[Show abstract][Hide abstract] ABSTRACT: We present the first results of the joint Polish-Czech observational campaign devoted to simultaneous observations of prominence oscillations. As was shown earlier by other authors, not all of the observed periodicities in the Doppler signal come from solar sources (seeing and slight changes in the position of the spectrograph slit may have a significant influence). To exclude false signals, we performed simultaneous observations of the same object on the Sun using two independent telescopes. On 23 September 2010, a quiescent prominence on the north-eastern part of the solar limb was observed with two distant solar telescopes: the Large Coronagraph installed at the Białków Observatory, Poland, and the Horizontal Telescope at the Ondřejov Observatory, Czech Republic. Of the many detected periods, the periods of 26, 31, and 55 min unquestionably originate in the prominence, but other periodicities are spurious. Proper detection of periodicities in prominences is crucial for modelling wave propagation and movements in the solar plasma, as well as for seismologically inverting prominence structures and physical parameters.
Solar Physics 06/2015; 290(6). DOI:10.1007/s11207-015-0696-x · 4.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Long-term records of sunspot number and concentrations of cosmogenic
radionuclides (10Be and 14C) on the Earth reveal the variation of the Sun's
magnetic activity over hundreds and thousands of years. We identify several
clear periods in sunspot, 10Be, and 14C data as 1000, 500, 350, 200 and 100
years. We found that the periods of the first five spherical harmonics of the
slow magnetic Rossby mode in the presence of a steady toroidal magnetic field
of 1200-1300 G in the lower tachocline are in perfect agreement with the time
scales of observed variations. The steady toroidal magnetic field can be
generated in the lower tachocline either due to the steady dynamo magnetic
field for low magnetic diffusivity or due to the action of the latitudinal
differential rotation on the weak poloidal primordial magnetic field, which
penetrates from the radiative interior. The slow magnetic Rossby waves lead to
variations of the steady toroidal magnetic field in the lower tachocline, which
modulate the dynamo magnetic field and consequently the solar cycle strength.
This result constitutes a key point for long-term prediction of the cycle
strength. According to our model, the next deep minimum in solar activity is
expected during the first half of this century.
[Show abstract][Hide abstract] ABSTRACT: The formation and dynamics of coronal rain are currently not fully
understood. Coronal rain is the fall of cool and dense blobs formed by thermal
instability in the solar corona towards the solar surface with acceleration
smaller than gravitational free fall. We aim to study the observational
evidence of the formation of coronal rain and to trace the detailed dynamics of
individual blobs. We used time series of the 171 \AA\, and 304 \AA\, spectral
lines obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamic Observatory (SDO) above active region AR 11420 on February 22, 2012.
Observations show that a coronal loop disappeared in the 171 \AA\ channel and
appeared in the 304 \AA\ line$\text{}\text{}$ more than one hour later, which
indicates a rapid cooling of the coronal loop from 1 MK to 0.05 MK. An energy
estimation shows that the radiation is higher than the heat input, which
indicates so-called catastrophic cooling. The cooling was accompanied by the
formation of coronal rain in the form of falling cold plasma. We studied two
different sequences of falling blobs. The first sequence includes three
different blobs. The mean velocities of the blobs were estimated to be 50 km
s$^{-1}$, 60 km s$^{-1}$ and 40 km s$^{-1}$. A polynomial fit shows the
different values of the acceleration for different blobs, which are lower than
free-fall in the solar corona. The first and second blob move along the same
path, but with and without acceleration, respectively. We performed simple
numerical simulations for two consecutive blobs, which show that the second
blob moves in a medium that is modified by the passage of the first blob.
Therefore, the second blob has a relatively high speed and no acceleration, as
is shown by observations. The second sequence includes two different blobs with
mean velocities of 100 km s$^{-1}$ and 90 km s$^{-1}$, respectively.
Astronomy and Astrophysics 04/2015; 577. DOI:10.1051/0004-6361/201424101 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the nature of dissipative instability at the boundary (seen
here as tangential discontinuity) between the viscous corona and the partially
ionised prominence plasma in the incompressible limit. The importance of the
partial ionisation is investigated in terms of the ionisation fraction.
Matching the solutions for the transversal component of the velocity and total
pressure at the interface between the prominence and coronal plasmas, we derive
a dispersion relation whose imaginary part describes the evolution of the
instability. Results are obtained in the limit of weak dissipation. Using
simple analytical methods, we show that dissipative instabilities appear for
flow speeds that are lower than the Kelvin-Helmholtz instability threshold.
While viscosity tends to destabilise the plasma, the effect of partial
ionisation (through the Cowling resistivity) will act towards stabilising the
interface. For ionisation degrees closer to a neutral gas the interface will be
unstable for larger values of equilibrium flow. The same principle is assumed
when studying the appearance of instability at the interface between
prominences and dark plumes. The unstable mode appearing in this case has a
very small growth rate and dissipative instability cannot explain the
appearance of flows in plumes. The present study improves our understanding of
the complexity of dynamical processes at the interface of solar prominences and
solar corona, and the role partial ionisation can have on the stability of the
plasma. Our results clearly show that the problem of partial ionisation
introduces new aspects of plasma stability with consequences on the evolution
of solar prominences.
Astronomy and Astrophysics 03/2015; 577. DOI:10.1051/0004-6361/201423973 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cool and dense prominences found in the solar atmosphere are known to be
partially ionized because of their relative low temperature. In this Letter, we
address the long-standing problem of how the neutral component of the plasma in
prominences is supported against gravity. Using the multiple fluid approach we
solve the time-dependent equations in two dimensions considering the frictional
coupling between the neutral and ionized components of the magnetized plasma
representative of a solar prominence embedded in a hot coronal environment. We
demonstrate that given an initial density enhancement in the two fluids,
representing the body of the prominence, the system is able to relax in the
vicinity of magnetic dips to a stationary state in which both neutrals and
ionized species are dynamically suspended above the photosphere. Two different
coupling processes are considered in this study, collisions between ions and
neutrals and charge exchange interactions. We find that for realistic
conditions ions are essentially static while neutrals have a very small
downflow velocity. The coupling between ions and neutrals is so strong at the
prominence body that the behavior is similar to that of a single fluid with an
effective density equal to the sum of the ion and neutral species. We also find
that the charge exchange mechanism is about three times more efficient
sustaining neutrals than elastic scattering of ions with neutrals.
[Show abstract][Hide abstract] ABSTRACT: In this paper we present a numerical study of the time evolution of solar
prominences embedded in sheared magnetic arcades. The prominence is represented
by a density enhancement in a background stratified atmosphere and is connected
to the photosphere through the magnetic field. By solving the ideal
magnetohydrodynamic (MHD) equations in three dimensions we study the dynamics
for a range of parameters representative of real prominences. Depending on the
parameters considered, we find prominences that are suspended above the
photosphere, i.e., detached prominences, but also configurations resembling
curtain or hedgerow prominences whose material continuously connects to the
photosphere. The plasma$-\beta$ is an important parameter that determines the
shape of the structure. In many cases magnetic Rayleigh-Taylor (MRT)
instabilities and oscillatory phenomena develop. Fingers and plumes are
generated, affecting the whole prominence body and producing vertical
structures in an essentially horizontal magnetic field. However, magnetic shear
is able to reduce or even to suppress this instability.
The Astrophysical Journal 12/2014; 799(1). DOI:10.1088/0004-637X/799/1/94 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transverse oscillations of thin threads in solar prominences are
frequently reported in high-resolution observations. The typical periods
of the oscillations are in the range of 3 to 20 min. A peculiar feature
of the oscillations is that they are damped in time, with short damping
times corresponding to few periods. Theoretically, the oscillations are
interpreted as kink magnetohydrodynamic waves. However, the mechanism
responsible for the damping is not well known. Here we perform a
comparative study between different physical mechanisms that may damp
kink waves in prominence threads. The considered processes are thermal
conduction, cooling by radiation, resonant absorption, and ion-neutral
collisions. We find that thermal conduction and radiative cooling are
very inefficient for the damping of kink waves. The effect of
ion-neutral collisions is minor for waves with periods usually observed.
Resonant absorption is the only process that produces an efficient
damping. The damping times theoretically predicted by resonant
absorption are compatible with those reported in the observations.
Proceedings of the International Astronomical Union 06/2014; DOI:10.1017/S1743921313010739
[Show abstract][Hide abstract] ABSTRACT: Prominence oscillations have been mostly detected using Doppler
velocity, although there are also claimed detections by means of the
periodic variations of half-width or line intensity. Our main aim here
is to explore the relationship between spectral indicators such as
Doppler shift, line intensity and line half-width and the linear
perturbations excited in a simple prominence model.
Proceedings of the International Astronomical Union 06/2014; DOI:10.1017/S1743921313010740
[Show abstract][Hide abstract] ABSTRACT: The dispersion of small amplitude, impulsively excited wave trains
propagating along a magnetic flux tube is investigated. The initial disturbance
is a localized transverse displacement of the tube that excites a fast kink
wave packet. The spatial and temporal evolution of the perturbed variables
(density, plasma displacement, velocity, ...) is given by an analytical
expression containing an integral that is computed numerically. We find that
the dispersion of fast kink wave trains is more important for shorter initial
disturbances (i.e. more concentrated in the longitudinal direction) and for
larger density ratios (i.e. for larger contrasts of the tube density with
respect to the environment density). This type of excitation generates a wave
train whose signature at a fixed position along a coronal loop is a short event
(duration ~ 20 s) in which the velocity and density oscillate very rapidly with
typical periods of the order of a few seconds. The oscillatory period is not
constant but gradually declines during the course of this event. Peak values of
the velocity are of the order of 10 km/s and are accompanied by maximum density
variations of the order of 10-15% the unperturbed loop density.
The Astrophysical Journal 02/2014; 789(1). DOI:10.1088/0004-637X/789/1/48 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Context. Prominence oscillations have been mostly detected using Doppler
velocity, although there are also claimed detections by means of periodic
variations in half-width or line intensity. However, scarce observational
evidence exists about simultaneous detection of oscillations in several
spectral indicators. Aims. Our main aim here is to explore the relationship
between spectral indicators, such as Doppler shift, line intensity, and line
half-width, and the linear perturbations excited in a simple prominence model.
Methods. Our equilibrium background model consists of a bounded, homogeneous
slab, which is permeated by a transverse magnetic field, having prominence-like
physical properties. Assuming linear perturbations, the dispersion relation for
fast and slow modes has been derived, as well as the perturbations for the
different physical quantities. These perturbations have been used as the input
variables in a one-dimensional radiative transfer code, which calculates the
full spectral profile of the hydrogen H-alpha and H-beta lines. Results. We
have found that different oscillatory modes produce spectral indicator
variations in different magnitudes. Detectable variations in the Doppler
velocity were found for the fundamental slow mode only. Substantial variations
in the H-beta line intensity were found for specific modes. Other modes lead to
lower and even undetectable parameter variations. Conclusions. To perform
prominence seismology, analysis of the H-alpha and H-beta spectral line
parameters could be a good tool to detect and identify oscillatory modes.
Astronomy and Astrophysics 01/2014; 562. DOI:10.1051/0004-6361/201322346 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Seismology of coronal loops using observations of damped transverse
oscillations in combination with results from theoretical models is a tool to
indirectly infer physical parameters in the solar atmospheric plasma. Existing
seismology schemes based on approximations to the period and damping time of
kink oscillations are often used beyond their theoretical range of
applicability. These approximations assume that the variation of density across
the loop is confined to a nonuniform layer much thinner than the radius of the
loop, but the results of the inversion problem often do not satisfy this
preliminary hypothesis. Here, we determine the accuracy of the analytic
approximations to the period and damping time, and its impact on seismology
estimates, when largely nonuniform loops are considered. We find that the
accuracy of the approximations when used beyond their range of applicability is
strongly affected by the form of the density profile across the loop, that is
observationally unknown and so must be arbitrarily imposed as part of the
theoretical model. The error associated with the analytic approximations can be
larger than 50% even for relatively thin nonuniform layers. This error directly
affects the accuracy of approximate seismology estimates compared to actual
numerical inversions. In addition, assuming different density profiles can
produce noncoincident intervals of the seismic variables in inversions of the
same event. The ignorance about the true shape of density variation across the
loop is an important source of error that may dispute the reliability of
parameters seismically inferred assuming an ad hoc density profile.
The Astrophysical Journal 12/2013; 781(2). DOI:10.1088/0004-637X/781/2/111 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Observations of active regions and limb prominences often show cold, dense
blobs descending with an acceleration smaller than that of free fall. The
dynamics of these condensations falling in the solar corona is investigated in
this paper using a simple fully ionised plasma model. We find that the presence
of a heavy condensation gives rise to a dynamical rearrangement of the coronal
pressure that results in the formation of a large pressure gradient that
opposes gravity. Eventually this pressure gradient becomes so large that the
blob acceleration vanishes or even points upwards. Then, the blob descent is
characterised by an initial acceleration phase followed by an essentially
constant velocity phase. These two stages can be identified in published
time-distance diagrams of coronal rain events. Both the duration of the first
stage and the velocity attained by the blob increase for larger values of the
ratio of blob to coronal density, for larger blob mass, and for smaller coronal
temperature. Dense blobs are characterised by a detectable density growth (up
to 60% in our calculations) and by a steepening of the density in their lower
part, that could lead to the formation of a shock. They also emit sound waves
that could be detected as small intensity changes with periods of the order of
100 s and lasting between a few and about ten periods. Finally, the curvature
of the falling path is only relevant when a very dense blob falls along
inclined magnetic field lines.
The Astrophysical Journal 11/2013; 784(1). DOI:10.1088/0004-637X/784/1/21 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Solar prominence models used so far in the analysis of MHD waves in such
structures are quite elementary. In this work, we calculate numerically
magnetohydrostatic models in two-dimensional configurations under the presence
of gravity. Our interest is in models that connect the magnetic field to the
photosphere and include an overlying arcade. The method used here is based on a
relaxation process and requires solving the time-dependent nonlinear ideal MHD
equations. Once a prominence model is obtained, we investigate the properties
of MHD waves superimposed on the structure. We concentrate on motions purely
two-dimensional neglecting propagation in the ignorable direction. We
demonstrate how by using different numerical tools we can determine the period
of oscillation of stable waves. We find that vertical oscillations, linked to
fast MHD waves, are always stable and have periods in the 4-10 min range.
Longitudinal oscillations, related to slow magnetoacoustic-gravity waves, have
longer periods in the range of 28-40 min. These longitudinal oscillations are
strongly influenced by the gravity force and become unstable for short magnetic
arcades.
The Astrophysical Journal 09/2013; 778(1). DOI:10.1088/0004-637X/778/1/49 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetohydrodynamic (MHD) waves are ubiquitously observed in the solar
atmosphere. Kink waves are a type of transverse MHD waves in magnetic flux
tubes that are damped due to resonant absorption. The theoretical study of kink
MHD waves in solar flux tubes is usually based on the simplification that the
transverse variation of density is confined to a nonuniform layer much thinner
than the radius of the tube, i.e., the so-called thin boundary approximation.
Here, we develop a general analytic method to compute the dispersion relation
and the eigenfunctions of ideal MHD waves in pressureless flux tubes with
transversely nonuniform layers of arbitrary thickness. Results for kink waves
are produced and are compared with fully numerical resistive MHD eigenvalue
computations in the limit of small resistivity. We find that the frequency and
resonant damping rate are the same in both ideal and resistive cases. The
actual results for thick nonuniform layers deviate from the behavior predicted
in the thin boundary approximation and strongly depend on the shape of the
nonuniform layer. The eigenfunctions in ideal MHD are very different from those
in resistive MHD. The ideal eigenfunctions display a global character
regardless of the thickness of the nonuniform layer, while the resistive
eigenfunctions are localized around the resonance and are indistinguishable
from those of ordinary resistive Alfv\'en modes. Consequently, the spatial
distribution of wave energy in the ideal and resistive cases is dramatically
different. This poses a fundamental theoretical problem with clear
observational consequences.
The Astrophysical Journal 09/2013; 777(2). DOI:10.1088/0004-637X/777/2/158 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetohydrodynamic seismology aims to determine difficult to measure
physical parameters in the solar corona by a combination of observed and
theoretical properties of waves and oscillations. We describe relevant
examples of the application of seismology techniques to transversely
oscillating coronal loops and prominence fine structures. We also show
how the use of statistical techniques, based on Bayesian inference, can
be of high value in the determination of physical parameters in these
structures, by consistently taking into account the information from
observations.
[Show abstract][Hide abstract] ABSTRACT: We investigate the excitation of magnetoacoustic-gravity waves generated from localized pulses in the gas pressure as well as in vertical component of velocity. These pulses are initially launched at the top of the solar photosphere that is permeated by a weak magnetic field. We investigate three different configurations of the background magnetic field lines: horizontal, vertical and oblique to the gravitational force. We numerically model magnetoacoustic-gravity waves by implementing a realistic (VAL-C) model of solar temperature. We solve two-dimensional ideal magnetohydrodynamic equations numerically with the use of the FLASH code to simulate the dynamics of the lower solar atmosphere. The initial pulses result in shocks at higher altitudes. Our numerical simulations reveal that a small-amplitude initial pulse can produce magnetoacoustic-gravity waves, which are later reflected from the transition region due to the large temperature gradient. The atmospheric cavities in the lower solar atmosphere are found to be the ideal places that may act as a resonator for various oscillations, including their trapping and leakage into the higher atmosphere. Our numerical simulations successfully model the excitation of such wave modes, their reflection and trapping, as well as the associated plasma dynamics.
Solar Physics 04/2012; 283(2):383. DOI:10.1007/s11207-012-0202-7 · 4.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Prominences are intriguing, but poorly understood, magnetic structures
of the solar corona. The dynamics of solar prominences has been the
subject of a large number of studies, and of particular interest is the
study of prominence oscillations. Ground- and space-based observations
have confirmed the presence of oscillatory motions in prominences and
they have been interpreted in terms of magnetohydrodynamic (MHD) waves.
This interpretation opens the door to perform prominence seismology,
whose main aim is to determine physical parameters in magnetic and
plasma structures (prominences) that are difficult to measure by direct
means. Here, we review the observational information gathered about
prominence oscillations as well as the theoretical models developed to
interpret small amplitude oscillations and their temporal and spatial
attenuation. Finally, several prominence seismology applications are
presented.
Living Reviews in Solar Physics 04/2012; 9:2-. DOI:10.12942/lrsp-2012-2 · 17.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Many solar filaments and prominences show short-lived horizontal threads
lying parallel to the photosphere. In this work the possible link between
Rayleigh-Taylor instabilities and thread lifetimes is investigated. This is
done by calculating the eigenmodes of a thread modelled as a Cartesian slab
under the presence of gravity. An analytical dispersion relation is derived
using the incompressible assumption for the magnetohydrodynamic (MHD)
perturbations. The system allows a mode that is always stable, independently of
the value of the Alfv\'en speed in the thread. The character of this mode
varies from being localised at the upper interface of the slab when the
magnetic field is weak, to having a global nature and resembling the transverse
kink mode when the magnetic field is strong. On the contrary, the slab model
permits another mode that is unstable and localised at the lower interface when
the magnetic field is weak. The growth rates of this mode can be very short, of
the order of minutes for typical thread conditions. This Rayleigh-Taylor
unstable mode becomes stable when the magnetic field is increased, and in the
limit of strong magnetic field it is essentially a sausage magnetic mode. The
gravity force might have a strong effect on the modes of oscillation of
threads, depending on the value of the Alfv\'en speed. In the case of threads
in quiescent filaments, where the Alfv\'en speed is presumably low, very short
lifetimes are expected according to the slab model. In active region
prominences, the stabilising effect of the magnetic tension might be enough to
suppress the Rayleigh-Taylor instability for a wide range of wavelengths.
Astronomy and Astrophysics 03/2012; 541. DOI:10.1051/0004-6361/201219027 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Given the difficulty in directly determining prominence physical parameters
from observations, prominence seismology stands as an alternative method to
probe the nature of these structures. We show recent examples of the
application of magnetohydrodynamic (MHD) seismology techniques to infer
physical parameters in prominence plasmas. They are based on the application of
inversion techniques using observed periods, damping times, and plasma flow
speeds of prominence thread oscillations. The contribution of Hinode to the
subject has been of central importance. We show an example based on data
obtained with Hinode's Solar Optical Telescope. Observations show an active
region limb prominence, composed by a myriad of thin horizontal threads that
flow following a path parallel to the photosphere and display synchronous
vertical oscillations. The coexistence of waves and flows can be firmly
established. By making use of an interpretation based on transverse MHD kink
oscillations, a seismological analysis of this event is performed. It is shown
that the combination of high quality Hinode observations and proper theoretical
models allows flows and waves to become two useful characteristics for our
understanding of the nature of solar prominences.
[Show abstract][Hide abstract] ABSTRACT: We investigate the temporal evolution of impulsively generated perturbations
in a potential coronal arcade with an embedded loop. As the initial
configuration we consider a coronal loop, represented by a density enhancement,
which is unbounded in the ignorable direction of the arcade. The linearized
time-dependent magnetohydrodynamic equations have been numerically solved in
field-aligned coordinates and the time evolution of the initial perturbations
has been studied in the zero-beta approximation. For propagation constrained to
the plane of the arcade, the considered initial perturbations do not excite
trapped modes of the system. This weakness of the model is overcome by the
inclusion of wave propagation in the ignorable direction. The inclusion of
perpendicular propagation produces two main results. First, damping by wave
leakage is less efficient because the loop is able to act as a wave trap of
vertical oscillations. Second, the consideration of an inhomogeneous corona
enables the resonant damping of vertical oscillations and the energy transfer
from the interior of the loop to the external coronal medium.
The Astrophysical Journal 01/2012; 763(1). DOI:10.1088/0004-637X/763/1/16 · 5.99 Impact Factor