400 research outputs found
Bayesian Coronal Seismology
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.Comment: 19 pages, accepted in Advances in Space Researc
Wave Heating of the Solar Atmosphere
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 on 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 on 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 understanding and
quantifying magnetic wave heating of the solar atmosphere.Comment: Accepted for publication in Royal Society of London Philosophical
Transactions Series
Model comparison for the density structure along solar prominence threads
Quiescent solar prominence fine structures are typically modelled as density
enhancements, called threads, which occupy a fraction of a longer magnetic flux
tube. The profile of the mass density along the magnetic field is however
unknown and several arbitrary alternatives are employed in prominence wave
studies. We present a comparison of theoretical models for the field-aligned
density along prominence fine structures. We consider Lorentzian, Gaussian, and
parabolic profiles. We compare their theoretical predictions for the period
ratio between the fundamental transverse kink mode and the first overtone to
obtain estimates for the ratio of densities between the central part of the
tube and its foot-points and to assess which one would better explain observed
period ratio data. Bayesian parameter inference and model comparison techniques
are developed and applied. Parameter inference requires the computation of the
posterior distribution for the density gradient parameter conditional on the
observable period ratio. Model comparison involves the computation of the
marginal likelihood as a function of the period ratio to obtain the
plausibility of each density model and the computation of Bayes Factors to
quantify the relative evidence for each model, given a period ratio
observation. 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 large 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 larger period ratio values.Comment: 9 pages, 5 figures, accepted in A&
Determination of the cross-field density structuring in coronal waveguides using the damping of transverse waves
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. 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. The inference is performed
by computing the marginal posterior distributions for density contrast and
transverse inhomo- geneity length-scale using Bayesian analysis and damping
ratios for transverse oscillations under the assumption that damping is
produced by resonant absorption. The obtained distributions show that, for
damping times of a few oscillatory periods, low density contrasts and short
inho- mogeneity length scales are more plausible in explaining observations.
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.Comment: 5 pages, 3 figures, accepte
Determination of Transverse Density Structuring from Propagating MHD Waves in the Solar Atmosphere
We present a Bayesian seismology inversion technique for propagating
magnetohydrodynamic (MHD) 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 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
On the nature of transverse coronal waves revealed by wavefront dislocations
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?
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. 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. The observed dislocations can only be explained by the interference
of a kink and a 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.
Alfv\'en waves can be responsible of 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 smaller than the pressure amplitude
of the sausage mode, though its observed velocity is larger. This concentrates
dislocations on the top of the loop. 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.Comment: 11 pages. 5 figures. Accepted for publication in A&
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