10,058 research outputs found
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&
Time damping of non-adiabatic magnetohydrodynamic waves in a partially ionized prominence plasma: Effect of helium
Prominences are partially ionized, magnetized plasmas embedded in the solar
corona. Damped oscillations and propagating waves are commonly observed. These
oscillations have been interpreted in terms of magnetohydrodynamic (MHD) waves.
Ion-neutral collisions and non-adiabatic effects (radiation losses and thermal
conduction) have been proposed as damping mechanisms. We study the effect of
the presence of helium on the time damping of non-adiabatic MHD waves in a
plasma composed by electrons, protons, neutral hydrogen, neutral helium (He I),
and singly ionized helium (He II) in the single-fluid approximation. The
dispersion relation of linear non-adiabatic MHD waves in a homogeneous,
unbounded, and partially ionized prominence medium is derived. The period and
the damping time of Alfven, slow, fast, and thermal waves are computed. A
parametric study of the ratio of the damping time to the period with respect to
the helium abundance is performed. The efficiency of ion-neutral collisions as
well as thermal conduction is increased by the presence of helium. However, if
realistic abundances of helium in prominences (~10%) are considered, this
effect has a minor influence on the wave damping. The presence of helium can be
safely neglected in studies of MHD waves in partially ionized prominence
plasmas.Comment: Research note submitted in A&
Seismology of Standing Kink Oscillations of Solar Prominence Fine Structures
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 considered damping mechanisms.
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 the 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 min) and short-wavelength (700 - 8,000 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., Alfv\'en 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.Comment: Accepted for publication in Ap
Prominence seismology using the period ratio of transverse thread oscillations
The ratio of the period of the fundamental mode to that of the first overtone
of kink oscillations, from here on the "period ratio", is a seismology tool
that can be used to infer information about the spatial variation of density
along solar magnetic flux tubes. The period ratio is 2 in longitudinally
homogeneous thin tubes, but it differs from 2 due to longitudinal
inhomogeneity. In this paper we investigate the period ratio in longitudinally
inhomogeneous prominence threads and explore its implications for prominence
seismology. We numerically solve the two-dimensional eigenvalue problem of kink
oscillations in a model of a prominence thread. We take into account three
nonuniform density profiles along the thread. In agreement with previous works
that used simple piecewise constant density profiles, we find that the period
ratio is larger than 2 in prominence threads. When the ratio of the central
density to that at the footpoints is fixed, the period ratio depends strongly
on the form of the density profile along the thread. The more concentrated the
dense prominence plasma near the center of the tube, the larger the period
ratio. However, the period ratio is found to be independent of the specific
density profile when the spatially averaged density in the thread is the same
for all the profiles. An empirical fit of the dependence of the period ratio on
the average density is given and its use for prominence seismology is
discussed.Comment: Accepted for publication in A&
The Thermal Instability of Solar Prominence Threads
The fine structure of solar prominences and filaments appears as thin and
long threads in high-resolution images. In H-alpha observations of filaments,
some threads can be observed for only 5 - 20 minutes before they seem to fade
and eventually disappear, suggesting that these threads may have very short
lifetimes. The presence of an instability might be the cause of this quick
disappearance. Here, we study the thermal instability of prominence threads as
an explanation of their sudden disappearance from H-alpha observations. We
model a prominence thread as a magnetic tube with prominence conditions
embedded in a coronal environment. We assume a variation of the physical
properties in the transverse direction, so that the temperature and density
continuously change from internal to external values in an inhomogeneous
transitional layer representing the particular prominence-corona transition
region (PCTR) of the thread. We use the nonadiabatic and resistive
magnetohydrodynamic equations, which include terms due to thermal conduction
parallel and perpendicular to the magnetic field, radiative losses, heating,
and magnetic diffusion. We combine both analytical and numerical methods to
study linear perturbations from the equilibrium state, focusing on unstable
thermal solutions. We find that thermal modes are unstable in the PCTR for
temperatures higher than 80,000 K, approximately. These modes are related to
temperature disturbances that can lead to changes in the equilibrium due to
rapid plasma heating or cooling. For typical prominence parameters, the
instability time scale is of the order of a few minutes and is independent of
the form of the temperature profile within the PCTR of the thread. This result
indicates that thermal instability may play an important role for the short
lifetimes of threads in the observations.Comment: Accepted for publication in Ap
Abduction for (non-ominiscient) agents
Among the non-monotonic reasoning processes, abduction is one of the most important. Usually described as the process of looking florexplantions, it has been recognized as one of the most commonly used in our daily activities. Still, the traditional definitions of an abductive problem and an abductive solution mention only theories and formulas, leaving agency out of the picture. Our work proposes a study of abductive reasoning from an epistemic and dynamic perspective, making special emphasis on non-ideal agents. We begin by exploring what an abductive problema is in terms of an agent’s information, and what an abductive solution is in terms of the actions that modify it. Then we explore the different kinds of abductive problems and abductive solutions that arise when we consider agents whose information is not closed under logical consequence, and agents whose reasoning abilities are not complete
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