4,049 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&
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&
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
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