explanation blue bibcodes open ADS page with paths to full text
Author name code: arregui
ADS astronomy entries on 2022-09-14
author:"Arregui, Inigo"
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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.
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.
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Title: Recent Applications of Bayesian Methods to the Solar Corona
Authors: Arregui, Iñigo
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.
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Title: Methodology for Predicting the Probability Distribution of
the Amplitude of Solar Cycle 25
Authors: Arregui, Iñigo
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.
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Title: Editorial Appreciation
Authors: Arregui, Iñigo; Leibacher, John; Mandrini, Cristina H.;
van Driel-Gesztelyi, Lidia; Wheatland, Michael S.
2022SoPh..297...11A Altcode:
No abstract at ADS
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Title: Erratum: "Bayesian Evidence for a Nonlinear Damping Model
for Coronal Loop Oscillations" (2021, ApJL, 915, L25)
Authors: Arregui, Iñigo
2021ApJ...922L..22A Altcode:
No abstract at ADS
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Title: Bayesian Evidence for a Nonlinear Damping Model for Coronal
Loop Oscillations
Authors: Arregui, Iñigo
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.
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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
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.
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Title: Resonant absorption: Transformation of compressive motions
into vortical motions
Authors: Goossens, M.; Arregui, I.; Soler, R.; Van Doorsselaere, T.
2020A&A...641A.106G Altcode: 2020arXiv200908152G
This paper investigates the changes in spatial properties when
magnetohydrodynamic (MHD) waves undergo resonant damping in the
Alfvén continuum. The analysis is carried out for a 1D cylindrical
pressure-less plasma with a straight magnetic field. The effect of the
damping on the spatial wave variables is determined by using complex
frequencies that arise as a result of the resonant damping. Compression
and vorticity are used to characterise the spatial evolution of the
MHD wave. The most striking result is the huge spatial variation in the
vorticity component parallel to the magnetic field. Parallel vorticity
vanishes in the uniform part of the equilibrium. However, when the
MHD wave moves into the non-uniform part, parallel vorticity explodes
to values that are orders of magnitude higher than those attained by
the transverse components in planes normal to the straight magnetic
field. In the non-uniform part of the equilibrium plasma, the MHD wave
is controlled by parallel vorticity and resembles an Alfvén wave,
with the unfamiliar property that it has pressure variations even in
the linear regime.
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Title: Quantifying the evidence for resonant damping of coronal
waves with foot-point wave power asymmetry
Authors: Montes-Solís, M.; Arregui, I.
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.
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Title: Coronal Cooling as a Result of Mixing by the Nonlinear
Kelvin-Helmholtz Instability
Authors: Hillier, Andrew; Arregui, Iñigo
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 }<SUB>mixed</SUB>}=\sqrt{{ρ }<SUB>1</SUB>{ρ }<SUB>2</SUB>}
and {T}<SUB>mixed</SUB>}=\sqrt{{T}<SUB>1</SUB>{T}<SUB>2</SUB>}. 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 <SUB>mixed</SUB> =
10<SUP>5</SUP> 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.
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Title: Fundamental transverse vibrations of the active region
solar corona
Authors: Luna, M.; Oliver, R.; Antolin, P.; Arregui, I.
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. <BR /> Aims: We study these vibrations and the possible
observational effects. <BR /> 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. <BR />
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. <BR /> 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. <P />A movie associated to Fig. 10 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935850/olm">http://
https://www.aanda.org </A>
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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.
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.
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Title: Inference of magnetic field strength and density from damped
transverse coronal waves
Authors: Arregui, I.; Montes-Solís, M.; Asensio Ramos, A.
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.
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Title: Mixed properties of MHD waves in non-uniform plasmas
Authors: Goossens, Marcel L.; Arregui, Inigo; Van Doorsselaere, Tom
2019FrASS...6...20G Altcode:
This paper investigates the mixed properties of MHD waves in a
non-uniform plasma. It starts with a short revision of MHD waves in
a uniform plasma of infinite extent. In that case the MHD waves do
not have mixed properties. They can be separated in Alfvén waves and
magneto-sonic waves. The Alfvén waves propagate parallel vorticity
and are incompressible. In addition they have no parallel displacement
component. The magneto-sonic waves are compressible and in general do
have a parallel component of displacement but do not propagate parallel
vorticity. This clear separation has been the reason why there has
been a strong inclination in the literature to use this classification
in the study of MHD waves in non-uniform plasmas. The main part of
this paper is concerned with MHD waves in a non-uniform plasma. It is
shown that the MHD waves in that situation in general propagate both
vorticity and compression and hence have mixed properties. Finally,
the close connection between resonant absorption and MHD waves with
mixed properties is discussed.
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Title: Inference of physical parameters in solar prominence threads
Authors: Montes-Solís, M.; Arregui, I.
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.
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Title: Inferring physical parameters in solar prominence threads
Authors: Montes-Solís, M.; Arregui, I.
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. <BR /> Aims: We use prominence
seismology techniques to analyse transverse oscillations in threads by
comparing magnetohydrodynamic (MHD) models and observations. <BR />
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. <BR /> 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. <BR /> Conclusions:
Our results show that Bayesian analysis offers valuable methods to
perform parameter inference and a model comparison in the context of
prominence seismology.
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Title: No unique solution to the seismological problem of standing
kink magnetohydrodynamic waves
Authors: Arregui, I.; Goossens, M.
2019A&A...622A..44A Altcode: 2018arXiv181207266A
The aim of this paper is to point out that the classic seismological
problem using observations and theoretical expressions for the periods
and damping times of transverse standing magnetohydrodynamic waves
in coronal loops is better referred to as a reduced seismological
problem. "Reduced" emphasises the fact that only a small number
of characteristic quantities of the equilibrium profiles can be
determined. Reduced also implies that there is no unique solution to
the full seismological problem. Even the reduced seismological problem
does not allow a unique solution. Bayesian inference results support
our mathematical arguments and offer insight into the relationship
between the algebraic and the probabilistic inversions.
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Title: On Kelvin-Helmholtz and parametric instabilities driven by
coronal waves
Authors: Hillier, Andrew; Barker, Adrian; Arregui, Iñigo; Latter,
Henrik
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.
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Title: Temporal and Spatial Scales for Coronal Heating by Alfvén
Wave Dissipation in Transverse Loop Oscillations
Authors: Terradas, Jaume; Arregui, Iñigo
2018RNAAS...2..196T Altcode: 2018RNAAS...2d.196T
No abstract at ADS
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Title: Prominence oscillations
Authors: Arregui, Iñigo; Oliver, Ramón; Ballester, José Luis
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.
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Title: Bayesian coronal seismology
Authors: Arregui, Iñigo
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.
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Title: Comparison of Damping Mechanisms for Transverse Waves in
Solar Coronal Loops
Authors: Montes-Solís, María; Arregui, Iñigo
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.
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Title: Magnetic field strength in solar coronal waveguides
Authors: Arregui, I.; Asensio Ramos, A.
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.
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Title: Heating of the solar corona
Authors: Arregui, Iñigo
2017psio.confE..59A Altcode:
No abstract at ADS
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Title: Comparison of damping mechanisms for transverse waves in
coronal loops
Authors: Montes-Solís, M.; Arregui, I.
2017psio.confE.115M Altcode:
No abstract at ADS
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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.
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).
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Title: Model Comparison for the Density Structure across Solar
Coronal Waveguides
Authors: Arregui, I.; Soler, R.; Asensio Ramos, A.
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.
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Title: On the nature of transverse coronal waves revealed by wavefront
dislocations
Authors: López Ariste, A.; Luna, M.; Arregui, I.; Khomenko, E.;
Collados, M.
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? <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424340/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Model comparison for the density structure along solar
prominence threads
Authors: Arregui, I.; Soler, R.
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. <BR /> 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. <BR />
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. <BR /> 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
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.
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. <BR /> 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. <BR /> 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. <BR />
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. <BR />
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
2011A&A...533A..60A Altcode: 2010arXiv1011.5175A
<BR /> Aims: We analyse the oscillatory properties of resonantly
damped transverse kink oscillations in two-dimensional prominence
threads. <BR /> 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. <BR /> 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. <BR />
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
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.
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.
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.
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.
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.
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.
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.
2009A&A...503..213G Altcode: 2009arXiv0905.0425G
Context: Magnetohydrodynamic (MHD) waves are often reported in the
solar atmosphere and usually classified as slow, fast, or Alfvén. The
possibility that these waves have mixed properties is often ignored. <BR
/>Aims: The goal of this work is to study and determine the nature
of MHD kink waves. <BR />Methods: This is done by calculating the
frequency, the damping rate and the eigenfunctions of MHD kink waves
for three widely different MHD waves cases: a compressible pressure-less
plasma, an incompressible plasma and a compressible plasma which allows
for MHD radiation. <BR />Results: In all three cases the frequency and
the damping rate are for practical purposes the same as they differ
at most by terms proportional to (k<SUB>z</SUB> R)^2. In the magnetic
flux tube the kink waves are in all three cases, to a high degree of
accuracy incompressible waves with negligible pressure perturbations
and with mainly horizontal motions. The main restoring force of kink
waves in the magnetised flux tube is the magnetic tension force. The
total pressure gradient force cannot be neglected except when the
frequency of the kink wave is equal or slightly differs from the local
Alfvén frequency, i.e. in the resonant layer. <BR />Conclusions:
Kink waves are very robust and do not care about the details of the
MHD wave environment. The adjective fast is not the correct adjective
to characterise kink waves. If an adjective is to be used it should
be Alfvénic. However, it is better to realize that kink waves have
mixed properties and cannot be put in one single box.
---------------------------------------------------------
Title: The Nature of Kink MHD Waves in Magnetic Flux Tubes
Authors: Goossens, Marcel; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J.
2009SPD....40.1306G Altcode:
We examine the nature of MHD kink waves. This is done by determining
the frequency, the damping rate and the eigenfunctions of MHD kink waves
for three widely different MHD waves cases: a compressible pressure-less
plasma, an incompressible plasma and a compressible plasma with non-zero
plasma pressure which allows for MHD radiation. The overall conclusion
is that kink waves are very robust and do not care about the details
of the MHD wave environment. In all three cases the frequency and the
damping rate are for most practical purposes the same. In the magnetic
flux tube the kink waves are in all three cases, to a high degree of
accuracy incompressible waves with negligible pressure perturbations
and with mainly horizontal motions. The main restoring force of kink
waves in the magnetized flux tube is the magnetic tension force. The
gradient pressure force cannot be neglected except when the frequency
of the kink wave is equal or slightly differs from the local Alfvén
frequency, i.e. in the resonant layer. In a non-magnetic external
plasma the wave is of course acoustic. The adjective fast is not the
correct adjective to characterize kink waves. If an adjective is to
be used it should be Alfvénic. However, it is better to realize that
kink waves have mixed properties and cannot be put in one single box.
---------------------------------------------------------
Title: The influence of longitudinal density variation in coronal
loops on the eigenfunctions of kink-oscillation overtones
Authors: Andries, J.; Arregui, I.; Goossens, M.
2009A&A...497..265A Altcode:
Context: As coronal loops are spatially at least partially resolved
in the longitudinal direction, attempts have been made to use
the longitudinal profiles of the oscillation amplitudes as a
seismological tool. <BR />Aims: We aim to derive simple formulae to
assess which oscillation modes and which quantities of the oscillation
(displacement or compression) are most prone to modifications induced
by stratification of the equilibrium density along the loop. We
furthermore clarify and quantify that the potential of such a method
could be enhanced if observational profiles of the compression in the
oscillations could be determined. <BR />Methods: By means of a linear
expansion in the longitudinal stratification along with the “thin
tube” approximation, the modifications to the eigenfunctions are
calculated analytically. The results are validated by direct numerical
computations. <BR />Results: Higher axial overtones are found to be
more affected by equilibrium stratification and hence would provide a
much better tool if observed. For the k-1th overtone the compression
is found to be around (k + 2)^2/k<SUP>2</SUP> times more sensitive to
longitudinal density variation than the displacement. While the linear
formulae do give a good indication of the strength of the effects
of longitudinal density stratification, the numerical computations
indicate that the corrections to the approximate analytical results
are significant and cannot be neglected under the expected coronal
conditions.
---------------------------------------------------------
Title: Nonlinear Instability of Kink Oscillations due to Shear Motions
Authors: Terradas, J.; Andries, J.; Goossens, M.; Arregui, I.; Oliver,
R.; Ballester, J. L.
2008ApJ...687L.115T Altcode: 2008arXiv0809.3664T
First results from a high-resolution three-dimensional nonlinear
numerical study of the kink oscillation are presented. We show in
detail the development of a shear instability in an untwisted line-tied
magnetic flux tube. The instability produces significant deformations
of the tube boundary. An extended transition layer may naturally evolve
as a result of the shear instability at a sharp transition between
the flux tube and the external medium. We also discuss the possible
effects of the instability on the process of resonant absorption when an
inhomogeneous layer is included in the model. One of the implications
of these results is that the azimuthal component of the magnetic field
of a stable flux tube in the solar corona, needed to prevent the shear
instability, is probably constrained to be in a very specific range.
---------------------------------------------------------
Title: Damping of Fast Magnetohydrodynamic Oscillations in Quiescent
Filament Threads
Authors: Arregui, Iñigo; Terradas, Jaume; Oliver, Ramón; Ballester,
José Luis
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.
2008ApJ...679.1611T Altcode: 2008arXiv0802.0591T
We study the excitation and damping of transverse oscillations in a
multistranded model of a straight line-tied coronal loop. The transverse
geometry of our equilibrium configuration is quite irregular and
more realistic than the usual cylindrical loop model. By numerically
solving the time-dependent ideal magnetohydrodynamic equations in
two dimensions, we show how the global motion of the whole bundle of
strands, excited by an external disturbance, is converted into localized
Alfvénic motions due to the process of resonant absorption. This
process produces the attenuation of the transverse oscillations. At
any location in the structure, two dominant frequencies are found:
the frequency of the global mode or quasi-mode, and the local Alfvén
frequency. We find that the mechanism of mode conversion, due to the
coupling between fast and Alfvén waves, is not compromised by the
complicated geometry of the model. We also show that it is possible to
have energy conversion not only at the external edge of the composite
loop, but also inside the structure. The implications of these results
and their relationship with the observations are discussed.
---------------------------------------------------------
Title: Analytic approximate seismology of transversely oscillating
coronal loops
Authors: Goossens, M.; Arregui, I.; Ballester, J. L.; Wang, T. J.
2008A&A...484..851G Altcode: 2008arXiv0804.3877G
Aims: We present an analytic approximate seismic inversion scheme for
damped transverse coronal loop oscillations based on the thin tube and
thin boundary approximation for computing the period and the damping
time. <BR />Methods: Asymptotic expressions for the period and damping
rate are used to illustrate the process of seismological inversion in a
simple and easy to follow manner. The inversion procedure is formulated
in terms of two simple functions, which are given by simple closed
expressions. <BR />Results: The analytic seismic inversion shows that
an infinite amount of 1-dimensional equilibrium models can reproduce
the observed periods and damping times. It predicts a specific range of
allowable values for the Alfvén travel time and lower bounds for the
density contrast and the inhomogeneity length scale. When the results
of the present analytic seismic inversion are compared with those of
a previous numerical inversion, excellent agreement is found up to
the point that the analytic seismic inversion emerges as a tool for
validating results of numerical inversions. Actually it helped us to
identify and correct inaccuracies in a previous numerical investigation.
---------------------------------------------------------
Title: Resonant absorption in multi-stranded coronal loops
Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L.
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.
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.
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<SUP>-1</SUP>) 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
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
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.
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.
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.
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. <BR />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. <BR
/>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. <BR
/>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. <BR />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.
2007A&A...463..333A Altcode:
Aims:We combine the magnetohydrodynamic (MHD) theory of resonantly
damped quasi-mode kink oscillations with observational estimates of
the period and damping of transverse coronal loop oscillations to
extract information on physical parameters in oscillating loops. <BR
/>Methods: A numerical study of the quasi-mode period and damping,
in one-dimensional fully non-uniform flux tubes, is used to obtain
equilibrium models that reproduce the observed periods and damping
rates. This scheme is applied to 11 loop oscillation events. <BR
/>Results: When only the damping rate is used, the valid equilibrium
models form a one-dimensional solution curve in the two-dimensional
parameter space (density contrast, transverse inhomogeneity
length-scale). Lower limits to the transverse inhomogeneity are
obtained in the limit of high contrast loops. When both the period and
the damping rate are used, the equilibrium Alfvén speed (or Alfvén
travel time) comes into play. The valid equilibrium models then form
a one-dimensional solution curve in the three-dimensional parameter
space (density contrast, transverse inhomogeneity length-scale, Alfvén
speed or Alfvén travel time). The projection of these solutions onto
the Alfvén speed axis is found to be constrained to a rather limited
interval. Upper limits to the internal Alfvén speed are derived for
9 of the 11 analysed events.
---------------------------------------------------------
Title: Time Dependent Simulations of 2D Coronal Loop Models
Authors: van Doorsselaere, T.; Poedts, S.; Andries, J.; Arregui, I.
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.
2006ESASP.617E..81A Altcode: 2006soho...17E..81A
No abstract at ADS
---------------------------------------------------------
Title: Seismology of Transversely Oscillating Loops Using Periods
and Damping Times
Authors: Goossens, Marcel; Arregui, I.; Andries, J.; Van Doorsselaere,
T.
2006SPD....37.1804G Altcode: 2006BAAS...38..247G
Periods and damping times of quasi-mode fundamental kink oscillations
have been computed for non-uniform cylindrical models of coronal
loops. The radial inhomogeneity length-scale, the density contrast
and the internal Alfvén velocity are three equilibrium quantities
that determine the theoretical values of the period and damping
times in 1-D equilibrium models. From a seismological point of view
this means that observed values of period and damping time can be
recovered by an infinite number of equilibrium models. In other words,
observed values of period and damping time of the fundamental kink
oscillation mode do not allow a unique identification of even a 1-D
equilibrium model. Only if there is additional information on one of
the three equilibrium quantities, can we use the observed values of
period and damping time to determine the two remaining equilibrium
quantities. However, it is not all bad news. It turns out that, even
without additional information, we can determine upper limits to the
internal Alfvén velocity. We apply this scheme to the set of 11 loop
oscillation events studied in Goossens et al. 2002 and find constraints
on the equilibrium parameters for these 11 events.
---------------------------------------------------------
Title: Damping of magnetohydrodynamic waves by resonant absorption
in the solar atmosphere
Authors: Goossens, M.; Andries, J.; Arregui, I.
2006RSPTA.364..433G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasi-mode damping in two-dimensional fully non-uniform
coronal loops
Authors: Arregui, I.; Van Doorsselaere, T.; Andries, J.; Goossens,
M.; Poedts, S.
2006RSPTA.364..529A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Seismology of Coronal Loops Using Resonant Absorption
Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens,
M.; Poedts, S.
2005ESASP.600E..21A Altcode: 2005dysu.confE..21A; 2005ESPM...11...21A
No abstract at ADS
---------------------------------------------------------
Title: Building a Time Dependent Code to Simulate Oscillations of
Line-Tied Coronal Loops
Authors: van Doorsselaere, T.; Poedts, S.; Arregui, I.; Andries, J.
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.
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.
2005ESASP.596E..44V Altcode: 2005ccmf.confE..44V
No abstract at ADS
---------------------------------------------------------
Title: Resonantly damped fast MHD kink modes in longitudinally
stratified tubes with thick non-uniform transitional layers
Authors: Arregui, I.; Van Doorsselaere, T.; Andries, J.; Goossens,
M.; Kimpe, D.
2005A&A...441..361A Altcode:
Resonantly damped fast kink quasi-modes are computed in fully <P
/>resistive magnetohydrodynamics (MHD) for two-dimensional equilibrium
<P />models. The equilibrium model is a straight cylindrically symmetric
flux <P />tube with a plasma density that is non-uniform both across
and along the <P />loop. The non-uniform layer across the loop is not
<P />restricted to be thin, but its thickness can reach values up to
the <P />loop diameter. <P />Our results indicate that <P />the period
and damping of coronal loop oscillations mainly depend on the density
contrast <P />and the inhomogeneity length-scale and are independent
of the details of <P />longitudinal stratification, depending on the
weighted mean density, <P />weighted with the wave energy. For fully
non-uniform loops, quasi-modes can <P />interact with resistive Alfvén
eigenmodes leading to avoided crossings <P />and gaps in the complex
frequency plane. The present study extends previous <P />studies on
coronal loop oscillations in one-dimensional equilibrium models <P
/>with thick boundary layers and in equilibria with longitudinally
stratified loops under the <P />thin boundary approximation, and allow
for a better comparison between <P />observations and theory raising
the prospect of coronal seismology using <P />the time damping of
coronal loop oscillations.
---------------------------------------------------------
Title: Solar coronal loop oscillations: theory of resonantly damped
oscillations and comparison with observations
Authors: Goossens, M.; Andries, J.; Arregui, I.; Doorsselaere, T. V.;
Poedts, S.
2005AIPC..784..114G Altcode:
One of the proposed damping mechanisms of coronal transverse loop
oscillations in the kink mode is resonant absorption as a result
of the spatial variation of the Alfvén velocity in the equilibrium
configuration. Analytical expressions for the period and the damping
time exist for 1-D cylindrical equilibrium models with thin non-uniform
transitional layers. Comparison with observations indicates that the
assumption of thin non-uniform transitional layers is not a very
accurate approximation of reality. This contributions starts with
a short review of observations on transverse oscillations in solar
coronal loops. Then it presents results on periods and damping times
of resonantly damped kink mode oscillations for (i) fully non-uniform
1-D cylindrical equilibrium models in which the equilibrium quantities
vary in the radial direction across the magnetic field from the centre
of the loop up to its boundary and (ii) non-uniform 2-D cylindrical
equilibrium models in which the equilibrium quantities vary both in the
radial direction across the magnetic field and in the axial direction
along the magnetic field. An important point is that the periods and
damping times obtained for these fully non-uniform models can differ
substantially from those obtained for thin non-uniform transitional
layers. This contribution then reports on a consistency test between
theory and observations showing that there is a very good agreement
within the observational inaccuracies.
---------------------------------------------------------
Title: Determination of the Coronal Density Stratification from the
Observation of Harmonic Coronal Loop Oscillations
Authors: Andries, Jesse; Arregui, Inigo; Goossens, Marcel
2005ApJ...624L..57A Altcode:
The recent detection of multiple harmonic standing transverse
oscillations in coronal loops by Verwichte et al. is of special
importance, as it allows one to obtain information on the longitudinal
density variation in loops. Verwichte et al. detected the simultaneous
presence of both the fundamental and the first-overtone mode in two
coronal loops. Here we point out that the ratio of the period of the
fundamental mode to the period of the overtone mode differs from 2
in loops with longitudinal density stratification. Conversely, the
difference between this ratio and 2 can be used as a seismological
tool to obtain information about the density scale height in loops.
---------------------------------------------------------
Title: Coronal loop oscillations. Calculation of resonantly damped
MHD quasi-mode kink oscillations of longitudinally stratified loops
Authors: Andries, J.; Goossens, M.; Hollweg, J. V.; Arregui, I.;
Van Doorsselaere, T.
2005A&A...430.1109A Altcode:
The observed coronal loop oscillations and their damping are often
theoretically described by the use of a very simple coronal loop
model, viz. a straight, longitudinally invariant, axi-symmetric, and
pressureless flux tube with a different density inside and outside
of the loop. In this paper we generalize the model by including
longitudinal density stratification and we examine how the longitudinal
density stratification alters the linear eigenmodes of the system,
their oscillation frequencies, and the damping rates by resonant
absorption. <P />Appendix A is only available in electronic form at
http://www.edpsciences.org
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Title: Numerical Solutions for Resonantly Damped MHD Quasi-Modes in
Two-Dimensional Coronal Loops
Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens, M.
2004ESASP.575...85A Altcode: 2004soho...15...85A
No abstract at ADS
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Title: Magnetohydrodynamic waves in a sheared potential coronal arcade
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
2004A&A...425..729A Altcode:
We study the effects of magnetic field shear (B<SUB>y</SUB> ≠ 0)
and longitudinal propagation of perturbations (k<SUB>y</SUB> ≠ 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 <P />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.
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Title: Erratum: “Magnetohydrodynamic Waves in Sheared Coronal
Arcades” (<A href="/abs/2004ApJ...602.1006A">ApJ, 602, 1006
[2004]</A>)
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
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 <A>e</A><SUB>n</SUB>=∇A/∇A=-<A>e</A><SUB>z</SUB> should
read <A>e</A><SUB>n</SUB>=∇A/∇A; i.e., the final equivalence should
be omitted. <P />The Press sincerely regrets this error.
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Title: Magnetohydrodynamic Waves in Sheared Coronal Arcades
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
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, B<SUB>y</SUB>, and of
the longitudinal wavenumber, k<SUB>y</SUB>, in these structures. Our
results show that whenever B<SUB>y</SUB>≠0 and/or k<SUB>y</SUB>≠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, k<SUB>y</SUB>, and B<SUB>y</SUB>
are presented. We have found that the frequency of coupled modes is
real when either B<SUB>y</SUB>≠0 or k<SUB>y</SUB>≠0 but is complex
when both B<SUB>y</SUB>≠0 and k<SUB>y</SUB>≠0. Also, an important
result is that coupled modes may not be observable since many of them
leak energy away from the solar corona.
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Title: Coupled Fast and Alfvén MHD Waves in Sheared Coronal Arcades
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
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.
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Title: Magnetohydrodynamic waves in sheared coronal magnetic
structures
Authors: Arregui, I.
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.
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Title: Coupling of fast and Alfvén waves in a straight bounded
magnetic field with density stratification
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
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 (k<SUB>y</SUB>
!=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 <SUP>2</SUP>
varying linearly with k<SUB>y</SUB><SUP>2</SUP>, 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).
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Title: Magnetohydrodynamic waves in sheared coronal magnetic
structures
Authors: Arregui, Iñigo
2003PhDT.......149A Altcode:
No abstract at ADS
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Title: Magnetohydrodynamic waves in sheared coronal magnetic
structures
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
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.
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Title: Numerical simulations of linear magnetohydrodynamic waves in
two-dimensional force-free magnetic fields
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
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.
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Title: Fast and Alfvén MHD waves in sheared coronal arcades
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
2001hsa..conf..253A Altcode:
No abstract at ADS
