3,044 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&
The statistical significance of the N-S asymmetry of solar activity revisited
The main aim of this study is to point out the difficulties found when trying
to assess the statistical significance of the North-South asymmetry (hereafter
SSNSA) of the most usually considered time series of solar activity. First of
all, we distinguish between solar activity time series composed by integer or
non-integer and dimensionless data, or composed by non-integer and dimensional
data. For each of these cases, we discuss the most suitable statistical tests
which can be applied and highlight the difficulties to obtain valid information
about the statistical significance of solar activity time series. Our results
suggest that, apart from the need to apply the suitable statistical tests,
other effects such as the data binning, the considered units and the need, in
some tests, to consider groups of data, affect substantially the determination
of the statistical significance of the asymmetry. Our main conclusion is that
the assessment of the statistical significance of the N-S asymmetry of solar
activity is a difficult matter and that an absolute answer cannot be given,
since many different effects influence the results given by the statistical
tests. In summary, the quantitative results about the statistical significance
of the N-S asymmetry of solar activity provided by different authors, as well
as the studies about its behaviour, must be considered with care because they
depend from the chosen values of different parameters or from the considered
units.Comment: Astronomy and Astrophysics Latex, 9 pages, 4 figure
Effect of partial ionization on wave propagation in solar magnetic flux tubes
Observations show that waves are ubiquitous in the solar atmosphere and may
play an important role for plasma heating. The study of waves in the solar
corona is usually based on linear ideal magnetohydrodynamics (MHD) for a fully
ionized plasma. However, the plasma in the photosphere and the chromosphere is
only partially ionized. Here we investigate theoretically the impact of partial
ionization on MHD wave propagation in cylindrical flux tubes in the two-fluid
model. We derive the general dispersion relation that takes into account the
effects of neutral-ion collisions and the neutral gas pressure. We take the
neutral-ion collision frequency as an arbitrary parameter. Particular results
for transverse kink modes and slow magnetoacoustic modes are shown. We find
that the wave frequencies only depend on the properties of the ionized fluid
when the neutral-ion collision frequency is much lower that the wave frequency.
For high collision frequencies realistic of the solar atmosphere ions and
neutrals behave as a single fluid with an effective density corresponding to
the sum of densities of both fluids and an effective sound velocity computed as
the average of the sound velocities of ions and neutrals. The MHD wave
frequencies are modified accordingly. The neutral gas pressure can be neglected
when studying transverse kink waves but it has to be taken into account for a
consistent description of slow magnetoacoustic waves. The MHD waves are damped
due to neutral-ion collisions. The damping is most efficient when the wave
frequency and the collision frequency are of the same order of magnitude. For
high collision frequencies slow magnetoacoustic waves are more efficiently
damped than transverse kink waves. In addition, we find the presence of
cut-offs for certain combinations of parameters that cause the waves to become
non-propagating.Comment: Accepted for publication in A&
The spatial damping of magnetohydrodynamic waves in a flowing partially ionised prominence plasma
Solar prominences are partially ionised plasmas displaying flows and
oscillations. These oscillations show time and spatial damping and, commonly,
have been explained in terms of magnetohydrodynamic (MHD) waves. We study the
spatial damping of linear non-adiabatic MHD waves in a flowing partially
ionised plasma, having prominence-like physical properties. We consider single
fluid equations for a partially ionised hydrogen plasma including in the energy
equation optically thin radiation, thermal conduction by electrons and
neutrals, and heating. Keeping the frequency real and fixed, we have solved the
obtained dispersion relations for the complex wavenumber, k, and have analysed
the behaviour of the damping length, wavelength and the ratio of the damping
length to the wavelength, versus period, for Alfven, fast, slow and thermal
waves.Comment: 28 pages, 9 figure
Transverse oscillations of two coronal loops
We study transverse fast magnetohydrodynamic waves in a system of two coronal
loops modeled as smoothed, dense plasma cylinders in a uniform magnetic field.
The collective oscillatory properties of the system due to the interaction
between the individual loops are investigated from two points of view. Firstly,
the frequency and spatial structure of the normal modes are studied. The system
supports four trapped normal modes in which the loops move rigidly in the
transverse direction. The direction of the motions is either parallel or
perpendicular to the plane containing the axes of the loops. Two of these modes
correspond to oscillations of the loops in phase, while in the other two they
move in antiphase. Thus, these solutions are the generalization of the kink
mode of a single cylinder to the double cylinder case. Secondly, we analyze the
time-dependent problem of the excitation of the pair of tubes. We find that
depending on the shape and location of the initial disturbance, different
normal modes can be excited. The frequencies of normal modes are accurately
recovered from the numerical simulations. In some cases, because of the
simultaneous excitation of several eigenmodes, the system shows beating and the
phase lag between the loops is .Comment: Accepted for publication in The Astrophysical Journa
On the scaling of the damping time for resonantly damped oscillations in coronal loops
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
Generation of graph-state streams
We propose a protocol to generate a stream of mobile qubits in a graph state
through a single stationary parent qubit and discuss two types of its physical
implementation, namely, the generation of photonic graph states through an
atom-like qubit and those of flying atoms through a cavity-mode photonic qubit.
The generated graph states fall into an important class that can hugely reduce
the resource requirement of fault-tolerant linear optics quantum computation,
which was previously known to be far from realistic. In regard to the flying
atoms, we also propose a heralded generation scheme, which allows for
high-fidelity graph states even under the photon loss.Comment: Accepted for publication at PRA Rapid Communication
Who Contributes to the Knowledge Sharing Economy?
Information sharing dynamics of social networks rely on a small set of
influencers to effectively reach a large audience. Our recent results and
observations demonstrate that the shape and identity of this elite, especially
those contributing \emph{original} content, is difficult to predict.
Information acquisition is often cited as an example of a public good. However,
this emerging and powerful theory has yet to provably offer qualitative
insights on how specialization of users into active and passive participants
occurs.
This paper bridges, for the first time, the theory of public goods and the
analysis of diffusion in social media. We introduce a non-linear model of
\emph{perishable} public goods, leveraging new observations about sharing of
media sources. The primary contribution of this work is to show that
\emph{shelf time}, which characterizes the rate at which content get renewed,
is a critical factor in audience participation. Our model proves a fundamental
\emph{dichotomy} in information diffusion: While short-lived content has simple
and predictable diffusion, long-lived content has complex specialization. This
occurs even when all information seekers are \emph{ex ante} identical and could
be a contributing factor to the difficulty of predicting social network
participation and evolution.Comment: 15 pages in ACM Conference on Online Social Networks 201
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
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