In a comparison of the damping models of resonant absorption in the Alfvén continuum, resonant absorption in the slow continuum, and Cowling's diffusion due to partial ionisation of prominence plasma, the resonant absorption in the Alfvén continuum is the most plausible mechanism to explain the existing observations. Relations between periods of fundamental and first overtone kink modes with values around 1 are better explained by expressions of the period ratio in the long thread approximation, while the rest of the values are more probable in the short thread limit for the period ratio.
Conclusions: Our results show that Bayesian analysis offers valuable methods to perform parameter inference and a model comparison in the context of prominence seismology. Title: No unique solution to the seismological problem of standing kink magnetohydrodynamic waves Authors: Arregui, I.; Goossens, M. Bibcode: 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. Title: On Kelvin-Helmholtz and parametric instabilities driven by coronal waves Authors: Hillier, Andrew; Barker, Adrian; Arregui, Iñigo; Latter, Henrik Bibcode: 2019MNRAS.482.1143H Altcode: 2018MNRAS.tmp.2618H; 2018arXiv181002773H The Kelvin-Helmholtz instability has been proposed as a mechanism to extract energy from magnetohydrodynamic (MHD) kink waves in flux tubes, and to drive dissipation of this wave energy through turbulence. It is therefore a potentially important process in heating the solar corona. However, it is unclear how the instability is influenced by the oscillatory shear flow associated with an MHD wave. We investigate the linear stability of a discontinuous oscillatory shear flow in the presence of a horizontal magnetic field within a Cartesian framework that captures the essential features of MHD oscillations in flux tubes. We derive a Mathieu equation for the Lagrangian displacement of the interface and analyse its properties, identifying two different instabilities: a Kelvin-Helmholtz instability and a parametric instability involving resonance between the oscillatory shear flow and two surface Alfvén waves. The latter occurs when the system is Kelvin-Helmholtz stable, thus favouring modes that vary along the flux tube, and as a consequence provides an important and additional mechanism to extract energy. When applied to flows with the characteristic properties of kink waves in the solar corona, both instabilities can grow, with the parametric instability capable of generating smaller scale disturbances along the magnetic field than possible via the Kelvin-Helmholtz instability. The characteristic time-scale for these instabilities is ∼100 s, for wavelengths of 200 km. The parametric instability is more likely to occur for smaller density contrasts and larger velocity shears, making its development more likely on coronal loops than on prominence threads. Title: Temporal and Spatial Scales for Coronal Heating by Alfvén Wave Dissipation in Transverse Loop Oscillations Authors: Terradas, Jaume; Arregui, Iñigo Bibcode: 2018RNAAS...2..196T Altcode: 2018RNAAS...2d.196T No abstract at ADS Title: Prominence oscillations Authors: Arregui, Iñigo; Oliver, Ramón; Ballester, José Luis Bibcode: 2018LRSP...15....3A Altcode: Prominences are intriguing, but poorly understood, magnetic structures of the solar corona. The dynamics of solar prominences has been the subject of a large number of studies, and of particular interest is the study of prominence oscillations. Ground- and space-based observations have confirmed the presence of oscillatory motions in prominences and they have been interpreted in terms of magnetohydrodynamic waves. This interpretation opens the door to perform prominence seismology, whose main aim is to determine physical parameters in magnetic and plasma structures (prominences) that are difficult to measure by direct means. Here, we review the observational information gathered about prominence oscillations as well as the theoretical models developed to interpret small and large amplitude oscillations and their temporal and spatial attenuation. Finally, several prominence seismology applications are presented. Title: Bayesian coronal seismology Authors: Arregui, Iñigo Bibcode: 2018AdSpR..61..655A Altcode: 2017arXiv170908372A In contrast to the situation in a laboratory, the study of the solar atmosphere has to be pursued without direct access to the physical conditions of interest. Information is therefore incomplete and uncertain and inference methods need to be employed to diagnose the physical conditions and processes. One of such methods, solar atmospheric seismology, makes use of observed and theoretically predicted properties of waves to infer plasma and magnetic field properties. A recent development in solar atmospheric seismology consists in the use of inversion and model comparison methods based on Bayesian analysis. In this paper, the philosophy and methodology of Bayesian analysis are first explained. Then, we provide an account of what has been achieved so far from the application of these techniques to solar atmospheric seismology and a prospect of possible future extensions. Title: Comparison of Damping Mechanisms for Transverse Waves in Solar Coronal Loops Authors: Montes-Solís, María; Arregui, Iñigo Bibcode: 2017ApJ...846...89M Altcode: 2017arXiv170903347M We present a method to assess the plausibility of alternative mechanisms to explain the damping of magnetohydrodynamic transverse waves in solar coronal loops. The considered mechanisms are resonant absorption of kink waves in the Alfvén continuum, phase mixing of Alfvén waves, and wave leakage. Our methods make use of Bayesian inference and model comparison techniques. We first infer the values for the physical parameters that control the wave damping, under the assumption of a particular mechanism, for typically observed damping timescales. Then, the computation of marginal likelihoods and Bayes factors enable us to quantify the relative plausibility between the alternative mechanisms. We find that, in general, the evidence is not large enough to support a single particular damping mechanism as the most plausible one. Resonant absorption and wave leakage offer the most probable explanations in strong damping regimes, while phase mixing is the best candidate for weak/moderate damping. When applied to a selection of 89 observed transverse loop oscillations, with their corresponding measurements of damping timescales and taking into account data uncertainties, we find that positive evidence for a given damping mechanism is only available in a few cases. Title: Magnetic field strength in solar coronal waveguides Authors: Arregui, I.; Asensio Ramos, A. Bibcode: 2017hsa9.conf..587A Altcode: We applied Bayesian techniques to the problem of inferring the magnetic field strength in transversely oscillating solar coronal loops from observed periods and damping times. This was done by computing the marginal posterior probability density for parameters such as the waveguide density, the density contrast, the transverse inhomogeneity length scale, and the magnetic field strength under the assumption that the observed waves can be modelled as standing or propagating magnetohydrodynamic (MHD) kink modes of magnetic flux tubes. Our results indicate that the magnetic field strength can be inferred, even if the densities inside and outside the structure are largely unknown. When information on plasma density is available, the method enables to self-consistently include this knowledge to further constrain the inferred magnetic field strength. The inclusion of the observed oscillation damping enables to obtain information on the transverse density structuring and considerably alters the obtained posterior for the magnetic field strength. Title: Heating of the solar corona Authors: Arregui, Iñigo Bibcode: 2017psio.confE..59A Altcode: No abstract at ADS Title: Comparison of damping mechanisms for transverse waves in coronal loops Authors: Montes-Solís, M.; Arregui, I. Bibcode: 2017psio.confE.115M Altcode: No abstract at ADS Title: On the Magnetism and Dynamics of Prominence Legs Hosting Tornadoes Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.; Collados, M.; Beck, C.; de la Cruz Rodríguez, J. Bibcode: 2016ApJ...825..119M Altcode: 2016arXiv160501183M Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires an understanding of their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here we analyze four consecutive spectro-polarimetric scans of a prominence hosting tornadoes on its legs, which helps us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, which is probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation that is probably related to the motion of the fibrils. Concerning the dynamics, we argue that (1) if rotation exists, it is intermittent, lasting no more than one hour, and (2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves). Title: Model Comparison for the Density Structure across Solar Coronal Waveguides Authors: Arregui, I.; Soler, R.; Asensio Ramos, A. Bibcode: 2015ApJ...811..104A Altcode: 2015arXiv150902340A 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 model-averaging techniques to the inference of the cross-field density structuring in solar magnetic waveguides using information on periods and damping times for resonantly damped magnetohydrodynamic 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 model-averaging techniques enables the computation of an evidence-weighted inference that takes into account the plausibility of each model in the calculation of a combined inversion for the unknown physical parameters. Title: On the nature of transverse coronal waves revealed by wavefront dislocations Authors: López Ariste, A.; Luna, M.; Arregui, I.; Khomenko, E.; Collados, M. Bibcode: 2015A&A...579A.127L Altcode: 2015arXiv150503348L Context. Coronal waves are an important aspect of the dynamics of the plasma in the corona. Wavefront dislocations are topological features of most waves in nature and also of magnetohydrodynamic waves. Are there dislocations in coronal waves?
Aims: The finding and explanation of dislocations may shed light on the nature and characteristics of the propagating waves, their interaction in the corona, and in general on the plasma dynamics.
Methods: We positively identify dislocations in coronal waves observed by the Coronal Multi-channel Polarimeter (CoMP) as singularities in the Doppler shifts of emission coronal lines. We study the possible singularities that can be expected in coronal waves and try to reproduce the observed dislocations in terms of localization and frequency of appearance.
Results: The observed dislocations can only be explained by the interference of a kink and sausage wave modes propagating with different frequencies along the coronal magnetic field. In the plane transverse to the propagation, the cross-section of the oscillating plasma must be smaller than the spatial resolution, and the two waves result in net longitudinal and transverse velocity components that are mixed through projection onto the line of sight. Alfvén waves can be responsible for the kink mode, but a magnetoacoustic sausage mode is necessary in all cases. Higher (flute) modes are excluded. The kink mode has a pressure amplitude that is less than the pressure amplitude of the sausage mode, though its observed velocity is higher. This concentrates dislocations on the top of the loop.
Conclusions: To explain dislocations, any model of coronal waves must include the simultaneous propagation and interference of kink and sausage wave modes of comparable but different frequencies with a sausage wave amplitude much smaller than the kink one.

Appendix A is available in electronic form at http://www.aanda.org Title: Model comparison for the density structure along solar prominence threads Authors: Arregui, I.; Soler, R. Bibcode: 2015A&A...578A.130A Altcode: 2015arXiv150503448A Context. Quiescent solar prominence fine structures are typically modelled as density enhancements, called threads, which occupy a fraction of a longer magnetic flux tube. This is justified from the spatial distribution of the imaged plasma emission or absorption of prominences at small spatial scales. The profile of the mass density along the magnetic field is unknown, however, and several arbitrary alternatives are employed in prominence wave studies. The identification and measurement of period ratios from multiple harmonics in standing transverse thread oscillations offer a remote diagnostics method to probe the density variation of these structures.
Aims: We present a comparison of theoretical models for the field-aligned density along prominence fine structures. They aim to imitate density distributions in which the plasma is more or less concentrated around the centre of the magnetic flux tube. We consider Lorentzian, Gaussian, and parabolic profiles. We compare theoretical predictions based on these profiles for the period ratio between the fundamental transverse kink mode and the first overtone to obtain estimates for the density ratios between the central part of the tube and its foot-points and to assess which one would better explain observed period ratio data.
Methods: Bayesian parameter inference and model comparison techniques were developed and applied. To infer the parameters, we computed the posterior distribution for the density gradient parameter that depends on the observable period ratio. The model comparison involved computing the marginal likelihood as a function of the period ratio to obtain the plausibility of each density model as a function of the observable. We also computed the Bayes factors to quantify the relative evidence for each model, given a period ratio observation.
Results: A Lorentzian density profile, with plasma density concentrated around the centre of the tube, seems to offer the most plausible inversion result. A Gaussian profile would require unrealistically high values of the density gradient parameter, and a parabolic density distribution does not enable us to obtain well-constrained posterior probability distributions. However, our model comparison results indicate that the evidence points to the Gaussian and parabolic profiles for period ratios in between 2 and 3, while the Lorentzian profile is preferred for higher period ratio values. The method we present can be used to obtain information on the plasma structure along threads, provided period ratio measurements become widely available. Title: Wave heating of the solar atmosphere Authors: Arregui, Iñigo Bibcode: 2015RSPTA.37340261A Altcode: 2015arXiv150106708A Magnetic waves are a relevant component in the dynamics of the solar atmosphere. Their significance has increased because of their potential as a remote diagnostic tool and their presumed contribution to plasma heating processes. We discuss our current understanding of coronal heating by magnetic waves, based on recent observational evidence and theoretical advances. The discussion starts with a selection of observational discoveries that have brought magnetic waves to the forefront of the coronal heating discussion. Then, our theoretical understanding of the nature and properties of the observed waves and the physical processes that have been proposed to explain observations are described. Particular attention is given to the sequence of processes that link observed wave characteristics with concealed energy transport, dissipation and heat conversion. We conclude with a commentary on how the combination of theory and observations should help us to understand and quantify magnetic wave heating of the solar atmosphere. Title: Determination of the cross-field density structuring in coronal waveguides using the damping of transverse waves Authors: Arregui, I.; Asensio Ramos, A. Bibcode: 2014A&A...565A..78A Altcode: 2014arXiv1404.0584A Context. Time and spatial damping of transverse magnetohydrodynamic (MHD) kink oscillations is a source of information on the cross-field variation of the plasma density in coronal waveguides.
Aims: We show that a probabilistic approach to the problem of determining the density structuring from the observed damping of transverse oscillations enables us to obtain information on the two parameters that characterise the cross-field density profile.
Methods: The inference is performed by computing the marginal posterior distributions for density contrast and transverse inhomogeneity length-scale using Bayesian analysis and damping ratios for transverse oscillations under the assumption that damping is produced by resonant absorption.
Results: The obtained distributions show that, for damping times of a few oscillatory periods, low density-contrasts and short inhomogeneity length scales are more plausible to explain observations.
Conclusions: This means that valuable information on the cross-field density profile can be obtained even if the inversion problem, with two unknowns and one observable, is a mathematically ill-posed problem. Title: The promise of Bayesian analysis for prominence seismology Authors: Arregui, Iñigo; Ramos, Andrés Asensio; Díaz, Antonio J. Bibcode: 2014IAUS..300..393A Altcode: We propose and use Bayesian techniques for the determination of physical parameters in solar prominence plasmas, combining observational and theoretical properties of waves and oscillations. The Bayesian approach also enables to perform model comparison to assess how plausible alternative physical models/mechanisms are in view of data. Title: Coronal loop physical parameters from the analysis of multiple observed transverse oscillations Authors: Asensio Ramos, A.; Arregui, I. Bibcode: 2013A&A...554A...7A Altcode: 2013arXiv1304.1921A The analysis of quickly damped transverse oscillations of solar coronal loops using magneto-hydrodynamic seismology allows us to infer physical parameters that are difficult to measure otherwise. Under the assumption that such damped oscillations are due to the resonant conversion of global modes into Alfvén oscillations of the tube surface, we carry out a global seismological analysis of a large set of coronal loops. A Bayesian hierarchical method is used to obtain distributions for coronal loop physical parameters by means of a global analysis of a large number of observations. The resulting distributions summarize global information and constitute data-favoured information that can be used for the inversion of individual events. The results strongly suggest that internal Alfvén travel times along the loop are longer than 100 s and shorter than 540 s with 95% probability. Likewise, the density contrast between the loop interior and the surrounding is larger than 2.3 and below 6.9 with 95% probability. Title: Multipactor-resistant low-pass harmonic filters with wide-band higher-order mode suppression Authors: Arregui, I.; Teberio, F.; Arnedo, I.; Lujambio, A.; Chudzik, M.; Benito, D.; Lopetegi, T.; Jost, R.; Gortz, F. -J.; Gil, J.; Vicente, C.; Gimeno, B.; Boria, V. E.; Raboso, D.; Laso, M. A. G. Bibcode: 2013imsd.conf97606A Altcode: No abstract at ADS Title: Determination of Transverse Density Structuring from Propagating Magnetohydrodynamic Waves in the Solar Atmosphere Authors: Arregui, I.; Asensio Ramos, A.; Pascoe, D. J. Bibcode: 2013ApJ...769L..34A Altcode: 2013arXiv1304.6869A We present a Bayesian seismology inversion technique for propagating magnetohydrodynamic transverse waves observed in coronal waveguides. The technique uses theoretical predictions for the spatial damping of propagating kink waves in transversely inhomogeneous coronal waveguides. It combines wave amplitude damping length scales along the waveguide with theoretical results for resonantly damped propagating kink waves to infer the plasma density variation across the oscillating structures. Provided that the spatial dependence of the velocity amplitude along the propagation direction is measured and the existence of two different damping regimes is identified, the technique would enable us to fully constrain the transverse density structuring, providing estimates for the density contrast and its transverse inhomogeneity length scale. Title: Bayesian Analysis of Multiple Harmonic Oscillations in the Solar Corona Authors: Arregui, I.; Asensio Ramos, A.; Díaz, A. J. Bibcode: 2013ApJ...765L..23A Altcode: 2013arXiv1302.3393A The detection of multiple mode harmonic kink oscillations in coronal loops enables us to obtain information on coronal density stratification and magnetic field expansion using seismology inversion techniques. The inference is based on the measurement of the period ratio between the fundamental mode and the first overtone and theoretical results for the period ratio under the hypotheses of coronal density stratification and magnetic field expansion of the wave guide. We present a Bayesian analysis of multiple mode harmonic oscillations for the inversion of the density scale height and magnetic flux tube expansion under each of the hypotheses. The two models are then compared using a Bayesian model comparison scheme to assess how plausible each one is given our current state of knowledge. Title: Application of Bayesian Inference and Model Comparison Techniques to Solar Atmospheric Seismology Authors: Arregui, I.; Asensio Ramos, A.; Pascoe, D. J.; Diaz, A. J. Bibcode: 2013enss.confE.131A Altcode: Wave activity is ubiquitous in the solar atmosphere. The detection and analysis of waves and oscillations is important for our understanding of solar atmospheric physics. Seismology deals with the determination of difficult to measure physical parameters by a comparison between observed and theoretical wave properties. We present recent results form the application of Bayesian inference and model comparison techniques to seismology. In the first example, quickly damped transverse coronal loop oscillation data are inverted to obtain estimates for the density contrast, the transverse inhomogeneity length scale, and the Alfvén speed in the oscillating loops. In the second example, we use the detection of multiple mode harmonic kink oscillations in coronal loops to obtain information on coronal density stratification and magnetic field expansion. The inference is based on the measurement of period ratios and their deviation due to the hypotheses of either coronal density stratification or magnetic field expansion of the wave guide. The two models are compared using a Bayesian model comparison scheme to assess how plausible each one is, given our current state of knowledge. In the last example, the spatial damping of propagating waves and the characteristic damping length scales are used to obtain estimates for the plasma density variation across the oscillating structures. This method could be applied to extended regions in the corona for obtaining information about the cross-field density structuring of the medium where these waves propagate. Title: Wave Leakage and Resonant Absorption in a Loop Embedded in a Coronal Arcade Authors: Rial, S.; Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2013ApJ...763...16R Altcode: 2012arXiv1201.4042R We investigate the temporal evolution of impulsively generated perturbations in a potential coronal arcade with an embedded loop. For the initial configuration we consider a coronal loop, represented by a density enhancement, which is unbounded in the ignorable direction of the arcade. The linearized time-dependent magnetohydrodynamic equations have been numerically solved in field-aligned coordinates and the time evolution of the initial perturbations has been studied in the zero-β approximation. For propagation constrained to the plane of the arcade, the considered initial perturbations do not excite trapped modes of the system. This weakness of the model is overcome by the inclusion of wave propagation in the ignorable direction. Perpendicular propagation produces two main results. First, damping by wave leakage is less efficient because the loop is able to act as a better wave trap of vertical oscillations. Second, the consideration of an inhomogeneous corona enables the resonant damping of vertical oscillations and the energy transfer from the interior of the loop to the external coronal medium. Title: Analytic Approximate Seismology of Propagating Magnetohydrodynamic Waves in the Solar Corona Authors: Goossens, M.; Soler, R.; Arregui, I.; Terradas, J. Bibcode: 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: Surface Alfvén Waves in Solar Flux Tubes Authors: Goossens, M.; Andries, J.; Soler, R.; Van Doorsselaere, T.; Arregui, I.; Terradas, J. Bibcode: 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: Prominence seismology using ground- and space-based observations Authors: Ballester, J. L.; Arregui, I.; Oliver, R.; Terradas, J.; Soler, R.; Lin, Y.; Engvold, O.; Langagen, O.; Rouppe van der Voort, L. H. M. Bibcode: 2012EAS....55..169B Altcode: Ground- and space-based observations have confirmed the presence of oscillatory motions in prominences and they have been interpreted in terms of magnetohydrodynamic (MHD) waves. This interpretation opens the door to perform prominence seismology, whose main aim is to determine physical parameters in magnetic and plasma structures (prominences) that are difficult to measure by direct means. Here, two prominence seismology applications are presented. Title: Prominence Seismology Authors: Arregui, I.; Ballester, J. L.; Oliver, R.; Soler, R.; Terradas, J. Bibcode: 2012ASPC..455..211A Altcode: 2012arXiv1201.4557A Given the difficulty in directly determining prominence physical parameters from observations, prominence seismology stands as an alternative method to probe the nature of these structures. We show recent examples of the application of magnetohydrodynamic (MHD) seismology techniques to infer physical parameters in prominence plasmas. They are based on the application of inversion techniques using observed periods, damping times, and plasma flow speeds of prominence thread oscillations. The contribution of Hinode to the subject has been of central importance. We show an example based on data obtained with Hinode's Solar Optical Telescope. Observations show an active region limb prominence, composed by a myriad of thin horizontal threads that flow following a path parallel to the photosphere and display synchronous vertical oscillations. The coexistence of waves and flows can be firmly established. By making use of an interpretation based on transverse MHD kink oscillations, a seismological analysis of this event is performed. It is shown that the combination of high quality Hinode observations and proper theoretical models allows flows and waves to become two useful characteristics for our understanding of the nature of solar prominences. Title: Inversion of Physical Parameters in Solar Coronal Magnetic Structures Authors: Arregui, I. .; Ballester, J.; Goossens, M.; Oliver, R.; Ramos, A. Bibcode: 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: Prominence Oscillations Authors: Arregui, Iñigo; Oliver, Ramón; Ballester, José Luis Bibcode: 2012LRSP....9....2A Altcode: Prominences are intriguing, but poorly understood, magnetic structures of the solar corona. The dynamics of solar prominences has been the subject of a large number of studies, and of particular interest is the study of prominence oscillations. Ground- and space-based observations have confirmed the presence of oscillatory motions in prominences and they have been interpreted in terms of magnetohydrodynamic (MHD) waves. This interpretation opens the door to perform prominence seismology, whose main aim is to determine physical parameters in magnetic and plasma structures (prominences) that are difficult to measure by direct means. Here, we review the observational information gathered about prominence oscillations as well as the theoretical models developed to interpret small amplitude oscillations and their temporal and spatial attenuation. Finally, several prominence seismology applications are presented. 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. Bibcode: 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: Inversion of Physical Parameters in Solar Atmospheric Seismology Authors: Arregui, Iñigo Bibcode: 2012ASSP...33..159A Altcode: 2012arXiv1202.0231A; 2012msdp.book..159A Magnetohydrodynamic (MHD) wave activity is ubiquitous in the solar atmosphere. MHD seismology aims to determine difficult to measure physical parameters in solar atmospheric magnetic and plasma structures by a combination of observed and theoretical properties of MHD waves and oscillations. This technique, similar to seismology or helio-seismology, demands the solution of two problems. The direct problem involves the computation of wave properties of given theoretical models. The inverse problem implies the calculation of unknown physical parameters, by means of a comparison of observed and theoretical wave properties. Solar atmospheric seismology has been successfully applied to different structures such as coronal loops, prominence plasmas, spicules, or jets. However, it is still in its infancy. Far more is there to come. We present an overview of recent results, with particular emphasis in the inversion procedure. Title: Bayesian Magnetohydrodynamic Seismology of Coronal Loops Authors: Arregui, I.; Asensio Ramos, A. Bibcode: 2011ApJ...740...44A Altcode: 2011arXiv1107.3943A We perform a Bayesian parameter inference in the context of resonantly damped transverse coronal loop oscillations. The forward problem is solved in terms of parametric results for kink waves in one-dimensional flux tubes in the thin tube and thin boundary approximations. For the inverse problem, we adopt a Bayesian approach to infer the most probable values of the relevant parameters, for given observed periods and damping times, and to extract their confidence levels. The posterior probability distribution functions are obtained by means of Markov Chain Monte Carlo simulations, incorporating observed uncertainties in a consistent manner. We find well-localized solutions in the posterior probability distribution functions for two of the three parameters of interest, namely the Alfvén travel time and the transverse inhomogeneity length scale. The obtained estimates for the Alfvén travel time are consistent with previous inversion results, but the method enables us to additionally constrain the transverse inhomogeneity length scale and to estimate real error bars for each parameter. When observational estimates for the density contrast are used, the method enables us to fully constrain the three parameters of interest. These results can serve to improve our current estimates of unknown physical parameters in coronal loops and to test the assumed theoretical model. Title: Magnetohydrodynamic kink waves in two-dimensional non-uniform prominence threads Authors: Arregui, I.; Soler, R.; Ballester, J. L.; Wright, A. N. Bibcode: 2011A&A...533A..60A Altcode: 2010arXiv1011.5175A
Aims: We analyse the oscillatory properties of resonantly damped transverse kink oscillations in two-dimensional prominence threads.
Methods: The fine structures are modelled as cylindrically symmetric magnetic flux tubes with a dense central part with prominence plasma properties and an evacuated part, both surrounded by coronal plasma. The equilibrium density is allowed to vary non-uniformly in both the transverse and the longitudinal directions. We examine the influence of longitudinal density structuring on periods, damping times, and damping rates for transverse kink modes computed by numerically solving the linear resistive magnetohydrodynamic (MHD) equations.
Results: The relevant parameters are the length of the thread and the density in the evacuated part of the tube, two quantities that are difficult to directly estimate from observations. We find that both of them strongly influence the oscillatory periods and damping times, and to a lesser extent the damping ratios. The analysis of the spatial distribution of perturbations and of the energy flux into the resonances allows us to explain the obtained damping times.
Conclusions: Implications for prominence seismology, the physics of resonantly damped kink modes in two-dimensional magnetic flux tubes, and the heating of prominence plasmas are discussed. Title: Damping Mechanisms for Oscillations in Solar Prominences Authors: Arregui, Iñigo; Ballester, José Luis Bibcode: 2011SSRv..158..169A Altcode: 2010SSRv..tmp..173A; 2010arXiv1002.3489A 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 non-adiabatic processes, mass flows, resonant damping in non-uniform 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. Title: Seismology of Transversely Oscillating Coronal Loops with Siphon Flows Authors: Terradas, J.; Arregui, I.; Verth, G.; Goossens, M. Bibcode: 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: Seismology of Standing Kink Oscillations of Solar Prominence Fine Structures Authors: Soler, R.; Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2010ApJ...722.1778S Altcode: 2010arXiv1007.1959S We investigate standing kink magnetohydrodynamic (MHD) oscillations in a prominence fine structure modeled as a straight and cylindrical magnetic tube only partially filled with the prominence material and with its ends fixed at two rigid walls representing the solar photosphere. The prominence plasma is partially ionized and a transverse inhomogeneous transitional layer is included between the prominence thread and the coronal medium. Thus, ion-neutral collisions and resonant absorption are the damping mechanisms considered. Approximate analytical expressions of the period, the damping time, and their ratio are derived for the fundamental mode in the thin tube and thin boundary approximations. We find that the dominant damping mechanism is resonant absorption, which provides damping ratios in agreement with the observations, whereas ion-neutral collisions are irrelevant for damping. The values of the damping ratio are independent of both the prominence thread length and its position within the magnetic tube, and coincide with the values for a tube fully filled with the prominence plasma. The implications of our results in the context of the MHD seismology technique are discussed, pointing out that the reported short-period (2-10 minutes) and short-wavelength (700-8000 km) thread oscillations may not be consistent with a standing mode interpretation and could be related to propagating waves. Finally, we show that the inversion of some prominence physical parameters, e.g., Alfvén speed, magnetic field strength, transverse inhomogeneity length scale, etc., is possible using observationally determined values of the period and damping time of the oscillations along with the analytical approximations of these quantities. Title: Three-dimensional Propagation of Magnetohydrodynamic Waves in Solar Coronal Arcades Authors: Rial, S.; Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2010ApJ...713..651R Altcode: 2010arXiv1002.0469R We numerically investigate the excitation and temporal evolution of oscillations in a two-dimensional coronal arcade by including the three-dimensional propagation of perturbations. The time evolution of impulsively generated perturbations is studied by solving the linear, ideal magnetohydrodynamic (MHD) equations in the zero-β approximation. As we neglect gas pressure, the slow mode is absent and therefore only coupled fast MHD and Alfvén modes remain. Two types of numerical experiments are performed. First, the resonant wave energy transfer between a fast normal mode of the system and local Alfvén waves is analyzed. It is seen how, because of resonant coupling, the fast wave with global character transfers its energy to Alfvénic oscillations localized around a particular magnetic surface within the arcade, thus producing the damping of the initial fast MHD mode. Second, the time evolution of a localized impulsive excitation, trying to mimic a nearby coronal disturbance, is considered. In this case, the generated fast wavefront leaves its energy on several magnetic surfaces within the arcade. The system is therefore able to trap energy in the form of Alfvénic oscillations, even in the absence of a density enhancement such as that of a coronal loop. These local oscillations are subsequently phase-mixed to smaller spatial scales. The amount of wave energy trapped by the system via wave energy conversion strongly depends on the wavelength of perturbations in the perpendicular direction, but is almost independent from the ratio of the magnetic to density scale heights. Title: The Sun and the Solar System Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2010ASSP...14..446A Altcode: 2010hsa5.conf..446A High-resolution observations provide evidence of the existence of small-amplitude transverse oscillations in solar filament fine structures. These oscillations are believed to represent fast magnetohydrodynamic (MHD) waves and the disturbances are seen to be damped in short timescales of the order of 1-4 periods. We propose that, due to the highly inhomogeneous nature of the filament plasma at the fine-structure spatial scale, the phenomenon of resonant absorption is likely to operate in the temporal attenuation of fast MHD oscillations. By considering transverse inhomogeneity in a straight flux tube model we find that, for density inhomogeneities typical of filament threads, the decay times are of a few oscillatory periods only. Title: The Temporal Evolution of Linear Fast and Alfvén MHD Waves in Solar Coronal Arcades Authors: Rial, S.; Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2010ASSP...14..459R Altcode: 2010hsa5.conf..459R The excitation and temporal evolution of fast and Alfvén magnetohydrodynamic oscillations in a two-dimensional coronal arcade are investigated. The approach is to consider an equilibrium magnetic and plasma structure and then to introduce a perturbation trying to mimic a nearby disturbance, such as a flare or filament eruption. By numerically solving the time-dependent linearized MHD wave equations, the properties of the solutions have been studied. First, the properties of uncoupled fast and Alfvén waves are described. Then, longitudinal propagation of perturbations is included, and the properties of coupled waves are determined. Title: Damping of Fast Magnetohydrodynamic Oscillations in Quiescent Filament Threads Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2009ASPC..415...71A Altcode: High-resolution observations provide evidence of the existence of small-amplitude transverse oscillations in solar filament fine structures. These oscillations are believed to represent fast magnetohydrodynamic (MHD) waves and the disturbances are seen to be damped in short timescales of the order of 1 to 4 periods. We propose that, due to the highly inhomogeneous nature of the filament plasma at the fine-structure spatial scale, the phenomenon of resonant absorption is likely to operate in the temporal attenuation of fast MHD oscillations. By considering transverse inhomogeneity in a straight flux tube model we find that, for density inhomogeneities typical of filament threads, the decay times are of a few oscillatory periods only. Title: On the nature of kink MHD waves in magnetic flux tubes Authors: Goossens, M.; Terradas, J.; Andries, J.; Arregui, I.; Ballester, J. L. Bibcode: 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.
Aims: The goal of this work is to study and determine the nature of MHD kink waves.
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.
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 (kz 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.
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. Bibcode: 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. Bibcode: 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.
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.
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.
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/k2 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: Nonlinear Instability of Kink Oscillations due to Shear Motions Authors: Terradas, J.; Andries, J.; Goossens, M.; Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 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: Damping of Fast Magnetohydrodynamic Oscillations in Quiescent Filament Threads Authors: Arregui, Iñigo; Terradas, Jaume; Oliver, Ramón; Ballester, José Luis Bibcode: 2008ApJ...682L.141A Altcode: 2008arXiv0806.2728A High-resolution observations provide evidence of the existence of small-amplitude transverse oscillations in solar filament fine structures. These oscillations are believed to represent fast magnetohydrodynamic (MHD) waves, and the disturbances are seen to be damped on short timescales of the order of 1-4 periods. In this Letter, we propose that, due to the highly inhomogeneous nature of the filament plasma at the fine-structure spatial scale, the phenomenon of resonant absorption is likely to operate in the temporal attenuation of fast MHD oscillations. By considering transverse inhomogeneity in a straight flux tube model, we find that, for density inhomogeneities typical of filament threads, the decay times are of a few oscillatory periods only. 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. Bibcode: 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. Bibcode: 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.
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.
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: Resonant absorption in multi-stranded coronal loops Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2008IAUS..247..116T Altcode: 2007IAUS..247..116T We study the excitation and damping of transverse oscillations in a complex multi-stranded model of a coronal loop. By numerically solving the time-dependent magnetohydrodynamic (MHD) equations in two dimensions, we show how the global motion of the whole bundle of tubes, produced by an external disturbance, is converted into localised motions due to the process of resonant absorption. At any location in the structure two dominant frequencies are found, the frequency of the global mode (different from the kink frequency of the individual strands) and the local Alfvén frequency. The mechanism of mode conversion is not affected by the complicated geometry of the system and for certain configurations the energy conversion does not only take place at the external edge of the composite loop but also inside the structure. Title: Damped oscillations of two interacting coronal loops Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2008IAUS..247..133A Altcode: 2007IAUS..247..133A 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 modelled, 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 due to resonant absorption is computed. 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. Title: Transverse Oscillations of Flowing Prominence Threads Observed with Hinode Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2008ApJ...678L.153T Altcode: 2008arXiv0803.2649T Recent observations with the Hinode Solar Optical Telescope display an active region prominence whose fine threads oscillate in the vertical direction as they move along a path parallel to the photosphere. A seismological analysis of this event is carried out by taking advantage of the small radius of these structures compared to the total length of magnetic field lines, i.e., by using the thin-tube approximation. This analysis reveals that the oscillatory period is only slightly modified by the existence of the flow and that the difference between the period of a flowing thread and a static one is below the error bars of these observations. Moreover, although it is not possible to obtain values of the physical parameters, a lower bound for the Alfvén speed (ranging between 120 and 350 km s-1) is obtained for each of the threads. Such Alfvén speeds agree with the intense magnetic fields and large densities usually found in active region prominences. Title: On the Scaling of the Damping Time for Resonantly Damped Oscillations in Coronal Loops Authors: Arregui, Iñigo; Ballester, José Luis; Goossens, Marcel Bibcode: 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: The Resonant Damping of Fast Magnetohydrodynamic Oscillations in a System of Two Coronal Slabs Authors: Arregui, Iñigo; Terradas, Jaume; Oliver, Ramón; Ballester, José Luis Bibcode: 2008ApJ...674.1179A Altcode: 2007arXiv0708.1251A Observations of transverse 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 of 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 transverse 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. Title: Resonantly Damped Surface and Body MHD Waves in a Solar Coronal Slab with Oblique Propagation Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2007SoPh..246..213A Altcode: 2007arXiv0708.3783A The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs in a zero-β 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 wavenumber 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 by resonant absorption. In this paper, first, a normal-mode analysis is performed and the period, the damping rate, and the spatial structure of the 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. Title: Transverse Oscillations in Coronal Loops Authors: Arregui, I.; Luna, M.; Oliver, R.; Terradas, J.; Ballester, J. L. Bibcode: 2007AIPC..934...54A Altcode: During last years, direct evidence about oscillations in different coronal structures has been obtained thanks to the detailed observations made by SoHO and TRACE. With the help of magnetohydrodynamic (MHD) wave theory, we can explain these observations as due to the excitation and propagation of MHD waves in the solar corona. In spite that there are many solar coronal structures in which oscillations have been detected (prominences, loops, plumes, coronal holes, etc.), in the following we will concentrate on transverse oscillations of coronal loops, reviewing some theoretical models developed to understand these oscillations in terms of MHD waves. Title: The influence of the internal structuring of coronal loops on the properties of their damped transverse oscillations Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L. Bibcode: 2007A&A...466.1145A Altcode: Context: The geometry and physical conditions in solar coronal loops are complicated and still not understood well. 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 the previous theoretical models used for the study of transversal coronal loop oscillations and of their damping due to resonant conversion of energy.
Aims: 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.
Methods: The equilibrium configuration of a single coronal loop with internal density structuring is modelled by considering the loop as composed of two very close, parallel, identical coronal slabs in Cartesian geometry. The period of the oscillation and the damping time are computed for the resonantly damped fundamental kink mode. These quantities are then compared to those obtained for two models for a single equivalent slab without internal density structuring.
Results: 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.
Conclusions: Therefore the internal density structuring of a coronal loop does not affect its oscillatory properties very much. However, the sub-resolution structuring of a coronal loop with different densities in its components or with different widths could vary these results. 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. Bibcode: 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.
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.
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: Time Dependent Simulations of 2D Coronal Loop Models Authors: van Doorsselaere, T.; Poedts, S.; Andries, J.; Arregui, I. Bibcode: 2006ESASP.617E.113V Altcode: 2006soho...17E.113V No abstract at ADS 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2006RSPTA.364..529A Altcode: No abstract at ADS Title: Seismology of Coronal Loops Using Resonant Absorption Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens, M.; Poedts, S. Bibcode: 2005ESASP.600E..21A Altcode: 2005dysu.confE..21A; 2005ESPM...11...21A No abstract at ADS Title: Building a Time Dependent Code to Simulate Oscillations of Line-Tied Coronal Loops Authors: van Doorsselaere, T.; Poedts, S.; Arregui, I.; Andries, J. Bibcode: 2005ESASP.600E..83V Altcode: 2005dysu.confE..83V; 2005ESPM...11...83V 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. Bibcode: 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: Dynamics of Coronal Loop Oscillations Authors: van Doorsselaere, T.; Arregui, I.; Andries, J.; Goossens, M.; Poedts, S. Bibcode: 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. Bibcode: 2005A&A...441..361A Altcode: Resonantly damped fast kink quasi-modes are computed in fully

resistive magnetohydrodynamics (MHD) for two-dimensional equilibrium

models. The equilibrium model is a straight cylindrically symmetric flux

tube with a plasma density that is non-uniform both across and along the

loop. The non-uniform layer across the loop is not

restricted to be thin, but its thickness can reach values up to the

loop diameter.

Our results indicate that

the period and damping of coronal loop oscillations mainly depend on the density contrast

and the inhomogeneity length-scale and are independent of the details of

longitudinal stratification, depending on the weighted mean density,

weighted with the wave energy. For fully non-uniform loops, quasi-modes can

interact with resistive Alfvén eigenmodes leading to avoided crossings

and gaps in the complex frequency plane. The present study extends previous

studies on coronal loop oscillations in one-dimensional equilibrium models

with thick boundary layers and in equilibria with longitudinally stratified loops under the

thin boundary approximation, and allow for a better comparison between

observations and theory raising the prospect of coronal seismology using

the time damping of coronal loop oscillations. 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. Bibcode: 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: Determination of the Coronal Density Stratification from the Observation of Harmonic Coronal Loop Oscillations Authors: Andries, Jesse; Arregui, Inigo; Goossens, Marcel Bibcode: 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: 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. Bibcode: 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.

Appendix A is only available in electronic form at http://www.edpsciences.org Title: Numerical Solutions for Resonantly Damped MHD Quasi-Modes in Two-Dimensional Coronal Loops Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens, M. Bibcode: 2004ESASP.575...85A Altcode: 2004soho...15...85A No abstract at ADS Title: Magnetohydrodynamic waves in a sheared potential coronal arcade Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2004A&A...425..729A Altcode: We study the effects of magnetic field shear (By ≠ 0) and longitudinal propagation of perturbations (ky ≠ 0) on the linear and adiabatic magnetohydrodynamic (MHD) normal modes of oscillation of a potential coronal arcade. In a cold plasma, the inclusion of these two effects produces the linear coupling of discrete fast modes, characterised by a discrete spectrum of frequencies and a global velocity structure, and Alfvén continuum modes, characterised by a continuous spectrum of frequencies and with a velocity perturbation confined to given magnetic surfaces in such a way that modes with mixed properties arise \citep{Arregui04}. The wave equations governing the velocity perturbations have been solved numerically and

our results show that the couplings between fast and Alfvén modes are governed by some parity rules for the symmetry of the eigenfunctions of fast and Alfvén modes in the direction along the equilibrium magnetic field. The nature of the coupling between fast and Alfvén modes can be resonant or non-resonant depending on the location of the fast mode frequency within the different Alfvén continua. Also, an important result is that in this kind of configurations coupled modes could be difficult to observe since when both magnetic field shear and longitudinal propagation are present the spatial distribution of the velocity may not be confined to low heights in the solar corona. Title: Erratum: ``Magnetohydrodynamic Waves in Sheared Coronal Arcades'' (ApJ, 602, 1006 [2004]) Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2004ApJ...607.1070A Altcode: Proof corrections were not carried faithfully to the printed version of this paper, resulting in an error in the text below equation (12). The equation en=∇A/∇A=-ez should read en=∇A/∇A; i.e., the final equivalence should be omitted.

The Press sincerely regrets this error. Title: Magnetohydrodynamic Waves in Sheared Coronal Arcades Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2004ApJ...602.1006A Altcode: The magnetohydrodynamic (MHD) normal modes of oscillation of sheared coronal arcades including longitudinal wave propagation are studied. Solutions have been computed to ascertain the effects of the longitudinal magnetic field component, By, and of the longitudinal wavenumber, ky, in these structures. Our results show that whenever By≠0 and/or ky≠0, fast modes, characterized by a global velocity structure and a discrete spectrum of frequencies, and Alfvén continuum modes, characterized by a velocity perturbation confined to given magnetic surfaces, get coupled and no pure fast modes or pure Alfvén modes exist, but modes with mixed properties arise. These oscillatory modes display a global spatial distribution, together with a nonsquare integrable singular behavior on certain magnetic surfaces. Moreover, this mode coupling is such that under some circumstances the Alfvénic contribution is also in the form of a smooth velocity profile covering a range of magnetic surfaces instead of a singularity on a fixed magnetic surface. The coupling between fast and Alfvén modes is governed by the parity of their eigenfunctions in the direction along the equilibrium magnetic field. The parity rules determining the features of coupled modes in terms of their spatial structure, ky, and By are presented. We have found that the frequency of coupled modes is real when either By≠0 or ky≠0 but is complex when both By≠0 and ky≠0. Also, an important result is that coupled modes may not be observable since many of them leak energy away from the solar corona. Title: Coupled Fast and Alfvén MHD Waves in Sheared Coronal Arcades Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2004ESASP.547..447A Altcode: 2004soho...13..447A The magnetohydrodynamic (MHD) normal modes of oscillation of sheared coronal arcades including longitudinal wave propagation are studied. Solutions have been computed in order to ascertain the effects of the longitudinal magnetic field component, By, and of the longitudinal propagation, ky in these structures. Our results show that whenever By = 0 and/or ky = 0 fast modes, characterized by a global velocity structure and a discrete spectrum of frequencies, and Alfvén continuum modes, characterized by a velocity perturbation confined to given magnetic surfaces, get coupled and no pure fast modes nor pure Alfvén modes exist, but modes with mixed properties arise. These oscillatory modes display a global spatial distribution together with a non-square integrable singular behaviour on certain magnetic surfaces. Under certain circumstances, moreover, this mode coupling is such that the Alfvénic contribution is also in the form of a smooth velocity profile covering a range of magnetic surfaces instead of a singularity on a fixed magnetic surface. The coupling between fast and Alfvén modes is governed by the parity of their eigenfunctions in the direction along the equilibrium magnetic field. The "parity rules" determining the features of coupled modes in terms of their parity, ky and By are presented. Title: Magnetohydrodynamic waves in sheared coronal magnetic structures Authors: Arregui, I. Bibcode: 2003PhDT.........7A Altcode: The MHD normal modes of oscillation of coronal magnetic structures are studied by including two aspects that have not been considered in previous theoretical works of this kind, namely the effects of the magnetic field shear and of the longitudinal propagation in the oscillatory properties of solar coronal structures. We show that, in a cold plasma, the inclusion of these two aspects produces the coupling of fast modes, characterized by a global velocity structure and a discrete spectrum of frequencies and Alfven continuum modes, characterized by a velocity perturbation confined to given magnetic surfaces, in such a way that no pure fast modes nor pure Alfven modes exist, but modes with mixed properties arise. These oscillatory modes display a global spatial distribution together with a non-square integrable singular behaviour on certain magnetic surfaces. Moreover, the mode coupling is such that, under some circumstances, the Alfvenic contribution is also in the form of a smooth velocity profile covering a range of magnetic surfaces instead of a singularity on a fixed magnetic surface. The coupling between fast and Alfven modes is governed by the parity of their eigenfunctions in the direction along the equilibrium magnetic field. The ``parity rules'' determining the features of coupled modes in terms of their spatial structure and the values of shear and longitudinal propagation are presented. The conditions for the resonant or non-resonant interaction between modes are also discussed. We have found that the frequency of coupled modes is real when only shear or longitudinal propagation are present, but is complex when both effects exist. Also, an important result is that coupled modes may not be observable since many of them leak energy away from the solar corona. Title: Coupling of fast and Alfvén waves in a straight bounded magnetic field with density stratification Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2003A&A...402.1129A Altcode: The theoretical understanding of the linear standing or propagating magnetohydrodynamic waves in a variety of solar coronal structures is far from complete since analytical solutions to the linearised MHD equations can only be found for very simple magnetic configurations. In this paper, we use a numerical code to solve the linear fast and Alfvén wave equations in a very simple, bounded magnetic configuration that incorporates two features that are not usually considered in similar works, namely the longitudinal magnetic field component and wave propagation in the longitudinal direction (ky !=q 0). We use a numerical code (Arregui et al. \cite{Arregui01}) that has been modified by including a staggered mesh that allows us to properly capture the spatial behaviour of solutions to the wave equations. Coupling between fast and Alfvén modes has been studied in detail and it has been found that it does not take place when the longitudinal field component is zero and the frequency of the fast mode is outside the Alfvén continuum with the same spatial structure along field lines. Under these circumstances, fast modes retain their global spatial behaviour and are also characterised by omega 2 varying linearly with ky2, such as in a uniform medium (although here the Alfvén speed changes exponentially in the direction normal to field lines). Regarding mode coupling, its main feature is the blend of fast and Alfvén solutions with close frequencies in some modes with a mixture of their properties, namely discontinuities or jumps around certain magnetic surfaces (such as in pure Alfvén waves), global spatial distribution of the normal velocity component and non-zero density perturbations (such as in fast waves). Title: Magnetohydrodynamic waves in sheared coronal magnetic structures Authors: Arregui, Iñigo Bibcode: 2003PhDT.......149A Altcode: No abstract at ADS Title: Magnetohydrodynamic waves in sheared coronal magnetic structures Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2002ESASP.506..535A Altcode: 2002svco.conf..535A; 2002ESPM...10..535A The theoretical understanding of the linear standing or propagating magnetohydrodynamic (MHD) waves in a variety of solar coronal structures is far from complete since analytical solutions to the linearised MHD equations can only be found for very simple magnetic configurations. In this work, a numerical code is used to solve the linear MHD wave equations in a coronal magnetic arcade with a longitudinal magnetic field component and considering propagation in the longitudinal direction. In a cold plasma, the inclusion of these two effects leads to the coupling of fast and Alfvén modes. The numerical code provides us with solutions for these coupled fast and Alfvén modes in the form of the two-dimensional distribution of the perpendicular and normal velocity components together with the oscillatory frequency. Solutions have been computed in order to ascertain the effects of the longitudinal magnetic field component and of the longitudinal propagation on the properties of fast and Alfvén modes. Title: Numerical simulations of linear magnetohydrodynamic waves in two-dimensional force-free magnetic fields Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2001A&A...369.1122A Altcode: High resolution observations of the solar corona made with instruments onboard the SOHO and TRACE spacecrafts have provided new evidence for the presence of oscillations in a variety of coronal magnetic structures. Most of these observations have been interpreted in terms of linear standing or propagating magnetohydrodynamic (MHD) waves, but the theoretical understanding is far from complete since analytical solutions to the linearised MHD wave equations can only be found for very simple magnetic configurations. Taking into account that the solar corona is basically structured by force-free magnetic fields, our purpose in this paper is to present the derivation of the linear MHD wave equations for a two-dimensional force-free magnetic field configuration having longitudinal invariance, as well as to introduce a numerical code to solve the resulting system of coupled partial differential equations. The accuracy of the code has been checked by numerically solving two cases for which analytical or simple numerical solutions exist. To our knowledge, this is the only two-dimensional code developed to study the normal MHD modes of oscillation of a general force-free field with longitudinal invariance. Title: Fast and Alfvén MHD waves in sheared coronal arcades Authors: Arregui, I.; Oliver, R.; Ballester, J. L. Bibcode: 2001hsa..conf..253A Altcode: No abstract at ADS