9,050 research outputs found
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&
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&
Efficient implementation of a van der Waals density functional: Application to double-wall carbon nanotubes
We present an efficient implementation of the van der Waals density
functional of Dion et al [Phys. Rev. Lett. 92, 246401 (2004)], which expresses
the nonlocal correlation energy as a double spacial integral. We factorize the
integration kernel and use fast Fourier transforms to evaluate the
selfconsistent potential, total energy, and atomic forces, in N log(N)
operations. The resulting overhead in total computational cost, over semilocal
functionals, is very moderate for medium and large systems. We apply the method
to calculate the binding energies and the barriers for relative translation and
rotation in double-wall carbon nanotubes.Comment: 4 pages, 1 figure, 1 tabl
Torsional Alfv\'en waves in solar partially ionized plasma: effects of neutral helium and stratification
Ion-neutral collisions may lead to the damping of Alfven waves in
chromospheric and prominence plasmas. Neutral helium atoms enhance the damping
in certain temperature interval, where the ratio of neutral helium and neutral
hydrogen atoms is increased. Therefore, the height-dependence of ionization
degrees of hydrogen and helium may influence the damping rate of Alfven waves.
We aim to study the effect of neutral helium in the damping of Alfven waves in
stratified partially ionized plasma of the solar chromosphere. We consider a
magnetic flux tube, which is expanded up to 1000 km height and then becomes
vertical due to merging with neighboring tubes, and study the dynamics of
linear torsional Alfven waves in the presence of neutral hydrogen and neutral
helium atoms. We start with three-fluid description of plasma and consequently
derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven
waves. Thin flux tube approximation allows to obtain the dispersion relation of
the waves in the lower part of tubes, while the spatial dependence of
steady-state Alfven waves is governed by Bessel type equation in the upper part
of tubes. Consecutive derivation of single-fluid MHD equations results in a new
Cowling diffusion coefficient in the presence of neutral helium which is
different from previously used one. We found that shorter-period (< 5 s)
torsional Alfven waves damp quickly in the chromospheric network due to
ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not
reach the transition region as they become evanescent at lower heights in the
network cores. Propagation of torsional Alfven waves through the chromosphere
into the solar corona should be considered with caution: low-frequency waves
are evanescent due to the stratification, while high-frequency waves are damped
due to ion neutral collisions.Comment: 9 pages, 7 figures (accepted in A&A
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