Author name code: ballester
ADS astronomy entries on 2022-09-14
author:"Ballester, Jose Luis"
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Title: Theory of Fluid Instabilities in Partially Ionized Plasmas:
An Overview
Authors: Soler, Roberto; Ballester, José Luis
Bibcode: 2022FrASS...9.9083S
Altcode:
Partially ionized plasmas (PIP) are essential constituents of
many astrophysical environments, including the solar atmosphere,
the interstellar medium, molecular clouds, accretion disks, planet
ionospheres, cometary tails, etc., where the ionization degree
may vary from very weak ionization to almost full ionization. The
dynamics of PIP is heavily affected by the interactions between the
various charged and neutral species that compose the plasma. It has
been shown that partial ionization effects influence the triggering
and development of fluid instabilities as, e.g., Kelvin-Helmholtz,
Rayleigh-Taylor, thermal, and magneto-rotational instabilities, among
others. Here we review the theory of some classic fluid instabilities
that are present in PIP and highlight the unique effects introduced
by partial ionization. The main emphasis of the review is put on
instabilities in the partially ionized solar atmospheric plasma,
although other astrophysical applications are also mentioned. We
focus on the mathematical and theoretical investigation of the onset
and exponential growth of the instabilities. Results of the nonlinear
evolution obtained from full numerical simulations are also discussed.
Title: Construction of coronal hole and active region
magnetohydrostatic solutions in two dimensions: Force and energy
balance
Authors: Terradas, J.; Soler, R.; Oliver, R.; Antolin, P.; Arregui,
I.; Luna, M.; Piantschitsch, I.; Soubrié, E.; Ballester, J. L.
Bibcode: 2022A&A...660A.136T
Altcode: 2022arXiv220206800T; 2022arXiv220206800J
Coronal holes and active regions are typical magnetic structures
found in the solar atmosphere. We propose several magnetohydrostatic
equilibrium solutions that are representative of these structures in
two dimensions. Our models include the effect of a finite plasma-β and
gravity, but the distinctive feature is that we incorporate a thermal
structure with properties similar to those reported by observations. We
developed a semi-analytical method to compute the equilibrium
configuration. Using this method, we obtain cold and under-dense
plasma structures in open magnetic fields representing coronal holes,
while in closed magnetic configurations, we achieve the characteristic
hot and over-dense plasma arrangements of active regions. Although
coronal holes and active regions seem to be antagonistic structures,
we find that they can be described using a common thermal structure
that depends on the flux function. In addition to the force balance,
the energy balance is included in the constructed models using an a
posteriori approach. From the two-dimensional computation of thermal
conduction and radiative losses in our models, we infer the required
heating function to achieve energy equilibrium. We find that the
temperature dependence on height is an important parameter that may
prevent the system from accomplishing thermal balance at certain spatial
locations. The implications of these results are discussed in detail.
Title: Extension and validation of the pendulum model for longitudinal
solar prominence oscillations
Authors: Luna, M.; Terradas, J.; Karpen, J.; Ballester, J. L.
Bibcode: 2022A&A...660A..54L
Altcode: 2022arXiv220207957L
Context. Longitudinal oscillations in prominences are common phenomena
on the Sun. These oscillations can be used to infer the geometry and
intensity of the filament magnetic field. Previous theoretical studies
of longitudinal oscillations made two simplifying assumptions: uniform
gravity and semicircular dips on the supporting flux tubes. However, the
gravity is not uniform and realistic dips are not semicircular.
Aims: Our aim is to understand the effects of including the nonuniform
solar gravity on longitudinal oscillations and explore the validity
of the pendulum model with different flux-tube geometries.
Methods: We first derived the equation describing the motion of the
plasma along the flux tube including the effects of nonuniform gravity,
yielding corrections to the original pendulum model. We also computed
the full numerical solutions for the normal modes and compared them
with the new pendulum approximation.
Results: We find that
the nonuniform gravity introduces a significant modification in the
pendulum model. We also found a cut-off period; i.e., the longitudinal
oscillations cannot have a period longer than 167 min. In addition,
considering different tube geometries, the period depends almost
exclusively on the radius of curvature at the bottom of the dip.
Conclusions: We conclude that nonuniform gravity significantly modifies
the pendulum model. These corrections are important for prominence
seismology, because the inferred values of the radius of curvature
and minimum magnetic-field strength differ substantially from those
of the old model. However, we find that the corrected pendulum model
is quite robust and is still valid for noncircular dips.
Title: The first adiabatic exponent in a partially ionized prominence
plasma: Effect on the period of slow waves
Authors: Ballester, J. L.; Soler, R.; Carbonell, M.; Terradas, J.
Bibcode: 2021A&A...656A.159B
Altcode:
Partially ionized plasmas are found in many different astrophysical
environments. The study of partially ionized plasmas is of great
interest for solar physics because some layers of the solar atmosphere
(photosphere and chromosphere) as well as solar structures, such as
spicules and prominences, are made of these kinds of plasmas. To our
knowledge, despite it being known that the adiabatic coefficient, γ, or
the first adiabatic exponent, Γ1, depend on the ionization
degree, this fact has been disregarded in all the studies related to
magnetohydrodynamic waves in solar partially ionized plasmas. However,
in other astrophysical areas, the dependence of γ or Γ1
on the plasma ionization degree has been taken into account. Therefore,
our aim here is to study how, in a plasma with prominence physical
properties, the joint action of the temperature, density, and ionization
degree modifies the numerical values of the first adiabatic exponent
Γ1 which affects the adiabatic sound speed and the period of
slow waves. In our computations, we have used two different approaches;
first of all, we assume local thermodynamic equilibrium (LTE) and,
later, we consider a non-local thermodynamic equilibrium (non-LTE)
model. When comparing the results in the LTE and non-LTE cases, the
numerical values of Γ1 are clearly different for both
and they are probably strongly dependent on the assumed model which
determines how the ionization degree evolves with temperature. Finally,
the effect of the ionization degree dependence of Γ1 on
the period of slow waves has been determined showing that it can be
of great importance for seismological studies of partially ionized
solar structures.
Title: One-dimensional prominence threads. I. Equilibrium models
Authors: Terradas, J.; Luna, M.; Soler, R.; Oliver, R.; Carbonell,
M.; Ballester, J. L.
Bibcode: 2021A&A...653A..95T
Altcode: 2021arXiv210606327T
Context. Threads are the building blocks of solar prominences and
very often show longitudinal oscillatory motions that are strongly
attenuated with time. The damping mechanism responsible for the
reported oscillations is not fully understood yet.
Aims: To
understand the oscillations and damping of prominence threads we must
first investigate the nature of the equilibrium solutions that arise
under static conditions and under the presence of radiative losses,
thermal conduction, and background heating. This provides the basis
to calculate the eigenmodes of the thread models.
Methods: The
non-linear ordinary differential equations for hydrostatic and thermal
equilibrium under the presence of gravity are solved using standard
numerical techniques and simple analytical expressions are derived
under certain approximations. The solutions to the equations represent
a prominence thread, a dense and cold plasma region of a certain length
that connects with the corona through a prominence corona transition
region (PCTR). The solutions can also match with a chromospheric-like
layer if a spatially dependent heating function localised around the
footpoints is considered.
Results: We have obtained static
solutions representing prominence threads and have investigated in
detail the dependence of these solutions on the different parameters
of the model. Among other results, we show that multiple condensations
along a magnetic field line are possible, and that the effect of partial
ionisation in the model can significantly modify the thermal balance
in the thread, and therefore their length. This last parameter is also
shown to be comparable to that reported in the observations when the
radiative losses are reduced for typical thread temperatures.
Title: Alfvén wave heating in partially ionized thin threads of
solar prominences
Authors: Melis, Llorenç; Soler, Roberto; Ballester, José Luis
Bibcode: 2021A&A...650A..45M
Altcode: 2021arXiv210316599M
There is observational evidence of the presence of small-amplitude
transverse magnetohydrodynamic (MHD) waves with a wide range of
frequencies in the threads of solar prominences. It is believed that
the waves are driven at the photosphere and propagate along the magnetic
field lines up to prominences suspended in the corona. The dissipation
of MHD wave energy in the partially ionized prominence plasma is a
heating mechanism whose relevance needs to be explored. Here we consider
a simple 1D model for a non-uniform thin thread and investigate
the heating associated with dissipation of Alfvén waves. The
model assumes an ad hoc density profile and a uniform pressure,
while the temperature and ionization degree are self-consistently
computed considering either local themodynamical equilibrium (LTE)
or non-LTE approximations for the hydrogen ionization. A broadband
driver for Alfvén waves is placed at one end of the magnetic field
line, representing photospheric excitation. The Alfvénic perturbations
along the thread are obtained by solving the linearized MHD equations
for a partially ionized plasma in the single-fluid approximation. We
find that wave heating in the partially ionized part of the thread is
significant enough to compensate for energy losses due to radiative
cooling. A greater amount of heating is found in the LTE case because
the ionization degree for core prominence temperatures is lower than
that in the non-LTE approximation. This results in a greater level of
dissipation due to ambipolar diffusion in the LTE case. Conversely,
in the hot coronal part of the model, the plasma is fully ionized
and wave heating is negligible. The results of this simple model
suggest that MHD wave heating can be relevant for the energy balance
in prominences. Further studies based on more elaborate models are
required.
Title: Chromospheric Heating by Magnetohydrodynamic Waves and
Instabilities
Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson,
M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.;
Murawski, K.; Zaqarashvili, T. V.
Bibcode: 2021JGRA..12629097S
Altcode: 2021arXiv210402010S
The importance of the chromosphere in the mass and energy transport
within the solar atmosphere is now widely recognized. This review
discusses the physics of magnetohydrodynamic waves and instabilities
in large-scale chromospheric structures as well as in magnetic flux
tubes. We highlight a number of key observational aspects that have
helped our understanding of the role of the solar chromosphere
in various dynamic processes and wave phenomena, and the heating
scenario of the solar chromosphere is also discussed. The review
focuses on the physics of waves and invokes the basics of plasma
instabilities in the context of this important layer of the solar
atmosphere. Potential implications, future trends and outstanding
questions are also delineated.
Title: Resonances in a Coronal Loop Driven by Torsional Alfvén
Waves Propagating from the Photosphere
Authors: Soler, Roberto; Terradas, Jaume; Oliver, Ramón; Ballester,
José Luis
Bibcode: 2021ApJ...909..190S
Altcode:
There is increasing evidence that magnetohydrodynamic waves play an
important role in the propagation and dissipation of energy in the solar
atmosphere. Here we investigate how torsional Alfvén waves driven at
the photosphere can transport energy to an overlying coronal magnetic
loop and explore their ability to heat the plasma. We consider a coronal
loop whose feet are embedded in the partially ionized chromosphere. A
broadband driver at the photosphere excites torsional Alfvén waves
that propagate upward to the coronal loop. By means of numerical
computations under the stationary-state assumption, we study the
transmission of wave energy to the loop and the heating associated
with ohmic diffusion and ion-neutral collisions. We find that wave
transmission to the loop is heavily affected by the presence of cavity
resonances when the frequency of the driver matches an eigenfrequency
of the loop. A tremendous amount of wave energy is channeled to the
coronal loop for those particular frequencies. The transmitted energy
surpasses by many orders of magnitude the requirements to balance
thermal radiation. However, dissipation is so weak in the coronal plasma
that only a tiny percentage of the energy budget is converted into
heat, which is not enough to compensate for radiative losses. Most
of the energy simply leaks back to the chromosphere. Conversely,
dissipation is much more efficient in the lower atmosphere, and wave
heating can locally balance a significant fraction of radiation in
the chromosphere. We argue that nonlinear effects such as turbulence
triggered by the Kelvin-Helmholtz instability should enhance the
heating efficiency at coronal heights.
Title: Rossby Waves in Astrophysics
Authors: Zaqarashvili, T. V.; Albekioni, M.; Ballester, J. L.;
Bekki, Y.; Biancofiore, L.; Birch, A. C.; Dikpati, M.; Gizon, L.;
Gurgenashvili, E.; Heifetz, E.; Lanza, A. F.; McIntosh, S. W.; Ofman,
L.; Oliver, R.; Proxauf, B.; Umurhan, O. M.; Yellin-Bergovoy, R.
Bibcode: 2021SSRv..217...15Z
Altcode:
Rossby waves are a pervasive feature of the large-scale motions of the
Earth's atmosphere and oceans. These waves (also known as planetary
waves and r-modes) also play an important role in the large-scale
dynamics of different astrophysical objects such as the solar
atmosphere and interior, astrophysical discs, rapidly rotating stars,
planetary and exoplanetary atmospheres. This paper provides a review
of theoretical and observational aspects of Rossby waves on different
spatial and temporal scales in various astrophysical settings. The
physical role played by Rossby-type waves and associated instabilities
is discussed in the context of solar and stellar magnetic activity,
angular momentum transport in astrophysical discs, planet formation,
and other astrophysical processes. Possible directions of future
research in theoretical and observational aspects of astrophysical
Rossby waves are outlined.
Title: Nonlinear coupling of Alfvén and slow magnetoacoustic waves
in partially ionized solar plasmas
Authors: Ballester, J. L.; Soler, R.; Terradas, J.; Carbonell, M.
Bibcode: 2020A&A...641A..48B
Altcode:
Context. Partially ionized plasmas constitute an essential ingredient
of the solar atmosphere since layers such as the chromosphere and the
photosphere and structures such as prominences and spicules are made
of this plasma. On the other hand, ground- and space-based observations
have indicated the presence of oscillations in partially ionized layers
and structures of the solar atmosphere, which have been interpreted
in terms of magnetohydrodynamic (MHD) waves.
Aims: Our aim is
to study the temporal behavior of nonlinear Alfvén waves, and the
subsequent excitation of field-aligned motions and perturbations, in a
partially ionized plasma when dissipative mechanisms such as ambipolar
diffusion, radiative losses, and thermal conduction are taken into
account.
Methods: First, we applied the regular perturbations
method for small-amplitude initial perturbations to obtain the temporal
behavior of perturbations. Then we solved the full set of nonlinear
MHD equations for larger values of the initial amplitude.
Results: We obtain analytical and numerical solutions to first-,
second-, and third-order systems of equations and study the effects
produced by ambipolar diffusion and thermal mechanisms on the temporal
behavior of Alfvén and slow waves. We also study how the majority of
the energy is transferred from the Alfvén waves to plasma internal
energy. After numerically solving the full nonlinear equations when a
large amplitude is assumed, the profile of the perturbations displays
the typical sawtooth profile characteristic of associated shocks.
Conclusions: When ambipolar diffusion is taken into account, first-order
Alfvén waves are damped in time, while second-order perturbations are
undamped. However, due to the release of heat produced by ambipolar
diffusion, other physical effects that modify the physical conditions
in the spatial domain under consideration appear. On the other hand, the
second-order perturbations are damped by thermal effects with a damping
time that can be longer or shorter than that of Afvén waves. Therefore,
after the initial excitation, Alfvén waves can be quickly damped,
while slow waves remain in the plasma for a longer time, and vice versa.
Title: Erratum: "Magneto-Rossby Waves in the Solar Tachocline and
the Annual Variations in Solar Activity" (2019, ApJ, 874, 162)
Authors: Gachechiladze, Tamar; Zaqarashvili, Teimuraz V.;
Gurgenashvili, Eka; Ramishvili, Giorgi; Carbonell, Marc; Oliver,
Ramon; Ballester, Jose Luis
Bibcode: 2020ApJ...895..149G
Altcode:
No abstract at ADS
Title: Magneto-Rossby Waves in the Solar Tachocline and the Annual
Variations in Solar Activity
Authors: Gachechiladze, Tamar; Zaqarashvili, Teimuraz V.;
Gurgenashvili, Eka; Ramishvili, Giorgi; Carbonell, Marc; Oliver,
Ramon; Ballester, Jose Luis
Bibcode: 2019ApJ...874..162G
Altcode: 2019arXiv190412788G
Annual oscillations have been detected in many indices of solar activity
during many cycles. Recent multi-spacecraft observations of coronal
bright points revealed slow retrograde toroidal phase drift (with the
speed of ∼3 m s-1) of 1 yr oscillations, which naturally
suggested their connection with Rossby-type waves in the interior. We
have studied, from a theoretical point of view, the dynamics of global
magneto-Kelvin and magneto-Rossby waves in the solar tachocline with
toroidal magnetic field. Using spherical coordinates, the dispersion
relations of the waves and latitudinal structure of solutions were
obtained analytically. We have also obtained the spectrum of unstable
magneto-Rossby wave harmonics in the presence of the latitudinal
differential rotation. Estimated periods and phase speeds show that
the magneto-Rossby waves rather than the Kelvin waves match with the
observations of 1 yr oscillations. On the other hand, Morlet wavelet
analysis of Greenwich Royal Observatory sunspot areas for the solar
cycle 23 has revealed multiple periodicities with periods of 450-460,
370-380, 310-320, 240-270, and 150-175 days in hemispheric and full
disk data. Comparison of theoretical results with the observations
allow us to conclude that the global magneto-Kelvin waves in the upper
overshoot tachocline may be responsible for the periodicity of 450-460
days (∼1.3 yr), while the remaining periods can be connected with
different harmonics of global fast magneto-Rossby waves.
Title: Energy Transport and Heating by Torsional Alfvén Waves
Propagating from the Photosphere to the Corona in the Quiet Sun
Authors: Soler, Roberto; Terradas, Jaume; Oliver, Ramón; Ballester,
José Luis
Bibcode: 2019ApJ...871....3S
Altcode: 2018arXiv181201323S
In the solar atmosphere, Alfvén waves are believed to play an important
role in the transfer of energy from the photosphere to the corona and
solar wind, and in the heating of the chromosphere. We perform numerical
computations to investigate the energy transport and dissipation
associated with torsional Alfvén waves propagating in magnetic flux
tubes that expand from the photosphere to the corona in quiet-Sun
conditions. We place a broadband driver at the photosphere that injects
a wave energy flux of 107 erg cm-2 s-1
and consider Ohm’s magnetic diffusion and ion-neutral collisions
as dissipation mechanisms. We find that only a small fraction of the
driven flux, ∼105 erg cm-2 s-1,
is able to reach coronal heights, but it may be sufficient to partly
compensate the total coronal energy loss. The frequency of maximal
transmittance is ∼5 mHz for a photospheric field strength of 1 kG
and is shifted to smaller/larger frequencies for weaker/stronger
fields. Lower frequencies are reflected at the transition region,
while higher frequencies are dissipated, producing enough heat to
balance chromospheric radiative losses. Heating in the low and middle
chromosphere is due to Ohmic dissipation, while ion-neutral friction
dominates in the high chromosphere. Ohmic diffusion is enhanced by
phase mixing because of the expansion of the magnetic field. This
effect has the important consequence of increasing the chromospheric
dissipation and, therefore, reducing the energy flux that reaches the
corona. We provide empirical fits of the transmission coefficient that
could be used as input for coronal models.
Title: Dynamics and morphology of solar prominences from MHD
simulations
Authors: Terradas, Jaume; Ballester, Jose-Luis; Soler, Roberto;
Oliver, Ramon; Luna, Manuel
Bibcode: 2018cosp...42E3357T
Altcode:
The numerical solution of the magnetohydrodynamic (MHD) equations
allows us to investigate prominences/filaments represented by cold
and dense plasmas suspended against gravity, and supported by the
magnetic field. Our interest is in models that connect the magnetic
field to the photosphere and include an overlying arcade. Two basic
types of such structures are considered, prominences incrusted in shear
magnetic arcades, and prominences embedded in three-dimensional magnetic
flux ropes. Depending on the parameters, we find structures that are
suspended above the photosphere, but also configurationsresembling
curtain or hedgerow prominences. During their evolution magnetic
Rayleigh-Taylor instabilities and Kelvin-Helmholtz instabilities
develop, producing a reach dynamics in the system. Special attention
is devoted to all the oscillatory phenomena that takes place during
the time evolution of the prominences. Finally, the problem of how the
neutral component of the plasma in prominences is supported against
gravity is addressed. We find that the systemis 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.
Title: GONG Catalog of Solar Filament Oscillations Near Solar Maximum
Authors: Luna, M.; Karpen, J.; Ballester, J. L.; Muglach, K.; Terradas,
J.; Kucera, T.; Gilbert, H.
Bibcode: 2018ApJS..236...35L
Altcode: 2018arXiv180403743L
We have cataloged 196 filament oscillations from the Global Oscillation
Network Group Hα network data during several months near the maximum
of solar cycle 24 (2014 January-June). Selected examples from the
catalog are described in detail, along with our statistical analyses of
all events. Oscillations were classified according to their velocity
amplitude: 106 small-amplitude oscillations (SAOs), with velocities
<10 {km} {{{s}}}-1, and 90 large-amplitude oscillations
(LAOs), with velocities >10 {km} {{{s}}}-1. Both SAOs
and LAOs are common, with one event of each class every two days on the
visible side of the Sun. For nearly half of the events, we identified
their apparent trigger. The period distribution has a mean value of
58 ± 15 minutes for both types of oscillations. The distribution
of the damping time per period peaks at τ/P = 1.75 and 1.25 for
SAOs and LAOs, respectively. We confirmed that LAO damping rates
depend nonlinearly on the oscillation velocity. The angle between the
direction of motion and the filament spine has a distribution centered
at 27° for all filament types. This angle agrees with the observed
direction of filament-channel magnetic fields, indicating that most
of the cataloged events are longitudinal (i.e., undergo field-aligned
motions). We applied seismology to determine the average radius of
curvature in the magnetic dips, R ≈ 89 Mm, and the average minimum
magnetic field strength, B ≈ 16 G. The catalog is available to the
community online and is intended to be expanded to cover at least 1
solar cycle.
Title: Prominence oscillations
Authors: Arregui, Iñigo; Oliver, Ramón; Ballester, José Luis
Bibcode: 2018LRSP...15....3A
Altcode:
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 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 and large amplitude oscillations and their temporal and
spatial attenuation. Finally, several prominence seismology applications
are presented.
Title: Partially Ionized Plasmas in Astrophysics
Authors: Ballester, José Luis; Alexeev, Igor; Collados, Manuel;
Downes, Turlough; Pfaff, Robert F.; Gilbert, Holly; Khodachenko,
Maxim; Khomenko, Elena; Shaikhislamov, Ildar F.; Soler, Roberto;
Vázquez-Semadeni, Enrique; Zaqarashvili, Teimuraz
Bibcode: 2018SSRv..214...58B
Altcode: 2017arXiv170707975B
Partially ionized plasmas are found across the Universe in many
different astrophysical environments. They constitute an essential
ingredient of the solar atmosphere, molecular clouds, planetary
ionospheres and protoplanetary disks, among other environments, and
display a richness of physical effects which are not present in fully
ionized plasmas. This review provides an overview of the physics of
partially ionized plasmas, including recent advances in different
astrophysical areas in which partial ionization plays a fundamental
role. We outline outstanding observational and theoretical questions
and discuss possible directions for future progress.
Title: The temporal behaviour of MHD waves in a partially ionized
prominence-like plasma: Effect of heating and cooling
Authors: Ballester, J. L.; Carbonell, M.; Soler, R.; Terradas, J.
Bibcode: 2018A&A...609A...6B
Altcode: 2017A&A...609A...6B
Context. During heating or cooling processes in prominences, the
plasma microscopic parameters are modified due to the change of
temperature and ionization degree. Furthermore, if waves are excited
on this non-stationary plasma, the changing physical conditions
of the plasma also affect wave dynamics.
Aims: Our aim is
to study how temporal variation of temperature and microscopic
plasma parameters modify the behaviour of magnetohydrodynamic (MHD)
waves excited in a prominence-like hydrogen plasma.
Methods:
Assuming optically thin radiation, a constant external heating, the
full expression of specific internal energy, and a suitable energy
equation, we have derived the profiles for the temporal variation of
the background temperature. We have computed the variation of the
ionization degree using a Saha equation, and have linearized the
single-fluid MHD equations to study the temporal behaviour of MHD
waves.
Results: For all the MHD waves considered, the period
and damping time become time dependent. In the case of Alfvén waves,
the cut-off wavenumbers also become time dependent and the attenuation
rate is completely different in a cooling or heating process. In the
case of slow waves, while it is difficult to distinguish the slow wave
properties in a cooling partially ionized plasma from those in an almost
fully ionized plasma, the period and damping time of these waves in
both plasmas are completely different when the plasma is heated. The
temporal behaviour of the Alfvén and fast wave is very similar in the
cooling case, but in the heating case, an important difference appears
that is related with the time damping.
Conclusions: Our results
point out important differences in the behaviour of MHD waves when
the plasma is heated or cooled, and show that a correct interpretation
of the observed prominence oscillations is very important in order to
put accurate constraints on the physical situation of the prominence
plasma under study, that is, to perform prominence seismology.
Title: North-South Asymmetry in Rieger-type Periodicity during Solar
Cycles 19-23
Authors: Gurgenashvili, Eka; Zaqarashvili, Teimuraz V.; Kukhianidze,
Vasil; Oliver, Ramon; Ballester, Jose Luis; Dikpati, Mausumi; McIntosh,
Scott W.
Bibcode: 2017ApJ...845..137G
Altcode: 2017arXiv170708615G
Rieger-type periodicity has been detected in different activity
indices over many solar cycles. It was recently shown that the
periodicity correlates with solar activity having a shorter period
during stronger cycles. Solar activity level is generally asymmetric
between northern and southern hemispheres, which could suggest the
presence of a similar behavior in the Rieger-type periodicity. We
analyze the sunspot area/number and the total magnetic flux data for
northern and southern hemispheres during solar cycles 19-23, which had
remarkable north-south asymmetry. Using wavelet analysis of sunspot
area and number during the north-dominated cycles (19-20), we obtained
the periodicity of 160-165 days in the stronger northern hemisphere
and 180-190 days in the weaker southern hemisphere. On the other hand,
south-dominated cycles (21-23) display the periodicity of 155-160 days
in the stronger southern hemisphere and 175-188 days in the weaker
northern hemisphere. Therefore, the Rieger-type periodicity has the
north-south asymmetry in sunspot area/number data during solar cycles
with strong hemispheric asymmetry. We suggest that the periodicity is
caused by magnetic Rossby waves in the internal dynamo layer. Using
the dispersion relation of magnetic Rossby waves and observed Rieger
periodicity, we estimated the magnetic field strength in the layer as
45-49 kG in more active hemispheres (north during cycles 19-20 and
south during cycles 21-23) and 33-40 kG in weaker hemispheres. The
estimated difference in the hemispheric field strength is around
10 kG, which provides a challenge for dynamo models. Total magnetic
flux data during cycles 20-23 reveals no clear north-south asymmetry,
which needs to be explained in the future.
Title: Propagation of Torsional Alfvén Waves from the Photosphere
to the Corona: Reflection, Transmission, and Heating in Expanding
Flux Tubes
Authors: Soler, Roberto; Terradas, Jaume; Oliver, Ramón; Ballester,
José Luis
Bibcode: 2017ApJ...840...20S
Altcode:
It has been proposed that Alfvén waves play an important role in
the energy propagation through the solar atmospheric plasma and its
heating. Here we theoretically investigate the propagation of torsional
Alfvén waves in magnetic flux tubes expanding from the photosphere
up to the low corona and explore the reflection, transmission, and
dissipation of wave energy. We use a realistic variation of the plasma
properties and the magnetic field strength with height. Dissipation
by ion-neutral collisions in the chromosphere is included using a
multifluid partially ionized plasma model. Considering the stationary
state, we assume that the waves are driven below the photosphere
and propagate to the corona, while they are partially reflected and
damped in the chromosphere and transition region. The results reveal
the existence of three different propagation regimes depending on the
wave frequency: low frequencies are reflected back to the photosphere,
intermediate frequencies are transmitted to the corona, and high
frequencies are completely damped in the chromosphere. The frequency
of maximum transmissivity depends on the magnetic field expansion rate
and the atmospheric model, but is typically in the range of 0.04-0.3
Hz. Magnetic field expansion favors the transmission of waves to the
corona and lowers the reflectivity of the chromosphere and transition
region compared to the case with a straight field. As a consequence,
the chromospheric heating due to ion-neutral dissipation systematically
decreases when the expansion rate of the magnetic flux tube increases.
Title: Synthetic Hydrogen Spectra of Oscillating Prominence Slabs
Immersed in the Solar Corona
Authors: Zapiór, M.; Oliver, R.; Ballester, J. L.; Heinzel, P.
Bibcode: 2016ApJ...827..131Z
Altcode:
We study the behavior of Hα and Hβ spectral lines and their spectral
indicators in an oscillating solar prominence slab surrounded by the
solar corona, using an MHD model combined with a 1D radiative transfer
code taken in the line of sight perpendicular to the slab. We calculate
the time variation of the Doppler shift, half-width, and maximum
intensity of the Hα and Hβ spectral lines for different modes of
oscillation. We find a non-sinusoidal time dependence of some spectral
parameters with time. Because Hα and Hβ spectral indicators have
different behavior for different modes, caused by differing optical
depths of formation and different plasma parameter variations in
time and along the slab, they may be used for prominence seismology,
especially to derive the internal velocity field in prominences.
Title: Rieger-type Periodicity during Solar Cycles 14-24: Estimation
of Dynamo Magnetic Field Strength in the Solar Interior
Authors: Gurgenashvili, Eka; Zaqarashvili, Teimuraz V.; Kukhianidze,
Vasil; Oliver, Ramon; Ballester, Jose Luis; Ramishvili, Giorgi;
Shergelashvili, Bidzina; Hanslmeier, Arnold; Poedts, Stefaan
Bibcode: 2016ApJ...826...55G
Altcode: 2016arXiv160504162G
Solar activity undergoes a variation over timescales of several months
known as Rieger-type periodicity, which usually occurs near maxima
of sunspot cycles. An early analysis showed that the periodicity
appears only in some cycles and is absent in other cycles. But the
appearance/absence during different cycles has not been explained. We
performed a wavelet analysis of sunspot data from the Greenwich Royal
Observatory and the Royal Observatory of Belgium during cycles 14-24. We
found that the Rieger-type periods occur in all cycles, but they are
cycle dependent: shorter periods occur during stronger cycles. Our
analysis revealed a periodicity of 185-195 days during the weak cycles
14-15 and 24 and a periodicity of 155-165 days during the stronger
cycles 16-23. We derived the dispersion relation of the spherical
harmonics of the magnetic Rossby waves in the presence of differential
rotation and a toroidal magnetic field in the dynamo layer near the
base of the convection zone. This showed that the harmonics of fast
Rossby waves with m = 1 and n = 4, where m (n) indicates the toroidal
(poloidal) wavenumbers, perfectly fit with the observed periodicity. The
variation of the toroidal field strength from weaker to stronger cycles
may lead to the different periods found in those cycles, which explains
the observed enigmatic feature of the Rieger-type periodicity. Finally,
we used the observed periodicity to estimate the dynamo field strength
during cycles 14-24. Our estimations suggest a field strength of ∼40
kG for the stronger cycles and ∼20 kG for the weaker cycles.
Title: The role of Alfvén wave heating in solar prominences
Authors: Soler, Roberto; Terradas, Jaume; Oliver, Ramon; Ballester,
Jose Luis
Bibcode: 2016A&A...592A..28S
Altcode: 2016arXiv160507048S
Observations have shown that magnetohydrodynamic waves over a large
frequency range are ubiquitous in solar prominences. The waves are
probably driven by photospheric motions and may transport energy up to
prominences suspended in the corona. Dissipation of wave energy can
lead to heating of the cool prominence plasma, thereby contributing
to the local energy balance within the prominence. Here we discuss
the role of Alfvén wave dissipation as a heating mechanism for the
prominence plasma. We consider a slab-like quiescent prominence model
with a transverse magnetic field embedded in the solar corona. The
prominence medium is modeled as a partially ionized plasma composed
of a charged ion-electron single fluid and two separate neutral fluids
corresponding to neutral hydrogen and neutral helium. Friction between
the three fluids acts as a dissipative mechanism for the waves. The
heating caused by Alfvén waves incident on the prominence slab is
analytically explored. We find that the dense prominence slab acts as
a resonant cavity for the waves. The fraction of incident wave energy
that is channeled into the slab strongly depends upon the wave period,
P. Using typical prominence conditions, we obtain that wave energy
trapping and associated heating are negligible when P ≳ 100 s, so that
it is unlikely that those waves have a relevant influence on prominence
energetics. When 1 s ≲ P ≲ 100 s the energy absorption into the slab
shows several sharp and narrow peaks that can reach up to ~100% when the
incident wave frequency matches a cavity resonance of the slab. Wave
heating is enhanced at those resonant frequencies. Conversely, when
P ≲ 1 s cavity resonances are absent, but the waves are heavily
damped by the strong dissipation. We estimate that wave heating may
compensate for about 10% of radiative losses of the prominence plasma.
Title: Prominence oscillations: Effect of a time-dependent background
temperature
Authors: Ballester, J. L.; Carbonell, M.; Soler, R.; Terradas, J.
Bibcode: 2016A&A...591A.109B
Altcode:
Context. Small amplitude oscillations in prominences have been
known about for a long time, and from a theoretical point of view,
these oscillations have been interpreted in terms of standing or
propagating linear magnetohydrodynamic (MHD) waves. In general,
these oscillations were studied by producing small perturbations in a
background equilibrium with stationary physical properties.
Aims:
Taking into account that prominences are dynamic plasma structures,
the assumption of a stationary equilibrium is not realistic. Therefore,
our main aim is to study the effects produced by a non-stationary
background on slow MHD waves, which could be responsible for prominence
oscillations.
Methods: Assuming that the radiation term is
proportional to temperature and constant external heating, we have
derived an expression for the temporal variation of the background
temperature, which depends on the imbalance between heating and cooling
processes. Furthermore, radiative losses, together with parallel
thermal conduction, have also been included as damping mechanisms
for the waves.
Results: As temperature increases with time,
the period of slow waves decreases and the amplitude of the velocity
perturbations is damped. The inclusion of radiative losses enhances the
damping. As temperature decreases with time, the period of slow waves
increases and the amplitude of velocity perturbations grows while, as
expected, the inclusion of radiative losses contributes to the damping
of oscillations.
Conclusions: There is observational evidence
that, in different locations of the same prominence, oscillations are
damped or amplified with time. This temporal damping or amplification
can be obtained by a proper combination of a variable background
temperature, together with radiative damping. Furthermore, decayless
oscillations can also be obtained with an appropriate choice of the
characteristic radiation time.
Title: Solar Prominences Embedded in Flux Ropes: Morphological
Features and Dynamics from 3D MHD Simulations
Authors: Terradas, J.; Soler, R.; Luna, M.; Oliver, R.; Ballester,
J. L.; Wright, A. N.
Bibcode: 2016ApJ...820..125T
Altcode: 2015arXiv151207096T
The temporal evolution of a solar prominence inserted in a
three-dimensional magnetic flux rope is investigated numerically. Using
the model of Titov & Démoulin under the regime of weak twist, the
cold and dense prominence counteracts gravity by modifying the initially
force-free magnetic configuration. In some cases a quasi-stationary
situation is achieved after the relaxation phase, characterized by the
excitation of standing vertical oscillations. These oscillations show
a strong attenuation with time produced by the mechanism of continuum
damping due to the inhomogeneous transition between the prominence and
solar corona. The characteristic period of the vertical oscillations
does not depend strongly on the twist of the flux rope. Nonlinearity is
responsible for triggering the Kelvin-Helmholtz instability associated
with the vertical oscillations and that eventually produces horizontal
structures. Contrary to other configurations in which the longitudinal
axis of the prominence is permeated by a perpendicular magnetic field,
like in unsheared arcades, the orientation of the prominence along the
flux rope axis prevents the development of Rayleigh-Taylor instabilities
and therefore the appearance of vertical structuring along this axis.
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
Bibcode: 2016arXiv160100587W
Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
Title: On the Spatial Scales of Wave Heating in the Solar Chromosphere
Authors: Soler, Roberto; Carbonell, Marc; Ballester, Jose Luis
Bibcode: 2015ApJ...810..146S
Altcode: 2015arXiv150801497S
Dissipation of magnetohydrodynamic (MHD) wave energy has been proposed
as a viable heating mechanism in the solar chromospheric plasma. Here,
we use a simplified one-dimensional model of the chromosphere to
theoretically investigate the physical processes and spatial scales
that are required for the efficient dissipation of Alfvén waves and
slow magnetoacoustic waves. We consider the governing equations for
a partially ionized hydrogen-helium plasma in the single-fluid MHD
approximation and include realistic wave damping mechanisms that may
operate in the chromosphere, namely, Ohmic and ambipolar magnetic
diffusion, viscosity, thermal conduction, and radiative losses. We
perform an analytic local study in the limit of small amplitudes to
approximately derive the lengthscales for critical damping and efficient
dissipation of MHD wave energy. We find that the critical dissipation
lengthscale for Alfvén waves depends strongly on the magnetic field
strength and ranges from 10 m to 1 km for realistic field strengths. The
damping of Alfvén waves is dominated by Ohmic diffusion for weak
magnetic field and low heights in the chromosphere, and by ambipolar
diffusion for strong magnetic field and medium/large heights in the
chromosphere. Conversely, the damping of slow magnetoacoustic waves
is less efficient, and spatial scales shorter than 10 m are required
for critical damping. Thermal conduction and viscosity govern the
damping of slow magnetoacoustic waves and play an equally important
role at all heights. These results indicate that the spatial scales at
which strong wave heating may work in the chromosphere are currently
unresolved by observations.
Title: Long-term variation in the Sun's activity caused by magnetic
Rossby waves in the tachocline
Authors: Zaqarashvili, Teimuraz V.; Oliver, Ramon; Hanslmeier, Arnold;
Carbonell, Marc; Ballester, Jose Luis; Gachechiladze, Tamar; Usoskin,
Ilya G.
Bibcode: 2015ApJ...805L..14Z
Altcode: 2015arXiv150502652Z
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 timescales 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.
Title: On the Support of Neutrals Against Gravity in Solar Prominences
Authors: Terradas, J.; Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2015ApJ...802L..28T
Altcode: 2015arXiv150305354T
Cool and dense prominences found in the solar atmosphere are
known to be partially ionized because of their relatively 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 at sustaining neutrals than elastic scattering
of ions with neutrals.
Title: Prominence seismology using the period ratio of transverse
thread oscillations
Authors: Soler, R.; Goossens, M.; Ballester, J. L.
Bibcode: 2015A&A...575A.123S
Altcode: 2015arXiv150105238S
The ratio of the period of the fundamental mode to that of the first
overtone of kink oscillations (hereafter 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 because
of 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.
Title: Morphology and Dynamics of Solar Prominences from 3D MHD
Simulations
Authors: Terradas, J.; Soler, R.; Luna, M.; Oliver, R.; Ballester,
J. L.
Bibcode: 2015ApJ...799...94T
Altcode: 2014arXiv1412.7438T
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 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-β is an important parameter
that determines the shape of the structure. In many cases magnetic
Rayleigh-Taylor 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.
Title: Overdamped Alfvén waves due to ion-neutral collisions in
the solar chromosphere
Authors: Soler, R.; Ballester, J. L.; Zaqarashvili, T. V.
Bibcode: 2015A&A...573A..79S
Altcode: 2014arXiv1411.5887S
Alfvénic waves are ubiquitous in the solar atmosphere and their
dissipation may play an important role in atmospheric heating. In
the partially ionized solar chromosphere, collisions between ions and
neutrals are an efficient dissipative mechanism for Alfvén waves with
frequencies near the ion-neutral collision frequency. The collision
frequency is proportional to the ion-neutral collision cross section
for momentum transfer. Here, we investigate Alfvén wave damping as a
function of height in a simplified chromospheric model and compare the
results for two sets of collision cross sections, namely those of the
classic hard-sphere model and those based on recent quantum-mechanical
computations. We find important differences between the results
for the two sets of cross sections. There is a critical interval of
wavelengths for which impulsively excited Alfvén waves are overdamped
as a result of the strong ion-neutral dissipation. The critical
wavelengths are in the range from 1 km to 50 km for the hard-sphere
cross sections, and from 1 m to 1 km for the quantum-mechanical cross
sections. Equivalently, for periodically driven Alfvén waves there
is an optimal frequency for which the damping is most effective. The
optimal frequency varies from 1 Hz to 102 Hz for the
hard-sphere cross sections, and from 102 Hz to 104
Hz for the quantum-mechanical cross sections. Future observations at
sufficiently high spatial or temporal resolution may show the importance
of high-frequency Alfvén waves for chromospheric heating. For instance,
the Atacama Large Millimeter/submillimeter Array (ALMA) may be able
to detect the critical wavelengths and optimal frequencies and so to
test the effective collision cross section in the chromospheric plasma.
Title: Magnetism and Dynamics of Prominences: MHD Waves
Authors: Ballester, José Luis
Bibcode: 2015ASSL..415..259B
Altcode:
Quiescent solar prominences are highly dynamic structures which,
among other features, display oscillatory motions. The presence
of these oscillations has been confirmed by means of ground- and
space-based observations, and they have been classified in small
and large amplitude oscillations. Since prominences are magnetized
structures, the theoretical interpretation of their oscillations has
been mostly done in terms of magnetohydrodynamic (MHD) waves. This
interpretation has allowed the development of prominence seismology as
a tool to determine prominence physical parameters (magnetic field,
Alfvén speed, inhomogeneity scale, etc.) which are difficult to
measure by direct means.
Title: Rayleigh-Taylor Instabilities with Sheared Magnetic Fields
Authors: Ruderman, M. S.; Terradas, J.; Ballester, J. L.
Bibcode: 2014ApJ...785..110R
Altcode: 2014arXiv1402.6593R
Magnetic Rayleigh-Taylor (MRT) instabilities may play a relevant role
in many astrophysical problems. In this work the effect of magnetic
shear on the growth rate of the MRT instability is investigated. The
eigenmodes of an interface and a slab model under the presence of
gravity are analytically calculated assuming that the orientation of
the magnetic field changes in the equilibrium, i.e., there is magnetic
shear. We solve the linearized magnetohydrodynamic equations in the
incompressible regime. We find that the growth rate is bounded under
the presence of magnetic shear. We have derived simple analytical
expressions for the maximum growth rate, corresponding to the most
unstable mode of the system. These expressions provide the explicit
dependence of the growth rate on the various equilibrium parameters. For
small angles the growth time is linearly proportional to the shear
angle, and in this regime the single interface problem and the slab
problem tend to the same result. On the contrary, in the limit of large
angles and for the interface problem the growth time is essentially
independent of the shear angle. In this regime we have also been able
to calculate an approximate expression for the growth time for the
slab configuration. Magnetic shear can have a strong effect on the
growth rates of the instability. As an application of the results
found in this paper we have indirectly determined the shear angle in
solar prominence threads using their lifetimes and the estimation of
the Alfvén speed of the structure.
Title: Synthetic hydrogen spectra of prominence oscillations
Authors: Heinzel, P.; Zapiór, M.; Oliver, R.; Ballester, J. L.
Bibcode: 2014A&A...562A.103H
Altcode: 2014arXiv1401.2131H
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 Hα and Hβ 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 Hβ 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 Hα
and Hβ spectral line parameters could be a good tool to detect and
identify oscillatory modes.
Title: The damping of transverse oscillations of prominence threads:
a comparative study
Authors: Soler, Roberto; Oliver, Ramon; Ballester, Jose Luis
Bibcode: 2014IAUS..300...48S
Altcode:
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.
Title: Prominence Seismology
Authors: Ballester, J. L.
Bibcode: 2014IAUS..300...30B
Altcode:
Quiescent solar prominences are cool and dense plasma clouds located
inside the hot and less dense solar corona. They are highly dynamic
structures displaying flows, instabilities, oscillatory motions,
etc. The oscillations have been mostly interpreted in terms of
magnetohydrodynamic (MHD) waves, which has allowed to perform
prominence seismology as a tool to determine prominence physical
parameters difficult to measure. Here, several prominence seismology
applications to large and small amplitude oscillations are reviewed.
Title: Non-LTE Modeling and Observations of Oscillating Prominences
Authors: Zapiór, M.; Heinzel, P.; Oliver, R.; Ballester, J. L.;
Kotrč, P.
Bibcode: 2014IAUS..300...52Z
Altcode:
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.
Title: Magnetohydrodynamic Waves in Two-dimensional Prominences
Embedded in Coronal Arcades
Authors: Terradas, J.; Soler, R.; Díaz, A. J.; Oliver, R.; Ballester,
J. L.
Bibcode: 2013ApJ...778...49T
Altcode: 2013arXiv1309.4934T
Solar prominence models used so far in the analysis of MHD waves in
two-dimensional 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 minute range. Longitudinal oscillations,
related to slow magnetoacoustic-gravity waves, have longer periods
in the range of 28-40 minutes. These longitudinal oscillations are
strongly influenced by the gravity force and become unstable for short
magnetic arcades.
Title: Magnetoacoustic Waves in a Partially Ionized Two-fluid Plasma
Authors: Soler, Roberto; Carbonell, Marc; Ballester, Jose Luis
Bibcode: 2013ApJS..209...16S
Altcode: 2013arXiv1309.7204S
Compressible disturbances propagate in a plasma in the form of
magnetoacoustic waves driven by both gas pressure and magnetic
forces. In partially ionized plasmas the dynamics of ionized and neutral
species are coupled due to ion-neutral collisions. As a consequence,
magnetoacoustic waves propagating through a partially ionized medium are
affected by ion-neutral coupling. The degree to which the behavior of
the classic waves is modified depends on the physical properties of the
various species and on the relative value of the wave frequency compared
to the ion-neutral collision frequency. Here, we perform a comprehensive
theoretical investigation of magnetoacoustic wave propagation in a
partially ionized plasma using the two-fluid formalism. We consider
an extensive range of values for the collision frequency, ionization
ratio, and plasma β, so that the results are applicable to a wide
variety of astrophysical plasmas. We determine the modification of the
wave frequencies and study the frictional damping due to ion-neutral
collisions. Approximate analytic expressions for the frequencies are
given in the limit case of strongly coupled ions and neutrals, while
numerically obtained dispersion diagrams are provided for arbitrary
collision frequencies. In addition, we discuss the presence of cutoffs
in the dispersion diagrams that constrain wave propagation for certain
combinations of parameters. A specific application to propagation of
compressible waves in the solar chromosphere is given.
Title: Kelvin-Helmholtz and Rayleigh-Taylor instabilities in partially
ionised prominences
Authors: Díaz, A. J.; Soler, R.; Ballester, J. L.; Goossens, M.
Bibcode: 2013hsa7.conf..776D
Altcode:
We study the modification of the classical criterion for the linear
onset and growing rate of the Kelvin-Helmholtz Instability (KHI) and the
Rayleigh-Taylor instability (RTI) in a partially ionised plasma in the
two-fluid description. The plasma is composed of a neutral fluid and
an electron-ion fluid, coupled by means of particle collisions. The
governing linear equations and appropriate boundary conditions,
including gravitational terms, are derived and applied to the case
a single interface between two partially ionised plasmas. For high
collision frequencies and low density contrasts the KHI is present for
super-Alfvénic velocity shear only. For high density contrasts the
threshold velocity shear can be reduced to sub-Alfvénic values. For
the particular case of turbulent plumes in prominences, we conclude
that sub-Alfvénic flow velocities can trigger the KHI thanks to the
ion-neutral coupling, but with long time scales. Ion-neutral collisions
have a strong impact on the RTI growth rate, which can be decreased
by an order of magnitude compared to the value in the collisionless
case. The time scale for the development of the instability is much
longer than in the classical incompressible fully ionised case. This
result may explain the existence of prominence fine structures with
life times of the order of 30 minutes.
Title: Alfvén Waves in a Partially Ionized Two-fluid Plasma
Authors: Soler, R.; Carbonell, M.; Ballester, J. L.; Terradas, J.
Bibcode: 2013ApJ...767..171S
Altcode: 2013arXiv1303.4297S
Alfvén waves are a particular class of magnetohydrodynamic waves
relevant in many astrophysical and laboratory plasmas. In partially
ionized plasmas the dynamics of Alfvén waves is affected by the
interaction between ionized and neutral species. Here we study Alfvén
waves in a partially ionized plasma from the theoretical point of
view using the two-fluid description. We consider that the plasma is
composed of an ion-electron fluid and a neutral fluid, which interact
by means of particle collisions. To keep our investigation as general
as possible, we take the neutral-ion collision frequency and the
ionization degree as free parameters. First, we perform a normal mode
analysis. We find the modification due to neutral-ion collisions of
the wave frequencies and study the temporal and spatial attenuation of
the waves. In addition, we discuss the presence of cutoff values of the
wavelength that constrain the existence of oscillatory standing waves
in weakly ionized plasmas. Later, we go beyond the normal mode approach
and solve the initial-value problem in order to study the time-dependent
evolution of the wave perturbations in the two fluids. An application
to Alfvén waves in the low solar atmospheric plasma is performed and
the implication of partial ionization for the energy flux is discussed.
Title: Effect of partial ionization on wave propagation in solar
magnetic flux tubes
Authors: Soler, R.; Díaz, A. J.; Ballester, J. L.; Goossens, M.
Bibcode: 2013A&A...551A..86S
Altcode: 2013arXiv1301.5214S
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 theoretically
investigate the impact of partial ionization on MHD wave propagation
in cylindrical flux tubes in a 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 assumed the neutral-ion
collision frequency to be an arbitrary parameter. Specific 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 that realistically
represent the solar atmosphere, ions and neutrals behave as a single
fluid with an effective density corresponding to the sum of densities
of fluids plus 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 included for a
consistent description of slow magnetoacoustic waves. The MHD waves are
damped by neutral-ion collisions. The damping is most efficient when
the wave frequency and the collision frequency are on 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.
Title: Wave Leakage and Resonant Absorption in a Loop Embedded in
a Coronal Arcade
Authors: Rial, S.; Arregui, I.; Terradas, J.; Oliver, R.; Ballester,
J. L.
Bibcode: 2013ApJ...763...16R
Altcode: 2012arXiv1201.4042R
We investigate the temporal evolution of impulsively generated
perturbations in a potential coronal arcade with an embedded loop. For
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-β
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. Perpendicular propagation
produces two main results. First, damping by wave leakage is less
efficient because the loop is able to act as a better 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.
Title: Cut-off wavenumber of Alfvén waves in partially ionized
plasmas of the solar atmosphere
Authors: Zaqarashvili, T. V.; Carbonell, M.; Ballester, J. L.;
Khodachenko, M. L.
Bibcode: 2012A&A...544A.143Z
Altcode: 2012arXiv1207.5377Z
Context. Alfvén wave dynamics in partially ionized plasmas of the
solar atmosphere shows that there is indeed a cut-off wavenumber,
i.e. the Alfvén waves with wavenumbers higher than the cut-off
value are evanescent. The cut-off wavenumber appears in single-fluid
magnetohydrodynamic (MHD) approximation but it is absent in a
multi-fluid approach. Up to now, an explanation for the existence of the
cut-off wavenumber is still missing.
Aims: The aim of this paper
is to point out the reason for the appearance of a cut-off wavenumber
in single-fluid MHD.
Methods: Beginning with three-fluid
equations (with electrons, protons and neutral hydrogen atoms),
we performed consecutive approximations until we obtained the usual
single-fluid description. We solved the dispersion relation of linear
Alfvén waves at each step and sought the approximation responsible
of the cut-off wavenumber appearance.
Results: We have found
that neglecting inertial terms significantly reduces the real part
of the Alfvén frequency although it never becomes zero. Therefore,
the cut-off wavenumber does not exist at this stage. However, when the
inertial terms together with the Hall term in the induction equation
are neglected, the real part of the Alfvén frequency becomes zero.
Conclusions: The appearance of a cut-off wavenumber, when Alfvén
waves in partially ionized regions of the solar atmosphere are studied,
is the result of neglecting inertial and Hall terms, therefore it has
no physical origin.
Title: Rayleigh-Taylor Instability in Partially Ionized Compressible
Plasmas
Authors: Díaz, A. J.; Soler, R.; Ballester, J. L.
Bibcode: 2012ApJ...754...41D
Altcode:
We study the modification of the classical criterion for the linear
onset and growing rate of the Rayleigh-Taylor instability (RTI) in a
partially ionized plasma in the two-fluid description. The plasma is
composed of a neutral fluid and an electron-ion fluid, coupled by means
of particle collisions. The governing linear equations and appropriate
boundary conditions, including gravitational terms, are derived and
applied to the case of the RTI in a single interface between two
partially ionized plasmas. The limits of collisionless, no gravity,
and incompressible fluids are checked before addressing the general
case. We find that both compressibility and ion-neutral collisions
lower the linear growth rate, but do not affect the critical threshold
of the onset of the RTI. The configuration is always unstable when a
lighter plasma is below a heavier plasma regardless the value of the
magnetic field strength, the ionization degree, and the ion-neutral
collision frequency. However, ion-neutral collisions have a strong
impact on the RTI growth rate, which can be decreased by an order of
magnitude compared to the value in the collisionless case. Ion-neutral
collisions are necessary to accurately describe the evolution of the
RTI in partially ionized plasmas such as prominences. The timescale
for the development of the instability is much longer than in the
classical incompressible fully ionized case. This result may explain
the existence of prominence fine structures with life times of the
order of 30 minutes. The timescales derived from the classical theory
are about one order of magnitude shorter and incompatible with the
observed life times.
Title: Prominence seismology using ground- and space-based
observations
Authors: Ballester, J. L.; Arregui, I.; Oliver, R.; Terradas, J.;
Soler, R.; Lin, Y.; Engvold, O.; Langagen, O.; Rouppe van der Voort,
L. H. M.
Bibcode: 2012EAS....55..169B
Altcode:
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, two prominence
seismology applications are presented.
Title: The role of Rayleigh-Taylor instabilities in filament threads
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2012A&A...541A.102T
Altcode: 2012arXiv1203.4396T
Context. Many solar filaments and prominences show short-lived
horizontal threads lying parallel to the photosphere.
Aims:
In this work the possible link between Rayleigh-Taylor instabilities
and thread lifetimes is investigated.
Methods: 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.
Results: The system allows a mode that is
always stable, independently of the value of the Alfvén 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.
Conclusions: The gravity force might have a strong
effect on the modes of oscillation of threads, depending on the value
of the Alfvén speed. In the case of threads in quiescent filaments,
where the Alfvén 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.
Title: Prominence Seismology
Authors: Arregui, I.; Ballester, J. L.; Oliver, R.; Soler, R.;
Terradas, J.
Bibcode: 2012ASPC..455..211A
Altcode: 2012arXiv1201.4557A
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.
Title: Inversion of Physical Parameters in Solar Coronal Magnetic
Structures
Authors: Arregui, I. .; Ballester, J.; Goossens, M.; Oliver, R.;
Ramos, A.
Bibcode: 2012ASPC..456..121A
Altcode:
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.
Title: Stability of thermal modes in cool prominence plasmas
Authors: Soler, R.; Ballester, J. L.; Parenti, S.
Bibcode: 2012A&A...540A...7S
Altcode: 2012arXiv1201.4668S
Magnetohydrodynamic thermal modes may play an important role
in the formation, plasma condensation, and evolution of solar
prominences. Unstable thermal modes due to unbalance between
radiative losses and heating can lead to rapid plasma cooling and
condensation. An accurate description of the radiative loss function
is therefore crucial for this process. We study the stability of
thermal modes in unbounded and uniform plasmas with properties akin to
those in solar prominences. Effects of partial ionization are taken
into account. Three different parametrizations of the radiative loss
function are used. By means of a normal mode analysis, we investigate
linear nonadiabatic perturbations superimposed on the equilibrium
state. We find an approximate instability criterion for thermal
modes, while the exact linear growth rate is obtained by numerically
solving the general dispersion relation. The stability of thermal
disturbances is compared for the three different loss functions that
we consider. Using up-to-date computations of radiative losses derived
from the CHIANTI atomic database, we find that thermal modes may be
unstable in prominences for lower temperatures than those predicted
with previously existing loss functions. Thermal instability can take
place for temperatures as low as about 15 000 K. The obtained linear
growth rates indicate that this instability might have a strong impact
on the dynamics and evolution of cool prominence condensations.
Title: Kelvin-Helmholtz Instability in Partially Ionized Compressible
Plasmas
Authors: Soler, R.; Díaz, A. J.; Ballester, J. L.; Goossens, M.
Bibcode: 2012ApJ...749..163S
Altcode: 2012arXiv1202.4274S
The Kelvin-Helmholtz instability (KHI) has been observed in the
solar atmosphere. Ion-neutral collisions may play a relevant role for
the growth rate and evolution of the KHI in solar partially ionized
plasmas such as in, e.g., solar prominences. Here, we investigate the
linear phase of the KHI at an interface between two partially ionized
magnetized plasmas in the presence of a shear flow. The effects
of ion-neutral collisions and compressibility are included in the
analysis. We obtain the dispersion relation of the linear modes and
perform parametric studies of the unstable solutions. We find that,
in the incompressible case, the KHI is present for any velocity
shear regardless of the value of the collision frequency. In the
compressible case, the domain of instability depends strongly on the
plasma parameters, especially the collision frequency and the density
contrast. For high collision frequencies and low density contrasts the
KHI is present for super-Alfvénic velocity shear only. For high density
contrasts the threshold velocity shear can be reduced to sub-Alfvénic
values. For the particular case of turbulent plumes in prominences,
we conclude that sub-Alfvénic flow velocities can trigger the KHI
thanks to the ion-neutral coupling.
Title: Prominence Oscillations
Authors: Arregui, Iñigo; Oliver, Ramón; Ballester, José Luis
Bibcode: 2012LRSP....9....2A
Altcode:
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.
Title: Magnetohydrodynamic Waves in Partially Ionized Prominence
Plasmas
Authors: Soler, Roberto; Ballester, Jose Luis
Bibcode: 2012ASSP...33..111S
Altcode: 2012arXiv1201.3752S; 2012msdp.book..111S
Prominences or filaments are cool clouds of partially ionized plasma
living in the solar corona. 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. Existing observational evidence points out that these oscillatory
motions are damped in short spatial and temporal scales by some still
not well known physical mechanism(s). Since prominences are partially
ionized plasmas, a potential mechanism able to damp these oscillations
could be ion-neutral collisions. Here, we will review the work done
on the effects of partial ionization on MHD waves in prominence plasmas.
Title: Polar spots in rapidly rotating stars: stellar wind and
evolution of exoplanets
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.; Carbonell,
M.; Khodachenko, M. L.; Lammer, H.; Leitzinger, M.; Odert, P.
Bibcode: 2011epsc.conf..236Z
Altcode: 2011DPS....43..236Z
We aim to study large-scale shallow water waves in the tachoclines
of rapidly rotating stars and their connection to the periodicity
and the formation of starspots at high latitudes. Shallow water
magnetohydrodynamic equations are used to study the dynamics of
largescale waves at the rapidly rotating stellar tachoclines in the
presence of toroidal magnetic field. We found that low frequency
magnetic Rossby waves tend to locate at poles, but high frequency
magnetic Poincaré waves are concentrated near the equator in rapidly
rotating stars. Unstable magnetic Rossby waves may lead to the local
enhancement of magnetic flux at high latitudes of tachoclines in rapidly
rotating stars. The enhanced magnetic flux may rise upwards due to
the magnetic buoyancy in the form of tubes and appear as starspots at
polar regions. Magnetic Rossby waves may also cause observed short
term periodicity in the stellar magnetic activity. These results
have important implications for the evolution of the stellar wind and
exoplanets in young Sun-like stars.
Title: Twisted magnetic tubes with field aligned flow. I. Linear
twist and uniform longitudinal field
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.; Soler, R.
Bibcode: 2011A&A...533A..95D
Altcode:
Aims: We study the equilibrium and stability of twisted magnetic
flux tubes with mass flows along the field lines. Then, we focus on the
stability and oscillatory modes of magnetic tubes with uniform twist
B0 = B0(r/p eϕ + ez)
in a zero-β plasma, surrounded by a uniform, purely longitudinal
field.
Methods: First we investigate the possible equilibriums,
and then consider the linearised MHD equations and obtain a system of
two first-order differential equations. These are solved numerically,
while analytical approximations involving confluent hypergeometric
functions are found in the thin tube limit. Finally, new appropriate
boundary conditions are deduced and the outer solution considered
(with the apparition of cut-off frequencies). We use this to derive
a dispersion relation, from which the frequencies of the normal modes
can be obtained.
Results: Regarding the equilibrium, the only
value of the flow that satisfies the equations for this magnetic field
configuration is a super-Alfvénic one. Then, we consider the normal
modes of this configuration. The thin-tube approximation proves accurate
for typical values, and it is used to prove that the equilibrium is
unstable, unless the pitch is large. The stability criteria for twisted
tubes are significantly lowered.
Conclusions: The twisted tube
is subject to the kink instability unless the pitch is very high, since
the Lundquist criterion is significantly lowered. This is caused by
the requirement of having a magnetic Mach number greater than 1, so the
magnetic pressure balances the magnetic tension and fluid inertia. This
type of instability might be observed in some solar atmospheric
structures, like surges. Appendix is available in electronic form
at http://www.aanda.org
Title: Magnetohydrodynamic kink waves in two-dimensional non-uniform
prominence threads
Authors: Arregui, I.; Soler, R.; Ballester, J. L.; Wright, A. N.
Bibcode: 2011A&A...533A..60A
Altcode: 2010arXiv1011.5175A
Aims: We analyse the oscillatory properties of resonantly
damped transverse kink oscillations in two-dimensional prominence
threads.
Methods: The fine structures are modelled as
cylindrically symmetric magnetic flux tubes with a dense central
part with prominence plasma properties and an evacuated part, both
surrounded by coronal plasma. The equilibrium density is allowed to vary
non-uniformly in both the transverse and the longitudinal directions. We
examine the influence of longitudinal density structuring on periods,
damping times, and damping rates for transverse kink modes computed
by numerically solving the linear resistive magnetohydrodynamic (MHD)
equations.
Results: The relevant parameters are the length of the
thread and the density in the evacuated part of the tube, two quantities
that are difficult to directly estimate from observations. We find that
both of them strongly influence the oscillatory periods and damping
times, and to a lesser extent the damping ratios. The analysis of the
spatial distribution of perturbations and of the energy flux into the
resonances allows us to explain the obtained damping times.
Conclusions: Implications for prominence seismology, the physics of
resonantly damped kink modes in two-dimensional magnetic flux tubes,
and the heating of prominence plasmas are discussed.
Title: Rossby waves and polar spots in rapidly rotating stars:
implications for stellar wind evolution
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.; Carbonell,
M.; Khodachenko, M. L.; Lammer, H.; Leitzinger, M.; Odert, P.
Bibcode: 2011A&A...532A.139Z
Altcode: 2011arXiv1107.3395Z
Context. Rapidly rotating stars show short-period oscillations in
magnetic activity and polar appearance of starspots.
Aims:
The aim of this paper is to study large-scale shallow water waves in
the tachoclines of rapidly rotating stars and their connection to the
periodicity and the formation of starspots at high latitudes.
Methods: Shallow-water magnetohydrodynamic equations were used to study
the dynamics of large-scale waves at the rapidly rotating stellar
tachoclines in the presence of toroidal magnetic field. Dispersion
relations and latitudinal distribution of wave modes were derived.
Results: We found that low-frequency magnetic Rossby waves tend to
be located at poles, but high-frequency magnetic Poincaré waves are
concentrated near the equator in rapidly rotating stars. These results
have important implications for the evolution of the stellar wind in
young Sun-like stars.
Conclusions: Unstable magnetic Rossby waves
may lead to the local enhancement of magnetic flux at high latitudes of
tachoclines in rapidly rotating stars. The enhanced magnetic flux may
rise upwards owing to the magnetic buoyancy in the form of tubes and
appear as starspots at polar regions. Magnetic Rossby waves may also
cause observed short-term periodicity in the stellar magnetic activity.
Title: Damping Mechanisms for Oscillations in Solar Prominences
Authors: Arregui, Iñigo; Ballester, José Luis
Bibcode: 2011SSRv..158..169A
Altcode: 2010SSRv..tmp..173A; 2010arXiv1002.3489A
Small amplitude oscillations are a commonly observed feature in
prominences/filaments. These oscillations appear to be of local
nature, are associated to the fine structure of prominence plasmas,
and simultaneous flows and counterflows are also present. The existing
observational evidence reveals that small amplitude oscillations, after
excited, are damped in short spatial and temporal scales by some as yet
not well determined physical mechanism(s). Commonly, these oscillations
have been interpreted in terms of linear magnetohydrodynamic (MHD)
waves, and this paper reviews the theoretical damping mechanisms
that have been recently put forward in order to explain the observed
attenuation scales. These mechanisms include thermal effects, through
non-adiabatic processes, mass flows, resonant damping in non-uniform
media, and partial ionization effects. The relevance of each mechanism
is assessed by comparing the spatial and time scales produced by each
of them with those obtained from observations. Also, the application
of the latest theoretical results to perform prominence seismology
is discussed, aiming to determine physical parameters in prominence
plasmas that are difficult to measure by direct means.
Title: The Thermal Instability of Solar Prominence Threads
Authors: Soler, R.; Ballester, J. L.; Goossens, M.
Bibcode: 2011ApJ...731...39S
Altcode: 2011arXiv1102.2317S
The fine structure of solar prominences and filaments appears as thin
and long threads in high-resolution images. In Hα 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 Hα 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 timescale
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.
Title: Time damping of non-adiabatic magnetohydrodynamic waves in
a partially ionised prominence medium: Effect of a background flow
Authors: Barceló, S.; Carbonell, M.; Ballester, J. L.
Bibcode: 2011A&A...525A..60B
Altcode:
Context. The simultaneous occurrence of flows and time damped
small-amplitude oscillations in solar prominences is a common
phenomenon. These oscillations are mostly interpreted in terms of
magnetohydrodynamic (MHD) waves.
Aims: We study the time damping
of linear non-adiabatic MHD waves in a flowing partially ionised
plasma with prominence-like physical conditions.
Methods:
Considering non-adiabatic single fluid equations for a partially
ionised hydrogen plasma, we have solved our dispersion relations for the
complex frequency, ω, and we have analysed the behavior of the period,
damping time and the ratio of the damping time to the period, versus the
real wavenumber k, for Alfvén, fast, slow, and thermal waves.
Results: While in the case without flow there is a critical wavenumber
at which the period of Alfvén and fast waves goes to infinite,
when a flow is present two different critical wavenumbers appear. The
smaller wavenumber depends on the flow speed and causes the period of
the high-period branch to go to infinite. When the second critical
wavenumber is attained the period of both branches become equal. In
general, the time damping of Alfvén and fast waves is dominated by
resistive effects, and its damping ratio is very inefficient when
compared to observations. The damping of slow and thermal waves is
basically dominated by non-adiabatic effects, and for slow waves it is
possible to obtain a damping ratio close to observations, although it
would correspond to long period oscillations with large damping times
not often observed. The consideration of a structured medium produces
new features such as the apparition of four critical wavenumbers for
Alfvén waves, and one critical wavenumber for slow waves. For fast
waves, constrained propagation substantially improves, within the range
of observed wavelengths, the ratio of the damping time to period.
Conclusions: The presence of a background flow in a partially ionised
plasma gives place to new interesting features when the time damping of
MHD waves is studied. In general, the results point out that ion-neutral
collisions are an inefficient mechanism to explain the observed time
damping of prominence oscillations if they are produced by Alfvén and
fast waves. If the oscillations are produced by slow waves, only long
period oscillations with large damping times produce damping ratios
in agreement with observations.
Title: Spatial Damping of Propagating Kink Waves in Prominence Threads
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2011ApJ...726..102S
Altcode: 2010arXiv1009.4871S
Transverse oscillations and propagating waves are frequently
observed in threads of solar prominences/filaments and have been
interpreted as kink magnetohydrodynamic (MHD) modes. We investigate
the spatial damping of propagating kink MHD waves in transversely
nonuniform and partially ionized prominence threads. Resonant
absorption and ion-neutral collisions (Cowling's diffusion) are the
damping mechanisms taken into account. The dispersion relation of
resonant kink waves in a partially ionized magnetic flux tube is
numerically solved by considering prominence conditions. Analytical
expressions of the wavelength and damping length as functions of the
kink mode frequency are obtained in the thin tube and thin boundary
approximations. For typically reported periods of thread oscillations,
resonant absorption is an efficient mechanism for the kink mode spatial
damping, while ion-neutral collisions have a minor role. Cowling's
diffusion dominates both the propagation and damping for periods
much shorter than those observed. Resonant absorption may explain
the observed spatial damping of kink waves in prominence threads. The
transverse inhomogeneity length scale of the threads can be estimated
by comparing the observed wavelengths and damping lengths with the
theoretically predicted values. However, the ignorance of the form of
the density profile in the transversely nonuniform layer introduces
inaccuracies in the determination of the inhomogeneity length scale.
Title: Prominence Thread Seismology Using the P 1/2P
2 Ratio
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2010ApJ...725.1742D
Altcode:
Prominence threads are expected to be cold plasma condensations in a
long magnetic tube. Because of this density inhomogeneity along the
magnetic field, the ratio of the fundamental transverse mode period
to twice that of its first overtone, P 1/2P 2,
must differ from 1. We investigate the dependence of this ratio on the
equilibrium parameters of prominence threads and its possible use as
a diagnostic tool for prominence seismology. Using the low-beta plasma
approximation, we follow the procedure of previous works to obtain the
frequencies and spatial distribution of the modes. We also check the
thin tube approximation and find it reasonably accurate. The period
ratio P 1/2P 2 is found to be greater than unity,
in contrast with coronal loops, for which the effect of inhomogeneities
is to make this ratio smaller than 1. The ratio is very sensitive to
the thread length, while the dependence on other parameters is less
important for threads than for coronal loops. Hence, the period ratio
can be used to obtain an estimation of the length of the supporting
magnetic tube, since the thread length is known from observations. The
obtained value of the tube length does not depend on other parameters,
so their potential for prominence seismology may be great.
Title: Quasi-biennial Oscillations in the Solar Tachocline Caused
by Magnetic Rossby Wave Instabilities
Authors: Zaqarashvili, Teimuraz V.; Carbonell, Marc; Oliver, Ramón;
Ballester, José Luis
Bibcode: 2010ApJ...724L..95Z
Altcode: 2010arXiv1011.1361Z
Quasi-biennial oscillations (QBOs) are frequently observed in solar
activity indices. However, no clear physical mechanism for the
observed variations has been suggested so far. Here, we study the
stability of magnetic Rossby waves in the solar tachocline using the
shallow water magnetohydrodynamic approximation. Our analysis shows
that the combination of typical differential rotation and a toroidal
magnetic field with a strength of >=105 G triggers the
instability of the m = 1 magnetic Rossby wave harmonic with a period of
~2 years. This harmonic is antisymmetric with respect to the equator
and its period (and growth rate) depends on the differential rotation
parameters and magnetic field strength. The oscillations may cause a
periodic magnetic flux emergence at the solar surface and consequently
may lead to the observed QBO in solar activity features. The period
of QBOs may change throughout a cycle, and from cycle to cycle, due
to variations of the mean magnetic field and differential rotation in
the tachocline.
Title: Seismology of Standing Kink Oscillations of Solar Prominence
Fine Structures
Authors: Soler, R.; Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2010ApJ...722.1778S
Altcode: 2010arXiv1007.1959S
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 damping mechanisms considered. 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 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 minutes)
and short-wavelength (700-8000 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., Alfvén 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.
Title: The damping of small-amplitude oscillations in quiescent
prominences
Authors: Ballester, J. L.
Bibcode: 2010AdSpR..46..364B
Altcode:
The presence of small-amplitude oscillations in prominences is
well-known from long time ago. These oscillations, whose exciters are
still unknown, seem to be of local nature and are interpreted in terms
of magnetohydrodynamic (MHD) waves. During last years, observational
evidence about the damping of these oscillations has grown and several
mechanisms able to damp these oscillations have been the subject of
intense theoretical modelling. Among them, the most efficient seem to
be radiative cooling and ion-neutral collisions. Radiative cooling is
able to damp slow MHD waves efficiently, while ion-neutral collisions,
in partially ionised plasmas like those of solar prominences, can also
damp fast MHD waves. In this paper, we plan to summarize our current
knowledge about the time and spatial damping of small-amplitude
oscillations in prominences.
Title: Instability of twisted magnetic tubes with axial mass flows
Authors: Zaqarashvili, T. V.; Díaz, A. J.; Oliver, R.; Ballester,
J. L.
Bibcode: 2010A&A...516A..84Z
Altcode:
Context. Recent observations of various kinds of jets in the solar
atmosphere motivate studying the influence of mass flow on the stability
of solar magnetic structures.
Aims: We study the influence of
axial mass flows on the stability of twisted magnetic flux tubes.
Methods: We use the incompressible magnetohydrodynamic equations to
get the dispersion relation governing the behaviour of normal modes
in uniformly twisted magnetic tubes with sub-Alfvénic flows. The
dispersion relation is then solved analytically and numerically to find
stability criteria for twisted tubes with flow.
Results: Two
main important results are found. First, the axial mass flow reduces
the threshold of kink instability in twisted magnetic tubes. Second,
the twist of magnetic tubes leads to the Kelvin-Helmholtz instability
of sub-Alfvénic flows for the harmonics with a large enough azimuthal
wave number -m.
Conclusions: The observed mass flow may
trigger the kink instability in magnetic configurations that are
near their stability threshold, leading to solar flares and coronal
mass ejections. The effect is more significant for photospheric
magnetic tubes than for coronal ones. Sub-Alfvénic flows undergo the
Kelvin-Helmholtz instability in slightly twisted magnetic tubes if
the azimuthal wavenumber is big enough.
Title: The spatial damping of magnetohydrodynamic waves in a flowing
partially ionised prominence plasma
Authors: Carbonell, M.; Forteza, P.; Oliver, R.; Ballester, J. L.
Bibcode: 2010A&A...515A..80C
Altcode: 2010arXiv1001.4962C
Context. Solar prominences are partially ionised plasmas displaying
flows and oscillations. These oscillations exhibit time and spatial
damping and have commonly been explained in terms of magnetohydrodynamic
(MHD) waves.
Aims: We study the spatial damping of linear
non-adiabatic MHD waves in a flowing partially ionised plasma with
prominence-like physical properties.
Methods: We consider single
fluid equations for a partially ionised hydrogen plasma by including
in the energy equation optically thin radiation, thermal conduction by
electrons and neutrals, and heating. By keeping ω real and fixed, we
solved the dispersion relations obtained for the complex wavenumber,
k, and analysed the behaviour of the damping length, wavelength and
the ratio of the damping length to the wavelength, versus period,
for Alfvén, fast, slow, and thermal waves.
Results: In the
presence of a background flow, the results indicate that new strongly
damped fast and Alfvén waves appear that depend on the joint action
of flow and resistivity. The damping lengths of adiabatic fast and slow
waves are strongly affected by partial ionisation, which also modifies
the ratio between damping lengths and wavelengths. The behaviour
of adiabatic fast waves also resembles that of Alfvén waves. For
non-adiabatic slow waves, the unfolding in both wavelength and damping
length induced by the flow allows efficient damping to be found for
periods compatible with those observed in prominence oscillations. This
effect is enhanced when low ionised plasmas are considered.
Conclusions: Since flows are ubiquitous in prominences, in the case of
non-adiabatic slow waves and within the range of periods of interest
for prominence oscillations, the joint effect of both flow and partial
ionisation leads to a ratio of damping length to wavelength denoting a
very efficient spatial damping. For fast and Alfvén waves, the most
efficient damping occurs at very short periods not compatible with
those observed in prominence oscillations.
Title: Transverse Oscillations of a Multi-stranded Loop
Authors: Luna, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2010ApJ...716.1371L
Altcode: 2009arXiv0912.4491L
We investigate the transverse oscillations of a line-tied multi-stranded
coronal loop composed of several parallel cylindrical strands. First,
the collective fast normal modes of the loop are found with the
T-matrix theory. There is a huge quantity of normal modes with very
different frequencies and a complex structure of the associated magnetic
pressure perturbation and velocity field. The modes can be classified
as bottom, middle, and top according to their frequencies and spatial
structure. Second, the temporal evolution of the velocity and magnetic
pressure perturbation after an initial disturbance are analyzed. We find
complex motions of the strands. The frequency analysis reveals that
these motions are a combination of low and high frequency modes. The
complexity of the strand motions produces a strong modulation of the
whole tube movement. We conclude that the presumed internal fine
structure of a loop influences its transverse oscillations and so
its transverse dynamics cannot be properly described by those of an
equivalent monolithic loop.
Title: Physics of Solar Prominences: II—Magnetic Structure and
Dynamics
Authors: Mackay, D. H.; Karpen, J. T.; Ballester, J. L.; Schmieder,
B.; Aulanier, G.
Bibcode: 2010SSRv..151..333M
Altcode: 2010SSRv..tmp...32M; 2010arXiv1001.1635M
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations
and models, (2) the dynamics of prominence plasmas (formation and
flows), (3) Magneto-hydrodynamic (MHD) waves in prominences and (4)
the formation and large-scale patterns of the filament channels in
which prominences are located. Finally, several outstanding issues in
prominence research are discussed, along with observations and models
required to resolve them.
Title: Three-dimensional Propagation of Magnetohydrodynamic Waves
in Solar Coronal Arcades
Authors: Rial, S.; Arregui, I.; Terradas, J.; Oliver, R.; Ballester,
J. L.
Bibcode: 2010ApJ...713..651R
Altcode: 2010arXiv1002.0469R
We numerically investigate the excitation and temporal evolution of
oscillations in a two-dimensional coronal arcade by including the
three-dimensional propagation of perturbations. The time evolution
of impulsively generated perturbations is studied by solving the
linear, ideal magnetohydrodynamic (MHD) equations in the zero-β
approximation. As we neglect gas pressure, the slow mode is absent
and therefore only coupled fast MHD and Alfvén modes remain. Two
types of numerical experiments are performed. First, the resonant
wave energy transfer between a fast normal mode of the system and
local Alfvén waves is analyzed. It is seen how, because of resonant
coupling, the fast wave with global character transfers its energy to
Alfvénic oscillations localized around a particular magnetic surface
within the arcade, thus producing the damping of the initial fast MHD
mode. Second, the time evolution of a localized impulsive excitation,
trying to mimic a nearby coronal disturbance, is considered. In this
case, the generated fast wavefront leaves its energy on several magnetic
surfaces within the arcade. The system is therefore able to trap energy
in the form of Alfvénic oscillations, even in the absence of a density
enhancement such as that of a coronal loop. These local oscillations are
subsequently phase-mixed to smaller spatial scales. The amount of wave
energy trapped by the system via wave energy conversion strongly depends
on the wavelength of perturbations in the perpendicular direction,
but is almost independent from the ratio of the magnetic to density
scale heights.
Title: Kelvin-Helmholtz Instability in Coronal Magnetic Flux Tubes
due to Azimuthal Shear Flows
Authors: Soler, R.; Terradas, J.; Oliver, R.; Ballester, J. L.;
Goossens, M.
Bibcode: 2010ApJ...712..875S
Altcode: 2009arXiv0912.3649S
Transverse oscillations of coronal loops are often observed and
have been theoretically interpreted as kink magnetohydrodynamic
(MHD) modes. Numerical simulations by Terradas et al. suggest that
shear flows generated at the loop boundary during kink oscillations
could give rise to a Kelvin-Helmholtz instability (KHI). Here, we
investigate the linear stage of the KHI in a cylindrical magnetic flux
tube in the presence of azimuthal shear motions. We consider the basic,
linearized MHD equations in the β = 0 approximation and apply them
to a straight and homogeneous cylindrical flux tube model embedded in
a coronal environment. Azimuthal shear flows with a sharp jump of the
velocity at the cylinder boundary are included in the model. We obtain
an analytical expression for the dispersion relation of the unstable
MHD modes supported by the configuration, and compute analytical
approximations of the critical velocity shear and the KHI growth rate
in the thin tube limit. A parametric study of the KHI growth rates is
performed by numerically solving the full dispersion relation. We find
that fluting-like modes can develop a KHI in timescales comparable to
the period of kink oscillations of the flux tube. The KHI growth rates
increase with the value of the azimuthal wavenumber and decrease with
the longitudinal wavenumber. However, the presence of a small azimuthal
component of the magnetic field can suppress the KHI. Azimuthal motions
related to kink oscillations of untwisted coronal loops may trigger
a KHI, but this phenomenon has not been observed to date. We propose
that the azimuthal component of the magnetic field is responsible for
suppressing the KHI in a stable coronal loop. The required twist is
small enough to prevent the development of the pinch instability.
Title: Time damping of non-adiabatic magnetohydrodynamic waves in
a partially ionized prominence plasma: effect of helium
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2010A&A...512A..28S
Altcode: 2009arXiv0910.2883S
Context. 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.
Aims: 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.
Methods: The dispersion relation of linear
non-adiabatic MHD waves in a homogeneous, unbounded, and partially
ionized prominence medium is derived. We compute the period and the
damping time of Alfvén, slow, fast, and thermal waves. A parametric
study of the ratio of the damping time to the period with respect to
the helium abundance is performed.
Results: 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 has a minor influence on
the wave damping.
Conclusions: The presence of helium can
be safely neglected in studies of MHD waves in partially ionized
prominence plasmas.
Title: Magnetic Rossby Waves in the Solar Tachocline and Rieger-Type
Periodicities
Authors: Zaqarashvili, Teimuraz V.; Carbonell, Marc; Oliver, Ramón;
Ballester, José Luis
Bibcode: 2010ApJ...709..749Z
Altcode: 2009arXiv0911.4591Z
Apart from the eleven-year solar cycle, another periodicity around
155-160 days was discovered during solar cycle 21 in high-energy solar
flares, and its presence in sunspot areas and strong magnetic flux
has been also reported. This periodicity has an elusive and enigmatic
character, since it usually appears only near the maxima of solar
cycles, and seems to be related with a periodic emergence of strong
magnetic flux at the solar surface. Therefore, it is probably connected
with the tachocline, a thin layer located near the base of the solar
convection zone, where a strong dynamo magnetic field is stored. We
study the dynamics of Rossby waves in the tachocline in the presence of
a toroidal magnetic field and latitudinal differential rotation. Our
analysis shows that the magnetic Rossby waves are generally unstable
and that the growth rates are sensitive to the magnetic field strength
and to the latitudinal differential rotation parameters. Variation of
the differential rotation and the magnetic field strength throughout
the solar cycle enhance the growth rate of a particular harmonic
in the upper part of the tachocline around the maximum of the solar
cycle. This harmonic is symmetric with respect to the equator and has
a period of 155-160 days. A rapid increase of the wave amplitude could
give rise to a magnetic flux emergence leading to observed periodicities
in solar activity indicators related to magnetic flux.
Title: The Sun and the Solar System
Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2010ASSP...14..446A
Altcode: 2010hsa5.conf..446A
High-resolution observations provide evidence of the existence of
small-amplitude transverse oscillations in solar filament fine
structures. These oscillations are believed to represent fast
magnetohydrodynamic (MHD) waves and the disturbances are seen to be
damped in short timescales of the order of 1-4 periods. We propose
that, due to the highly inhomogeneous nature of the filament plasma
at the fine-structure spatial scale, the phenomenon of resonant
absorption is likely to operate in the temporal attenuation of fast MHD
oscillations. By considering transverse inhomogeneity in a straight
flux tube model we find that, for density inhomogeneities typical of
filament threads, the decay times are of a few oscillatory periods only.
Title: The Temporal Evolution of Linear Fast and Alfvén MHD Waves
in Solar Coronal Arcades
Authors: Rial, S.; Arregui, I.; Terradas, J.; Oliver, R.; Ballester,
J. L.
Bibcode: 2010ASSP...14..459R
Altcode: 2010hsa5.conf..459R
The excitation and temporal evolution of fast and Alfvén
magnetohydrodynamic oscillations in a two-dimensional coronal arcade
are investigated. The approach is to consider an equilibrium magnetic
and plasma structure and then to introduce a perturbation trying to
mimic a nearby disturbance, such as a flare or filament eruption. By
numerically solving the time-dependent linearized MHD wave equations,
the properties of the solutions have been studied. First, the properties
of uncoupled fast and Alfvén waves are described. Then, longitudinal
propagation of perturbations is included, and the properties of coupled
waves are determined.
Title: Resonantly Damped Kink Magnetohydrodynamic Waves in a Partially
Ionized Filament Thread
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2009ApJ...707..662S
Altcode: 2009arXiv0909.3599S
Transverse oscillations of solar filament and prominence threads have
been frequently reported. These oscillations have the common features
of being of short period (2-10 minutes) and being damped after a few
periods. The observations are interpreted as kink magnetohydrodynamic
(MHD) wave modes, whereas resonant absorption in the Alfvén
continuum and ion-neutral collisions are candidates to be the damping
mechanisms. Here, we study both analytically and numerically the time
damping of kink MHD waves in a cylindrical, partially ionized filament
thread embedded in a coronal environment. The thread model is composed
of a straight and thin, homogeneous filament plasma, with a transverse
inhomogeneous transitional layer where the plasma physical properties
vary continuously from filament to coronal conditions. The magnetic
field is homogeneous and parallel to the thread axis. We find that
the kink mode is efficiently damped by resonant absorption for typical
wavelengths of filament oscillations, the damping times being compatible
with the observations. Partial ionization does not affect the process
of resonant absorption, and the filament plasma ionization degree is
only important for the damping for wavelengths much shorter than those
observed. To our knowledge, this is the first time that the phenomenon
of resonant absorption is studied in a partially ionized plasma.
Title: Damping of Fast Magnetohydrodynamic Oscillations in Quiescent
Filament Threads
Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2009ASPC..415...71A
Altcode:
High-resolution observations provide evidence of the existence of
small-amplitude transverse oscillations in solar filament fine
structures. These oscillations are believed to represent fast
magnetohydrodynamic (MHD) waves and the disturbances are seen to
be damped in short timescales of the order of 1 to 4 periods. We
propose that, due to the highly inhomogeneous nature of the filament
plasma at the fine-structure spatial scale, the phenomenon of resonant
absorption is likely to operate in the temporal attenuation of fast MHD
oscillations. By considering transverse inhomogeneity in a straight
flux tube model we find that, for density inhomogeneities typical of
filament threads, the decay times are of a few oscillatory periods only.
Title: Swaying Threads of a Solar Filament
Authors: Lin, Y.; Soler, R.; Engvold, O.; Ballester, J. L.; Langangen,
Ø.; Oliver, R.; Rouppe van der Voort, L. H. M.
Bibcode: 2009ApJ...704..870L
Altcode: 2009arXiv0909.2792L
From recent high-resolution observations obtained with the Swedish 1
m Solar Telescope in La Palma, we detect swaying motions of individual
filament threads in the plane of the sky. The oscillatory characters of
these motions are comparable with oscillatory Doppler signals obtained
from corresponding filament threads. Simultaneous recordings of motions
in the line of sight and in the plane of the sky give information
about the orientation of the oscillatory plane. These oscillations
are interpreted in the context of the magnetohydrodynamic (MHD)
theory. Kink MHD waves supported by the thread body are proposed as
an explanation of the observed thread oscillations. On the basis of
this interpretation and by means of seismological arguments, we give
an estimation of the thread Alfvén speed and magnetic field strength
by means of seismological arguments.
Title: On the nature of kink MHD waves in magnetic flux tubes
Authors: Goossens, M.; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J. L.
Bibcode: 2009A&A...503..213G
Altcode: 2009arXiv0905.0425G
Context: Magnetohydrodynamic (MHD) waves are often reported in the
solar atmosphere and usually classified as slow, fast, or Alfvén. The
possibility that these waves have mixed properties is often ignored.
Aims: The goal of this work is to study and determine the nature
of MHD kink waves.
Methods: This is done by calculating the
frequency, the damping rate and the eigenfunctions of MHD kink waves
for three widely different MHD waves cases: a compressible pressure-less
plasma, an incompressible plasma and a compressible plasma which allows
for MHD radiation.
Results: In all three cases the frequency and
the damping rate are for practical purposes the same as they differ
at most by terms proportional to (kz R)^2. In the magnetic
flux tube the kink waves are in all three cases, to a high degree of
accuracy incompressible waves with negligible pressure perturbations
and with mainly horizontal motions. The main restoring force of kink
waves in the magnetised flux tube is the magnetic tension force. The
total pressure gradient force cannot be neglected except when the
frequency of the kink wave is equal or slightly differs from the local
Alfvén frequency, i.e. in the resonant layer.
Conclusions:
Kink waves are very robust and do not care about the details of the
MHD wave environment. The adjective fast is not the correct adjective
to characterise kink waves. If an adjective is to be used it should
be Alfvénic. However, it is better to realize that kink waves have
mixed properties and cannot be put in one single box.
Title: Magnetohydrodynamic Waves in a Partially Ionized Filament
Thread
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2009ApJ...699.1553S
Altcode: 2009arXiv0904.3013S
Oscillations and propagating waves are commonly seen in high-resolution
observations of filament threads, i.e., the fine-structures of
solar filaments/prominences. Since the temperature of prominences is
typically of the order of 104 K, the prominence plasma is
only partially ionized. In this paper, we study the effect of neutrals
on the wave propagation in a filament thread modeled as a partially
ionized homogeneous magnetic flux tube embedded in an homogeneous and
fully ionized coronal plasma. Ohmic and ambipolar magnetic diffusion
are considered in the basic resistive magnetohydrodynamic (MHD)
equations. We numerically compute the eigenfrequencies of kink, slow,
and Alfvén linear MHD modes and obtain analytical approximations
in some cases. We find that the existence of propagating modes is
constrained by the presence of critical values of the longitudinal
wavenumber. In particular, the lower and upper frequency cutoffs of kink
and Alfvén waves owe their existence to magnetic diffusion parallel
and perpendicular to magnetic field lines, respectively. The slow mode
only has a lower frequency cutoff, which is caused by perpendicular
magnetic diffusion and is significantly affected by the ionization
degree. In addition, ion-neutral collision is the most efficient damping
mechanism for short wavelengths, while ohmic diffusion dominates in
the long-wavelength regime.
Title: The Nature of Kink MHD Waves in Magnetic Flux Tubes
Authors: Goossens, Marcel; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J.
Bibcode: 2009SPD....40.1306G
Altcode:
We examine the nature of MHD kink waves. This is done by determining
the frequency, the damping rate and the eigenfunctions of MHD kink waves
for three widely different MHD waves cases: a compressible pressure-less
plasma, an incompressible plasma and a compressible plasma with non-zero
plasma pressure which allows for MHD radiation. The overall conclusion
is that kink waves are very robust and do not care about the details
of the MHD wave environment. In all three cases the frequency and the
damping rate are for most practical purposes the same. In the magnetic
flux tube the kink waves are in all three cases, to a high degree of
accuracy incompressible waves with negligible pressure perturbations
and with mainly horizontal motions. The main restoring force of kink
waves in the magnetized flux tube is the magnetic tension force. The
gradient pressure force cannot be neglected except when the frequency
of the kink wave is equal or slightly differs from the local Alfvén
frequency, i.e. in the resonant layer. In a non-magnetic external
plasma the wave is of course acoustic. The adjective fast is not the
correct adjective to characterize kink waves. If an adjective is to
be used it should be Alfvénic. However, it is better to realize that
kink waves have mixed properties and cannot be put in one single box.
Title: Attenuation of small-amplitude oscillations in a prominence
corona model with a transverse magnetic field
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2009NewA...14..238S
Altcode: 2008arXiv0801.3744S
Observations show that small-amplitude prominence oscillations
are usually damped after a few periods. This phenomenon has been
theoretically investigated in terms of non-ideal magnetoacoustic
waves, non-adiabatic effects being the best candidates to explain
the damping in the case of slow modes. We study the attenuation of
non-adiabatic magnetoacoustic waves in a slab prominence embedded
in the coronal medium. We assume an equilibrium configuration with a
transverse magnetic field to the slab axis and investigate wave damping
by thermal conduction and radiative losses. The magnetohydrodynamic
equations are considered in their linearised form and terms representing
thermal conduction, radiation and heating are included in the energy
equation. The differential equations that govern linear slow and fast
modes are numerically solved to obtain the complex oscillatory frequency
and the corresponding eigenfunctions. We find that coronal thermal
conduction and radiative losses from the prominence plasma reveal as the
most relevant damping mechanisms. Both mechanisms govern together the
attenuation of hybrid modes, whereas prominence radiation is responsible
for the damping of internal modes and coronal conduction essentially
dominates the attenuation of external modes. In addition, the energy
transfer between the prominence and the corona caused by thermal
conduction has a noticeable effect on the wave stability, radiative
losses from the prominence plasma being of paramount importance for
the thermal stability of fast modes. We conclude that slow modes are
efficiently damped, with damping times compatible with observations. On
the contrary, fast modes are less attenuated by non-adiabatic effects
and their damping times are several orders of magnitude larger than
those observed. The presence of the corona causes a decrease of the
damping times with respect to those of an isolated prominence slab,
but its effect is still insufficient to obtain damping times of the
order of the period in the case of fast modes.
Title: Damping of Filament Thread Oscillations: Effect of the Slow
Continuum
Authors: Soler, R.; Oliver, R.; Ballester, J. L.; Goossens, M.
Bibcode: 2009ApJ...695L.166S
Altcode: 2009arXiv0902.0572S
Transverse oscillations of small amplitude are commonly seen in
high-resolution observations of filament threads, i.e., the fine
structures of solar filaments/prominences, and are typically damped
in a few periods. Kink wave modes supported by the thread body
offer a consistent explanation of these observed oscillations. Among
the proposed mechanisms to explain the kink mode damping, resonant
absorption in the Alfvén continuum seems to be the most efficient
as it produces damping times of about three periods. However, for a
nonzero-β plasma and typical prominence conditions, the kink mode is
also resonantly coupled to slow (or cusp) continuum modes, which could
further reduce the damping time. In this Letter, we explore for the
first time both analytically and numerically the effect of the slow
continuum on the damping of transverse thread oscillations. The thread
model is composed of a homogeneous and straight cylindrical plasma,
an inhomogeneous transitional layer, and the homogeneous coronal
plasma. We find that the damping of the kink mode due to the slow
resonance is much less efficient than that due to the Alfvén resonance.
Title: Time damping of non-adiabatic MHD slow and thermal waves in
a prominence medium: Effect of a background flow
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 2009NewA...14..277C
Altcode:
Material flows are typical features of prominences and are routinely
observed in H α, UV and EUV lines. Therefore, including a magnetic
field-aligned background flow, we study the effect of flows on
the damping of non-adiabatic magnetohydrodynamic (MHD) waves in a
magnetised unbounded prominence medium, and we explore the observational
implications. We have linearised the non-adiabatic MHD equations and,
considering only field-aligned propagation, we focus our study in the
behaviour of thermal and slow waves. When a flow with a constant speed
is present, two slow waves, with different periods, appear, while the
damping time remains unchanged. On the other hand, the thermal wave
becomes in this case a propagating wave, with finite period, while
its damping time remains also unmodified. As a consequence of the
changes in the periods produced by the flow, the damping per period of
the different waves is modified. In the case of slow waves, and for a
fixed flow speed, the damping per period of the high-period slow wave is
increased while the opposite happens for the low-period slow wave, and
the strongest finite damping per period, for the high-period slow wave,
is obtained for flow speeds close to the non-adiabatic sound speed. In
the case of the thermal wave, a finite value for the damping per period
is obtained for any non-zero flow speed, and in this case the strongest
finite damping per period is obtained for values of the flow speed close
to zero. Furthermore, we point out that there is the possibility to
have slow and thermal waves having the same period, the same damping
time, or both simultaneously, which makes the proper identification
of the waves for an external observer extremely difficult. Then, if
flows are ubiquitous in prominences the observational determinations
of periods and damping per period, made by an external observer,
include its effect, and for a proper identification, information about
the wavelength, flow speed and perturbations should be needed, which
constitutes a truly difficult observational task.
Title: Propagation of Nonadiabatic Magnetoacoustic Waves in a Threaded
Prominence With Mass Flows
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2009ApJ...693.1601S
Altcode: 2008arXiv0809.4765S
High-resolution observations of solar filaments suggest the
presence of groups of prominence threads, i.e., the fine structures
of solar prominences, which oscillate coherently (in phase). In
addition, mass flows along threads have been often observed. Here,
we investigate the effect of mass flows on the collective fast and
slow nonadiabatic magnetoacoustic wave modes supported by systems
of prominence threads. Prominence fine structures are modeled as
parallel, homogeneous, and infinite cylinders embedded in a coronal
environment. The magnetic field is uniform and parallel to the axis
of threads. Configurations of identical and nonidentical threads are
both explored. We apply the T-matrix theory of acoustic scattering
to obtain the oscillatory frequency and the eigenfunctions of linear
magnetosonic disturbances. We find that the existence of wave modes
with a collective dynamics, i.e., those that produce significant
perturbations in all threads, is only possible when the Doppler-shifted
individual frequencies of threads are very similar. This can be only
achieved for very particular values of the plasma physical conditions
and flow velocities within threads.
Title: Transverse Oscillations of Systems of Coronal loops
Authors: Luna, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2009ApJ...692.1582L
Altcode: 2008arXiv0809.4230L
We study the collective kinklike normal modes of a system of several
cylindrical loops using the T-matrix theory. Loops that have similar
kink frequencies oscillate collectively with a frequency which is
slightly different from that of the individual kink mode. On the
other hand, if the kink frequency of a loop is different from that of
the others, it oscillates individually with its own frequency. Since
the individual kink frequency depends on the loop density but not on
its radius for typical 1 MK coronal loops, a coupling between kink
oscillations of neighboring loops takes place when they have similar
densities. The relevance of these results in the interpretation of the
oscillations studied by Schrijver & Brown in 2000 and Verwichte et
al. in 2004, in which transverse collective loop oscillations seem to
be detected, is discussed. In the first case, two loops oscillating
in antiphase are observed; interpreting this motion as a collective
kink mode suggests that their densities are roughly equal. In the
second case, there are almost three groups of tubes that oscillate
with similar periods, and therefore their dynamics can be collective,
which again seems to indicate that the loops of each group share a
similar density. All the other loops seem to oscillate individually
and their densities can be different from the rest.
Title: Global Shallow Water Magnetohydrodynamic Waves in the Solar
Tachocline
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.
Bibcode: 2009ApJ...691L..41Z
Altcode: 2008arXiv0810.4227Z
We derive analytical solutions and dispersion relations of global
magnetic Poincaré (magneto-gravity) and magnetic Rossby waves in the
approximation of shallow water magnetohydrodynamics. The solutions
are obtained in a rotating spherical coordinate system for strongly
and weakly stable stratification separately in the presence of a
toroidal magnetic field. In both cases, magnetic Rossby waves split
into fast and slow magnetic Rossby modes. In the case of strongly
stable stratification (valid in the radiative part of the tachocline),
all waves are slightly affected by the layer thickness and the toroidal
magnetic field, while in the case of weakly stable stratification (valid
in the upper overshoot layer of the tachocline) magnetic Poincaré
and magnetic Rossby waves are found to be concentrated near the solar
equator, leading to equatorially trapped waves. The frequencies of
all waves are smaller in the upper weakly stable stratification region
than in the lower strongly stable stratification one.
Title: Time damping of non-adiabatic MHD waves in an unbounded
partially ionised prominence plasma
Authors: Forteza, P.; Oliver, R.; Ballester, J. L.
Bibcode: 2008A&A...492..223F
Altcode:
Aims: We study the joint effect of ion-neutral collisions and thermal
mechanisms on the damping of MHD waves in a partially ionised prominence
plasma.
Methods: Thermal conduction, radiation, and heating were
included in the energy equation of the one-fluid MHD set of equations
we derived earlier for an adiabatic, partially ionised plasma. Then,
assuming small perturbations, these equations were linearised and
the dispersion relation for magnetoacoustic, Alfvén, and thermal
waves obtained.
Results: Compared with the non-adiabatic,
fully ionised, and the adiabatic, partially ionised cases, the main
result is that there is an increase in the efficiency of the damping
of magnetoacoustic waves in prominence oscillations within the observed
range of wavelengths for the magnetoacoustic waves. On the other hand,
the Alfvén wave is only damped by the ion-neutral collision mechanism,
and this damping becomes important for almost neutral plasmas.
Conclusions: The coupling of non-adiabatic mechanisms with the
ion-neutral collisions mechanism offers a more complete model for
the damping of magnetoacoustic waves with those values compatible
with those observed in prominence oscillations. Furthermore, the
ion-neutral collisions mechanism is able to damp the Alfvén wave in
an efficient way.
Title: Nonlinear Instability of Kink Oscillations due to Shear Motions
Authors: Terradas, J.; Andries, J.; Goossens, M.; Arregui, I.; Oliver,
R.; Ballester, J. L.
Bibcode: 2008ApJ...687L.115T
Altcode: 2008arXiv0809.3664T
First results from a high-resolution three-dimensional nonlinear
numerical study of the kink oscillation are presented. We show in
detail the development of a shear instability in an untwisted line-tied
magnetic flux tube. The instability produces significant deformations
of the tube boundary. An extended transition layer may naturally evolve
as a result of the shear instability at a sharp transition between
the flux tube and the external medium. We also discuss the possible
effects of the instability on the process of resonant absorption when an
inhomogeneous layer is included in the model. One of the implications
of these results is that the azimuthal component of the magnetic field
of a stable flux tube in the solar corona, needed to prevent the shear
instability, is probably constrained to be in a very specific range.
Title: Nonadiabatic Magnetohydrodynamic Waves in a Cylindrical
Prominence Thread with Mass Flow
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2008ApJ...684..725S
Altcode: 2008arXiv0803.2600S
High-resolution observations show that oscillations and waves in
prominence threads are common and that they are attenuated in a few
periods. In addition, observers have also reported the presence of
material flows in such prominence fine-structures. Here we investigate
the time damping of nonleaky oscillations supported by a homogeneous
cylindrical prominence thread embedded in an unbounded corona and
with a steady mass flow. Thermal conduction and radiative losses
are taken into account as damping mechanisms, and the effect of
these nonideal effects and the steady flow on the attenuation of
oscillations is assessed. We solve the general dispersion relation
for linear, nonadiabatic magnetoacoustic and thermal waves supported
by the model and find that slow and thermal modes are efficiently
attenuated by nonadiabatic mechanisms. On the contrary, fast kink
modes are much less affected and their damping times are much larger
than those observed. The presence of flow has no effect on the damping
of slow and thermal waves, whereas fast kink waves are more (less)
attenuated when they propagate parallel (antiparallel) to the flow
direction. Although the presence of steady mass flows improves the
efficiency of nonadiabatic mechanisms on the attenuation of transverse,
kink oscillations for propagation parallel to the flow, its effect is
still not enough to obtain damping times compatible with observations.
Title: Oscillations in coronal structures: Recent Progress
Authors: Ballester, J. L.
Bibcode: 2008AIPC.1043..284B
Altcode:
The presence of small-amplitude oscillations in prominences/filaments
is well-known from long time ago. These oscillations, whose exciters
are still unknown, seem to be of local nature and are interpreted
in terms of magnetohydrodynamic (MHD) waves. During last years,
observational evidence about the damping of these oscillations has
grown and the mechanisms able to damp these oscillations have been
the subject of intense theoretical modelling. Here, we summarize
some of the theoretical results obtained about the time damping of
small-amplitude oscillations in prominence/corona medium.
Title: Collective Normal Modes of Ensembles of Magnetic Tubes:
Applications to Coronal Arcades and Multi-Stranded Loops
Authors: Luna, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2008ESPM...12.3.60L
Altcode:
Using the T-matrix theory, we have performed an analytical study
of the collective fast normal modes of an arbitrary system of
loops. The obtained results allow to assess the dynamical effects
of the interaction of loops and the frequency shifts with respect
to the individual loops in an arcade as a function of the number
of tubes. Furthermore, the frequency shifts of a multi-stranded
loop formed by several tens or hundreds of strands with respect to
a monolithic one, as well as the implications on the loop motion,
have been also investigated.
Title: Damping of Fast Magnetohydrodynamic Oscillations in Quiescent
Filament Threads
Authors: Arregui, Iñigo; Terradas, Jaume; Oliver, Ramón; Ballester,
José Luis
Bibcode: 2008ApJ...682L.141A
Altcode: 2008arXiv0806.2728A
High-resolution observations provide evidence of the existence of
small-amplitude transverse oscillations in solar filament fine
structures. These oscillations are believed to represent fast
magnetohydrodynamic (MHD) waves, and the disturbances are seen to be
damped on short timescales of the order of 1-4 periods. In this Letter,
we propose that, due to the highly inhomogeneous nature of the filament
plasma at the fine-structure spatial scale, the phenomenon of resonant
absorption is likely to operate in the temporal attenuation of fast
MHD oscillations. By considering transverse inhomogeneity in a straight
flux tube model, we find that, for density inhomogeneities typical of
filament threads, the decay times are of a few oscillatory periods only.
Title: Resonant Absorption in Complicated Plasma Configurations:
Applications to Multistranded Coronal Loop Oscillations
Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L.;
Andries, J.; Goossens, M.
Bibcode: 2008ApJ...679.1611T
Altcode: 2008arXiv0802.0591T
We study the excitation and damping of transverse oscillations in a
multistranded model of a straight line-tied coronal loop. The transverse
geometry of our equilibrium configuration is quite irregular and
more realistic than the usual cylindrical loop model. By numerically
solving the time-dependent ideal magnetohydrodynamic equations in
two dimensions, we show how the global motion of the whole bundle of
strands, excited by an external disturbance, is converted into localized
Alfvénic motions due to the process of resonant absorption. This
process produces the attenuation of the transverse oscillations. At
any location in the structure, two dominant frequencies are found:
the frequency of the global mode or quasi-mode, and the local Alfvén
frequency. We find that the mechanism of mode conversion, due to the
coupling between fast and Alfvén waves, is not compromised by the
complicated geometry of the model. We also show that it is possible to
have energy conversion not only at the external edge of the composite
loop, but also inside the structure. The implications of these results
and their relationship with the observations are discussed.
Title: Analytic approximate seismology of transversely oscillating
coronal loops
Authors: Goossens, M.; Arregui, I.; Ballester, J. L.; Wang, T. J.
Bibcode: 2008A&A...484..851G
Altcode: 2008arXiv0804.3877G
Aims: We present an analytic approximate seismic inversion scheme for
damped transverse coronal loop oscillations based on the thin tube and
thin boundary approximation for computing the period and the damping
time.
Methods: Asymptotic expressions for the period and damping
rate are used to illustrate the process of seismological inversion in a
simple and easy to follow manner. The inversion procedure is formulated
in terms of two simple functions, which are given by simple closed
expressions.
Results: The analytic seismic inversion shows that
an infinite amount of 1-dimensional equilibrium models can reproduce
the observed periods and damping times. It predicts a specific range of
allowable values for the Alfvén travel time and lower bounds for the
density contrast and the inhomogeneity length scale. When the results
of the present analytic seismic inversion are compared with those of
a previous numerical inversion, excellent agreement is found up to
the point that the analytic seismic inversion emerges as a tool for
validating results of numerical inversions. Actually it helped us to
identify and correct inaccuracies in a previous numerical investigation.
Title: Resonant absorption in multi-stranded coronal loops
Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2008IAUS..247..116T
Altcode: 2007IAUS..247..116T
We study the excitation and damping of transverse oscillations in
a complex multi-stranded model of a coronal loop. By numerically
solving the time-dependent magnetohydrodynamic (MHD) equations in
two dimensions, we show how the global motion of the whole bundle
of tubes, produced by an external disturbance, is converted into
localised motions due to the process of resonant absorption. At any
location in the structure two dominant frequencies are found, the
frequency of the global mode (different from the kink frequency of
the individual strands) and the local Alfvén frequency. The mechanism
of mode conversion is not affected by the complicated geometry of the
system and for certain configurations the energy conversion does not
only take place at the external edge of the composite loop but also
inside the structure.
Title: Influence of longitudinal structure in the fast modes of
prominence threads
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2008IAUS..247..167D
Altcode: 2007IAUS..247..167D
Recent high-resolution observations have pointed out that prominences
are made of small threads (also named fibrils) piled up to form the
body of the prominence. These fine structures also seem to support
their own oscillatory modes, while their effect on the global modes
of the prominences are less certain. We study the effect of adding a
smooth transition layer between the prominence material and the corona
along the magnetic field line, since previous studies have considered
a jump in density in this interface. Then we compare the results with
previous models and check that these transition layers do not affect
significantly the periods of the modes.
Title: Attenuation of non-adiabatic oscillations in a cartesian
prominence fibril
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2008IAUS..247..173S
Altcode: 2007IAUS..247..173S
One of the typical features shown by observations of solar prominence
oscillations is that they are quickly damped in time by one or several
not well-known mechanisms. In addition, recent high resolution
observations have revealed that the prominence fine structures,
called fibrils, can oscillate with their own periods, independently
from the rest of the prominence. The main aim of the present work
is to study the attenuation of oscillations supported by a single
prominence fibril. We consider an equilibrium made of a prominence
plasma Cartesian slab of finite width embedded in a coronal medium,
and assume non-adiabatic effects (thermal conduction, radiation losses
and heating) as damping mechanisms. The magnetic field is taken
uniform and parallel to the slab axis. We find that the efficiency
of the non-adiabatic effects as damping mechanisms is different for
each magnetoacoustic mode. The obtained values of the damping time
are compatible with those observed in the case of the slow modes,
but the fast modes are much less attenuated.
Title: Damped oscillations of two interacting coronal loops
Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2008IAUS..247..133A
Altcode: 2007IAUS..247..133A
We present results on the oscillatory properties (periods, damping
rates, and spatial distribution of perturbations) for resonantly damped
oscillations in a system of two inhomogeneous coronal slabs and compare
them to the properties found in single slab loop models. A system of two
identical coronal loops is modelled, in Cartesian geometry, as being
composed by two density enhancements. The linear magnetohydrodynamic
(MHD) wave equations for oblique propagation of waves are solved and the
damping due to resonant absorption is computed. Due to the interaction
between the loops, the normal modes of oscillation present in a single
slab split into symmetric and antisymmetric oscillations when a system
of two identical slabs is considered. The frequencies of these solutions
may differ from the single slab results when the distance between the
loops is of the order of a few slab widths. Oblique propagation of
waves weakens this interaction, since solutions become more confined
to the edges of the slabs. The damping is strong for surface-like
oscillations, while sausage body-like solutions are unaffected.
Title: Transverse Oscillations of Flowing Prominence Threads Observed
with Hinode
Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2008ApJ...678L.153T
Altcode: 2008arXiv0803.2649T
Recent observations with the Hinode Solar Optical Telescope display an
active region prominence whose fine threads oscillate in the vertical
direction as they move along a path parallel to the photosphere. A
seismological analysis of this event is carried out by taking advantage
of the small radius of these structures compared to the total length of
magnetic field lines, i.e., by using the thin-tube approximation. This
analysis reveals that the oscillatory period is only slightly modified
by the existence of the flow and that the difference between the
period of a flowing thread and a static one is below the error bars
of these observations. Moreover, although it is not possible to obtain
values of the physical parameters, a lower bound for the Alfvén speed
(ranging between 120 and 350 km s-1) is obtained for each
of the threads. Such Alfvén speeds agree with the intense magnetic
fields and large densities usually found in active region prominences.
Title: On the Properties of Low-β Magnetohydrodynamic Waves in
Curved Coronal Fields
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.; Keppens, R.
Bibcode: 2008ApJ...675..875T
Altcode:
The solar corona is a complex magnetic environment where several kinds
of waves can propagate. In this work, the properties of fast, Alfvén,
and slow magnetohydrodynamic waves in a simple curved structure are
investigated. We consider the linear regime, i.e., small-amplitude
waves. We study the time evolution of impulsively generated waves in a
coronal arcade by solving the ideal magnetohydrodynamic equations. We
use a numerical code specially designed to solve these equations in
the low-β regime. The results of the simulations are compared with
the eigenmodes of the arcade model. Fast modes propagate nearly
isotropically through the whole arcade and are reflected at the
photosphere, where line-tying conditions are imposed. On the other hand,
Alfvén and slow perturbations are very anisotropic and propagate
along the magnetic field lines. Because of the different physical
properties in different field lines, there is a continuous spectrum
of Alfvén and slow modes. Curvature can have a significant effect
on the properties of the waves. Among other effects, it considerably
changes the frequency of oscillation of the slow modes and enhances
the possible dissipation of the Alfvén modes due to phase mixing.
Title: Transverse Oscillations of Two Coronal Loops
Authors: Luna, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2008ApJ...676..717L
Altcode: 2007arXiv0707.0758L
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. First, 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. Second, 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.
Title: On the Scaling of the Damping Time for Resonantly Damped
Oscillations in Coronal Loops
Authors: Arregui, Iñigo; Ballester, José Luis; Goossens, Marcel
Bibcode: 2008ApJ...676L..77A
Altcode: 2008arXiv0802.1143A
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.
Title: The Resonant Damping of Fast Magnetohydrodynamic Oscillations
in a System of Two Coronal Slabs
Authors: Arregui, Iñigo; Terradas, Jaume; Oliver, Ramón; Ballester,
José Luis
Bibcode: 2008ApJ...674.1179A
Altcode: 2007arXiv0708.1251A
Observations of transverse coronal loop oscillations very often show
the excitation and damping of oscillations in groups of coronal loops
rather than in individual and isolated structures. We present results
on the oscillatory properties (periods, damping rates, and spatial
distribution of perturbations) for resonantly damped oscillations
in a system of two inhomogeneous coronal slabs and compare them to
the properties found in single-slab loop models. A system of two
identical coronal loops is modeled, in Cartesian geometry, as being
composed of two density enhancements. The linear magnetohydrodynamic
(MHD) wave equations for oblique propagation of waves are solved,
and the damping of the different solutions, due to the transverse
inhomogeneity of the density profile, is computed. The physics of
the obtained results is analyzed by an examination of the perturbed
physical variables. We find that, due to the interaction between
the loops, the normal modes of oscillation present in a single slab
split into symmetric and antisymmetric oscillations when a system of
two identical slabs is considered. The frequencies of these solutions
may differ from the single slab results when the distance between the
loops is of the order of a few slab widths. Oblique propagation of
waves weakens this interaction, since solutions become more confined
to the edges of the slabs. The damping is strong for surface-like
oscillations, while sausage body-like solutions are unaffected. For
some solutions, and small slab separations, the damping in a system
of two loops differs substantially from the damping of a single loop.
Title: The statistical significance of the North-South asymmetry of
solar activity revisited
Authors: Carbonell, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2007A&A...476..951C
Altcode: 2007arXiv0709.1901C
Aims:Many studies of the North-South asymmetry of solar activity and
its features have been performed. However, most of these studies
do not consider whether or not the asymmetry of the time series
under consideration is statistically significant. If the asymmetry
is statistically insignificant, any study about its behavior is
meaningless. Here, we discuss 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.
Methods: We distinguish between solar activity time
series composed of integer or non-integer and dimensionless data,
or composed of 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 in obtaining valid information
about the statistical significance of solar activity time series.
Results: Our results suggest that, apart from the need to apply
suitable statistical tests, other effects such as data binning,
the considered units and the need, in some tests, to consider groups
of data, substantially affect the determination of the statistical
significance of the asymmetry.
Conclusions: The assessment of
the statistical significance of the N-S asymmetry of solar activity is
difficult and an absolute answer cannot be given, since many different
effects influence the results given by the statistical tests. The
quantitative results about the statistical significance of the N-S
asymmetry of solar activity provided by different authors, as well
as studies of its behaviour, must be considered with care because
they depend on the chosen values of different parameters or on the
considered units.
Title: Preface: A Topical Issue in Honor of Professor Bernard Roberts
Authors: Ballester, J. L.; Erdélyi, R.; Hood, A. W.; Leibacher,
J. W.; Nakariakov, V. M.
Bibcode: 2007SoPh..246....1B
Altcode:
No abstract at ADS
Title: Oscillatory Modes of a Prominence PCTR Corona Slab Model
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2007SoPh..246...73S
Altcode: 2007arXiv0710.3122S
Oscillations of magnetic structures in the solar corona have often
been interpreted in terms of magnetohydrodynamic waves. We study
the adiabatic magnetoacoustic modes of a prominence plasma slab with
a uniform longitudinal magnetic field, surrounded by a prominence -
corona transition region (PCTR) and a coronal medium. Considering linear
small-amplitude oscillations, we deduce the dispersion relation for
the magnetoacoustic slow and fast modes by assuming evanescentlike
perturbations in the coronal medium. In the system without PCTR, a
classification of the oscillatory modes according to the polarisation
of their eigenfunctions is made to distinguish modes with fastlike or
slowlike properties. Internal and external slow modes are governed
by the prominence and coronal properties, respectively, and fast
modes are mostly dominated by prominence conditions for the observed
wavelengths. In addition, the inclusion of an isothermal PCTR does
not substantially influence the mode frequencies, but new solutions
(PCTR slow modes) are present.
Title: Resonantly Damped Surface and Body MHD Waves in a Solar
Coronal Slab with Oblique Propagation
Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2007SoPh..246..213A
Altcode: 2007arXiv0708.3783A
The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs
in a zero-β configuration and for parallel propagation of waves does
not allow the existence of surface waves. When oblique propagation
of perturbations is considered, both surface and body waves are
able to propagate. When the perpendicular wavenumber is larger
than a certain value, the body kink mode becomes a surface wave. In
addition, a sausage surface mode is found below the internal cutoff
frequency. When nonuniformity in the equilibrium is included, surface
and body modes are damped by resonant absorption. In this paper, first,
a normal-mode analysis is performed and the period, the damping rate,
and the spatial structure of the eigenfunctions are obtained. Then,
the time-dependent problem is solved, and the conditions under which
one or the other type of mode is excited are investigated.
Title: Transverse Oscillations in Coronal Loops
Authors: Arregui, I.; Luna, M.; Oliver, R.; Terradas, J.; Ballester,
J. L.
Bibcode: 2007AIPC..934...54A
Altcode:
During last years, direct evidence about oscillations in different
coronal structures has been obtained thanks to the detailed observations
made by SoHO and TRACE. With the help of magnetohydrodynamic (MHD) wave
theory, we can explain these observations as due to the excitation and
propagation of MHD waves in the solar corona. In spite that there are
many solar coronal structures in which oscillations have been detected
(prominences, loops, plumes, coronal holes, etc.), in the following we
will concentrate on transverse oscillations of coronal loops, reviewing
some theoretical models developed to understand these oscillations in
terms of MHD waves.
Title: The effect of the solar corona on the attenuation of
small-amplitude prominence oscillations. I. Longitudinal magnetic
field
Authors: Soler, R.; Oliver, R.; Ballester, J. L.
Bibcode: 2007A&A...471.1023S
Altcode: 2007arXiv0704.1566S
Context: One of the typical features shown by observations of solar
prominence oscillations is that they are damped in time and that
the values of the damping times are usually between one and three
times the corresponding oscillatory period. However, the mechanism
responsible for the attenuation is still not well-known.
Aims:
Thermal conduction, optically thin or thick radiation and heating
are taken into account in the energy equation, and their role on the
attenuation of prominence oscillations is evaluated.
Methods:
The dispersion relation for linear non-adiabatic magnetoacoustic waves
is derived considering an equilibrium made of a prominence plasma slab
embedded in an unbounded corona. The magnetic field is orientated along
the direction parallel to the slab axis and has the same strength in
all regions. By solving the dispersion relation for a fixed wavenumber,
a complex oscillatory frequency is obtained, and the period and the
damping time are computed.
Results: The effect of conduction
and radiation losses is different for each magnetoacoustic mode and
depends on the wavenumber. In the observed range of wavelengths the
internal slow mode is attenuated by radiation from the prominence
plasma, the fast mode by the combination of prominence radiation and
coronal conduction and the external slow mode by coronal conduction. The
consideration of the external corona is of paramount importance in the
case of the fast and external slow modes, whereas it does not affect
the internal slow modes at all. When a thinner slab representing
a filament thread is considered, the fast mode is less attenuated
whereas both internal and external slow modes are not affected.
Conclusions: Non-adiabatic effects are efficient damping mechanisms
for magnetoacoustic modes, and the values of the obtained damping
times are compatible with those observed.
Title: Rossby waves in ``shallow water'' magnetohydrodynamics
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.;
Shergelashvili, B. M.
Bibcode: 2007A&A...470..815Z
Altcode: 2007astro.ph..3105Z
Aims:The influence of a toroidal magnetic field on the dynamics of
Rossby waves in a thin layer of ideal conductive fluid on a rotating
sphere is studied in the “shallow water” magnetohydrodynamic
approximation for the first time.
Methods: Dispersion relations
for magnetic Rossby waves are derived analytically in Cartesian and
spherical coordinates.
Results: It is shown that the magnetic
field causes the splitting of low order (long wavelength) Rossby
waves into two different modes, here denoted fast and slow magnetic
Rossby waves. The high frequency mode (the fast magnetic Rossby mode)
corresponds to an ordinary hydrodynamic Rossby wave slightly modified
by the magnetic field, while the low frequency mode (the slow magnetic
Rossby mode) has new and interesting properties since its frequency is
significantly smaller than that of the same harmonics of pure Rossby
and Alfvén waves.
Title: The influence of the internal structuring of coronal loops
on the properties of their damped transverse oscillations
Authors: Arregui, I.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2007A&A...466.1145A
Altcode:
Context: The geometry and physical conditions in solar coronal loops
are complicated and still not understood well. Recent high-resolution
observations obtained with TRACE indicate the existence of
sub-resolution transverse structuring not accessible to direct
observation. This ingredient has not yet been taken into account in the
previous theoretical models used for the study of transversal coronal
loop oscillations and of their damping due to resonant conversion
of energy.
Aims: This study aims to assess the effect of the
possibly unresolved internal structure of a coronal loop on the
properties of its transverse oscillations and on the efficiency of
resonant absorption as a damping mechanism of these oscillations.
Methods: The equilibrium configuration of a single coronal loop
with internal density structuring is modelled by considering the loop
as composed of two very close, parallel, identical coronal slabs in
Cartesian geometry. The period of the oscillation and the damping time
are computed for the resonantly damped fundamental kink mode. These
quantities are then compared to those obtained for two models for
a single equivalent slab without internal density structuring.
Results: We find that the period and the damping time of a coronal
loop with internal density structuring change by less than 15%, when
compared to the same oscillatory properties of a single coronal loop
with either the same density contrast or a single coronal loop with
the same total mass.
Conclusions: Therefore the internal density
structuring of a coronal loop does not affect its oscillatory properties
very much. However, the sub-resolution structuring of a coronal loop
with different densities in its components or with different widths
could vary these results.
Title: MHD Coronal Seismology
Authors: Ballester, J. L.
Bibcode: 2007AIPC..895..125B
Altcode:
The presence of oscillations in solar coronal structures has been
known for more than seventy years. Observational reports about the
presence of oscillatory motions in solar filaments go back to 1930
while in the case of other coronal structures (loops, plumes, etc)
direct evidence has been recently obtained thanks to the detailed
observations made by SoHO and TRACE. Due to these satellites, as
well as to ground-based observations, evidence of magnetohydrodynamic
(MHD) waves in the solar corona has risen dramatically. MHD coronal
seismology provides with an indirect path to determine the physical
conditions and parameters of the solar corona [coronal magnetic field,
transport coefficients (viscosity, resistivity, thermal conductivity,
etc.), heating function, filling factors] which are difficult to measure
accurately. In essence, it is similar to the acoustic diagnostic of
the solar interior (Helioseismology) and to MHD spectroscopy used to
determine physical parameters of laboratory plasmas. In spite that
there are many solar coronal structures in which oscillations have
been detected (prominences, loops, plumes, coronal holes, etc.), in the
following we will concentrate in prominences and coronal loops trying
to summarize part of our current knowledge about their oscillations
as well as about the theoretical models developed to explain those
oscillations in terms of MHD waves.
Title: The Solar Corona
Authors: Ballester, José Luis
Bibcode: 2007RoAJ...17S..11B
Altcode:
No abstract at ADS
Title: Damping of oscillations by ion-neutral collisions in a
prominence plasma
Authors: Forteza, P.; Oliver, R.; Ballester, J. L.; Khodachenko, M. L.
Bibcode: 2007A&A...461..731F
Altcode:
Aims:The role of collisions between ions, electrons and neutrals in
a partially ionised plasma is assessed as a possible wave damping
mechanism. The relevance of this mechanism in the damping of small
amplitude prominence oscillations is evaluated.
Methods: A
one-fluid MHD set of equations taking into account various effects in
a partially ionised solar plasma (collisions between different species
and Joule dissipation) is derived. Assuming small perturbations, these
equations are next linearised about a uniform equilibrium configuration
and the dispersion relation of magnetoacoustic waves in an unbounded
medium is obtained.
Results: The presence of neutrals in the
plasma only affects the fast wave in a relevant way. An approximate
expression for the damping rate is obtained which shows that the
strongest damping takes place in a medium with strong magnetic field,
low density and low ionisation fraction. Wave attenuation arises mostly
from collisions between ions and neutrals.
Conclusions: .Given
the poor knowledge about the values of the density and ionisation
fraction in prominences, it is hard to judge the importance of the
physics of partial ionisation in the damping of fast waves in solar
prominences. Nevertheless, note that a very idealised case, with no
stratification and no equilibrium currents, is considered here, so
the addition of these features to the model may change the results of
this work.
Title: Solar Corona Seismology
Authors: Ballester, J. L.
Bibcode: 2006LNEA....2...91B
Altcode:
The presence of oscillations in solar coronal structures has been
known for more than seventy years. Observational reports about the
presence of oscillatory motions in solar filaments go back to 1930
while in the case of other coronal structures (loops, plumes, etc)
direct evidence has been recently obtained thanks to the detailed
observations made by SoHO and TRACE. Due to these satellites, as well
as to ground-based observations, evidence of magnetohydrodynamic (MHD)
waves in the solar corona has risen dramatically. MHD coronal seismology
provides with an indirect path to determine the physical conditions
and parameters of the solar corona [coronal magnetic field, transport
coefficients (viscosity, resistivity, thermal conductivity, etc.),
heating function, filling factors] which are difficult to measure
accurately. In essence, it is similar to the acoustic diagnostic
of the solar interior (Helioseismology) and to MHD spectroscopy
used to determine physical helioseismology parameters of laboratory
plasmas. In spite that there are many solar coronal structures in which
oscillations have been detected (prominences, loops, plumes, coronal
holes, etc.), in the following we will concentrate in prominences and
coronal loops trying to summarize part of our current knowledge about
their oscillations as well as about the theoretical models developed
to explain those oscillations in terms of MHD waves.
Title: Spatial damping of linear non-adiabatic magnetoacoustic waves
in a prominence medium
Authors: Carbonell, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2006A&A...460..573C
Altcode:
Aims.We study the spatial damping of linear non-adiabatic
magnetoacoustic waves in a homogeneous, isothermal, and unbounded medium
permeated by a uniform magnetic field, with physical properties akin
to those of solar prominences.
Methods: .We consider an energy
equation with optically thin radiative losses, thermal conduction,
and heating, and linearize the MHD equations to obtain a sixth-order
polynomial in the wavenumber k, which represents the dispersion relation
for slow, fast, and thermal MHD waves. Since we are interested in
the spatial damping, we have taken ω as real and have numerically
solved the dispersion relation to obtain complex solutions for the
wavenumber k corresponding to fast, slow, and thermal waves.
Results: .The thermal wave shows the strongest spatial damping, while
the fast wave shows the weakest spatial damping. At periods greater
than 1 s the spatial damping of magnetoacoustic waves is dominated by
radiation, while at shorter periods the spatial damping is dominated
by thermal conduction. For very short periods the isothermal regime
is attained and the damping length becomes almost constant.
Conclusions: .Radiative effects on linear magnetoacoustic slow waves
can be a viable mechanism for the spatial damping of short period
prominence oscillations, while thermal conduction does not play any
role. In particular, short-period oscillations (5-15 min) observed in
quiescent limb prominences, which seem to be due to internal fundamental
slow modes, have damping lengths in the range 104 - 5 ×
104 km, in good agreement with our results.
Title: Fast magnetohydrodynamic waves in a two-slab coronal structure:
collective behaviour
Authors: Luna, M.; Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2006A&A...457.1071L
Altcode:
Aims.We study fast magnetohydrodynamic waves in a system of two
coronal loops modeled as smoothed, dense plasma slabs in a uniform
magnetic field. This allows us to analyse in a simple configuration the
collective behaviour of the structure due to the interaction between
the slabs.
Methods: .We first calculate the normal modes of the
system and find analytical expressions for the dispersion relation of
the two-slab configuration. Next, we study the time-dependent problem of
the excitation of slab oscillations by numerically solving the initial
value problem. We investigate the behaviour of the system for several
shapes of the initial disturbances.
Results: .The symmetric mode
respect to the centre of the structure is the only trapped mode for
all distances between the slabs while the antisymmetric mode is leaky
for small slab separations. Nevertheless, there is a wide range of
slab separations for which the fundamental symmetric and antisymmetric
trapped modes are allowed and have very close frequencies. These modes
are excited according to the parity of the initial perturbation.
Conclusions: .We find that for any initial disturbance the slabs
oscillate with the normal modes of the coupled slab system, which
are different from the modes of the individual slabs. We show that it
is possible to excite the symmetric and antisymmetric trapped modes
at the same time. This kind of excitation can produce the beating
phenomenon, characterised by a continuous exchange of energy between
the individual slabs.
Title: Damping of Kink Oscillations in Curved Coronal Loops
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2006ApJ...650L..91T
Altcode:
We study the kink modes of oscillation of a curved coronal loop using a
toroidal model with a power-law density profile. We find that there are
two kink modes and that their velocity polarization is either mainly
horizontal or mainly vertical with respect to the photosphere. It is
found that the damping by resonant absorption in an inhomogeneous layer
between the loop and the coronal environment is slightly more efficient
than for the straight magnetic cylinder and that the period and damping
time of the two kink modes are very similar. In an equilibrium with
nonuniform Alfvén frequency, the kink modes display two additional
features, namely, the coupling to an Alfvén mode of the external
medium (which gives rise to resonant absorption in the corona) and
wave leakage by tunneling at some distance from the loop. Hence, in
general the modes are resonantly damped and leaky at the same time,
although our results point out that resonant absorption at the loop
boundary is the dominant damping mechanism.
Title: Spectral line width decrease in the solar corona: resonant
energy conversion from Alfvén to acoustic waves
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.
Bibcode: 2006A&A...456L..13Z
Altcode: 2007astro.ph..3297Z
Context: .Observations reveal an increase with height of the line
width of several coronal spectral lines probably caused by outwardly
propagating Alfvén waves. However, the spectral line width sometimes
shows a sudden decrease at a height ~0.1-0.2 R⊙, where
the ratio of sound to Alfvén speeds may approach unity.
Aims:
.To explain the observed line width reduction in terms of the energy
conversion from Alfvén to another type of wave motion.
Methods:
.Weakly non-linear wave-wave interaction in ideal MHD.
Results:
.Qualitative analysis shows that the resonant energy conversion from
Alfvén to acoustic waves near the region of the corona where the
plasma β approaches unity may explain the observed spectral line
width reduction.
Title: Fast Magnetohydrodynamic Oscillations in Coronal Loops with
Heating Profiles
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2006ApJ...645..766D
Altcode:
In this paper we study how the normal fast modes of a coronal loop
are modified by the addition of density structure along the loop
axis. Using isobaric and thermal equilibria with different heating
functions, a density profile for a loop is derived, and then its
oscillatory modes are studied with techniques similar to those
in our previous works. The main result is that the frequency and
spatial structure of the trapped modes are very sensitive to density
variations inside the loop, especially at the apex, where the density
depends strongly on the footpoint density and the ratio of heating to
thermal conduction. Moreover, different tested heating profiles do not
introduce important differences with respect to the case of uniform
heating along the loop.
Title: Damped Coronal Loop Oscillations: Time-dependent Results
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2006ApJ...642..533T
Altcode:
The excitation and damping of transverse coronal loop oscillations
is studied using a one-dimensional model of a line-tied cylindrical
loop. By solving the time-dependent magnetohydrodynamic (MHD) equations,
we show how an initial disturbance produced in the solar corona induces
kink-mode oscillations. We analyze the effect of the disturbance on
a loop with a nonuniform boundary layer and investigate the damping
of such a disturbance due to resonant absorption. We find that the
period and attenuation time of the time-dependent results agree
with the calculations of the corresponding quasi-mode (i.e., the
kink mode resonantly coupled to Alfvén modes) and that the resonant
absorption mechanism is capable of damping the oscillations almost
immediately after the excitation. We study in detail the behavior of
solutions in the inhomogeneous layer and show how the energy of the
global oscillation is converted into torsional oscillations in the
inhomogeneous layer. In addition, we estimate that the amplitude of
the torsional oscillations is, for large magnetic Reynolds numbers
and for thick layers, between 4 and 6 times the amplitude of the
initial transverse motions. The implications of these results and
their relationship with the observations are discussed.
Title: Seismology of Prominence-Fine structures: Observations
and Theory
Authors: Ballester, José Luis
Bibcode: 2006SSRv..122..129B
Altcode:
Prominence seismology is a rapidly developing topic which seeks to
infer the internal structure and properties of solar prominences
from the study of its oscillations. Two-dimensional high-resolution
observations suggest that filaments can be considered as made by
small scale fibrils, having a cool region, stacked one after another
in the vertical and horizontal directions. An extense observational
background about oscillations in filaments has been gathered during the
last 20 years and these observations point out that fibrils or groups of
fibrils can oscillate independently. From the theoretical point of view,
small amplitude oscillations in single and multifibril configurations
have been studied as a first step to explain observational features.
Title: Recent progress in prominence seismology
Authors: Ballester, José Luis
Bibcode: 2006RSPTA.364..405B
Altcode:
No abstract at ADS
Title: Excitation and damping of disturbances in cylindrical
coronal loops
Authors: Terradas, Jaume; Oliver, Ramón; Ballester, José Luis
Bibcode: 2006RSPTA.364..547T
Altcode:
No abstract at ADS
Title: Solar/ heliospheric dynamics and magnetism. Solar vision
2015-2025
Authors: Khodachenko, M. L.; Arber, T. D.; Ballester, J. L.; et al.
Bibcode: 2005ESASP.588..379K
Altcode: 2005tssc.conf..379K
No abstract at ADS
Title: Theoretical Advances in Prominence Seismology
Authors: Ballester, José Luis
Bibcode: 2005SSRv..121..105B
Altcode:
Prominence seismology is a rapidly developing topic which seeks to
infer the internal structure and properties of solar prominences from
the study of its oscillations. An extensive observational background
about oscillations in quiescent solar prominences has been gathered
during the last 70 years. These observations point out the existence
of two different types of oscillations: Flare-induced oscillations
(winking filaments) which affect the whole prominence and are of large
amplitude and small amplitude oscillations which seem to be of local
nature. From the theoretical point of view, few models have been set up
to explain the phenomenon of winking filaments while, on the contrary,
for small amplitude oscillations a large number of models trying to
explain the observed features have been proposed.
Title: On the excitation of trapped and leaky modes in coronal slabs
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2005A&A...441..371T
Altcode:
We study the excitation of fast waves in solar coronal loops modeled
as smoothed, dense plasma slabs in a uniform magnetic field. We analyse
the features of the trapped and leaky modes which result from an initial
disturbance generated in the loop. By solving the time-dependent problem
we find that the stationary state of the slab motion is described by
the trapped normal modes and that the transient between the initial
and the stationary phase is dominated by wave leakage. The period and
duration of this transient (what we call the impulsive leaky phase)
are in perfect agreement with the corresponding values of leaky modes
calculated from the normal mode analysis.
Title: Fast magnetohydrodynamic oscillations in a multifibril
Cartesian prominence model
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2005A&A...440.1167D
Altcode:
Observations of quiescent filaments show very fine structures
which suggests that they can be composed of small-scale threads or
fibrils. Two-dimensional, high-resolution observations point out that
individual fibrils or groups of fibrils may oscillate independently with
their own periods. In this paper, we study the fast magnetohydrodynamic
modes of oscillation of multifibril Cartesian systems to represent
the oscillations of the fibril structure of a real prominence. In the
case of a system made of equal fibrils, our results show that the only
non-leaky mode is the symmetric one, which means that all the fibrils
oscillate in spatial phase with the same frequency. On the other hand,
in a system made of non-equal fibrils, i.e. with different Alfvén
speeds, the results show that the amplitudes of oscillation are higher
in the denser fibrils, that the frequency of oscillation of the only
non-leaky mode is slightly smaller than that of the dominant fibril
considered alone, and that all the fibrils also oscillate in phase.
Title: Time damping of linear non-adiabatic magnetoacoustic waves
in a slab-like quiescent prominence
Authors: Terradas, J.; Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 2005A&A...434..741T
Altcode:
We study the time damping of linear non-adiabatic magnetoacoustic
waves in a homogeneous, isothermal and bounded magnetic slab of
plasma with physical properties akin to those of quiescent solar
prominences. Because of the chosen configuration, our results are
related to short or intermediate period prominence oscillations and
show that the damping times of fast modes are very long compared to
those of slow modes. In an attempt to mimic optically thick prominences,
different prominence regimes have been considered by reducing radiative
losses. Then, when the temperature and/or density of the prominence are
modified, the damping time varies in a complex way which also depends
on the prominence regime considered. In all the prominence regimes,
a minimum of the damping time can be obtained for a certain value of
temperature and density. Finally, the consideration of different heating
mechanisms, the case of no heating included, can modify the damping
times in a substantial way while the periods are only slightly affected.
Title: Parametric excitation of slow magnetoacoustic waves in the
solar corona due to photospheric periodic motions
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.
Bibcode: 2005A&A...433..357Z
Altcode:
We suggest that the periodic shaking of coronal magnetic field lines due
to photospheric periodic motions may induce the parametric excitation
of slow magnetoacoustic waves in short coronal magnetic structures
(with length <50{-}70 Mm). Considering the periodic shaking of field
lines as an external transversal periodic action on the coronal plasma
we show that the temporal behaviour of slow wave spatial Fourier
harmonics in the low plasma β limit is governed by the Mathieu
equation. Consequently, harmonics of slow magnetoacoustic waves with
half the frequency of the photospheric driver have an exponential
growth in time. The growth rate of slow waves is proportional
to the amplitude of photospheric motions and to the value of the
plasma β in the corona. The mechanism may explain the existence of
slow magnetoacoustic waves at least in short coronal loop systems
(Nightingale et al. [CITE], Sol. Phys., 190, 249).
Title: The periodic behaviour of the North-South asymmetry of sunspot
areas revisited
Authors: Ballester, J. L.; Oliver, R.; Carbonell, M.
Bibcode: 2005A&A...431L...5B
Altcode:
Up to now, the periodic behaviour of the N-S asymmetry of solar
activity has been analyzed by applying the power spectrum analysis
to the time series generated from a normalized definition of the
asymmetry. Using sunspot areas and the properties of the discrete
Fourier transform, we show that the use of the normalized time
series leads to misleading results, and that the correct asymmetry
time series to be used is generated from the difference between the
values of solar activity indicators in the Northern and Southern solar
hemispheres. However, in this case the found significant periodicities
correspond to periodicities already present in the hemispheric sunspot
areas time series, not providing with any interesting information
about the asymmetric behaviour of the solar activity. Of course, our
conclusions are extensive to all the studies of the periodic behaviour
of the N-S asymmetry of solar activity time series computed by means
of the normalized definition of the asymmetry, and independent of the
considered solar activity feature.
Title: The Excitation and Damping of Transversal Coronal Loop
Oscillations
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2005ApJ...618L.149T
Altcode:
The excitation and damping of transversal coronal loop oscillations is
studied using a simple line-tied one-dimensional model. The dynamics
of the loop and the coronal environment is governed by the well-known
Klein-Gordon equation, and so a localized disturbance (representing the
effect of a solar flare) gives rise to a perturbation that undergoes
dispersion as it propagates toward the loop. As a consequence, the loop
oscillates at the external cutoff frequency, and its motion attenuates
with time roughly as t-1/2 (with dense and wide loops having
even stronger damping profiles). Hence, the damping of transversal
loop oscillations is not related to any dissipation mechanism but is
simply produced by the wake of the traveling disturbance. In addition,
these damped oscillations are not related to the kink mode, although
this mode can be excited after the attenuation process by the energy
of the wave packet deposited in the loop.
Title: Instability of periodic MHD shear flows
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.;
Belvedere, G.
Bibcode: 2004AIPC..733..201Z
Altcode:
The stability of periodic MHD shear flows generated by an external
transversal periodic force in magnetized plasma is studied. It is
shown that the temporal behaviour of magnetosonic wave spatial Fourier
harmonics in such flows is governed by Mathieu equation. Consequently
the harmonics with the half frequency of the shear flows grow
exponentially in time. Therefore the periodic shear motions are unstable
to the perturbations of compressible magnetosonic waves. The motions
represent the kinetic part of the transversal oscillation in magnetized
plasma. Therefore due to the instability of periodic shear motions,
the transversal oscillations may quickly be damped, so transferring
their energy to compressible magnetosonic perturbations.
Title: Return of the Near 160 Day Periodicity in the Photospheric
Magnetic Flux during Solar Cycle 23
Authors: Ballester, J. L.; Oliver, R.; Carbonell, M.
Bibcode: 2004ApJ...615L.173B
Altcode:
A periodicity between 152 and 158 days was discovered in the daily
number of high-energy solar flares detected by the Solar Maximum Mission
(SMM) and the Geosynchronous Operational Environmental Satellites (GOES)
around the maximum of solar cycle 21. In a previous work we pointed
out that this periodicity had been time-coincident with a periodic
emergence of magnetic flux in the form of strong magnetic fields, which
suggested a causal relationship between both periodicities. Using the
Mount Wilson Sunspot Index, evidence is presented for the return of
the periodicity in the strong photospheric magnetic flux during the
current solar cycle. The periodicity has reappeared around the solar
activity maximum with a frequency similar to that of solar cycle 21,
but contrary to what happened during cycle 21, it is completely absent
in energetic flares. A tentative explanation for this feature is that
in the current solar cycle, part of the periodic emergence of magnetic
flux has taken place away from already developed sunspot groups and
so has not contributed to enhance their magnetic complexity, which
has prevented the triggering of periodic energetic flares.
Title: Application of Statistical Techniques to the Analysis of
Solar Coronal Oscillations
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ApJ...614..435T
Altcode:
In this work, the application of two different techniques to the
analysis of coronal time series is investigated. The first technique,
called empirical mode decomposition, was developed by Huang and
coworkers and can be used to decompose a signal in its characteristic
timescales, allowing, among other applications, efficient filtration of
the signal. The second technique, called complex empirical orthogonal
function (CEOF) analysis, is an extension of the well-known principal
component analysis, to which the Hilbert transform has been added. The
CEOF analysis allows identification of the dominant spatial and
temporal structures in a multivariate data set and is thus ideally
suited to the study of propagating and standing features that can be
associated with waves or oscillations. Here we apply both methods
to time series obtained from a coronal loop and obtain detailed
two-dimensional information about a propagating and a standing wave
with periods around 5 and 10 minutes, respectively.
Title: Magnetohydrodynamic waves in a sheared potential coronal arcade
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2004A&A...425..729A
Altcode:
We study the effects of magnetic field shear (By ≠ 0)
and longitudinal propagation of perturbations (ky ≠ 0)
on the linear and adiabatic magnetohydrodynamic (MHD) normal modes
of oscillation of a potential coronal arcade. In a cold plasma, the
inclusion of these two effects produces the linear coupling of discrete
fast modes, characterised by a discrete spectrum of frequencies and a
global velocity structure, and Alfvén continuum modes, characterised by
a continuous spectrum of frequencies and with a velocity perturbation
confined to given magnetic surfaces in such a way that modes with
mixed properties arise \citep{Arregui04}. The wave equations governing
the velocity perturbations have been solved numerically and our
results show that the couplings between fast and Alfvén modes are
governed by some parity rules for the symmetry of the eigenfunctions
of fast and Alfvén modes in the direction along the equilibrium
magnetic field. The nature of the coupling between fast and Alfvén
modes can be resonant or non-resonant depending on the location of the
fast mode frequency within the different Alfvén continua. Also, an
important result is that in this kind of configurations coupled modes
could be difficult to observe since when both magnetic field shear
and longitudinal propagation are present the spatial distribution of
the velocity may not be confined to low heights in the solar corona.
Title: Fast MHD oscillations in line-tied homogeneous coronal loops
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.; Roberts, B.
Bibcode: 2004A&A...424.1055D
Altcode:
Loop oscillations have been abundantly reported in recent years. Earlier
analytical studies of loop oscillations consider freely propagating
waves, allowing for line-tying by a quantization of the wavenumber. Here
we consider the rich spectrum of fast MHD modes (both standing and
leaky) in coronal loops, allowing for line-tying and performing some
comparisons with observational data. We point out that in a straight
and homogeneous cylindrical flux tube there should be observational
signatures of the excitation of higher order harmonics. Our results
indicate that these modes become leaky with the addition of the
chromospheric structure at the base of the loop. Leakage can be quite
efficient in damping the oscillations for many of these high frequency
(compared to fundamental) modes. Appendix A is only available in
electronic form at http://www.edpsciences.org
Title: Erratum: ``Magnetohydrodynamic Waves in Sheared Coronal
Arcades'' (ApJ, 602, 1006
[2004])
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ApJ...607.1070A
Altcode:
Proof corrections were not carried faithfully to the printed version of
this paper, resulting in an error in the text below equation (12). The
equation en=∇A/∇A=-ez should
read en=∇A/∇A; i.e., the final equivalence should
be omitted. The Press sincerely regrets this error.
Title: Magnetohydrodynamic Waves in Sheared Coronal Arcades
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ApJ...602.1006A
Altcode:
The magnetohydrodynamic (MHD) normal modes of oscillation of
sheared coronal arcades including longitudinal wave propagation
are studied. Solutions have been computed to ascertain the effects
of the longitudinal magnetic field component, By, and of
the longitudinal wavenumber, ky, in these structures. Our
results show that whenever By≠0 and/or ky≠0,
fast modes, characterized by a global velocity structure and a discrete
spectrum of frequencies, and Alfvén continuum modes, characterized
by a velocity perturbation confined to given magnetic surfaces, get
coupled and no pure fast modes or pure Alfvén modes exist, but modes
with mixed properties arise. These oscillatory modes display a global
spatial distribution, together with a nonsquare integrable singular
behavior on certain magnetic surfaces. Moreover, this mode coupling is
such that under some circumstances the Alfvénic contribution is also
in the form of a smooth velocity profile covering a range of magnetic
surfaces instead of a singularity on a fixed magnetic surface. The
coupling between fast and Alfvén modes is governed by the parity of
their eigenfunctions in the direction along the equilibrium magnetic
field. The parity rules determining the features of coupled modes in
terms of their spatial structure, ky, and By
are presented. We have found that the frequency of coupled modes is
real when either By≠0 or ky≠0 but is complex
when both By≠0 and ky≠0. Also, an important
result is that coupled modes may not be observable since many of them
leak energy away from the solar corona.
Title: Time damping of linear non-adiabatic magnetohydrodynamic
waves in an unbounded plasma with solar coronal properties
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 2004A&A...415..739C
Altcode:
In this paper, we study the time damping of magnetoacoustic waves when
the adiabaticity assumption is removed by means of an energy equation
which includes optically thin radiative losses, thermal conduction
and heating. For the sake of simplicity, this study has been done for
a homogeneous, isothermal and unbounded medium permeated by a uniform
magnetic field, with physical properties akin to those of the corona,
the prominence-corona transition region (PCTR) and prominences. In
some PCTR regimes and in the coronal regime wave instabilities appear,
which prevents the computation of the damping time and the damping per
period for part of the wavenumber interval considered. Furthermore,
except for one of the PCTR regimes, in the rest of the regimes the
apparition of those instabilities depends on the heating mechanism
considered. Also, for the same heating mechanism, the behaviour of the
damping time for the different considered regimes changes significantly
when going from very small to very large wavenumbers and, in all the
regimes, it becomes almost constant for very large wavenumbers.
Title: Fast MHD Oscillations in Coronal Loops with Heating Profiles
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..489D
Altcode: 2004soho...13..489D
In this work we consider the rich spectrum of fast MHD modes in
low-beta, line-tied coronal loops (standing and leaky). One of the most
important conclusions is that in a straight and homogeneous cylindrical
flux tube there should be observational signatures of the excitation
of higher order harmonics. Nevertheless, our results indicate that
these modes become leaky with the addition of some structure along
the loop and that leakage can be quite efficient in damping the
oscillations. After that, we study the effects in the modes of the
addition of density structure along the loop axis: using an isobaric and
thermal equilibrium with different heating functions, a density profile
for a loop is derived, and then its oscillatory modes are studied. The
main result is that the frequency and spatial structure of the trapped
modes are very sensitive to variations of density inside the loop,
specially in the apex, which depends strongly on the footpoint density
and the ratio of heating to thermal conduction. Moreover, different
tested heating profiles give more or less the same results that for
the case of constant heating all along the loop.
Title: Fast MHD Oscillations in Multifibril Cartesian Systems
Authors: Díaz, A. J.; Olivier, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..205D
Altcode: 2004soho...13..205D
Observations of quiescent filaments show very fine structures suggesting
that they can be composed by smallscale threads or fibrils and,
on the other hand, twodimensional, high-resolution observations have
pointed out that individual fibrils or groups of fibrils may oscillate
independently with their own periods. Using Cartesian geometry, Díaz
et al. (2001) studied the fast magnetohydrodynamic oscillations of
a single and isolated prominence fibril showing that for reasonable
values of the fibril's width the perturbations extend far away from its
axis. This study can be looked at as incomplete since, by considering
only one fibril, the interaction between the different fibrils composing
the prominence was not taken into account. In this work, we study
the fast MHD modes of oscillation of homogeneous and inhomogeneous
multifibril Cartesian systems trying to represent the oscillations of
the fibril structure of a real prominence. In the case of an homogeneous
multifibril system, our results show that, for a realistic separation
between fibrils, the only surviving mode is the symmetric one, which
means that, at the end, all the fibrils oscillate in spatial phase with
the same frequency. An inhomogeneous multifibril system can be obtained
by varying the Alfvén velocity within each considered fibril, and the
results show that there are not symmetric or antisymmetric modes, that
the amplitudes of oscillation are higher in the more dense fibrils,
that the frequency of oscillation of the fibrils, due to the only
non-leaky mode, is slightly smaller than that of the dominant fibril
considered alone, and that all the fibrils oscillate in phase. Finally,
the introduction of a wavenumber results in a better confinement,
a lower interaction between fibrils and a decrease in frequencies.
Title: Coupled Fast and Alfvén MHD Waves in Sheared Coronal Arcades
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..447A
Altcode: 2004soho...13..447A
The magnetohydrodynamic (MHD) normal modes of oscillation of
sheared coronal arcades including longitudinal wave propagation
are studied. Solutions have been computed in order to ascertain the
effects of the longitudinal magnetic field component, By, and of the
longitudinal propagation, ky in these structures. Our results show
that whenever By = 0 and/or ky = 0 fast modes, characterized by a
global velocity structure and a discrete spectrum of frequencies,
and Alfvén continuum modes, characterized by a velocity perturbation
confined to given magnetic surfaces, get coupled and no pure fast
modes nor pure Alfvén modes exist, but modes with mixed properties
arise. These oscillatory modes display a global spatial distribution
together with a non-square integrable singular behaviour on certain
magnetic surfaces. Under certain circumstances, moreover, this mode
coupling is such that the Alfvénic contribution is also in the form of
a smooth velocity profile covering a range of magnetic surfaces instead
of a singularity on a fixed magnetic surface. The coupling between fast
and Alfvén modes is governed by the parity of their eigenfunctions in
the direction along the equilibrium magnetic field. The "parity rules"
determining the features of coupled modes in terms of their parity,
ky and By are presented.
Title: New Damping Mechanism for Coronal Loop Kink Oscillation
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..465Z
Altcode: 2004soho...13..465Z
We propose a new damping mechanism for the coronal loop kink
oscillation. We show that the periodic shearing motion near the xed
footpoints of the coronal loop, oscillating in the global kink mode,
is unstable and leads to the resonant generation of slow magnetosonic
waves when the system tries to smooth out the spatial inhomogeneity of
the motion. Therefore, the kink oscillation transfers its energy into
many harmonics of slow magnetosonic waves propagating with di erent
angles about the loop axis and consequently it can be quickly damped,
as observed by TRACE. The mechanism can be tested by the observation
of slow magnetosonic waves at the loop footpoints during and/or after
the damping of kink oscillations.
Title: Damping of Standing Alfvén Waves
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..193Z
Altcode: 2004soho...13..193Z
A new damping mechanism of standing Alfven waves is proposed. The
mechanism is based on the instability of periodic shearing motion near
the velocity nodes of Alfven waves. The slow magnetosonic waves are
found to be ampli ed due to the instability when the system tries to
smooth out the ow inhomogeneity. The amplitudes of spatial Fourier
harmonics of the slow magnetosonic waves are governed by Mathieu's
equation, therefore those harmonics with half the frequency of Alfven
waves have an exponential growth. As a result, the standing Alfven
waves can be rapidly damped, resonantly transforming the energy into
many harmonics of slow magnetosonic waves. The mechanism can be of
importance for the damping of standing Alfven waves in the solar
atmosphere and other astrophysical situations.
Title: 3D Numerical Simulations of Impulsively Generated MHD Waves
in Solar Coronal Loops
Authors: Selwa, M.; Murawski, K.; Kowal, G.; Nakariakov, V.;
Aschwanden, M.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..495S
Altcode: 2004soho...13..495S
Impulsively generated magnetohydrodynamic waves in a typical EUV solar
coronal loop are studied numerically with a use of the three-dimensional
FLASH code. Our results reveal several 3D effects such as distinctive
time signatures which are collected at a detection point inside the
loop. A slow magnetosonic wave generates a significant variation in a
mass density profile with a time-scale of the order of s. A fast kink
wave affects a mass density too but its magnitude is much lower than
in the case of a slow wave. Time-scales which are associated with the
fast kink wave are generally lower than in the case of a slow wave;
they are in the range of a dozen or so seconds. Temporal signatures
of a fast sausage wave reveal s oscillations in the quasi-periodic
phase. Impulses which are launched outside the loop excite few seconds
oscillations in the mass density. Time-signatures depend on a position
of the detection point; they are usually more complex further out from
the exciter.
Title: Coupling of fast and Alfvén waves in a straight bounded
magnetic field with density stratification
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2003A&A...402.1129A
Altcode:
The theoretical understanding of the linear standing or propagating
magnetohydrodynamic waves in a variety of solar coronal structures
is far from complete since analytical solutions to the linearised MHD
equations can only be found for very simple magnetic configurations. In
this paper, we use a numerical code to solve the linear fast and
Alfvén wave equations in a very simple, bounded magnetic configuration
that incorporates two features that are not usually considered in
similar works, namely the longitudinal magnetic field component
and wave propagation in the longitudinal direction (ky
!=q 0). We use a numerical code (Arregui et al. \cite{Arregui01})
that has been modified by including a staggered mesh that allows us
to properly capture the spatial behaviour of solutions to the wave
equations. Coupling between fast and Alfvén modes has been studied
in detail and it has been found that it does not take place when the
longitudinal field component is zero and the frequency of the fast mode
is outside the Alfvén continuum with the same spatial structure along
field lines. Under these circumstances, fast modes retain their global
spatial behaviour and are also characterised by omega 2
varying linearly with ky2, such as in a uniform
medium (although here the Alfvén speed changes exponentially in
the direction normal to field lines). Regarding mode coupling, its
main feature is the blend of fast and Alfvén solutions with close
frequencies in some modes with a mixture of their properties, namely
discontinuities or jumps around certain magnetic surfaces (such as in
pure Alfvén waves), global spatial distribution of the normal velocity
component and non-zero density perturbations (such as in fast waves).
Title: Fast MHD oscillations of a 3-dimensional prominence fibril
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2003A&A...402..781D
Altcode:
High-resolution observations of quiescent filaments point out that
their fine structure is made of small-scale threads or fibrils. These
fibrils can be represented as thin loops having cool tops, i.e. the
prominence material, while the rest of the loop displays coronal
temperatures. Then, the stacking of these thin loops in the vertical and
horizontal directions gives place to the fine structure of filaments. On
the other hand, two-dimensional, high-resolution observations of
oscillations in filaments suggest that individual fibrils or groups
of fibrils may oscillate independently with their own periods (Yi
et al. \cite{yi}). Díaz et al. (\cite{doeb01}, hereafter Paper I)
studied the fast magnetohydrodynamic oscillations of a single and
isolated slab prominence fibril using a two-dimensional model with no
dependence in the y-direction. Here, we introduce a completely different
approach which allows us to build up a 3-dimensional model for the fast
magnetohydrodynamic oscillations of the fibril configuration used in
Paper I. As compared to the results obtained in Paper I, two relevant
new features appear; first of all, the cut-off frequency varies with
the longitudinal wavenumber, so more modes can be trapped within the
fibril; secondly, a much better confinement of the energy of the modes
within the fibril appears, diminishing the leakage of energy towards
neighbouring fibrils and so difficulting their mutual excitation.
Title: Fast MHD oscillations in cylindrical prominence fibrils
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2002ESASP.506..593D
Altcode: 2002svco.conf..593D; 2002ESPM...10..593D
Some observations suggest that quiescent solar prominences can
be considered as composed by small-scale loops, or fibrils, which
are stacked one after another in both the vertical and horizontal
directions. In a previous work we studied, in Cartensian geometry,
the propagation of fast MHD waves in a two-dimensional magnetostatic
model representing one of these fibrils. In this paper we use a more
realistic model based on a cylindrically symmetric flux tube and
study the propagation of fast MHD waves in this structure. Among
other conclusions, our results show that all sausage modes (m=0)
possess a cutoff frequency, while the fundamental kink and fluting
modes (m>0) do not show such a cut-off. Moreover, the spatial
structure of the modes below the cut-off frequency is such that in this
geometry perturbations are confined in the dense part of the fibril,
the leakage of energy towards the coronal medium being very small,
which may prevent the excitation of neighbouring fibrils. Finally,
diagnostic diagrams displaying the oscillatory period in terms of some
equilibrium parameters are provided in order to allow for a comparison
between our theoretical results and those coming from observations.
Title: Magnetohydrodynamic waves in sheared coronal magnetic
structures
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2002ESASP.506..535A
Altcode: 2002svco.conf..535A; 2002ESPM...10..535A
The theoretical understanding of the linear standing or propagating
magnetohydrodynamic (MHD) waves in a variety of solar coronal structures
is far from complete since analytical solutions to the linearised MHD
equations can only be found for very simple magnetic configurations. In
this work, a numerical code is used to solve the linear MHD wave
equations in a coronal magnetic arcade with a longitudinal magnetic
field component and considering propagation in the longitudinal
direction. In a cold plasma, the inclusion of these two effects leads
to the coupling of fast and Alfvén modes. The numerical code provides
us with solutions for these coupled fast and Alfvén modes in the form
of the two-dimensional distribution of the perpendicular and normal
velocity components together with the oscillatory frequency. Solutions
have been computed in order to ascertain the effects of the longitudinal
magnetic field component and of the longitudinal propagation on the
properties of fast and Alfvén modes.
Title: Fast Magnetohydrodynamic Oscillations in Cylindrical Prominence
Fibrils
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2002ApJ...580..550D
Altcode:
Some observations suggest that quiescent solar prominences can
be considered as composed by small-scale loops, or fibrils, which
are stacked one after another in both the vertical and horizontal
directions. In a previous work we studied, in Cartesian geometry,
the propagation of fast MHD waves in a two-dimensional magnetostatic
model representing one of these fibrils. Since this is a crude model
to represent a real fibril, in this paper we use a more realistic
model based on a cylindrically symmetric flux tube and study the
propagation of fast MHD waves in this structure. A new array of modes
of oscillation, together with their periods and spatial properties,
is described, showing several important differences with respect
to the properties of modes in Cartesian geometry. Among other
conclusions, our results show that all sausage modes (m=0) possess
a cutoff frequency, while the fundamental kink and fluting modes
(m>0) do not show such a cutoff. In addition, the frequency of
these modes is independent of the azimuthal wavenumber (m) and of the
fibril thickness for a wide range of values of this parameter, which
is an important fact for prominence seismology. Moreover, the spatial
structure of the modes below the cutoff frequency is such that in this
geometry perturbations are confined in the dense part of the fibril,
the leakage of energy toward the coronal medium being very small,
which may prevent the excitation of neighboring fibrils. Finally,
diagnostic diagrams displaying the oscillatory period in terms of some
equilibrium parameters are provided in order to allow for a comparison
between our theoretical results and those coming from observations.
Title: Fast MHD oscillations in cylindrical prominence fibrils
Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.
Bibcode: 2002ESASP.505..393D
Altcode: 2002IAUCo.188..393D; 2002solm.conf..393D
Some observations suggest that quiescent solar prominences can
be considered as composed by small-scale loops, or fibrils, which
are stacked one after another in both the vertical and horizontal
directions. In a previous work we studied, in Cartesian geometry,
the propagation of fast MHD waves in a two-dimensional magnetostatic
model representing one of these fibrils. In this paper we use a more
realistic model based on a cylindrically symmetric flux tube and
study the propagation of fast, MHD waves in this structure. Among
other conclusions, our results show that all sausage modes (m = 0)
possess a cut-off frequency, while the fundamental kink and fluting
modes (m > 0) do not show such a cut-off. Moreover, the spatial
structure of the modes below the cut-off frequency is such that in this
geometry perturbations are confined in the dense part of the fibril,
the leakage of energy towards the coronal medium being very small,
which may prevent the excitation of neighbouring fibrils. Finally,
diagnostic diagrams displaying the oscillatory period in terms of some
equilibrium parameters are provided in order to allow for a comparison
between our theoretical results and those coming from observations.
Title: Two-dimensional distribution of oscillations in a quiescent
solar prominence
Authors: Terradas, J.; Molowny-Horas, R.; Wiehr, E.; Balthasar, H.;
Oliver, R.; Ballester, J. L.
Bibcode: 2002A&A...393..637T
Altcode:
Using time series of two-dimensional Dopplergrams, a temporal and
spatial analysis of oscillations in a quiescent prominence has been
performed. The presence of an outstanding oscillatory signal in the
acquired data has allowed us to study the two-dimensional distribution
of wave motions and, in particular, to detect the location of wave
generation and the anisotropic propagation of perturbations from that
place. Moreover, a strong damping of oscillations has been observed,
with damping times between two and three times the wave period. The
direction of propagation, wavelength and phase speed, together with
the damping time and wave period, have been quantified and their
spatial arrangement has been analysed. Thanks to the goodness of the
observational data, the image alignment procedure applied during
the data reduction stage and the analysis tools employed, it has
been possible to carry out a novel and far-reaching observational
study of prominence oscillations. The five movies are available at
http://www.edpsciences.org
Title: Parametric Amplification of Magnetosonic Waves by an External,
Transversal, Periodic Action
Authors: Zaqarashvili, T. V.; Oliver, R.; Ballester, J. L.
Bibcode: 2002ApJ...569..519Z
Altcode:
The influence of an external, transversal, periodic force on
the local dynamics of magnetosonic waves is studied. We show that
periodic, transversal plasma motions generated by this force lead to
a periodical drift of the wavevector of the spatial Fourier harmonics
in the local frame and, as a result, their amplitude is governed by
Mathieu's equation. Thus, those harmonics with half the frequency of
the external force have an exponential growth, and in consequence the
energy of transversal oscillations can be transformed into the energy
of magnetosonic waves. Some of the many applications of this mechanism
to different astrophysical situations are discussed.
Title: Forecasting the solar cycle with genetic algorithms
Authors: Orfila, A.; Ballester, J. L.; Oliver, R.; Alvarez, A.;
Tintoré, J.
Bibcode: 2002A&A...386..313O
Altcode:
In the past, it has been postulated that the irregular dynamics of
the solar cycle may embed a low order chaotic process (Weiss 1988,
1994; Spiegel 1994) which, if true, implies that the future behaviour
of solar activity should be predictable. Here, starting from the
historical record of Zürich sunspot numbers, we build a dynamical
model of the solar cycle which allows us to make a long-term forecast
of its behaviour. Firstly, the deterministic part of the time series
has been reconstructed using the Singular Spectrum Analysis and then
an evolutionary algorithm (Alvarez et al. 2001), based on Darwinian
theories of natural selection and survival and ideally suited
for non-linear time series, has been applied. Then, the predictive
capability of the algorithm has been tested by comparing the behaviour
of solar cycles 19-22 with forecasts made with the algorithm, obtaining
results which show reasonable agreement with the known behaviour of
those cycles. Next, the forecast of the future behaviour of solar
cycle 23 has been performed and the results point out that the level
of activity during this cycle will be somewhat smaller than in the
two previous ones.
Title: The 158-day periodicity in the rising phase of cycle 23
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 2002ESASP.477..155O
Altcode: 2002scsw.conf..155O
A near 158-day periodicity was discovered in the solar flare
occurrence rate during solar cycle 21. It has later become clear that
this periodicity was caused by the periodic emergence of magnetic
flux that gave rise to the formation of sunspot groups with complex
magnetic topology. The magnetic complexity of sunspot groups in turn
leads to a suitable scenario for the occurrence of energetic flares
by means of periodic episodes of magnetic reconnection. In this work,
various solar activity indicators are used to study the presence of
the 158-day periodicity during the rising phase of solar cycle 23.
Title: Oscillations in Quiescent Solar Prominences Observations and
Theory (Invited Review)
Authors: Oliver, Ramón; Ballester, José Luis
Bibcode: 2002SoPh..206...45O
Altcode:
An extensive observational background about the existence of
oscillations in quiescent solar prominences has been gathered during
the last twenty years. From these observations, information about
different oscillatory parameters such as period, wavelength, phase
speed, damping time, etc., has been obtained. This observational
background, combined with a growing number of theoretical studies
about magneto-hydrodynamic waves in prominences, should allow the
development of prominence seismology which, following helioseismology's
approach, seeks to infer the internal structure and properties of solar
prominences. The most recent observational and theoretical developments
on prominence oscillations are reviewed here, with an emphasis on the
aspects suitable to develop an observation versus theory feedback,
but also pointing out key topics which should be the subject of future
research for a further advancement of this field.
Title: The Near 160 Day Periodicity in the Photospheric Magnetic Flux
Authors: Ballester, J. L.; Oliver, R.; Carbonell, M.
Bibcode: 2002ApJ...566..505B
Altcode:
A periodicity near 154 days was discovered in the number of high-energy
solar flares detected by Solar Maximum Mission (SMM) and Geosynchronous
Operational Environmental Satellites (GOES) during the time interval
1980-1984 (Rieger et al.; Dennis). In this paper, we analyze the
historical records of photospheric magnetic flux to show that during
solar cycle 21 the periodicity appeared in the photospheric magnetic
flux linked to strong magnetic fields, while it was absent during solar
cycle 22. We also show that there was a time and frequency coincidence
between both periodicities during solar cycle 21, which suggests the
existence of a causal link between them. Taking into account that
high-energy flares are triggered in regions of enhanced magnetic
complexity (Kurokawa Ishii et al.), we propose that the appearance of
the periodicity in the magnetic flux materializes through the formation
of new sunspots within already formed sunspot groups, setting up a
suitable scenario for the occurrence of energetic flares. This scenario
leads to the occurrence of periodic episodes of magnetic reconnection
between old and new emergent magnetic flux, able to trigger the periodic
occurrence of energetic flares recorded by SMM and GOES.
Title: Fast MHD oscillations in prominence fine structures
Authors: Díaz, A. J.; Oliver, R.; Erdélyi, R.; Ballester, J. L.
Bibcode: 2001A&A...379.1083D
Altcode:
High-resolution observations suggest that quiescent solar prominences
are made of small-scale fibrils stacked one after another in both the
vertical and horizontal directions. These fibrils are interpreted as the
cool, highermost part of much larger coronal loops which are rooted in
the solar photosphere. On the other hand, there is some evidence showing
that small amplitude oscillations in prominences can affect individual
or groups of fibrils, which vibrate with their own periods. Using
a simple magnetostatic model to represent the fibril structure
of quiescent solar prominences, Joarder et al. (\cite{joarder})
investigated some oscillatory properties of the Alfvén and fast
magnetohydrodynamic modes. In this paper, with a proper treatment of
boundary conditions, we reexamine their configuration and explore more
deeply the basic features (mainly frequency and spatial structure)
of the fast mode. The main conclusion is that, for reasonable values
of the fibril's width, perturbations extend far away from its axis
and, therefore, a single oscillating fibril can excite oscillations
in neighbouring ones.
Title: Radiative damping of quiescent prominence oscillations
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2001A&A...378..635T
Altcode:
Observations of quiescent prominence oscillations point out their
finite lifetime, which suggests the presence of time damping. Recent
analysis of ground-based observations of prominence oscillations
(Molowny-Horas et al. \cite{molo99}) has revealed for the first time
the temporal damping of velocity perturbations at different spatial
locations within a quiescent prominence. Although the damping of
wave motions can be explained using a variety of mechanisms, here we
have adopted a very simple one, namely a radiative loss term based on
Newton's law of cooling with constant relaxation time (τR),
to analyse the influence of this type of radiative dissipation on
the modes of oscillation of Kippenhahn-Schlüter and Menzel quiescent
prominence models. Among other results, it is shown that slow modes
are characterised by short damping times, which indicates that these
oscillations are heavily damped, whereas fast modes are practically
unaffected by this radiative dissipation and have very long damping
times. Moreover, for a range of values of the radiative relaxation
time the fundamental slow mode attains very large values of the period
because of the destabilising effect of gravity. On the other hand,
three-dimensional dispersion diagrams (i.e. plots of the real and
imaginary parts of the frequency versus the wavenumber) are used to
investigate the coupling between slow and fast modes. It turns out that
far from adiabatic and isothermal conditions, the radiation mechanism
can effectively decouple the two magnetoacoustic modes.
Title: Nonlinear fast magnetosonic waves in solar coronal holes
Authors: Murawski, K.; Oliver, R.; Ballester, J. L.
Bibcode: 2001A&A...375..264M
Altcode:
A coronal hole is modeled as a slab of cold plasma threaded by a
vertical, uniform magnetic field. A periodic driver acting at the
coronal base is assumed to drive the velocity component normal
to the equilibrium magnetic field. Previous works indicate that,
in the linear regime, only fast mode perturbations propagate, since
Alfvén waves are excluded from the model and the slow wave is absent
in the cold plasma limit. However, in this work, it is shown that
nonlinear terms in the magnetohydrodynamic (MHD) equations give rise
to excitation of the velocity component parallel to the equilibrium B,
with a lower amplitude than the normal component. Another consequence
of nonlinearities is the generation of higher-frequency Fourier modes,
which can be detected by Fourier analyzing the velocity variations
above the photosphere. The nature of the nonlinear interactions in
the MHD equations determines the frequency of those modes. These
interactions are quadratic in the case of the parallel component,
while they are cubic in the case of the normal component. Therefore,
nonlinearly excited frequencies 2ωd, 4ωd,
6ωd, \ldots are present in the parallel velocity, whereas
frequencies 3ωd, 5ωd, 7ωd, \ldots
are present in the normal velocity, with ωd the driving
frequency.
Title: Numerical simulations of linear magnetohydrodynamic waves in
two-dimensional force-free magnetic fields
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2001A&A...369.1122A
Altcode:
High resolution observations of the solar corona made with instruments
onboard the SOHO and TRACE spacecrafts have provided new evidence
for the presence of oscillations in a variety of coronal magnetic
structures. Most of these observations have been interpreted in terms
of linear standing or propagating magnetohydrodynamic (MHD) waves, but
the theoretical understanding is far from complete since analytical
solutions to the linearised MHD wave equations can only be found for
very simple magnetic configurations. Taking into account that the
solar corona is basically structured by force-free magnetic fields,
our purpose in this paper is to present the derivation of the linear
MHD wave equations for a two-dimensional force-free magnetic field
configuration having longitudinal invariance, as well as to introduce
a numerical code to solve the resulting system of coupled partial
differential equations. The accuracy of the code has been checked by
numerically solving two cases for which analytical or simple numerical
solutions exist. To our knowledge, this is the only two-dimensional
code developed to study the normal MHD modes of oscillation of a
general force-free field with longitudinal invariance.
Title: Fast and Alfvén MHD waves in sheared coronal arcades
Authors: Arregui, I.; Oliver, R.; Ballester, J. L.
Bibcode: 2001hsa..conf..253A
Altcode:
No abstract at ADS
Title: Fast magnetohydrodynamic oscillations in prominence fine
structures
Authors: Díaz, A.; Oliver, R.; Ballester, J. L.
Bibcode: 2001hsa..conf..369D
Altcode:
No abstract at ADS
Title: Non adiabatic effects in magnetohydrodynamic waves in the
solar atmosphere
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 2001hsa..conf..249T
Altcode:
No abstract at ADS
Title: Cross-sections for Electron Capture by Interstellar PAH Cations
Authors: Ballester, J. L.; Tielens, A. G. G. M.
Bibcode: 2000AAS...197.4202B
Altcode: 2000BAAS...32.1465B
There is significant evidence for the presence of polycyclic aromatic
hydrocarbons (PAHs) in interstellar space. They have been suggested
as carriers of diffuse interstellar bands and could also play a
significant role in heating the interstellar gas. Some theoretical
models which incorporate PAHs must consider the charge state as an
important variable. In order to help address the PAH charge issue we
have developed a model for the electron-PAH cation interaction and are
using it to calculate electron PAH cation recombination cross-sections
and rates. We present results for coronene, a representative PAH and
discuss the significance of various model details. JLB gratefully
acknowledges support through Kansas NASA EPSCoR and a spring 2000
sabbatical leave from Emporia State University.
Title: Cyclical Variability of Prominences, CMEs and Flares
Authors: Ballester, J. L.
Bibcode: 2000JApA...21..221B
Altcode:
No abstract at ADS
Title: Slow surface wave damping in plasmas with anisotropic viscosity
and thermal conductivity
Authors: Ruderman, M. S.; Oliver, R.; Erdélyi, R.; Ballester, J. L.;
Goossens, M.
Bibcode: 2000A&A...354..261R
Altcode:
This paper studies the damping of slow surface MHD waves propagating
along the equilibrium magnetic field on a finite-thickness magnetic
interface. The plasma is assumed to be strongly magnetised, and the
full Braginskii's expressions for viscosity and the heat flux are
used. The primary focus of the paper is on the competition between
resonant absorption in the thin dissipative layer embracing the ideal
resonant position and the bulk wave damping due to viscosity and thermal
conductivity as damping mechanisms for surface MHD waves. The dependence
of the wave damping decrement on the wave length and the dissipative
coefficients is studied. Application of the obtained results to the
surface MHD wave damping in the solar chromosphere is discussed.
Title: Fast and Alfvén MHD Waves in Sheared Coronal Arcades
Authors: Arregi, I.; Oliver, R.; Ballester, J. L.
Bibcode: 1999ESASP.448..221A
Altcode: 1999ESPM....9..221A; 1999mfsp.conf..221A
No abstract at ADS
Title: Non-Adiabatic Effects in Magneto-Acoustic-Gravity Waves in
the Solar Atmosphere
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 1999ESASP.448..385T
Altcode: 1999mfsp.conf..385T; 1999ESPM....9..385T
No abstract at ADS
Title: Coronal potential magnetic fields from photospheric sources
with finite width
Authors: Oliver, R.; Čadež , V. M.; Carbonell, M.; Ballester, J. L.
Bibcode: 1999A&A...351..733O
Altcode:
Finite width photospheric sources are used to generate coronal
potential magnetic field configurations. The prescription of a
suitable distribution of the magnetic flux function within such
regions allows to solve the Grad-Shafranov equation and, using the
superposition principle, to obtain magnetic field configurations related
to arbitrary combinations of photospheric sources and sinks. Following
this approach, we have focused our attention on bipolar and quadrupolar
magnetic configurations in a background horizontal magnetic field,
which creates the conditions for complex magnetic field topologies with
magnetic X-points and local dips (minima). These configurations, with
infinitely thin and point sources, have been previously invoked (Priest
et al. \cite{PP}; \cite{PB}) to explain the process of photospheric flux
cancellation (cancelling magnetic features) and prominence formation
from photospheric material. We have investigated how the different
parameters of the model (i.e. source width and magnetic strength)
influence the magnetic field topology and have compared our results
to previous ones.
Title: MHD Waves in Slab-Like Prominences: Non Adiabatic Effects
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 1999ESASP.446..651T
Altcode: 1999soho....8..651T
No abstract at ADS
Title: Nonlinear Fast MHD Waves in Open Coronal Magnetic Structures
Authors: Oliver, R.; Ballester, J. L.; Murawski, K.
Bibcode: 1999ASPC..184..253O
Altcode:
An open coronal magnetic structure is modeled as a slab of cold plasma
threaded by a vertical, uniform magnetic field. A periodic driver
acting at the coronal base is assumed to drive the velocity component
normal to the equilibrium magnetic field. Previous works indicate
that, in the linear regime, only fast mode perturbations propagate,
since Alfvén waves are excluded from the model and the slow wave is
absent in the cold plasma limit. However, in this work it is shown that
nonlinear terms in the magnetohydrodynamic (MHD) equations give rise
to excitation of the velocity component parallel to the equilibrium B,
with a lower amplitude than the normal component. Another consequence
of nonlinearities is the generation of higher-frequency Fourier modes,
which can be detected by Fourier analyzing the velocity variations
above the photosphere. The nature of the nonlinear interactions in
the MHD equations determines the frequency of those modes. These
interactions are quadratic in the case of the parallel component,
while they are cubic in the case of the normal component. Therefore,
nonlinearly excited frequencies 2ω_d, 4ω_d, 6ω_d, ... are present
in the parallel velocity, whereas frequencies 3ω_d, 5ω_d, 7ω_d,
... are present in the normal velocity, with ω_d the driving frequency.
Title: Discovery of the Near 158 Day Periodicity in Group Sunspot
Numbers during the Eighteenth Century
Authors: Ballester, J. L.; Oliver, R.; Baudin, F.
Bibcode: 1999ApJ...522L.153B
Altcode:
A new record of solar activity, made by compiling the daily number of
sunspot groups visible on the Sun's surface between 1610 and 1995,
has recently been made available by Hoyt & Schatten. Wavelet
analysis of this record shows that an episode of the periodicity near
158 days occurred during the eighteenth century, around the maximum
of solar cycle 2, and that episodes of the periodicity, much weaker
than that in solar cycle 2, have appeared around the maxima of solar
cycles 16-21 (covering the interval 1923-1986). The presence of the
periodicity in the group sunspot number confirms that it is caused by
a periodic emergence of magnetic flux. On the other hand, periodogram
analysis allows one to compare the behavior of the periodicity in
both sunspot groups and sunspot areas, and the results suggest that,
at least during the twentieth century, the periodic emergence of
magnetic flux has adopted two different forms. In solar cycles 16
and 17, new sunspot groups were periodically formed, simultaneously
increasing the number of sunspot groups and the total sunspot area
on the Sun's surface, while during solar cycles 18, 19, 20, and 21
the periodicity has occurred within already formed sunspot groups,
increasing sunspot areas only. We point out that this second type of
emergence, which enhances the magnetic complexity of sunspot groups, is
responsible for the appearance of the periodicity in high-energy solar
flares as detected by the Solar Maximum Mission during solar cycle 21.
Title: Magnetohydrodynamic Waves in Coronal Magnetostatic Arcades
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.
Bibcode: 1999ApJ...517..488T
Altcode:
The solar corona is structured by the magnetic field, which has the
shape of loops and arcades. These structures remain stable from days
to weeks, and there is some evidence, such as sporadic or periodic
brightenings detected in UV and soft X-rays, pointing to the existence
of periodic oscillations or waves in the corona. Recently Oliver
et al. studied, both analytically and numerically, the modes of
oscillation of potential and nonpotential magnetic arcades; here we
investigate the fast mode properties in a fully magnetostatic arcade
(Zweibel & Hundhausen) in which the gravity force is present. The
results show that in this equilibrium the fast and slow modes are
essentially decoupled and that the spatial structure of the perturbed
velocity field is determined by the behavior of the Alfvén speed with
height. An arcade in which the Alfvén speed increases with height is
characterized by vertically evanescent fast mode solutions. These are
the kind of modes that could be more easily detected since their energy
is confined within the solar corona. On the other hand, an equilibrium
in which the Alfvén speed decreases with height has solutions whose
energy tends to escape toward large heights, making difficult the
detection of these modes. Also, it is found that the frequencies of
fast modes in a nonpotential equilibrium are considerably reduced with
respect to the potential case.
Title: Prominence Doppler oscillations.
Authors: Molowny-Horas, R.; Wiehr, E.; Balthasar, H.; Oliver, R.;
Ballester, J. L.
Bibcode: 1999joso.proc..126M
Altcode:
Longitudinal velocity maps have been constructed from time series
of Hβ filtergrams, obtained at three positions along the emission
line profile. Several examples of velocity perturbations with a
periodic behaviour have been found at different spatial locations in
the prominence. Periods have been determined to vary approximately
between 28 and 95 minutes. The presence of damping (and in one case,
of excitation) in those oscillatory motions is discussed.
Title: Electrostatic Interactions Between Electrons and PAH Molecules
Authors: Ballester, J. L.; Tielens, A. G. G. M.
Bibcode: 1998AAS...193.6518B
Altcode: 1998BAAS...30.1342B
We report on our models for the interaction between an electron and a
neutral or charged PAH molecule. The primary electrostatic interaction
is due to the total charge. Near the PAH the spatial extent of the
molecule leads to modifications in the interaction potential. These
modifications can be described in terms of one or more quadrupole
moments. It also possible to formulate constrained model potentials that
are more amenable to calculation. An improved picture of the interaction
potential will be used to calculate electron PAH recombination cross
sections in interstellar environments.
Title: Is there memory in solar activity?
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1998PhRvE..58.5650O
Altcode:
The Hurst effect is a presumed and unexpected behavior of
geoastrophysical time series by which these time series have persistence
or ``memory.'' The application of Hurst analysis to monthly sunspot
numbers [B. B. Mandelbrot and J. R. Wallis, Water Resour. Res. 5,
321 (1969)] yielded a Hurst exponent H=0.86+/-0.05, suggesting that
solar activity shows persistence and that the underlying responsible
mechanism can guarantee a positive correlation of solar activity
during long time lapses, raising, at the same time, the possibility of
the existence of long-term memory in solar activity. More recently,
radiocarbon data have been used for a similar study [A. Ruzmaikin,
J. Feynmann, and P. Robinson, Sol. Phys. 149, 395 (1994)] resulting
in a constant value H=0.84 between 100 and 3000 years, which indicates
persistence of solar activity in such time scales. Furthermore, Mount
Wilson rotation measurements have also been analyzed in the same way
[R. W. Komm, Sol. Phys. 156, 17 (1996)] and the results indicate that
temporal variations of solar rotation on time scales shorter than the
11-year cycle are caused by a stochastic process which is characterized
by persistence. Here, we have followed the scale of fluctuation approach
to show that there is no incontrovertible evidence for the presence
of the Hurst effect in sunspot areas and, therefore, that there is no
proof of the existence of long-term memory in solar activity.
Title: Emergence of magnetic flux on the Sun as the cause of a
158-day periodicity in sunspot areas
Authors: Oliver, R.; Ballester, J. L.; Baudin, F.
Bibcode: 1998Natur.394..552O
Altcode:
The temporal behaviour of solar activity (as manifested in sunspots)
has long been debated. The 11-year periodicity in the total number of
sunspots is well established observationally, as is a periodicity of
152-158 days in the occurrence of high-energy solar flares that was
seen during cycle 21 (refs 1-7). The cause of the latter periodicity
is not clear, although several mechanisms have been proposed. Here
we report a time-frequency analysis, using the wavelet technique, of
sunspot areas between 1874 and 1993, which reveals a 158-day periodicity
coincident with that of energetic solar flares. The signature of this
periodicity is strongest in cycle 19, which was the most intense cycle
of the century. The periodicity disappears after cycle 21. The analysis
shows that the 158-day periodicity in both high-energy solar flares
and sunspots is related to a periodic emergence of magnetic flux which
only appears near the maxima of some solar cycles.
Title: Numerical simulations of impulsively generated MHD waves in
a potential coronal arcade
Authors: Oliver, R.; Murawski, K.; Ballester, J. L.
Bibcode: 1998A&A...330..726O
Altcode:
Impulsively generated waves in coronal arcades are simulated
numerically by an application of nonlinear ideal magnetohydrodynamic
(MHD) equations. The simulations are performed in the (x,z)-plane on
a non-uniform Cartesian mesh. In this geometry the magnetic field can
be expressed in terms of the vector potential. The governing equations,
which are applied in the limit of low plasma-beta , are solved by a flux
corrected transport method. The model excludes the Alfven waves and,
since the slow mode is absent in the cold plasma limit, the excited
disturbances are fast magnetosonic waves. Numerical results show that
for short times after the impulse is launched (i. e., in the linear
regime), only motions normal to the equilibrium magnetic field get
propagated away from the position of the initial displacement and
that any velocity parallel to the unperturbed magnetic field lines
remains essentially unchanged in time. In the nonlinear regime there
is conversion between normal and parallel flow and the two velocity
components propagate from the site of the initial impulse. In addition,
nonlinearities that are built in the MHD equations modify the shape
and speed of the propagating wavefront, an effect that becomes
most noticeable where the wave amplitude is larger. The effect of
nonlinearity on down-going perturbations is to speed up positive
wave amplitudes and to slow down negative wave amplitudes (positive
and negative refers to the sign of the normal velocity component). On
the contrary, up-going positive and negative waves are slowed down and
speeded up, respectively. Impulsively generated waves exhibit temporal
signatures with characteristic time scales of the order of 10 s. Similar
scales have been recently reported in radio observations, microwaves,
and hard X-rays.
Title: A Fibril Structure Model for Stellar Prominences
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1998ASPC..150..247O
Altcode: 1998npsp.conf..247O; 1998IAUCo.167..247O
No abstract at ADS
Title: Infrared Doppler Oscillations in a Solar Filament
Authors: Molowny-Horas, R.; Oliver, R.; Ballester, J. L.; Baudin, F.
Bibcode: 1998ASPC..150..139M
Altcode: 1998IAUCo.167..139M; 1998npsp.conf..139M
No abstract at ADS
Title: The Prominence-Corona Transition Region and the Problem of
Prominence Oscillations
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1998ASPC..150..143O
Altcode: 1998IAUCo.167..143O; 1998npsp.conf..143O
No abstract at ADS
Title: He I 10830 AA Doppler Oscillations in a Solar Filament
Authors: Molowny-Horas, R.; Baudin, F.; Oliver, R.; Ballester, J. L.
Bibcode: 1998ASPC..154..650M
Altcode: 1998csss...10..650M
The results of a high spatial resolution investigation of Doppler
oscillations in a solar filament, using the He 1 10830 AA infrared
spectral line, are presented. Fourier periodograms, and the so-called
wavelet analysis, have been employed to study the existence of periodic
signals. We have focused on the analysis of two conspicuous features
along the slit, showing oscillatory motions with typical periods of
2.7 min and 12.5 min. An estimate of their spatial size gives 2.7
arcsec and 4.75 arcsec, respectively. Their approximate lifetimes,
yielded by the wavelet technique, are also 10 and 20 minutes.
Title: Recombination Rates of Electrons With Interstellar PAH
Molecules
Authors: Ballester, J. L.; Tielens, A. G. G. M.
Bibcode: 1998BAAS...30..766B
Altcode:
We are developing models for the recombination of electrons
with neutral and charged compact PAH molecules in interstellar
environments. Our immediate objective is to determine the relative
importance of simplifying assumptions which are made in the calculation
of electron capture cross-sections from sphere or disk models. Electron
recombination rates calculated for representative interstellar
conditions will lead to a prediction of steady state relative
abundances of PAH charge-states. We will present preliminary results
which highlight the effect of the de-localized PAH cation charge.
Title: Resonant Alfven waves in coronal arcades driven by footpoint
motions
Authors: Ruderman, M. S.; Goossens, M.; Ballester, J. L.; Oliver, R.
Bibcode: 1997A&A...328..361R
Altcode:
X-ray spectroscopy performed from different astronomical spacecrafts
has shown that the solar corona is structured by magnetic fields
having the shape of loops and arcades. These structures are formed
by stretching and reconnection of magnetic fields, and remain stable
from days to weeks. Also, sporadic or periodic brightenings of such
structures have been detected in UV and soft X-ray observations,
suggesting the existence of propagating waves and plasma heating
within them. In this paper, a mechanism for the deposition of Alfven
wave energy and heating of coronal arcades via resonant absorption is
investigated. An analytical solution to the linear viscous, resistive
MHD equations that describes the steady state of resonant shear Alfven
oscillations in coronal arcades driven by toroidal footpoint motions is
obtained. General expressions for the total amount of dissipated wave
energy and for its spatial distribution within the resonant magnetic
surface is derived.
Title: Magnetic arcades in stellar coronae I. Cylindrical geometry
Authors: Čadež, V. M.; Oliver, R.; Ballester, J. L.
Bibcode: 1997Ap&SS.254...67C
Altcode:
X-ray spectroscopy performed by different astronomical spacecrafts has
shown that many active late-type stars possess coronae. For such reason,
the magnetic structure of stellar coronae has raised considerable
interest and, by analogy with the Sun, it is generally assumed that
stellar coronae are structured by magnetic fields having the shape of
arcades. Most of those coronal magnetic field configurations assume
translational symmetry and are based in planar source surfaces. However,
as soon as either the length or the width of such source surfaces become
non negligible as compared to the stellar radius, the application of
the cylindrical geometry seems to be more appropriate. Then, one way
of obtaining coronal magnetic configurations is to deal with source
domains extended over a cylindrical surface. In this paper we generate
potential coronal arcades based on cylindrical source surfaces with
non negligible length or width compared to the stellar radius. The
flux function, the magnetic field components, the shape of magnetic
field lines and other characteristic magnitudes have been obtained
and analyzed for both cases.
Title: Observations of Doppler oscillations in a solar prominence
Authors: Molowny-Horas, Roberto; Oliver, Ramón; Ballester, José Luis;
Baudin, Frédéric
Bibcode: 1997SoPh..172..181M
Altcode: 1997ESPM....8..181M
We report on the observation of Doppler oscillations in a quiescent
limb prominence. Fourier analysis of the data reveals an oscillatory
period of 7.5 min, whose phase varies linearly at 16 consecutive
points (7280 km) along the slit. This yields an upper limit for the
perturbation wavelength of ≈ 20000 km. Wavelet analysis confirms the
above period and indicates an oscillation lifetime of 12 min. Moreover,
the disturbance appears to travel at a speed greater than 4.4 km
s-1. A comparison of these results with the predictions of
some theoretical models is made.
Title: Detection and Characterization of Cold Interstellar Dust and
Polycyclic Aromatic Hydrocarbon Emission, from COBE Observations
Authors: Dwek, E.; Arendt, R. G.; Fixsen, D. J.; Sodroski, T. J.;
Odegard, N.; Weiland, J. L.; Reach, W. T.; Hauser, M. G.; Kelsall,
T.; Moseley, S. H.; Silverberg, R. F.; Shafer, R. A.; Ballester, J.;
Bazell, D.; Isaacman, R.
Bibcode: 1997ApJ...475..565D
Altcode: 1996astro.ph.10198D
Using data obtained by the DIRBE instrument on the COBE spacecraft, we
present the mean 3.5-240 μm spectrum of high-latitude dust. Combined
with a spectrum obtained by the FIRAS instrument, these data represent
the most comprehensive wavelength coverage of dust in the diffuse
interstellar medium, spanning the 3.5-1000 μm wavelength regime. At
wavelengths shorter than ~60 μm the spectrum shows an excess of
emission over that expected from dust heated by the local interstellar
radiation field and radiating at an equilibrium temperature. The DIRBE
data thus extend the observations of this excess, first detected by
the IRAS satellite at 25 and 12 μm, to shorter wavelengths. The
excess emission arises from very small dust particles undergoing
temperature fluctuations. However, the 3.5-4.9 μm intensity ratio
cannot be reproduced by very small silicate or graphite grains. The
DIRBE data strongly suggest that the 3.5-12 μm emission is produced
by carriers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 μm solid
state emission features that have been detected in a wide variety
of astrophysical objects. The carriers of these features have been
widely identified with polycyclic aromatic hydrocarbons (PAHs). Our dust model consists of a mixture of PAH molecules and bare
astronomical silicate and graphite grains with optical properties
given by Draine & Lee. We obtain a very good fit to the DIRBE
spectrum, deriving the size distribution, abundances relative to the
total hydrogen column density, and relative contribution of each dust
component to the observed IR emission. At wavelengths above 140 μm the
model is dominated by emission from T ~ 17-20 K graphite and 15-18 K
silicate grains. The model provides a good fit to the FIRAS spectrum
in the 140-500 μm wavelength regime but leaves an excess Galactic
emission component at 500-1000 μm. The nature of this component is
still unresolved. We find that (C/H) is equal to (7.3 +/- 2.2) ×
10-5 for PAHs and equal to (2.5 +/- 0.8) × 10-4
for graphite grains, requiring about 20% of the cosmic abundance of
carbon to be locked up in PAHs, and about 70% in graphite grains [we
adopt (C/H)⊙ = 3.6 × 10-4]. The model also
requires all of the available magnesium, silicon, and iron to be locked
up in silicates. The power emitted by PAHs is 1.6 × 10-31
W per H atom, by graphite grains 3.0 × 10-31 W per H atom,
and by silicates 1.4 × 10-31 W per H atom, adding up to a
total infrared intensity of 6.0 × 10-31 W per H atom, or
~2 L⊙ M-1solar. The [C II] 158
μm line emission detected by the FIRAS provides important information
on the gas phase abundance of carbon in the diffuse ISM. The 158 μm
line arises predominantly from the cold neutral medium (CNM) and shows
that for typical CNM densities and temperatures C+/H =
(0.5-1.0) × 10-4, which is ~14%-28% of the cosmic carbon
abundance. The remaining carbon abundance in the CNM, which must be
locked up in dust, is about equal to that required to provide the
observed IR emission, consistent with notion that most (>~75%) of
this emission arises from the neutral component of the diffuse ISM. The model provides a good fit to the general interstellar extinction
curve. However, at UV wavelengths it predicts a larger extinction. The
excess extinction may be the result of the UV properties adopted for
the PAHs. If real, the excess UV extinction may be accounted for by
changes in the relative abundances of PAHs and carriers of the 2200
Å extinction bump.
Title: Rescaled Range Analysis of the Asymmetry of Solar Activity
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1996SoPh..169..215O
Altcode:
Previous studies of the north-south asymmetry of solar activity (e.g.,
Carbonell, Oliver, and Ballester, 1993; Oliver and Ballester, 1994)
suggest that the asymmetry time series can be represented by means of
a multicomponent model made up of a long-term trend, a weak sinusoidal
component (with a period close to 12.1 years) and a dominant random
process. Here, we have used the rescaled range analysis to study the
valuation of the stochastic component of the asymmetry. To avoid the
influence of the trend and the sinusoidal component on the result,
we have removed both from the original time series. The value obtained
for the Hurst exponent (0.717 ± 0.002) suggests that the non-periodic
component is a correlated random process.
Title: Propagation of fast MHD perturbations in coronal potential
arcades.
Authors: Cadez, V. M.; Oliver, R.; Ballester, J. L.
Bibcode: 1996A&A...314..636C
Altcode:
We present an analytical approach, using Fourier transformations,
to investigate the phenomenon of wave propagation in a coronal
potential magnetic field. The system is initially at rest and later
set into motion by a photospheric perturber with specified spatial
and temporal properties. The disturbances thus excited at the base of
the arcade are transmitted into the corona by the fast mode, which is
characterised by motions in the direction normal to the unperturbed
magnetic field. Under the assumption of a spatially periodic perturber,
the time-dependent partial differential equation that arises is shown
to be identical to the Klein-Gordon equation. Therefore, the system
is dispersive and modes in the spectrum of the exciter with different
frequencies travel upwards at different speeds. Furthermore, normal
modes with frequencies below the cut-off frequency become evanescent,
being unable to propagate into the corona. The method used results in
the need of computing numerically a semi-infinite integral, which turns
out to be considerably less computer-time consuming than integrating
numerically the fast mode partial differential equation.
Title: Quasiperpendicular wave propagation in coronal magnetostatic
arcades.
Authors: Cadez, V. M.; Oliver, R.; Ballester, J. L.
Bibcode: 1996A&A...307..947C
Altcode:
Applying 2D linear isothermal, perturbations to a magnetostatic
arcade, we have obtained the MHD equations for the induced velocity
field. We have considered the limit of very large wavenumbers in the
direction perpendicular to the equilibrium magnetic field, which gives
rise to what can be described as quasiperpendicular propagation. At
comparatively low frequencies, the MHD wave equations decouple into
two independent ordinary differential equations that can be easily
solved either by numerical means or analytically, using the WKB
method. The solutions are derived for an isothermal basic state when
both the magnetic field intensity and the plasma density have arbitrary
dependences on the flux function ψ. The obtained waves represent the
Alfven and the slow magnetoacoustic modes, both modified by gravity
and by the profile of the magnetic field. On the other hand, the high
frequency domain yields only the fast MHD mode, that propagates along
the tunnel of the arcade. A characteristic feature of these waves is
that they are stable in the presence of a magnetic arcade, contrary
to the case of a purely horizontal magnetic field when the magnetic
buoyancy instability can set in.
Title: The Influence of the Temperature Profile on the
Magnetohydrodynamic Modes of a Prominence-Corona System
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1996ApJ...456..393O
Altcode:
To explain the observational evidence gathered during recent years
about periodic oscillations in quiescent solar prominences the modes of
oscillation of some theoretical models for solar prominences have been
studied. The main drawback of these models is the lack of a realistic
temperature profile for the prominence- corona system, which should be
obtained from the coupling between magnetostatics and energetics once
the physical properties of the prominence and the prominence-corona
transition region (PCTR) are known. However, this seems to be far from
our present possibilities since there is a lack of knowledge about the
physical processes occurring in both. To make further progress
in the study of MHD waves in prominences, we have adopted an "ad hoc"
temperature profile that can be adjusted to give different runs of
the temperature, from prominence to coronal values. This profile
allows us to modify the thickness of the PCTR while modifying the
steepness of the temperature variation within it. Also, by including
this profile in the model proposed by Poland & Anzer, we are able
to construct an equilibrium model for the prominence-corona system
and to study the linear, adiabatic MHD waves of such configuration. Among the results obtained we highlight that the presence of a PCTR
does not eliminate the subdivision of modes into hybrid, external,
and internal and that its existence is linked to the presence of two
temperature plateaus. A change in the thickness of the PCTR produces
a modification of the mode frequency and also affects the horizontal
velocity component of internal modes by diminishing its amplitude
in the prominence region. For a thin PCTR, because of the velocity
amplitude inside the prominence, the modes likely to be detected in
prominence oscillations are the internal and hybrid ones, although
as a consequence of the effect already pointed out, the existence of
a thick PCTR could make difficult or even impossible the detection of
internal modes. In summary, our results point out the importance of
the PCTR to the oscillations of quiescent solar prominences and to the
identification of modes through the amplitudes of the eigenfunctions in
the prominence. This indicates the strong need for accurate knowledge
of the physical properties of this region, in order to be able to make
accurate theoretical predictions about the amplitudes and frequencies
of oscillations in quiescent prominences. Probably, that knowledge
can be obtained in the near future by means of the UV instruments of
the SOHO spacecraft.
Title: Resonant absorption of MHD surface waves in an arcade with
a continuous boundary.
Authors: Cadez, V. M.; Ballester, J. L.
Bibcode: 1996A&A...305..977C
Altcode:
We consider an isolated potential magnetic arcade with a thick boundary
forming a layer with strongly nonpotential field, that separates the
interior of the arcade from the ambient nonmagnetized plasma of the
solar corona. It is shown that, in this case, local surface waves,
having a relatively large wavelength compared to the layer thickness,
can resonantly generate a continuous MHD slow mode spectrum within the
layer. The obtained rate coefficient depends on the value of the field
gradient at the place where the resonance occurs and it vanishes when
the gradient tends to infinity.
Title: Oscillations in a Quiescent Solar Prominence
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1996mpsa.conf..463O
Altcode: 1996IAUCo.153..463O
No abstract at ADS
Title: Coronal MHD Perturbation Field Generated by Localized
Perturbers in a Photospheric Active Region
Authors: Čadež, V. M.; Oliver, R.; Ballester, J. L.
Bibcode: 1995SoPh..159..229C
Altcode:
We investigate the two-dimensional boundary value problem of the linear
wave excitation in the solar corona by a pair of periodic perturbers
localized at the photosphere. The physical properties of the corona
allow us to consider a magnetic configuration such as a potential arcade
which is in a magneto-hydrostatic equilibrium with the surrounding
plasma. The model excludes the A1fvén mode and since the slow mode
is absent in a potential arcade, the excited waves are then the fast
magneto-acoustic modes. The characteristic magnetic field scale length
is twice the scale height of the coronal plasma, assumed isothermal
and the induced fluid motions are in the direction perpendicular to
the magnetic field, in the cross-sectional plane of the arcade.
Title: Magnetohydrodynamic Waves in a Bounded Inhomogeneous Medium
with Prominence-Corona Properties
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1995ApJ...448..444O
Altcode:
Short- and long-period oscillations in quiescent solar prominences
have been abundantly reported during recent years. In this paper,
we investigate the magnetoacoustic-gravity modes of vibration of an
inhomogeneous medium in which the temperature and density vary smoothly
from prominence to coronal values. The differential equations of slow,
fast, and Alfvén modes have been solved numerically, and the main
properties of these modes have been studied. The perturbations
to the plasma pressure, magnetic pressure, magnetic tension, and
gravitational forces have been computed because these forces are
responsible for the different nature of fast and slow MHD modes. It
has been found that motions produced by slow modes are driven by the
horizontal component of the pressure gradient, while the main driver
of fast modes is the magnetic tension which is practically vertical in
most, but not all, cases. As for the association of different modes
with different polarizations of the velocity of oscillation, we have
found that slow and Alfvén modes present a dominant component in
the horizontal plane, in a direction transverse to and along the
prominence axis, respectively. However, and contrary to what was
found in previous works, some fast modes produce both vertical and
horizontal motions. When the Doppler shift of a spectral line is used,
this implies that all three MHD modes could be detected in a limb
prominence and that only the fast mode could be detected in a filament
located in the disk center. No evidence for hybrid, external, and
internal modes has been found, which suggests that this subdivision,
previously reported in Oliver et al. (1993) and Joarder & Roberts
(1992b), disappears when the temperature suffers a smooth transition
from prominence to coronal conditions.
Title: MHD waves in coronal arcades.
Authors: Čadež, V. M.; Ballester, J. L.; Oliver, R.
Bibcode: 1995POBeo..49..109C
Altcode:
The MHD wave behavior in the solar corona with magnetic field having
the shape of arcades is investigated. It is shown that a particular
analytical solution to the linearized MHD equations can be obtained for
perturbations with short wavelengths in the direction of the arcade
tunnel. Two possibilities are considered regarding the related wave
frequency: the high frequency domain yields MHD waves propagating along
the tunnel of the arcade as a fast MHD mode while the low frequencies
produce two decoupled wave modes representing the Alfvén and the slow
magnetoacoustic wave, both modified by the gravity and the profile of
the magnetic field. All these wave are stable, contrary to the case
when the magnetic field is purely horizontal and when the magnetic
buoyancy instabilities can set in.
Title: MHD disturbances in a coronal potential arcade generated by
localized perturbers.
Authors: Cadez, V. M.; Ballester, J. L.
Bibcode: 1995A&A...296..537C
Altcode:
We present an analytical approach which combines Fourier and Laplace
transformations to solve the two dimensional boundary value problem of
the linear wave excitation in the solar corona by a distinct periodic
perturber localized at an arbitrary surface. Due to the physical
properties of the corona, we consider a magnetic configuration such as
a potential arcade. Depending on the shape of the coronal potential
magnetic arcade, the surface on which the perturber is located
can be the photosphere itself, the magnetic field surface or any
other surface connecting two foot-lines of the arcade which is in a
magneto-hydrostatic equilibrium with the surrounding plasma. We do not
consider Alfven waves and since the slow mode is absent in a potential
arcade, the excited waves are, then, the fast magneto-acoustic modes
causing fluid motions in the direction perpendicular to the magnetic
field in the cross sectional plane of the arcade. We give a particular
example wich deals with a magnetic arcade whose characteristic magnetic
field scale length is twice the isothermal scale height of the coronal
plasma. The boundary conditions are imposed at the photosphere, in the
form of periodic perturbers with various strength distributions along
the horizontal direction.The results show a feature of wave channeling
which certainly is an effect of interest in coronal heating mechanisms.
Title: Time evolution of MHD disturbances impulsively excited by a
localized perturber in a potential coronal arcade.
Authors: Cadez, V. M.; Ballester, J. L.
Bibcode: 1995A&A...296..550C
Altcode:
An analytical approach to the initial value problem of MHD wave
excitation in the solar corona is presented. We use the combined
Fourier and Laplace transformations to solve the two-dimensional
initial value problem of linear wave excitation. By this procedure,
we have been able to obtain the solution for the time dependent
two dimensional perturbation velocity field above the solar
photosphere. Due to the physical properties of the corona, the low
beta approximation has been applied to the considered processes. The
perturber is initially located at a given surface in a particularly
chosen frame of reference. Depending on the shape of the coronal
potential magnetic arcade, this surface can be the photosphere itself,
a magnetic field surface, or any other inside a magnetic arcade in
hydrostatic equilibrium with the surrounding plasma. The considered
excited perturbations are the fast magneto-acoustic waves which cause
fluid motions, in the normal direction to the magnetic field, in the
cross sectional plane of the arcade. We present an example of a given
perturber located at the photosphere, showing the induced velocity
field time evolution.
Title: Short-term periodicities in sunspot areas during solar cycle 22
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1995SoPh..156..145O
Altcode:
We have analyzed the daily record of sunspot areas during the current
cycle 22 looking for the short-term periodicity of around 155 days which
was present during some previous solar cycles. Two different methods
have been used to compute the power spectra and the results indicate
that such periodicity has been absent during the current solar cycle,
which confirms the results obtained by other authors who used flares
or flare-related data.
Title: Activities in prominences prior to eruption
Authors: Ballester, J. L.
Bibcode: 1994ESASP.373...13B
Altcode: 1994soho....3...13B
No abstract at ADS
Title: Local waves in a magnetic arcade embedded in the solar corona.
Authors: Cadez, V. M.; Ballester, J. L.
Bibcode: 1994A&A...292..669C
Altcode:
We consider a particular type of a magnetic non potential 2D
arcade which is in static equilibrium with the ambient, isothermal
atmosphere. By applying linear 2D perturbations to it, we obtain the
relevant MHD equations that describe the resulting wave velocity field,
which can be reduced to a set of two coupled second order differential
equations for the velocity components parallel and normal to the
magnetic surfaces. Those equations have been solved analytically
in local approximation and under the assumption of constant Alfven
speed. The results show the existence of a surface wave, in addition
to the propagating waves (the fast and the slow mode), when a single
boundary is present. The slow mode cannot escape the arcade while the
fast mode can leave it provided the speed of sound is not negligible
if compared with the Alfven speed. In the case of two boundaries,
i.e. for a magnetic arch, the obtained wave behaviour resembles that
for a horizontal slab.
Title: A search for chaotic behaviour in solar activity.
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 1994A&A...290..983C
Altcode:
We present an analysis of the behaviour of solar activity and look
for the presence of low-dimensional deterministic chaos within it. The
original databases for such analysis have been the daily sunspot number
(1818-1990) and daily sunspot areas (1874-1989) from which we have
constructed twenty different data sets, raw and filtered, displaying
the solar and magnetic cycle. We have used the Grassberger-Procaccia
algorithm to compute the correlation dimension which, also, has allowed
us to obtain the K_2_ entropy and, for some time series, the maximum
Lyapunov exponent has also been computed. Our results show that in none
of the twenty time series considered does evidence appear of chaotic
behaviour, since there is no saturation of the correlation dimension
with the embedding dimension and the K_2_ entropy shows a divergent
behaviour. A study of previous works which claim this kind of behaviour
to be present in solar activity suggests that such a conclusion has
been derived from very short scaling regions obtained using low time
delays in the computations of the correlation dimension. The behaviour
of solar cycle, with periods of low activity, suggests the presence
of deterministic chaos and some of its features can be reproduced by
means of simple nonlinear dynamo models. However, it seems that for
an unambiguous detection of such behaviour, from solar activity data
or proxy records, we will have to wait for the availability of longer
and reliable data sets covering the periods of reduced activity.
Title: The North-South Asymmetry of Sunspot Areas during
SOLAR-CYCLE-22
Authors: Oliver, R.; Ballester, J. L.
Bibcode: 1994SoPh..152..481O
Altcode:
We have analyzed the asymmetry of sunspot areas during the current solar
cycle 22, finding that it has been statistically significant and that
the shape of the underlying trend within the full asymmetry time series
(1874-1993) indicates that the dominance of solar activity has started
to shift, during the current cycle, from the northern hemisphere to
the southern one.
Title: The shape of a magnetic arcade embedded in the solar corona
Authors: Cadez, V. M.; Oliver, R.; Ballester, J. L.
Bibcode: 1994A&A...282..934C
Altcode:
We have generated two-dimensional magnetic arcades, with invariance
along the longitudinal axis, by solving, with invariance along the
longitudinal axis, by solving, together, the Grad-Sharanov equation
between the arcade and the corona. We have used a particular class of
magnetic fields in which the slope of the magnetic field lines only
depends on coordinate and, with this approach, we have been able to
obtain general expressions for the magnetic field components, which
only depend on the gas pressure profile, without the need to know the
analytical expression for the flux function. These expressions for
the magnetic field components have allowed us to generate potential
and non-potential fields and to obtain some conclusions about the
half-width, height and magnetic field flux of the different arcades.
Title: The asymmetric behaviour of solar activity
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 1994LNP...432...85C
Altcode: 1994LNPM...11...85C
We present a thorough study of the North-South asymmetry of solar
activity made with the daily sunspot areas since they are good
indicators of magnetic activity. To perform the study, we have
constructed an asymmetry time series and analyzed it by different
methods. First of all, its significance and statistical behaviour
have been studied. Secondly, using a Monte Carlo simulation we have
generated synthetic asymmetries and compared them to the real one
and, thirdly, we have used modern techniques, developed to study the
chaotic behaviour of time series, to search for low-dimensional chaos
in the asymmetry time series. Our results show that in most cases the
north-south asymmetry is statistically highly significant and that it
cannot be obtained from a distribution of sunspots areas generated, in
a random way, from a binomial or uniform distribution of probability
between hemispheres. Moreover, the real asymmetry time series can be
represented by a means of a multi-component model made up of a long
term trend, a sinusoidal component with a period of 12.1 yr and a
dominant purely random component; while chaotic analysis does not
reveal the existence of a strange attractor in the time series.
Title: On the asymmetry of solar activity
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 1993A&A...274..497C
Altcode:
Although the existence of a north-south asymmetry in solar activity is
generally accepted, a detailed study of its significance and behaviour
is still missing. In this paper, we present a thorough study of the
north-south asymmetry of solar activity made with the daily sunspot
areas, since they are a good indicator of magnetic activity. To
perform the study, we have constructed an asymmetry time series and
analysed it by different methods. First of all, its significance and
statistical behaviour has been studied. Secondly, using a Montecarlo
simulation we have generated synthetic asymmetries and compared them to
the real one by means of an χ2-test of goodness of fit, and
thirdly, we have used modern techniques, developed to study the chaotic
behaviour of time series, to search for low-dimensional deterministic
chaos in the asymmetry time series. Our results show that in most
cases the north-south asymmetry is statistically highly significant,
i.e. it is a real feature of sunspot areas distribution, and that it
cannot be obtained from a distribution of sunspot areas generated, in
a random way, from a binomial or uniform distribution of probability
between hemispheres; moreover, synthetic asymmetries obtained from the
Morfill et al. (1991) model, which seems to reproduce realistically the
sunspot cycle, do not agree with the real one and the real asymmetry
time series can be represented by means of a multicomponent model made
up of a long-term trend, a sinusoidal component with a period of 12.1
yr and a dominant purely random component. Also, the chaotic analysis
does not reveal the existence of a strange attractor in the time series
and its multifractal properties indicate that intermittency or bursts
of activity are present in it.
Title: Oscillations of a Quiescent Solar Prominence Embedded in a
Hot Corona
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
Bibcode: 1993ApJ...409..809O
Altcode:
The magneto-acoustic-gravity modes of vibration of a
Kippenhahn-Schlueter prominence model, surrounded by an external medium
representing the solar corona, are investigated. Modes which do not
disappear when the coronal region is removed are called internal modes,
while modes which do not disappear when the prominence is removed are
called external modes. Three modes were found which could be labeled
as either internal or external modes, since they do not disappear
when either region is removed. The results of this study suggest that
short-period oscillations in limb prominences may be caused by the
fundamental and first harmonics of the internal magneto-acoustic-gravity
slow and Alfven modes, while the reported long-period oscillations in
limb prominences may be the result of the hybrid slow mode. Short-period
oscillations lasting a few minutes and detected in filaments may be
related to internal fast modes, while the hybrid fast mode causes
vertical oscillations with periods of approximately 20 min.
Title: Magnetohydrodynamic waves in a potential coronal arcade
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
Bibcode: 1993A&A...273..647O
Altcode:
An important result obtained from solar observations made in coronal
emission lines has been to establish the existence of periodic
oscillations or waves in the corona. These are detected, although
not always simultaneously, in line intensity, line-width or Doppler
velocity with oscillation periods of several minutes. For this reason,
we have studied the magnetohydrodynamic modes of oscillation of a
coronal potential arcade under different boundary conditions, solving
the wave equations for different density profiles. Our results show
that only the Alfvén and the fast mode exist in such a structure
and that the slow mode is absent, because of the assumed low-β
limit. The Alfvén mode produces a continuous spectrum of frequencies
with periods which can vary from zero to infinity, depending on the
assumed density profile. Due to the polarization direction of the plasma
motions, these waves could in principle be detected in front-view
observations of coronal arcades at the limb. Concerning the fast
mode, we have obtained several diagnostic diagrams using different
density profiles and boundary conditions. These diagrams would be
useful for comparing the predicted and observed periods. However,
the observational identification of fast modes could be harder since
we would need lateral observations of the coronal arcades.
Title: MHD Waves in a Solar Prominence
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
Bibcode: 1993ASSL..183..191O
Altcode: 1993pssc.symp..191O
No abstract at ADS
Title: Asymmetry and periodicities in solar activity.
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 1993ASPC...40...84C
Altcode: 1993IAUCo.137...84C; 1993ist..proc...84C
The historical record of daily sunspot areas (1874-1989) has been
analysed, looking for the short (155 d) and intermediate (323 and 540 d)
term periodicities. Also the North-South asymmetry during those years
has been studied.
Title: Quiescent Solar Prominences: A Two-Dimensional Model
Authors: Oliver, R.; Ballester, J. L.; Priest, E. R.
Bibcode: 1993ASSL..183..187O
Altcode: 1993pssc.symp..187O
No abstract at ADS
Title: Magnetohydrodynamic Waves in a Solar Prominence
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
Bibcode: 1992ApJ...400..369O
Altcode:
The presence of oscillations in solar prominences has been known for
a long time, and at first they were thought to be initiated by shock
waves emitted by a flare. However, short- and long-period oscillations,
not related to flares, have been abundantly reported during recent
years. In limb prominences they have been detected mainly in the
velocity field, whereas in filaments they are hardly detected at
all. In this paper we try to provide a theoretical explanation for
these oscillations by investigating the magneto-acoustic-gravity modes
of vibration of a Kippenhahn-Schlueter prominence model. First of
all, we obtain the magneto-acoustic modes for a very thin prominence
with horizontal magnetic field, and later we solve the full problem
numerically. Our results suggest that short-period oscillations detected
in limb prominences could be due to the fundamental and first harmonic
of the magneto-acoustic-gravity slow modes of the prominence itself,
while the long-period oscillations could be due to global oscillations
of the whole surrounding arcade.
Title: Power spectra of gapped time series - A comparison of several
methods
Authors: Carbonell, M.; Oliver, R.; Ballester, J. L.
Bibcode: 1992A&A...264..350C
Altcode:
Gapped time series appear frequently in astronomy due to many different
causes. Our aim in this paper has been to test four different methods
using synthetic and real signals in which we have generated gaps of
different types and sizes. This procedure has allowed us to compare
the recovered spectrum with the real one and to draw conclusions about
the more efficient method.
Title: The Equilibrium Shape of Slender Flux Tubes in a Linear
Force-Free Magnetic Field
Authors: Juan, J.; Ballester, J. L.
Bibcode: 1992SoPh..137..257J
Altcode:
In this paper we extend previous work of Browning and Priest (1984,
1986) by studying the equilibrium path of twisted and untwisted
thin flux tubes in a stratified, isothermal atmosphere using as the
ambient field a linear force-free field. When an untwisted flux tube
is considered, we find that shearing the magnetic arcade provides
a different form to change the parameter λ which characterizes the
external atmosphere, but at the same time this introduces a limitation
in the width allowed for the external arcade. Also, the critical width
found for the different analytical cases considered is always greater
than one arch of the ambient arcade which prevents an eruption inside
the arcade. In the case of twisted flux tubes, an analytical solution
can be found for the critical λc, which separates regimes
of strong and weak gravity, and the shape of the flux tube is now
dependent on β, a parameter which represents the magnetic field
enhancement of the loop at the photosphere.
Title: The periodic behaviour of solar activity - The near 155-day
periodicity in sunspot areas
Authors: Carbonell, M.; Ballester, J. L.
Bibcode: 1992A&A...255..350C
Altcode:
The historical record of daily sunspot areas (1878-1982), covering
cycles 12 to 21, has been analyzed, looking for the periodicity
around 155 d found in other indicators of solar activity, mainly
solar flares. The results indicate that a periodicity between 150-160d
(77-72 nHz) seems to be significant during solar cycles 16-21, while it
cannot be detected in solar cycles 12 to 15. However, a sliding-window
analysis reveals some time intervals within each of most of the cycles
considered, in which a strong periodicity appears at 155.6 d. This
feature suggests an intermittent character. Also, some evidence is
found suggesting that the periodicity appears or is more important
in the hemisphere where sunspot areas have been dominant during the
solar cycle.
Title: The Equilibrium of Coronal Flux Tubes Under Toroidal Forces
Authors: Juan, J.; Ballester, J. L.
Bibcode: 1992Ap&SS.188..279J
Altcode:
The properties of slender isolated flux tubes, taking into account
curvature effects, were investigated by Parker (1975, 1979) and Spruit
(1981), and many studies have been made concerning the equilibrium of
slender flux tubes in the solar corona. In this paper we use a different
approach considering the coronal loop as a part of a circular torus
and studying the position of its top when the loop is in equilibrium
under toroidal forces. Toroidal forces were considered by Shafranov
(1966) for toroidal pinches and the equilibrium can be studied for
different values of the toroidal current intensity and external magnetic
field. The results show that it is possible to have a coronal flux
tube in equilibrium without considering gravity and external magnetic
field. Furthermore, the total twist of the flux tube and its variation
with the toroidal intensity has been studied.
Title: Intermediate-Term Periodicities in Solar Activity
Authors: Oliver, R.; Carbonell, M.; Ballester, J. L.
Bibcode: 1992SoPh..137..141O
Altcode:
The presence of intermediate-term periodicities in solar activity,
at approximately 323 and 540 days, has been claimed by different
authors. In this paper, we have performed a search for them in the
historical records of two main indices of solar activity, namely,
the daily sunspot areas (cycles 12-21) and the daily Zürich sunspot
number (cycles 6-21). Two different methods to compute power spectra
have been used, one of them being especially appropriate to deal with
gapped time series. The results obtained for the periodicity near
323 days indicate that it has only been present in cycle 21, while in
previous cycles no significant evidence for it has been found. On the
other hand, a significant periodicity at 350 days is found in sunspot
areas and Zürich sunspot number during cycles 12-21 considered all
together, also having been detected in some individual cycles. However,
this last periodicity must be looked into with care due to the lack of
confirmation for it coming from other features of solar activity. The
periodicity around 540 days is found in cycles 12, 14, and 17 in sunspot
areas, while during cycles 18 and 19 it is present, with a very high
significance, in sunspot areas and Zürich sunspot number. It also
appears at 528 days in sunspot areas during cycles 12-21. On the other
hand, it is important to note the coincidence between the asymmetry,
favouring the northern hemisphere, of sunspot areas and solar flares
during cycle 19, and the fact that the periodicity at 540 days was
only present, with high significance, in that hemisphere during that
solar cycle.
Title: Search Algorithm for Weak Periodicities in Definite Time
Intervals
Authors: Carbonell, M.; Ballester, J. L.
Bibcode: 1991A&A...249..295C
Altcode:
An algorithm allowing the computation of the Lomb-Scargle periodogram
in any predefined frequency interval with any desired resolution is
presented. When the search of periodicities is restricted to definite
time intervals, a significant saving of CPU time is obtained.
Title: A Two-Dimensional Model for a Solar Prominence - Effect of
an External Magnetic Field
Authors: Oliver, R.; Ballester, J. L.; Priest, E. R.
Bibcode: 1991SoPh..134..123O
Altcode:
Using analytical approximations we study the effects of different
external magnetic configurations on the half-width, mass, and internal
magnetic structure of a quiescent solar prominence, modelled as a thin
vertical sheet of cool plasma. Firstly, we build up a zeroth-order
model and analyse the effects produced by a potential coronal field
or a constant-α force-free field. This model allows us to obtain
the half-width and mass of the prominence for different values of
the external field, pressure and shear angle. Secondly, the effects
of these external magnetic configurations on a two-dimensional model
proposed by Ballester and Priest (1987) are studied. The main effects
are a change of the half-width with height, an increase of the mass,
a decrease of the magnetic field strength with height and a change in
the shape of the magnetic field lines.
Title: A short-term periodicity near 155 day in sunspot areas
Authors: Carbonell, M.; Ballester, J. L.
Bibcode: 1990A&A...238..377C
Altcode:
Evidence is presented of a nearby 155-day periodicity in the historical
record of sunspot areas from 1904 to 1976, rotation by rotation,
covering cycles 14 to 20. This suggests that earlier periodicities
around 155 days found in other indicators of solar activity during the
last three cycles could be connected to the one presented here. Due
to the fact that the amount of emerging magnetic flux is directly
proportional to sunspot areas, this result also suggests the existence
of a periodicity in the emergence of magnetic flux through the solar
photosphere. Another peak of about 510-540 days, pointed out in solar
flares by several authors, is also present in these results; however,
it is not significant at all, because it remains below the level of the
'null continuum'.
Title: Prominence models with line currents - Stabilisation by
flux conservation
Authors: Anzer, U.; Ballester, J. L.
Bibcode: 1990A&A...238..365A
Altcode:
A model of prominence eruption, based on a filament with circular cross
section as different from the filament of a single line current used in
earlier models, is presented. For configurations with normal and inverse
polarity, the model is considered to be stable with respect to the
vertical displacements. This implies that simple configurations cannot
model prominence eruptions unless the effect of flux conservation is
also included. It is found that if the magnetic flux below a finite size
filament is conserved, a run-away instability no longer exists. The
results apply to a limited class of 2-D equilibria containing a
cylindrical filament and subjected to strictly 2-D perturbation. Models
that are more realistic can be unstable.
Title: Atoms in Carbon Cages as a Source of Interstellar Diffuse Lines
Authors: Ballester, J. L.; Antoniewicz, P. R.; Smoluchowski, R.
Bibcode: 1990ApJ...356..507B
Altcode:
A model to describe the resonance absorption lines of various atoms
trapped in closed carbon cages is presented. These systems may be
responsible for some of the as yet unexplained diffuse interstellar
bands. Model potentials for possible atom-C60 systems are obtained and
used to calculate the resonance lines. The trapped atoms considered are
O, N, Si, Mg, Al, Na, and S, and in all cases the resonance lines are
shifted toward the red as compared to the isolated atoms. The calculated
wavelengths are compared to the range of wavelengths observed for the
diffuse interstellar bands, and good agreement is found for Mg and
Si resonance lines. Other lines may be caused by other than resonance
transitions or by trapped molecules. The oscillator strengths and the
abundances are evaluated and compared with observation. Mechanisms
to explain the observed band width of the lines and the existence of
certain correlated pairs of lines are discussed.
Title: The north-south asymmetry of sunspots
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1990A&A...229..540V
Altcode:
An exhaustive study has been performed of the N-S asymmetry of sunspot
areas during the period 1874-1976. The asymmetry of the yearly values
is statistically significant; thus, the asymmetry is not due to random
fluctuations. The highest values of the asymmetry coefficient are
obtained around solar minimum. The change in the slope of the regression
lines fitted to the yearly values of the asymmetry every four cycles
in the cycles 12 to 21 suggests a long-term periodic behavior in the
asymmetry around eight cycles in which the activity in one hemisphere
is more important during the ascending branch of the cycle and becomes
more important in the opposite hemisphere during the descending branch.
Title: A Short Term Periodicity near 155 Day in Sunspot Areas
Authors: Carbonell, M.; Ballester, J. L.
Bibcode: 1990PDHO....7..100C
Altcode: 1990dysu.conf..100C; 1990ESPM....6..100C
The authors present evidence of a nearby 155 days periodicity in
the historical record of sunspot areas from 1904 to 1976, rotation by
rotation, covering cycles 14 to 20. Another peak around 510 - 540 days,
pointed out in solar flares by several authors, seems also present.
Title: Fibril Structure of Solar Prominences
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1990LNP...363..241B
Altcode: 1990doqp.coll..241B; 1990IAUCo.117..241B
No abstract at ADS
Title: Effect of an External Magnetic Field on Prominence Properties
Authors: Oliver, R.; Ballester, J. L.; Priest, E. R.
Bibcode: 1990PDHO....7..170O
Altcode: 1990dysu.conf..170O; 1990ESPM....6..170O
No abstract at ADS
Title: Fibril structure of solar prominences
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1990GMS....58..321B
Altcode:
While previous modelings of solar active-region prominences have taken
the form of single, cool loops, observations suggest a more accurate
interpretation in terms of many loops of plasma that are inclined to
the filament. An effort is presently made to follow those suggestions
in the construction of a model for the fibrillar structure of solar
prominences in terms of slender flux tubes reproducing the observed
parameters of both quiescent and active region prominences. The model
structure encompasses a hot component and a depressed cool component;
the most realistic results include the effect of an external magnetic
field in the corona.
Title: Prominence Models with Line Currents: Stabilization by Flux
Conservation
Authors: Anzer, U.; Ballester, J. L.
Bibcode: 1990PDHO....7..156A
Altcode: 1990ESPM....6..156A; 1990dysu.conf..156A
No abstract at ADS
Title: Model for the fibril structure of solar prominences
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1989A&A...225..213B
Altcode:
Limb observations of quiescent solar prominences have revealed them to
be composed of many fine structures. Also observations in H-alpha and
UV lines suggest that quiescent filaments are made up of many clusters
of small-scale loops at different temperatures inclined to the filament
axis, with the C IV structures more extended than the H-alpha ones. In
H-alpha, the dimensions of these structures are about 7000 km long
and 1000 km thick, and they evolve over a typical time scale of about
8 min. The aim has been to take into account such observations and to
construct a model for the fibril structure of solar prominences in terms
of slender magnetic flux tubes, in which the main observed parameters
of both quiescent and active-region prominences are reproduced.
Title: Periodicities in the North-South Asymmetry of Solar Activity
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1989SoPh..119..411V
Altcode:
This paper tries to cast additional evidence on the proposed periodic
behaviour of the N-S asymmetry in sudden disappearances (SD) of
solar prominences (Vizoso and Ballester, 1987). We have performed a
Blackman-Tukey power spectrum of the values of the SD N-S asymmetry
and the results shows a significant peak, above 95% confidence level,
at 12.4 years, another peak at 2.3 years fails to be statistically
significant. Moreover, power spectrum performed with the values of
N-S asymmetry of flare number and flare index (Vizoso and Ballester
(1987) display significant peaks, above 95% confidence level, around
3.1-3.2 years.
Title: A 2D prominence model.
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1988dssp.conf..137B
Altcode:
A two-dimensional magnetohydrostatic model of a vertical prominence
sheet is set up by allowing slow variations of the magnetic field and
plasma properties with height. The width of the prominence is found
to decrase with height and in many cases the field lines become less
curved, while the strength of the horizontal magnetic field increases
with height, in agreement with some observations.
Title: Dynamics and structure of solar prominences. Proceedings of
the workshop held November 18 - 20, 1987, at the Universitat de les
Illes Balears, Palma de Mallorca, Spain
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1988dssp.conf.....B
Altcode:
No abstract at ADS
Title: Periodic behaviour in sudden disappearances of solar
prominences.
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1988dssp.conf..167V
Altcode:
Looking for periodicities in sudden disappearances of solar prominences,
the authors have performed FFT of equatorial SD during cycles 19
and 20, both together and one by one. The results suggest that some
periodicities could be present during these solar cycles.
Title: North-south asymmetry in sudden disappearances of solar
prominences.
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1988dssp.conf..171V
Altcode:
The authors present the results of a study of the N-S asymmetry
in SD of solar prominences during solar cycles 18 - 21. The N-S SD
asymmetry curve is not in phase with the solar cycle and peaks about
the time of solar minimum, the asymmetry reverses in sign during the
solar maximum. This change of sign is coincident with the reversal of
the Sun's magnetic dipole. The SD asymmetry curve can be fitted by a
sinusoidal function with a period of eleven years.
Title: A Two-Dimensional Model for a Solar Prominence
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1987SoPh..109..335B
Altcode:
A two-dimensional magnetohydrostatic model of a vertical prominence
sheet is set up by allowing slow variations of the magnetic field and
plasma properties with height. The width of the prominence is found
to decrease with height and in many cases the field lines become
less curved, while the strength of the horizontal magnetic field
increases with height, in agreement with observations. Since we are
only considering a local analysis, the model applies to a general
prominence sheet, whether of Kippenhahn-Schlüter or Kuperus-Raadu
type. The challenge in the future is to understand the detailed
fine-scale microstructure which takes place in the mould formed by
the present global macro-models.
Title: North-South Asymmetry in Sudden Disappearances of Solar
Prominences
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1987SoPh..112..317V
Altcode:
This paper presents the results of a study of the N-S asymmetry in
sudden disappearances (SD) of solar prominences during solar cycles
18-21, obtained as a part of a more extensive research on SD and
reappearances during years 1931-1985 (Ballester, 1984). As can be seen,
the N-S SD asymmetry curve is not in phase with the solar cycle and
peaks about the time of solar minimum, the asymmetry reverses in sign
during the solar maximum, being, this change of sign, coincident with
the reversal of the Sun's magnetic dipole. The SD asymmetry curve can
be fitted by a sinusoidal function with a period of eleven years. On
the other hand, the SD asymmetry curve shows a strong coincidence with
the N-S asymmetries presented by other solar activity manifestations
as studied by different authors.
Title: North-South Asymmetry in Sudden Disappearances of Solar
Prominences
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1987dssp.work..172V
Altcode: 1987ASSL..150..172V
No abstract at ADS
Title: A Two-Dimensional Prominence Model
Authors: Ballester, J. L.; Priest, E. R.
Bibcode: 1987dssp.work..137B
Altcode: 1987ASSL..150..137B
No abstract at ADS
Title: Periodic Behaviour in Sudden Disappearances of Solar
Prominences
Authors: Vizoso, G.; Ballester, J. L.
Bibcode: 1987dssp.work..167V
Altcode: 1987ASSL..150..167V
No abstract at ADS
Title: Diffuse Interstellar Bands and Carbon Shells
Authors: Smoluchowski, R.; Antoniewicz, P. R.; Ballester, J. L.
Bibcode: 1986BAAS...18.1008S
Altcode:
No abstract at ADS
Title: Uvby-beta photometry of active-chromosphere binaries. I. The
system TZ Coronae Borealis.
Authors: Gimenez, A.; Ballester, J. L.; Reglero, V.;
Fernandez-Figueroa, M. J.; de Castro, E.
Bibcode: 1986AJ.....92..131G
Altcode:
Simultaneous uvby and H-beta photometry of thee noneclipsing
double-lined spectroscopic binary TZ CrB, an active binary system
with almost unevolved components, is presented. A small amplitude
distortion wave (0.012 mag in y) has been found with maximum light
at phase about 0.75. No variations in color or the beta index during
the orbital cycle have been detected within the precision of the
observations. The resulting color indices in the standard uvby system
allow the estimation of some basic stellar parameters, such as the
average effective temperature about 6000 K and the stellar radii of
the component stars about 1.1 solar radii. These results, together
with available spectroscopic data, permit a consistent picture for
this interesting binary to be obtained.
Title: Stromgren uvby Photometry of the RS CVn-like Binaries sigma
CrB and HD 166181
Authors: Gimenez, A.; Reglero, V.; Ballester, J. L.;
Fernandez-Figueroa, M. J.; de Castro, E.
Bibcode: 1985IBVS.2797....1G
Altcode:
No abstract at ADS
Title: Photoelectric Observations of the Flare Star AD Leo
Authors: Reglero, V.; Fuensalida, J. J.; Arevalo, M. J.; Ballester,
J. L.
Bibcode: 1985IBVS.2752....1R
Altcode:
No abstract at ADS
Title: About sudden disappearances and reappearances.
Authors: Ballester, J. L.
Bibcode: 1984ESASP.220..275B
Altcode: 1984ESPM....4..275B
An extensive study (1931 - 1981) of sudden disappearances (SD) and
reappearances, and their relationships with active centers, maximum and
minimum of solar cycle, latitude, coronal holes, flares, emerging flux
and age of neutral lines is now in progress at the University of Palma
de Mallorca, trying to obtain a picture of the external causes that
might be involved in the production of SD and the circumstances that
might affect reappearances. In this paper, some preliminary results
are presented.
Title: A Note on Magnetic Fields and Electric Currents in Solar
Prominences
Authors: Ballester, J. L.
Bibcode: 1984SoPh...94..151B
Altcode:
We determine magnetic fields and electric currents in three prominences
without simplifications used in our previous papers.
Title: Prominence Motions and Their Implications for Magnetic Fields
Authors: Ballester, J. L.; Kleczek, J.
Bibcode: 1984SoPh...90...37B
Altcode:
Using results obtained in our earlier paper (Ballester and Kleczek,
1983) and the equipartition principle we attempt to calculate the
lower limits of magnetic fields in three solar prominences. The values
are then compared with the magnetic fields found by experimental
methods. Furthermore, we have calculated by Ampère's law the lower
limits of electric currents inside the conical surface where the
knot's motion is located. The results obtained are compared with a few
determinations of electric currents in prominences, that can be found
in the bibliography. An attempt is made to use a three-currents system
to explain the configuration of magnetic fields in solar prominences.
Title: Kinematics of solar prominences.
Authors: Ballester, J. L.; Kleczek, J.
Bibcode: 1983SoPh...87..261B
Altcode:
Using a 16-mm film recorded by R. B. Dunn at Sacramento Peak Observatory
with a 15-inch coronagraph, we have studied motions of three solar
prominences from the kinematic point of view. We applied three methods
proposed earlier (Billings and Pecker, 1954; Palus, 1972; Makhmudov
et al., 1980) to three prominences of the film. A new method based on
geometrical considerations is introduced. The results of all the four
methods are compared and discussed.
Title: Neutrino Transport with Isoenergetic and Neutrino / Electron
Scattering
Authors: Bruenn, S. W.; Ballester, J. L.
Bibcode: 1981Ap&SS..78..247B
Altcode:
Solution to steady-state, bigroup neutrino transport with isoenergetic
and neutrino-electron scattering have been obtained for idealized
conditions representative of the outer core or envelope of a
star whose inner core is undergoing gravitational collapse. The
solutions are presented in a form suitable for comparison with other
numerical schemes. The incident neutrino distribution was chosen to
be nonequilibrium, and the solutions clearly show the tendency of the
neutrinos to equilibrate via neutrino-electron scattering as they
propagate through the material. Care is taken to compare solutions
obtained by two completely different techniques to ensure that the
solutions presented here are accurate and error free. The solutions
are compared with those obtained by a simple diffusion scheme, and
by a flux limited diffusion scheme. The solutions given by the latter
agree well with the solutions derived here, except when the neutrino
angular distribution becomes oblate.