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Author name code: moreno-insertis
ADS astronomy entries on 2022-09-14
author:"Moreno-Insertis, Fernando"
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Title: A 2D Model for Coronal Bright Points: Association with
Spicules, UV Bursts, Surges, and EUV Coronal Jets
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.
2022ApJ...935L..21N Altcode: 2022arXiv220804308N
Coronal bright points (CBPs) are ubiquitous structures in the
solar atmosphere composed of hot small-scale loops observed in
extreme-ultraviolet (EUV) or X-rays in the quiet Sun and coronal
holes. They are key elements to understanding the heating of
the corona; nonetheless, basic questions regarding their heating
mechanisms, the chromosphere underneath, or the effects of flux
emergence in these structures remain open. We have used the
Bifrost code to carry out a 2D experiment in which a coronal-hole
magnetic null-point configuration evolves perturbed by realistic
granulation. To compare with observations, synthetic SDO/AIA, Solar
Orbiter EUI-HRI, and IRIS images have been computed. The experiment
shows the self-consistent creation of a CBP through the action of
stochastic granular motions alone, mediated by magnetic reconnection
in the corona. The reconnection is intermittent and oscillatory,
and it leads to coronal and transition-region temperature loops that
are identifiable in our EUV/UV observables. During the CBP lifetime,
convergence and cancellation at the surface of its underlying opposite
polarities takes place. The chromosphere below the CBP shows a number
of peculiar features concerning its density and the spicules in
it. The final stage of the CBP is eruptive: Magnetic flux emergence
at the granular scale disrupts the CBP topology, leading to different
ejections, such as UV bursts, surges, and EUV coronal jets. Apart
from explaining observed CBP features, our results pave the way for
further studies combining simulations and coordinated observations in
different atmospheric layers.
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Title: Ambipolar diffusion: Self-similar solutions and MHD code
testing. Cylindrical symmetry
Authors: Moreno-Insertis, F.; Nóbrega-Siverio, D.; Priest, E. R.;
Hood, A. W.
2022A&A...662A..42M Altcode: 2022arXiv220306272M
Context. Ambipolar diffusion is a process occurring in partially
ionised astrophysical systems that imparts a complicated mathematical
and physical nature to Ohm's law. The numerical codes that solve the
magnetohydrodynamic (MHD) equations have to be able to deal with the
singularities that are naturally created in the system by the ambipolar
diffusion term. <BR /> Aims: The global aim is to calculate a set of
theoretical self-similar solutions to the nonlinear diffusion equation
with cylindrical symmetry that can be used as tests for MHD codes which
include the ambipolar diffusion term. <BR /> Methods: First, following
the general methods developed in the applied mathematics literature,
we obtained the theoretical solutions as eigenfunctions of a nonlinear
ordinary differential equation. Phase-plane techniques were used to
integrate through the singularities at the locations of the nulls,
which correspond to infinitely sharp current sheets. In the second
half of the paper, we consider the use of these solutions as tests
for MHD codes. To that end, we used the Bifrost code, thereby testing
the capabilities of these solutions as tests as well as (inversely) the
accuracy of Bifrost's recently developed ambipolar diffusion module. <BR
/> Results: The obtained solutions are shown to constitute a demanding,
but nonetheless viable, test for MHD codes that incorporate ambipolar
diffusion. Detailed tabulated runs of the solutions have been made
available at a public repository. The Bifrost code is able to reproduce
the theoretical solutions with sufficient accuracy up to very advanced
diffusive times. Using the code, we also explored the asymptotic
properties of our theoretical solutions in time when initially perturbed
with either small or finite perturbations. <BR /> Conclusions: The
functions obtained in this paper are relevant as physical solutions
and also as tests for general MHD codes. They provide a more stringent
and general test than the simple Zeldovich-Kompaneets-Barenblatt-Pattle
solution. <P />Movies associated to Figs. 4 and 7 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141449/olm">https://www.aanda.org</A>
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Title: Large-amplitude Prominence Oscillations following Impact by
a Coronal Jet
Authors: Luna, Manuel; Moreno-Insertis, Fernando
2021ApJ...912...75L Altcode: 2021arXiv210302661L
Observational evidence shows that coronal jets can hit prominences and
set them in motion. The impact leads to large-amplitude oscillations
(LAOs) in the prominence. In this paper, we attempt to understand this
process via 2.5D MHD numerical experiments. In our model, the jets
are generated in a sheared magnetic arcade above a parasitic bipolar
region located in one of the footpoints of the filament channel (FC)
supporting the prominence. The shear is imposed at velocities not
far above the observed photospheric values; this leads to a multiple
reconnection process, as obtained in previous jet models. Both a
fast Alfvénic perturbation and a slower supersonic front preceding a
plasma jet are issued from the reconnection site; in the later phase,
a more violent (eruptive) jet is produced. The perturbation and jets
run along the FC; they are partially reflected at the prominence,
and partially transmitted through it. This results in a pattern
of counter-streaming flows along the FC, and oscillations in the
prominence. The oscillations are LAOs (i.e., with amplitudes above
10 km s<SUP>-1</SUP>) in some areas of the prominence, both in the
longitudinal and transverse directions. In some field lines, the impact
is so strong that the prominence mass is brought out of the dip and
down to the chromosphere along the FC. Two cases are studied, with
respect to arcades at different heights above the parasitic bipolar
region, leading to different heights for the region of the prominence
perturbed by the jets. The obtained oscillation amplitudes and periods
are in general agreement with the observations.
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Title: The chromospheric component of coronal bright points. Coronal
and chromospheric responses to magnetic-flux emergence
Authors: Madjarska, Maria S.; Chae, Jongchul; Moreno-Insertis,
Fernando; Hou, Zhenyong; Nóbrega-Siverio, Daniel; Kwak, Hannah;
Galsgaard, Klaus; Cho, Kyuhyoun
2021A&A...646A.107M Altcode: 2020arXiv201209426M
Context. We investigate the chromospheric counterpart of small-scale
coronal loops constituting a coronal bright point (CBP) and its
response to a photospheric magnetic-flux increase accompanied by
co-temporal CBP heating. <BR /> Aims: The aim of this study is
to simultaneously investigate the chromospheric and coronal layers
associated with a CBP, and in so doing, provide further understanding on
the heating of plasmas confined in small-scale loops. <BR /> Methods:
We used co-observations from the Atmospheric Imaging Assembly and
Helioseismic Magnetic Imager on board the Solar Dynamics Observatory,
together with data from the Fast Imaging Solar Spectrograph taken
in the Hα and Ca II 8542.1 Å lines. We also employed both linear
force-free and potential field extrapolation models to investigate
the magnetic topology of the CBP loops and the overlying corona,
respectively. We used a new multi-layer spectral inversion technique
to derive the temporal variations of the temperature of the Hα loops
(HLs). <BR /> Results: We find that the counterpart of the CBP, as
seen at chromospheric temperatures, is composed of a bundle of dark
elongated features named in this work Hα loops, which constitute an
integral part of the CBP loop magnetic structure. An increase in the
photospheric magnetic flux due to flux emergence is accompanied by
a rise of the coronal emission of the CBP loops, that is a heating
episode. We also observe enhanced chromospheric activity associated
with the occurrence of new HLs and mottles. While the coronal emission
and magnetic flux increases appear to be co-temporal, the response of
the Hα counterpart of the CBP occurs with a small delay of less than
3 min. A sharp temperature increase is found in one of the HLs and
in one of the CBP footpoints estimated at 46% and 55% with respect
to the pre-event values, also starting with a delay of less than 3
min following the coronal heating episode. The low-lying CBP loop
structure remains non-potential for the entire observing period. The
magnetic topological analysis of the overlying corona reveals the
presence of a coronal null point at the beginning and towards the end
of the heating episode. <BR /> Conclusions: The delay in the response
of the chromospheric counterpart of the CBP suggests that the heating
may have occurred at coronal heights. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039329/olm">https://www.aanda.org</A>
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Title: Case study of multi-temperature coronal jets for emerging
flux MHD models
Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte;
Moreno-Insertis, Fernando; Aulanier, Guillaume; Nóbrega-Siverio,
Daniel; Devi, Pooja
2020A&A...639A..22J Altcode: 2020arXiv200506064J
Context. Hot coronal jets are a basic observed feature of the solar
atmosphere whose physical origin is still actively debated. <BR />
Aims: We study six recurrent jets that occurred in active region NOAA
12644 on April 4, 2017. They are observed in all the hot filters
of AIA as well as cool surges in IRIS slit-jaw high spatial and
temporal resolution images. <BR /> Methods: The AIA filters allow us
to study the temperature and the emission measure of the jets using
the filter ratio method. We studied the pre-jet phases by analysing
the intensity oscillations at the base of the jets with the wavelet
technique. <BR /> Results: A fine co-alignment of the AIA and IRIS
data shows that the jets are initiated at the top of a canopy-like
double-chambered structure with cool emission on one and hot emission
on the other side. The hot jets are collimated in the hot temperature
filters, have high velocities (around 250 km s<SUP>-1</SUP>) and
are accompanied by cool surges and ejected kernels that both move
at about 45 km s<SUP>-1</SUP>. In the pre-phase of the jets, we find
quasi-periodic intensity oscillations at their base that are in phase
with small ejections; they have a period of between 2 and 6 min,
and are reminiscent of acoustic or magnetohydrodynamic waves. <BR />
Conclusions: This series of jets and surges provides a good case study
for testing the 2D and 3D magnetohydrodynamic emerging flux models. The
double-chambered structure that is found in the observations corresponds
to the regions with cold and hot loops that are in the models below
the current sheet that contains the reconnection site. The cool surge
with kernels is comparable with the cool ejection and plasmoids that
naturally appears in the models. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037806/olm">https://www.aanda.org</A>
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Title: Ambipolar diffusion in the Bifrost code
Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis,
F.; Carlsson, M.
2020A&A...638A..79N Altcode: 2020arXiv200411927N
Context. Ambipolar diffusion is a physical mechanism related to the
drift between charged and neutral particles in a partially ionized
plasma that is key to many different astrophysical systems. However,
understanding its effects is challenging due to basic uncertainties
concerning relevant microphysical aspects and the strong constraints it
imposes on the numerical modeling. <BR /> Aims: Our aim is to introduce
a numerical tool that allows us to address complex problems involving
ambipolar diffusion in which, additionally, departures from ionization
equilibrium are important or high resolution is needed. The primary
application of this tool is for solar atmosphere calculations, but the
methods and results presented here may also have a potential impact
on other astrophysical systems. <BR /> Methods: We have developed a
new module for the stellar atmosphere Bifrost code that improves its
computational capabilities of the ambipolar diffusion term in the
generalized Ohm's law. This module includes, among other things,
collision terms adequate to processes in the coolest regions in
the solar chromosphere. As the main feature of the module, we have
implemented the super time stepping (STS) technique, which allows an
important acceleration of the calculations. We have also introduced
hyperdiffusion terms to guarantee the stability of the code. <BR />
Results: We show that to have an accurate value for the ambipolar
diffusion coefficient in the solar atmosphere it is necessary to
include as atomic elements in the equation of state not only hydrogen
and helium, but also the main electron donors like sodium, silicon,
and potassium. In addition, we establish a range of criteria to set
up an automatic selection of the free parameters of the STS method
that guarantees the best performance, optimizing the stability and
speed for the ambipolar diffusion calculations. We validate the STS
implementation by comparison with a self-similar analytical solution.
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Title: Quasi Periodic Oscillations in the Pre Phases of Recurrent
Jets Highlighting Plasmoids in Current Sheet
Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte;
Aulanier, Guillaume; Devi, Pooja; Moreno-Insertis, Fernando;
Nóbrega-Siverio, Daniel
2020EGUGA..2222351J Altcode:
Solar jets observed at the limb are important to determine the location
of reconnection sites in the corona. In this study, we investigate
six recurrent hot and cool jets occurring in the active region NOAA
12644 as it is crossing the west limb on April 04, 2017. These jets
are observed in all the UV/EUV filters of SDO/AIA and in cooler
temperature formation lines in IRIS slit jaw images. The jets are
initiated at the top of a double chamber vault with cool loops on one
side and hot loops on the other side. The existence of such double
chamber vaults suggests the presence of emerging flux with cool
loops, the hot loops being the reconnected loops similarly as in the
models of Moreno-Insertiset al. 2008, 2013 and Nóbrega-Siverio et
al. 2016. In the preliminary phase of the main jets, quasi periodic
intensity oscillations accompanied by smaller jets are detected in the
bright current sheet between the vault and the preexisting magnetic
field. Individual kernels and plasmoids are ejected in open field lines
along the jets. Plasmoids may launch torsional Alfven waves and the
kernels would be the result of the untwist of the plasmoids in open
magnetic field as proposed in the model of Wyper et al. 2016.
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Title: Nonequilibrium ionization and ambipolar diffusion in solar
magnetic flux emergence processes
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.; Carlsson, M.; Szydlarski, M.
2020A&A...633A..66N Altcode: 2019arXiv191201015N
Context. Magnetic flux emergence from the solar interior has
been shown to be a key mechanism for unleashing a wide variety of
phenomena. However, there are still open questions concerning the
rise of the magnetized plasma through the atmosphere, mainly in
the chromosphere, where the plasma departs from local thermodynamic
equilibrium (LTE) and is partially ionized. <BR /> Aims: We aim to
investigate the impact of the nonequilibrium (NEQ) ionization and
recombination and molecule formation of hydrogen, as well as ambipolar
diffusion, on the dynamics and thermodynamics of the flux emergence
process. <BR /> Methods: Using the radiation-magnetohydrodynamic
Bifrost code, we performed 2.5D numerical experiments of magnetic flux
emergence from the convection zone up to the corona. The experiments
include the NEQ ionization and recombination of atomic hydrogen, the NEQ
formation and dissociation of H<SUB>2</SUB> molecules, and the ambipolar
diffusion term of the generalized Ohm's law. <BR /> Results: Our
experiments show that the LTE assumption substantially underestimates
the ionization fraction in most of the emerged region, leading to an
artificial increase in the ambipolar diffusion and, therefore, in the
heating and temperatures as compared to those found when taking the
NEQ effects on the hydrogen ion population into account. We see that
LTE also overestimates the number density of H<SUB>2</SUB> molecules
within the emerged region, thus mistakenly magnifying the exothermic
contribution of the H<SUB>2</SUB> molecule formation to the thermal
energy during the flux emergence process. We find that the ambipolar
diffusion does not significantly affect the amount of total unsigned
emerged magnetic flux, but it is important in the shocks that cross
the emerged region, heating the plasma on characteristic times ranging
from 0.1 to 100 s. We also briefly discuss the importance of including
elements heavier than hydrogen in the equation of state so as not to
overestimate the role of ambipolar diffusion in the atmosphere. <P
/>Movies associated to Figs. 2-5, 8, 9, and A.1 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936944/olm">https://www.aanda.org</A>
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Title: Self-similar Approach for Rotating Magnetohydrodynamic Solar
and Astrophysical Structures
Authors: Luna, M.; Priest, E.; Moreno-Insertis, F.
2018ApJ...863..147L Altcode: 2018arXiv180702473L
Rotating magnetic structures are common in astrophysics,
from vortex tubes and tornadoes in the Sun all the way to
jets in different astrophysical systems. The physics of these
objects often combine inertial, magnetic, gas pressure, and
gravitational terms. Also, they often show approximate symmetries
that help simplify the otherwise rather intractable equations
governing their morphology and evolution. Here we propose a
general formulation of the equations assuming axisymmetry and a
self-similar form for all variables: in spherical coordinates (r,
θ, ϕ), the magnetic field and plasma velocity are taken to be of
the form {\boldsymbol{B}}={\boldsymbol{f}}(θ )/{r}<SUP>n</SUP>
and {\boldsymbol{v}}={\boldsymbol{g}}(θ )/{r}<SUP>m</SUP>, with
corresponding expressions for the scalar variables like pressure
and density. Solutions are obtained for potential, force-free, and
non-force-free magnetic configurations. Potential field solutions
can be found for all values of n. Nonpotential force-free solutions
possess an azimuthal component B <SUB> ϕ </SUB> and exist only for n
≥ 2 the resulting structures are twisted and have closed field lines
but are not collimated around the system axis. In the non-force-free
case, including gas pressure, the magnetic field lines acquire an
additional curvature to compensate for an outward pointing pressure
gradient force. We have also considered a pure rotation situation
with no gravity, in the zero-β limit: the solution has cylindrical
geometry and twisted magnetic field lines. The latter solutions can be
helpful in producing a collimated magnetic field structure; but they
exist only when n < 0 and m < 0: for applications they must be
matched to an external system at a finite distance from the origin.
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Title: Small-scale Magnetic Flux Emergence in the Quiet Sun
Authors: Moreno-Insertis, F.; Martinez-Sykora, J.; Hansteen, V. H.;
Muñoz, D.
2018ApJ...859L..26M Altcode: 2018arXiv180600489M
Small bipolar magnetic features are observed to appear in the interior
of individual granules in the quiet Sun, signaling the emergence of
tiny magnetic loops from the solar interior. We study the origin
of those features as part of the magnetoconvection process in the
top layers of the convection zone. Two quiet-Sun magnetoconvection
models, calculated with the radiation-magnetohydrodynamic (MHD)
Bifrost code and with domain stretching from the top layers of the
convection zone to the corona, are analyzed. Using 3D visualization
as well as a posteriori spectral synthesis of Stokes parameters,
we detect the repeated emergence of small magnetic elements in the
interior of granules, as in the observations. Additionally, we identify
the formation of organized horizontal magnetic sheets covering whole
granules. Our approach is twofold, calculating statistical properties
of the system, like joint probability density functions (JPDFs), and
pursuing individual events via visualization tools. We conclude that
the small magnetic loops surfacing within individual granules in the
observations may originate from sites at or near the downflows in the
granular and mesogranular levels, probably in the first 1 or 1.5 Mm
below the surface. We also document the creation of granule-covering
magnetic sheet-like structures through the sideways expansion of a
small subphotospheric magnetic concentration picked up and pulled out
of the interior by a nascent granule. The sheet-like structures that we
found in the models may match the recent observations of Centeno et al.
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Title: On the Importance of the Nonequilibrium Ionization of Si IV
and O IV and the Line of Sight in Solar Surges
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.
2018ApJ...858....8N Altcode: 2018arXiv180310251N
Surges are ubiquitous cool ejections in the solar atmosphere that often
appear associated with transient phenomena like UV bursts or coronal
jets. Recent observations from the Interface Region Imaging Spectrograph
show that surges, although traditionally related to chromospheric
lines, can exhibit enhanced emission in Si IV with brighter spectral
profiles than for the average transition region (TR). In this paper,
we explain why surges are natural sites to show enhanced emissivity
in TR lines. We performed 2.5D radiative-MHD numerical experiments
using the Bifrost code including the nonequilibrium (NEQ) ionization of
silicon and oxygen. A surge is obtained as a by-product of magnetic flux
emergence; the TR enveloping the emerged domain is strongly affected
by NEQ effects: assuming statistical equilibrium would produce an
absence of Si IV and O IV ions in most of the region. Studying the
properties of the surge plasma emitting in the Si IV λ1402.77 and O IV
λ1401.16 lines, we find that (a) the timescales for the optically thin
losses and heat conduction are very short, leading to departures from
statistical equilibrium, and (b) the surge emits in Si IV more and has
an emissivity ratio of Si IV to O IV larger than a standard TR. Using
synthetic spectra, we conclude the importance of line-of-sight effects:
given the involved geometry of the surge, the line of sight can cut the
emitting layer at small angles and/or cross it multiple times, causing
prominent, spatially intermittent brightenings in both Si IV and O IV.
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Title: Surges and Si IV Bursts in the Solar Atmosphere: Understanding
IRIS and SST Observations through RMHD Experiments
Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis,
F.; Rouppe van der Voort, L.
2017ApJ...850..153N Altcode: 2017arXiv171008928N
Surges often appear as a result of the emergence of magnetized
plasma from the solar interior. Traditionally, they are observed
in chromospheric lines such as Hα 6563 \mathringA and Ca II 8542
\mathringA . However, whether there is a response to the surge
appearance and evolution in the Si IV lines or, in fact, in many
other transition region lines has not been studied. In this paper,
we analyze a simultaneous episode of an Hα surge and a Si IV burst
that occurred on 2016 September 03 in active region AR 12585. To that
end, we use coordinated observations from the Interface Region Imaging
Spectrograph and the Swedish 1-m Solar Telescope. For the first time,
we report emission of Si IV within the surge, finding profiles that
are brighter and broader than the average. Furthermore, the brightest
Si IV patches within the domain of the surge are located mainly near
its footpoints. To understand the relation between the surges and the
emission in transition region lines like Si IV, we have carried out 2.5D
radiative MHD (RMHD) experiments of magnetic flux emergence episodes
using the Bifrost code and including the nonequilibrium ionization of
silicon. Through spectral synthesis, we explain several features of
the observations. We show that the presence of Si IV emission patches
within the surge, their location near the surge footpoints and various
observed spectral features are a natural consequence of the emergence of
magnetized plasma from the interior to the atmosphere and the ensuing
reconnection processes.
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Title: Magnetic topological analysis of coronal bright points
Authors: Galsgaard, K.; Madjarska, M. S.; Moreno-Insertis, F.; Huang,
Z.; Wiegelmann, T.
2017A&A...606A..46G Altcode: 2017arXiv170704174G
Context. We report on the first of a series of studies on coronal
bright points which investigate the physical mechanism that generates
these phenomena. <BR /> Aims: The aim of this paper is to understand
the magnetic-field structure that hosts the bright points. <BR />
Methods: We use longitudinal magnetograms taken by the Solar Optical
Telescope with the Narrowband Filter Imager. For a single case,
magnetograms from the Helioseismic and Magnetic Imager were added
to the analysis. The longitudinal magnetic field component is used
to derive the potential magnetic fields of the large regions around
the bright points. A magneto-static field extrapolation method is
tested to verify the accuracy of the potential field modelling. The
three dimensional magnetic fields are investigated for the presence
of magnetic null points and their influence on the local magnetic
domain. <BR /> Results: In nine out of ten cases the bright point
resides in areas where the coronal magnetic field contains an opposite
polarity intrusion defining a magnetic null point above it. We find that
X-ray bright points reside, in these nine cases, in a limited part of
the projected fan-dome area, either fully inside the dome or expanding
over a limited area below which typically a dominant flux concentration
resides. The tenth bright point is located in a bipolar loop system
without an overlying null point. <BR /> Conclusions: All bright points
in coronal holes and two out of three bright points in quiet Sun regions
are seen to reside in regions containing a magnetic null point. An as
yet unidentified process(es) generates the brigh points in specific
regions of the fan-dome structure. <P />The movies are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201731041/olm">http://www.aanda.org</A>
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Title: Solar Coronal Jets: Observations, Theory, and Modeling
Authors: Raouafi, N. E.; Patsourakos, S.; Pariat, E.; Young, P. R.;
Sterling, A. C.; Savcheva, A.; Shimojo, M.; Moreno-Insertis, F.;
DeVore, C. R.; Archontis, V.; Török, T.; Mason, H.; Curdt, W.;
Meyer, K.; Dalmasse, K.; Matsui, Y.
2016SSRv..201....1R Altcode: 2016arXiv160702108R; 2016SSRv..tmp...31R
Coronal jets represent important manifestations of ubiquitous solar
transients, which may be the source of significant mass and energy
input to the upper solar atmosphere and the solar wind. While
the energy involved in a jet-like event is smaller than that of
"nominal" solar flares and coronal mass ejections (CMEs), jets
share many common properties with these phenomena, in particular,
the explosive magnetically driven dynamics. Studies of jets could,
therefore, provide critical insight for understanding the larger,
more complex drivers of the solar activity. On the other side of the
size-spectrum, the study of jets could also supply important clues on
the physics of transients close or at the limit of the current spatial
resolution such as spicules. Furthermore, jet phenomena may hint to
basic process for heating the corona and accelerating the solar wind;
consequently their study gives us the opportunity to attack a broad
range of solar-heliospheric problems.
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Title: Mini-CME eruptions in a flux emergence event in a coronal
hole environment
Authors: Galsgaard, K.; Moreno-Insertis, F.
2016usc..confE..64G Altcode:
Small scale jets are observed to take place at the interface between
the open magnetic field in coronal holes and bipolar magnetic field
concentrations. A fraction of these shows an eruptive behavior, where
a combination of cold dense and hot light plasma has been observed
to propagate out along the jet region, combining traditional jets
with what looks like the eruption of mini-CMEs. Here we discuss a
simple model scenario for the explosive energy release process that
leads to a mixture of hot and cold plasma being accelerated upwards
simultaneously. The model explains both the typical steady state
inverted-Y jet and the subsequent mini-CME eruptions found in blowout
jets. The numerical experiment consists of a buoyant unstable flux
rope that emerges into an overlying slanted coronal field, thereby
creating a bipolar magnetic field distribution in the photosphere
with coronal loops linking the polarities. Reconnection between the
emerged and preexisting magnetic systems including the launching of
a classical inverted-Y jet. The experiment shows that this simple
model provides for a very complicated dynamical behavior in its late
phases. Five independent mini-CME eruptions follow the initial near
steady-state jet phase. The first one is a direct consequence of the
reconnection of the emerged magnetic flux, is mediated by the formation
of a strongly sheared arcade followed by a tether-cutting reconnection
process, and leads to the eruption of a twisted flux rope. The final
four explosive eruptions, instead, are preceded by the formation of a
twisted torus-like flux rope near the strong magnetic concentrations
at the photosphere. As the tube center starts emerging an internal
current sheet is formed below it. This sheet experiences a tether
cutting process that provides the important upwards kick of the
newly formed mini-CME structure. As the fast rising cold and dense
tube interacts with the overlying magnetic field, it reconnects at
different spatial locations, either through a null region or through
a local strong shear region without nulls. The restructuring of the
magnetic field lines generate magneto-acoustic waves that transport
twist and cold plasma out along the less stressed parts of the newly
reconnected field lines. The emphasis of the talk will be on the
physical forces responsible for the initial flux tube rising and the
effects and reasons for the following destruction of the mini-CMEs.
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Title: The Cool Surge Following Flux Emergence in a Radiation-MHD
Experiment
Authors: Nóbrega-Siverio; D.; Moreno-Insertis, F.; Martínez-Sykora,
J.
2016usc..confE..68N Altcode:
Cool and dense ejections, typically Hα surges, often appear alongside
EUV or X-ray coronal jets as a result of the emergence of magnetized
plasma from the solar interior. Idealized numerical experiments explain
those ejections as being indirectly associated with the magnetic
reconnection taking place between the emerging and preexisting
systems. However, those experiments miss basic elements that can
importantly affect the surge phenomenon. In this paper we study the
cool surges using a realistic treatment of the radiation transfer and
material plasma properties. To that end, the Bifrost code is used,
which has advanced modules for the equation of state of the plasma,
photospheric and chromospheric radiation transfer, heat conduction,
and optically thin radiative cooling. We carry out a 2.5D experiment of
the emergence of magnetized plasma through (meso) granular convection
cells and the low atmosphere to the corona. Through detailed Lagrange
tracing we study the formation and evolution of the cool ejection and,
in particular, the role of the entropy sources; this allows us to
discern families of evolutionary patterns for the plasma elements. In
the launch phase, many elements suffer accelerations well in excess
of gravity; when nearing the apex of their individual trajectories,
instead, the plasma elements follow quasi-parabolic trajectories with
accelerations close to the solar gravity . We show how the formation
of the cool ejection is mediated by a wedge-like structure composed
of two shocks, one of which leads to the detachment of the surge from
the original emerged plasma dome.
---------------------------------------------------------
Title: The Cool Surge Following Flux Emergence in a Radiation-MHD
Experiment
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.
2016ApJ...822...18N Altcode: 2016arXiv160104074N
Cool and dense ejections, typically Hα surges, often appear alongside
EUV or X-ray coronal jets as a result of the emergence of magnetized
plasma from the solar interior. Idealized numerical experiments explain
those ejections as being indirectly associated with the magnetic
reconnection taking place between the emerging and preexisting
systems. However, those experiments miss basic elements that can
importantly affect the surge phenomenon. In this paper we study the
cool surges using a realistic treatment of the radiation transfer and
material plasma properties. To that end, the Bifrost code is used,
which has advanced modules for the equation of state of the plasma,
photospheric and chromospheric radiation transfer, heat conduction,
and optically thin radiative cooling. We carry out a 2.5D experiment of
the emergence of magnetized plasma through (meso) granular convection
cells and the low atmosphere to the corona. Through detailed Lagrange
tracing we study the formation and evolution of the cool ejection and,
in particular, the role of the entropy sources; this allows us to
discern families of evolutionary patterns for the plasma elements. In
the launch phase, many elements suffer accelerations well in excess
of gravity; when nearing the apex of their individual trajectories,
instead, the plasma elements follow quasi-parabolic trajectories with
accelerations close to {g}<SUB>⊙ </SUB>. We show how the formation
of the cool ejection is mediated by a wedge-like structure composed
of two shocks, one of which leads to the detachment of the surge from
the original emerged plasma dome.
---------------------------------------------------------
Title: Commission 12: Solar Radiation and Structure
Authors: Cauzzi, Gianna; Shchukina, Nataliya; Kosovichev, Alexander;
Bianda, Michele; Brandenburg, Axel; Chou, Dean-Yi; Dasso, Sergio; Ding,
Ming-De; Jefferies, Stuart; Krivova, Natalie; Kuznetsov, Vladimir D.;
Moreno-Insertis, Fernando
2016IAUTA..29..278C Altcode:
Commission 12 of the International Astronomical Union encompasses
investigations of the internal structure and dynamics of the Sun,
the quiet solar atmosphere, solar radiation and its variability, and
the nature of relatively stable magnetic structures like sunspots,
faculae and the magnetic network. The Commission sees participation
of over 300 scientists worldwide.
---------------------------------------------------------
Title: Are Tornado-Like Magnetic Structures Able to Support Solar
Prominence Plasma?
Authors: Ogunjo, S. T.; Luna Bennasar, M.; Moreno-Insertis, F.;
Priest, E. R.
2015AGUFMSH53B2483O Altcode:
Recent high-resolution and high-cadence observations have surprisingly
suggested that prominence barbs exhibit apparent rotating motions
suggestive of a tornado-like structure. Additional evidence has
been provided by Doppler measurements. The observations reveal
opposite velocities for both hot and cool plasma on the two sides of
a prominence barb. This motion is persistent for several hours and
has been interpreted in terms of rotational motion of prominence
feet. Several authors suggest that such barb motions are rotating
helical structures around a vertical axis similar to tornadoes
on Earth. One of the difficulties of such a proposal is how to
support cool prominence plasma in almost-vertical structures against
gravity. In this work we model analytically a tornado-like structure
and try to determine possible mechanisms to support the prominence
plasma. We have found that the Lorentz force can indeed support the
barb plasma provided the magnetic structure is sufficiently twisted
and/or significant poloidal flows are present.
---------------------------------------------------------
Title: Multi-parametric Study of Rising 3D Buoyant Flux Tubes in an
Adiabatic Stratification Using AMR
Authors: Martínez-Sykora, Juan; Moreno-Insertis, Fernando; Cheung,
Mark C. M.
2015ApJ...814....2M Altcode: 2015arXiv150701506M
We study the buoyant rise of magnetic flux tubes embedded in
an adiabatic stratification using two-and three-dimensional,
magnetohydrodynamic simulations. We analyze the dependence of the tube
evolution on the field line twist and on the curvature of the tube axis
in different diffusion regimes. To be able to achieve a comparatively
high spatial resolution we use the FLASH code, which has a built-in
Adaptive Mesh Refinement (AMR) capability. Our 3D experiments reach
Reynolds numbers that permit a reasonable comparison of the results
with those of previous 2D simulations. When the experiments are run
without AMR, hence with a comparatively large diffusivity, the amount
of longitudinal magnetic flux retained inside the tube increases
with the curvature of the tube axis. However, when a low-diffusion
regime is reached by using the AMR algorithms, the magnetic twist is
able to prevent the splitting of the magnetic loop into vortex tubes
and the loop curvature does not play any significant role. We detect
the generation of vorticity in the main body of the tube of opposite
sign on the opposite sides of the apex. This is a consequence of the
inhomogeneity of the azimuthal component of the field on the flux
surfaces. The lift force associated with this global vorticity makes
the flanks of the tube move away from their initial vertical plane in
an antisymmetric fashion. The trajectories have an oscillatory motion
superimposed, due to the shedding of vortex rolls to the wake, which
creates a Von Karman street.
---------------------------------------------------------
Title: Are Tornado-like Magnetic Structures Able to Support Solar
Prominence Plasma?
Authors: Luna, M.; Moreno-Insertis, F.; Priest, E.
2015ApJ...808L..23L Altcode: 2015arXiv150701455L
Recent high-resolution and high-cadence observations have surprisingly
suggested that prominence barbs exhibit apparent rotating motions
suggestive of a tornado-like structure. Additional evidence has
been provided by Doppler measurements. The observations reveal
opposite velocities for both hot and cool plasma on the two sides of
a prominence barb. This motion is persistent for several hours and
has been interpreted in terms of rotational motion of prominence
feet. Several authors suggest that such barb motions are rotating
helical structures around a vertical axis similar to tornadoes
on Earth. One of the difficulties of such a proposal is how to
support cool prominence plasma in almost-vertical structures against
gravity. In this work we model analytically a tornado-like structure
and try to determine possible mechanisms to support the prominence
plasma. We have found that the Lorentz force can indeed support the
barb plasma provided the magnetic structure is sufficiently twisted
and/or significant poloidal flows are present.
---------------------------------------------------------
Title: Continuum Intensity and [O I] Spectral Line Profiles in Solar
3D Photospheric Models: The Effect of Magnetic Fields
Authors: Fabbian, D.; Moreno-Insertis, F.
2015ApJ...802...96F Altcode: 2015arXiv150106916F
The importance of magnetic fields in three-dimensional (3D)
magnetoconvection models of the Sun’s photosphere is investigated
in terms of their influence on the continuum intensity at different
viewing inclination angles and on the intensity profile of two [O i]
spectral lines. We use the RH numerical radiative transfer code to
perform a posteriori spectral synthesis on the same time series of
magnetoconvection models used in our publications on the effect of
magnetic fields on abundance determination. We obtain a good match of
the synthetic disk-center continuum intensity to the absolute continuum
values from the Fourier Transform Spectrometer (FTS) observational
spectrum; the match of the center-to-limb variation synthetic data
to observations is also good, thanks, in part, to the 3D radiation
transfer capabilities of the RH code. The different levels of magnetic
flux in the numerical time series do not modify the quality of the
match. Concerning the targeted [O i] spectral lines, we find, instead,
that magnetic fields lead to nonnegligible changes in the synthetic
spectrum, with larger average magnetic flux causing both of the lines
to become noticeably weaker. The photospheric oxygen abundance that
one would derive if instead using nonmagnetic numerical models would
thus be lower by a few to several centidex. The inclusion of magnetic
fields is confirmed to be important for improving the current modeling
of the Sun, here in particular in terms of spectral line formation
and of deriving consistent chemical abundances. These results may shed
further light on the still controversial issue regarding the precise
value of the solar oxygen abundance.
---------------------------------------------------------
Title: The power spectrum of solar convection flows from
high-resolution observations and 3D simulations
Authors: Yelles Chaouche, L.; Moreno-Insertis, F.; Bonet, J. A.
2014A&A...563A..93Y Altcode: 2014arXiv1402.2293Y
Context. Understanding solar surface magnetoconvection requires
the study of the Fourier spectra of the velocity fields. Nowadays,
observations are available that resolve very small spatial scales,
well into the subgranular range, almost reaching the scales routinely
resolved in numerical magnetoconvection simulations. Comparison of
numerical and observational data at present can provide an assessment
of the validity of the observational proxies. <BR /> Aims: Our aims are:
(1) to obtain Fourier spectra for the photospheric velocity fields using
the spectropolarimetric observations with the highest spatial resolution
so far (~120 km), thus reaching for the first time spatial scales well
into the subgranular range; (2) to calculate corresponding Fourier
spectra from realistic 3D numerical simulations of magnetoconvection
and carry out a proper comparison with their observational counterparts
considering the residual instrumental degradation in the observational
data; and (3) to test the observational proxies on the basis of
the numerical data alone, by comparing the actual velocity field
in the simulations with synthetic observations obtained from the
numerical boxes. <BR /> Methods: (a) For the observations, data from
the SUNRISE/IMaX spectropolarimeter are used. (b) For the simulations,
we use four series of runs obtained with the STAGGER code for different
average signed vertical magnetic field values (0, 50, 100, and 200
G). Spectral line profiles are synthesized from the numerical boxes for
the same line observed by IMaX (Fe I 5250.2 Å) and degraded to match
the performance of the IMaX instrument. Proxies for the velocity field
are obtained via Dopplergrams (vertical component) and local correlation
tracking (LCT, for the horizontal component). Fourier power spectra are
calculated and a comparison between the synthetic and observational data
sets carried out. (c) For the internal comparison of the numerical data,
velocity values on constant optical depth surfaces are used instead
of on horizontal planes. <BR /> Results: A very good match between
observational and simulated Fourier power spectra is obtained for the
vertical velocity data for scales between 200 km and 6 Mm. Instead,
a clear vertical shift is obtained when the synthetic observations are
not degraded to emulate the degradation in the IMaX data. The match
for the horizontal velocity data is much less impressive because
of the inaccuracies of the LCT procedure. Concerning the internal
comparison of the direct velocity values of the numerical boxes with
those from the synthetic observations, a high correlation (0.96) is
obtained for the vertical component when using the velocity values on
the log τ<SUB>500</SUB> = -1 surface in the box. The corresponding
Fourier spectra are near each other. A lower maximum correlation (0.5)
is reached (at log τ<SUB>500</SUB> = 0) for the horizontal velocities
as a result of the coarseness of the LCT procedure. Correspondingly,
the Fourier spectra for the LCT-determined velocities is well below that
from the actual velocity components. <BR /> Conclusions: As measured
by the Fourier spectra, realistic numerical simulations of surface
magnetoconvection provide a very good match to the observational
proxies for the photospheric velocity fields at least on scales from
several Mm down to around 200 km. Taking into account the spatial and
spectral instrumental blurring is essential for the comparison between
simulations and observations. Dopplergrams are an excellent proxy for
the vertical velocities on constant-τ isosurfaces, while LCT is a
much less reliable method of determining the horizontal velocities.
---------------------------------------------------------
Title: Twisting solar coronal jet launched at the boundary of an
active region
Authors: Schmieder, B.; Guo, Y.; Moreno-Insertis, F.; Aulanier, G.;
Yelles Chaouche, L.; Nishizuka, N.; Harra, L. K.; Thalmann, J. K.;
Vargas Dominguez, S.; Liu, Y.
2013A&A...559A...1S Altcode: 2013arXiv1309.6514S
<BR /> Aims: A broad jet was observed in a weak magnetic field area
at the edge of active region NOAA 11106 that also produced other
nearby recurring and narrow jets. The peculiar shape and magnetic
environment of the broad jet raised the question of whether it was
created by the same physical processes of previously studied jets
with reconnection occurring high in the corona. <BR /> Methods:
We carried out a multi-wavelength analysis using the EUV images
from the Atmospheric Imaging Assembly (AIA) and magnetic fields
from the Helioseismic and Magnetic Imager (HMI) both on-board the
Solar Dynamics Observatory, which we coupled to a high-resolution,
nonlinear force-free field extrapolation. Local correlation tracking
was used to identify the photospheric motions that triggered the jet,
and time-slices were extracted along and across the jet to unveil its
complex nature. A topological analysis of the extrapolated field was
performed and was related to the observed features. <BR /> Results:
The jet consisted of many different threads that expanded in around 10
minutes to about 100 Mm in length, with the bright features in later
threads moving faster than in the early ones, reaching a maximum speed
of about 200 km s<SUP>-1</SUP>. Time-slice analysis revealed a striped
pattern of dark and bright strands propagating along the jet, along with
apparent damped oscillations across the jet. This is suggestive of a
(un)twisting motion in the jet, possibly an Alfvén wave. Bald patches
in field lines, low-altitude flux ropes, diverging flow patterns, and a
null point were identified at the basis of the jet. <BR /> Conclusions:
Unlike classical λ or Eiffel-tower-shaped jets that appear to be caused
by reconnection in current sheets containing null points, reconnection
in regions containing bald patches seems to be crucial in triggering
the present jet. There is no observational evidence that the flux
ropes detected in the topological analysis were actually being ejected
themselves, as occurs in the violent phase of blowout jets; instead,
the jet itself may have gained the twist of the flux rope(s) through
reconnection. This event may represent a class of jets different from
the classical quiescent or blowout jets, but to reach that conclusion,
more observational and theoretical work is necessary.
---------------------------------------------------------
Title: Thermodynamic fluctuations in solar photospheric
three-dimensional convection simulations and observations
(Corrigendum)
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
Rezaei, R.
2013A&A...559C...1B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermodynamic fluctuations in solar photospheric
three-dimensional convection simulations and observations
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
Rezaei, R.
2013A&A...557A.109B Altcode: 2013arXiv1306.6093B
Context. Numerical three-dimensional (3D) radiative
(magneto-)hydrodynamical [(M)HD] simulations of solar convection
are nowadays used to understand the physical properties of the solar
photosphere and convective envelope, and, in particular, to determine
the Sun's photospheric chemical abundances. To validate this approach,
it is important to check that no excessive thermodynamic fluctuations
arise as a consequence of the partially incomplete treatment of
radiative transfer causing radiative damping that is too modest. <BR
/> Aims: We investigate the realism of the thermodynamics in recent
state-of-the-art 3D convection simulations of the solar atmosphere
carried out with the Stagger code. <BR /> Methods: We compared the
characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565
nm) and one Si i line at 1082.7 nm in solar disc-centre observations
of different spatial resolution with spectra synthesized from 3D
convection simulations. The observations were taken with ground-based
(Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI),
POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all
at the Vacuum Tower Telescope on Tenerife) and space-based instruments
(Hinode/Spectropolarimeter). We degraded the synthetic spectra to
the spatial resolution of the observations, based on the distribution
of the continuum intensity I<SUB>c</SUB>. We estimated the spectral
degradation to be applied to the simulation results by comparing atlas
spectra with averaged observed spectra. In addition to deriving a set
of line parameters directly from the intensity profiles, we used the
SIR (Stokes Inversion based on Response functions) code to invert
the spectra. <BR /> Results: The spatial degradation kernels yield
a similar generic spatial stray-light contamination of about 30%
for all instruments. The spectral stray light inside the different
spectrometers is found to be between 2% and 20%. Most of the line
parameters from the observational data are matched by the degraded
HD simulation spectra. The inversions predict a macroturbulent
velocity v<SUB>mac</SUB> below 10 m s<SUP>-1</SUP> for the HD
simulation spectra at full spatial resolution, whereas they yield
v<SUB>mac</SUB> ≲ 1000 m s<SUP>-1</SUP> at a spatial resolution of
0.″3. The temperature fluctuations in the inversion of the degraded
HD simulation spectra do not exceed those from the observational data
(of the order of 100-200 K rms for -2 ⪉ log τ<SUB>500 nm</SUB>
⪉ -0.5). The comparison of line parameters in spatially averaged
profiles with the averaged values of line parameters in spatially
resolved profiles indicates a significant change in (average) line
properties on a spatial scale between 0.″13 and 0.″3. <BR />
Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra),
we find no indications of excessive thermodynamic fluctuations
in the 3D HD simulation. To definitely confirm that simulations
without spatial degradation contain fully realistic thermodynamic
fluctuations requires observations at even higher spatial resolution
(i.e. <0.″13). <P />Appendices A and B are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Plasma Jets and Eruptions in Solar Coronal Holes: A
Three-dimensional Flux Emergence Experiment
Authors: Moreno-Insertis, F.; Galsgaard, K.
2013ApJ...771...20M Altcode: 2013arXiv1305.2201M
A three-dimensional (3D) numerical experiment of the launching of
a hot and fast coronal jet followed by several violent eruptions is
analyzed in detail. These events are initiated through the emergence
of a magnetic flux rope from the solar interior into a coronal
hole. We explore the evolution of the emerging magnetically dominated
plasma dome surmounted by a current sheet and the ensuing pattern
of reconnection. A hot and fast coronal jet with inverted-Y shape is
produced that shows properties comparable to those frequently observed
with EUV and X-ray detectors. We analyze its 3D shape, its inhomogeneous
internal structure, and its rise and decay phases, lasting for some
15-20 minutes each. Particular attention is devoted to the field line
connectivities and the reconnection pattern. We also study the cool
and high-density volume that appears to encircle the emerged dome. The
decay of the jet is followed by a violent phase with a total of five
eruptions. The first of them seems to follow the general pattern of
tether-cutting reconnection in a sheared arcade, although modified by
the field topology created by the preceding reconnection evolution. The
two following eruptions take place near and above the strong-field
concentrations at the surface. They show a twisted, Ω-loop-like rope
expanding in height, with twist being turned into writhe, thus hinting
at a kink instability (perhaps combined with a torus instability)
as the cause of the eruption. The succession of a main jet ejection
and a number of violent eruptions that resemble mini-CMEs and their
physical properties suggest that this experiment may provide a model
for the blowout jets recently proposed in the literature.
---------------------------------------------------------
Title: MHD simulations of flux emergence in an open field region:
Jet formation and explosive events.
Authors: Galsgaard, Klaus; Moreno-Insertis, Fernando
2013enss.confE..32G Altcode:
The launch of Hinode in 2006 was the start of a new interest in the jet
phenomena in open field regions. Since then observations by the Hinode,
SDO and Stereo satellites have shown that one characteristic jet type
dominates, namely the so-called Eiffeltower or inverted-Y jet. The names
arise from the jet's appearance in X-ray, where they are seen having
two small "legs" below a long monolith structure representing the
jet. This structure is interpreted as the result of the interaction
between a newly emerged bipolar field into an unipolar magnetic
field region. This picture naturally leads to magnetic reconnection
between the two flux regions, where two the high velocity outflows
from the diffusion region forms both the long jet structure and the
underlying loop structure. To investigate this scenario in detail,
we have preformed new MHD experiments of the emergence of a magnetic
dipole region into an uniform open field region. The new experiments
represent a significant extension of both the domain size, the duration
of the experiment and the details of the analysis compared to the
one presented in Moreno-Insertis et al. 2008. We find the initial jet
phase to last on the order of 10 minutes, showing a smoothly evolving
structure which, for a part of the evolution, closely resemblance the
inverted-y structure. A number of characteristic structures arises
around the footpoint region of the loop that may be compared with
observations. Towards the end of this "steady state" inverted-y jet
phase, the amount of flux in the emerged bipolar region is being
exhausted by the reconnection process and the dynamical evolution
enters a new phase. In this phase we find five explosive eruption from
different parts of the remaining structure. These eruptions arises from
only three main areas of the emerged flux region, implying that the same
physical region can host repeated instabilities in the magnetic field.
---------------------------------------------------------
Title: Solar Fe abundance and magnetic fields. Towards a consistent
reference metallicity
Authors: Fabbian, D.; Moreno-Insertis, F.; Khomenko, E.; Nordlund, Å.
2012A&A...548A..35F Altcode: 2012arXiv1209.2771F
<BR /> Aims: We investigate the impact on Fe abundance determination of
including magnetic flux in series of 3D radiation-magnetohydrodynamics
(MHD) simulations of solar convection, which we used to synthesize
spectral intensity profiles corresponding to disc centre. <BR />
Methods: A differential approach is used to quantify the changes
in theoretical equivalent width of a set of 28 iron spectral lines
spanning a wide range in wavelength, excitation potential, oscillator
strength, Landé factor, and formation height. The lines were computed
in local thermodynamic equilibrium (LTE) using the spectral synthesis
code LILIA. We used input magnetoconvection snapshots covering 50 min
of solar evolution and belonging to series having an average vertical
magnetic flux density of ⟨ B<SUB>vert</SUB> ⟩ = 0,50,100, and
200 G. For the relevant calculations we used the Copenhagen Stagger
code. <BR /> Results: The presence of magnetic fields causes both a
direct (Zeeman-broadening) effect on spectral lines with non-zero
Landé factor and an indirect effect on temperature-sensitive
lines via a change in the photospheric T - τ stratification. The
corresponding correction in the estimated atomic abundance ranges
from a few hundredths of a dex up to |Δlog ɛ(Fe)<SUB>⊙</SUB>|
~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the
IR. The largest modification to previous solar abundance determinations
based on visible spectral lines is instead due to the indirect effect,
i.e., the line-weakening caused by a warmer stratification as seen on
an optical depth scale. Our results indicate that the average solar
iron abundance obtained when using magnetoconvection models can be ~
0.03-0.11 dex higher than when using the simpler hydrodynamics (HD)
convection approach. <BR /> Conclusions: We demonstrate that accounting
for magnetic flux is important in state-of-the-art solar photospheric
abundance determinations based on 3D convection simulations.
---------------------------------------------------------
Title: The emergence of magnetized plasma from the solar interior
into the atmosphere
Authors: Moreno-Insertis, Fernando
2012cosp...39.1275M Altcode: 2012cosp.meet.1275M
Magnetized plasma is continually emerging from the solar interior
into the atmosphere on a very large range of space- and timescales,
extending from, e.g., the tiny bipolar magnetic elements observed
to emerge within granules, all the way up to the largest active
regions. Magnetic flux emergence is one of the fundamental processes
that shape the solar atmosphere. However, it poses an enormous challenge
for detailed theoretical understanding given that, among other things,
it typically affects all atmospheric layers from the photosphere to
the corona, that it requires the simultaneous solution of the plasma
physics and radiation transfer problems, and that proper check with
the observations requires the use of data from detectors in the
visible/IR, the EUV and X-Rays. In this lecture, a number recent
results concerning the modeling of flux emergence events in three
dimensions are reviewed. Recent 3D numerical experiments are able
to follow the emergence of small to intermediate bipolar regions
from the topmost thousands of km below the surface into the low
atmosphere and the corona. The most complete models are now carried
out using massively-parallel radiation-MHD codes and can cope with
selected NLTE aspects of the radiation transfer problem; other models
sacrifice the radiation transfer side of the problem and concentrate
onto the magnetodynamical aspects (including the topology/connectivity
changes following reconnection, eruption episodes, etc). With the
breathtaking pace of advance of supercomputing installations (now
already beyond the Petaflop level and heading toward the Exaflop mark)
and with the impressive number of ongoing or planned space missions
and new-generation ground-based telescopes, we can expect substantial
advances in this field in the coming years.
---------------------------------------------------------
Title: Sources and Removal of Magnetic Flux in the Solar Atmosphere
Authors: Moreno-Insertis, F.
2012ASPC..455...91M Altcode:
Recent advances in the observation and numerical modeling of magnetic
flux emergence on small-scales are reviewed. The high-resolution limit
of solar photospheric observations has reached scales of order 0".2,
or 100-200 km, in recent years. Observations with that resolution show
individual flux tubes emerging within single granules in the quiet Sun
as small bipolar features of flux as low as 10<SUP>16</SUP> Mx. Also,
high-resolution observations of emerging ephemeral active regions have
been carried out simultaneously at heights from the photosphere to
the corona using different instruments in space and on the ground, and
providing views of the emergence process with unprecedented detail. This
paper starts with a brief review of some of the highest-resolution
flux emergence observations. On the theory side, there is an increasing
number of realistic numerical simulations of flux emergence that solve
the equations of magnetohydrodynamics and radiation transfer. Various
groups have studied different aspects of the radiation-MHD modeling
of flux emergence, but their simulations in part cover the same
processes. In this paper, a number of conclusions of the models are
discussed with special focus on the comparison between the results of
the different groups. The removal of magnetic fields from the surface is
a less explored field than the inverse process, both observationally
and theoretically. Yet, there is a good number of observations of
flux disappearance from the photosphere and other atmospheric layers,
typically in the form of cancellation of colliding flux elements of
opposite polarity. On the simulation side, various numerical experiments
of emerging flux regions find clear instances of flux cancellation
and removal in the runs. In those cases, reconnection of field lines
of opposite polarity is taking place and leads to phenomena akin to
those reported in some of the observations. In this review a number
of recent results from theory and observation are discussed which help
understand the removal of flux from the solar atmosphere.
---------------------------------------------------------
Title: Magnetic flux emergence into the atmosphere: 3D numerical
models.
Authors: Moreno-Insertis, Fernando
2012decs.confE.114M Altcode:
The emergence of magnetic flux from the solar interior is one of the
fundamental processes that shape the solar photosphere, chromosphere and
corona. Taking place on a bewildering range of space- and timescales,
it has defied detailed understanding for a long time, among other
things due to insufficient observational and computing/modeling
power. With the current golden age of solar space missions and with
the advent of Petaflop massively parallel computing, the situation
is quickly improving. Recent 3D numerical experiments are able to
follow the emergence of small to intermediate bipolar regions from the
topmost thousands of km below the surface into the low atmosphere and
the corona. Some of those models include the simultaneous solution
of the MHD and radiation transfer problems, with the divergence of
the radiation flux then being used as entropy source in the plasma
physics problem. In all cases, post-facto spectral synthesis based
on the computed data permits comparison with observational data in
the visible/IR, EUV and X-ray ranges. The interaction between theory
and observation is thus reaching an excellent level and it must be
strengthened for the benefit of future solar physics research. In this
lecture, a review of recent modeling efforts of flux emergence processes
will be provided. Although with a theoretical bias, the lecture will
also provide results concerning the comparison with observations. A
number of shortcomings of the current modeling capabilities will
be discussed.
---------------------------------------------------------
Title: The Three-dimensional Structure of an Active Region Filament
as Extrapolated from Photospheric and Chromospheric Observations
Authors: Yelles Chaouche, L.; Kuckein, C.; Martínez Pillet, V.;
Moreno-Insertis, F.
2012ApJ...748...23Y Altcode: 2012arXiv1201.2456Y
The three-dimensional structure of an active region filament is studied
using nonlinear force-free field extrapolations based on simultaneous
observations at a photospheric and a chromospheric height. To that end,
we used the Si I 10827 Å line and the He I 10830 Å triplet obtained
with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope
(Tenerife). The two extrapolations have been carried out independently
from each other and their respective spatial domains overlap in
a considerable height range. This opens up new possibilities for
diagnostics in addition to the usual ones obtained through a single
extrapolation from, typically, a photospheric layer. Among those
possibilities, this method allows the determination of an average
formation height of the He I 10830 Å signal of ≈2 Mm above the
surface of the Sun. It allows, as well, a cross-check of the obtained
three-dimensional magnetic structures to verify a possible deviation
from the force-free condition, especially at the photosphere. The
extrapolations yield a filament formed by a twisted flux rope whose
axis is located at about 1.4 Mm above the solar surface. The twisted
field lines make slightly more than one turn along the filament within
our field of view, which results in 0.055 turns Mm<SUP>-1</SUP>. The
convex part of the field lines (as seen from the solar surface)
constitutes dips where the plasma can naturally be supported. The
obtained three-dimensional magnetic structure of the filament depends
on the choice of the observed horizontal magnetic field as determined
from the 180° solution of the azimuth. We derive a method to check
for the correctness of the selected 180° ambiguity solution.
---------------------------------------------------------
Title: On the Origin of the Type II Spicules: Dynamic
Three-dimensional MHD Simulations
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Moreno-Insertis,
Fernando
2011ApJ...736....9M Altcode: 2010arXiv1011.4703M
Recent high temporal and spatial resolution observations of the
chromosphere have forced the definition of a new type of spicule, "type
II's," that are characterized by rising rapidly, having short lives,
and by fading away at the end of their lifetimes. Here, we report on
features found in realistic three-dimensional simulations of the outer
solar atmosphere that resemble the observed type II spicules. These
features evolve naturally from the simulations as a consequence of
the magnetohydrodynamical evolution of the model atmosphere. The
simulations span from the upper layer of the convection zone to
the lower corona and include the emergence of a horizontal magnetic
flux. The state-of-art Oslo Staggered Code is used to solve the full
MHD equations with non-gray and non-LTE radiative transfer and thermal
conduction along the magnetic field lines. We describe in detail the
physics involved in a process which we consider a possible candidate
for the driver mechanism that produces type II spicules. The modeled
spicule is composed of material rapidly ejected from the chromosphere
that rises into the corona while being heated. Its source lies in
a region with large field gradients and intense electric currents,
which lead to a strong Lorentz force that squeezes the chromospheric
material, resulting in a vertical pressure gradient that propels the
spicule along the magnetic field, as well as Joule heating, which
heats the jet material, forcing it to fade.
---------------------------------------------------------
Title: Mesogranulation and the Solar Surface Magnetic Field
Distribution
Authors: Yelles Chaouche, L.; Moreno-Insertis, F.; Martínez Pillet,
V.; Wiegelmann, T.; Bonet, J. A.; Knölker, M.; Bellot Rubio, L. R.;
del Toro Iniesta, J. C.; Barthol, P.; Gandorfer, A.; Schmidt, W.;
Solanki, S. K.
2011ApJ...727L..30Y Altcode: 2010arXiv1012.4481Y
The relation of the solar surface magnetic field with mesogranular
cells is studied using high spatial (≈100 km) and temporal (≈30
s) resolution data obtained with the IMaX instrument on board
SUNRISE. First, mesogranular cells are identified using Lagrange
tracers (corks) based on horizontal velocity fields obtained through
local correlation tracking. After ≈20 minutes of integration, the
tracers delineate a sharp mesogranular network with lanes of width
below about 280 km. The preferential location of magnetic elements in
mesogranular cells is tested quantitatively. Roughly 85% of pixels with
magnetic field higher than 100 G are located in the near neighborhood
of mesogranular lanes. Magnetic flux is therefore concentrated in
mesogranular lanes rather than intergranular ones. Second, magnetic
field extrapolations are performed to obtain field lines anchored in
the observed flux elements. This analysis, therefore, is independent
of the horizontal flows determined in the first part. A probability
density function (PDF) is calculated for the distribution of distances
between the footpoints of individual magnetic field lines. The PDF has
an exponential shape at scales between 1 and 10 Mm, with a constant
characteristic decay distance, indicating the absence of preferred
convection scales in the mesogranular range. Our results support
the view that mesogranulation is not an intrinsic convective scale
(in the sense that it is not a primary energy-injection scale of solar
convection), but also give quantitative confirmation that, nevertheless,
the magnetic elements are preferentially found along mesogranular lanes.
---------------------------------------------------------
Title: Solar Abundance Corrections Derived Through Three-dimensional
Magnetoconvection Simulations
Authors: Fabbian, D.; Khomenko, E.; Moreno-Insertis, F.; Nordlund, Å.
2010ApJ...724.1536F Altcode: 2010arXiv1006.0231F
We explore the effect of the magnetic field when using realistic
three-dimensional convection experiments to determine solar element
abundances. By carrying out magnetoconvection simulations with a
radiation-hydro code (the Copenhagen stagger code) and through a
posteriori spectral synthesis of three Fe I lines, we obtain evidence
that moderate amounts of mean magnetic flux cause a noticeable
change in the derived equivalent widths compared with those for a
non-magnetic case. The corresponding Fe abundance correction for a
mean flux density of 200 G reaches up to ~0.1 dex in magnitude. These
results are based on space- and time-averaged line profiles over a time
span of 2.5 solar hours in the statistically stationary regime of the
convection. The main factors causing the change in equivalent widths,
namely the Zeeman broadening and the modification of the temperature
stratification, act in different amounts and, for the iron lines
considered here, in opposite directions; yet, the resulting |Δlog
epsilon<SUB>sun</SUB>(Fe)| coincides within a factor of 2 in all
of them, even though the sign of the total abundance correction
is different for the visible and infrared lines. We conclude that
magnetic effects should be taken into account when discussing precise
values of the solar and stellar abundances and that an extended study
is warranted.
---------------------------------------------------------
Title: Galaxies at High Redshift
Authors: Pérez-Fournon, I.; Balcells, M.; Moreno-Insertis, F.;
Sánchez, F.
2010gahr.book.....P Altcode:
Participants; Group photograph; Preface; Acknowledgements; 1. Galaxy
formation and evolution: recent progress R. Ellis; 2. Galaxies at high
redshift M. Dickinson; 3. High-redshift galaxies: the far-infrared
and sub-millimeter view A. Franceschini; 4. Quasar absorption lines
J. Bechtold; 5. Stellar population synthesis models at low and high
redshift G. Bruzual A.; 6. Elliptical galaxies K. C. Freeman; 7. Disk
galaxies K. C. Freeman; 8. Dark matter in disk galaxies K. C. Freeman.
---------------------------------------------------------
Title: Comparison Of Observations And Advanced Numerical Simulations
Of Type II Spicules
Authors: Martinez-Sykora, Juan; De Pontieu, B.; Hansteen, V.;
Moreno-Insertis, F.
2010AAS...21640306M Altcode: 2010BAAS...41..878M
We have performed realistic 3D radiation MHD simulations of the
solar atmosphere. These simulations show jet-like features that
are similar to the type II spicules discovered with Hinode's Solar
Optical Telescope. These type II spicules have been associated with
so-called rapid blueshifted events (RBE's) on the solar disk, and with
significant blueward asymmetries in transition region and coronal
lines at the footpoints of coronal loops (discovered with Hinode's
EIS). These observational results and their ubiquity suggest they may
play a significant role in providing the corona with hot plasma. We
will present a detailed comparison of the properties of the simulated
jets, with those of type II spicules (observed with Hinode) and RBE's
(with ground-based instruments). We will present analysis of a wide
variety of synthetic emission lines from the simulations covering
temperatures from 10,000 K to several million K, and compare their
intensities, velocities, line widths and asymmetry with those of the
observed phenomena. We will also show how the formation mechanism of
these jets (reconnection at tangential discontinuities) complicates
efforts to establish a firm link between observations of magnetic
fields and of chromospheric flows, and suggests that magnetic field
observations at chromospheric heights may be crucial to establish from
observations how these jets are formed.
---------------------------------------------------------
Title: X-Ray Jets in Coronal Holes: Numerical Simulation and Hinode
Observations
Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I.
2009ASPC..415...51M Altcode:
We report on our recent 3D numerical models of the launching of hot,
high-speed jets in a coronal hole following the emergence of magnetized
plasma from the solar interior. As part of the same research, we have
also analyzed Hinode (EIS and XRT) and Soho-MDI observational data
of an actual process of flux emergence followed by jet launching in
a coronal hole. From the observations, we reconstruct the magnetic
topology at the emergence site and calculate velocity and further
physical properties of the observed event. The 3D model was calculated
for realistic conditions in a coronal hole, including, in particular,
a low-density (10<SUP>8</SUP> particles cm<SUP>-3</SUP>), high Alfven
speed plasma prior to the emergence. After emergence, a ribbon-like
current sheet is created at the site of collision of the emerging and
preexisting magnetic systems. Field line reconnection ensues, which
leads to the ejection of the X-Ray jet. We analyze the global magnetic
topology, and the temperature, velocity and current distribution in
the 3D experiment. The numerical results provide a good match to the
observed features of the coronal hole jets. This is meant regarding
both our own observational results as well as the ranges and average
values of the statistical study by Savcheva et al. (2007).
---------------------------------------------------------
Title: Magnetic flux emergence into the solar photosphere and
chromosphere
Authors: Tortosa-Andreu, A.; Moreno-Insertis, F.
2009A&A...507..949T Altcode:
Aims: We model the emergence of magnetized plasma across granular
convection cells and the low atmosphere, including layers up to the
mid-chromosphere. <BR />Methods: Three-dimensional numerical experiments
are carried out in which the equations of MHD and radiative transfer
are solved self-consistently. We use the MURaM code, which assumes local
thermodynamic equilibrium between plasma and radiation. <BR />Results:
In the photosphere, we find good agreement between our simulation
predictions and observational results obtained with the Hinode satellite
for the velocity and magnetic fields. We also confirm earlier simulation
results by other authors. Our experiments reveal a natural mechanism
of formation of twisted magnetic flux tubes that results from the
retraction of photospheric horizontal fields at new intergranular
lanes in decaying granules. In the chromosphere, we present evidence
for the non-radiative heating of the emerging magnetized plasma due to
the passage of shocks and/or ohmic dissipation. We study the formation
of high-temperature points in the magnetic domain. We detect two types
of points, classified according to whether they have a photospheric
counterpart or otherwise. We also find evidence of those two types in
Hinode observations. Using Lagrangian tracing of a large statistical
sample of fluid particles, we detect and study episodes of convective
collapse of magnetic elements returning to the photosphere. On the
other hand, we study the maximum heights reached by all tracers,
magnetized or otherwise. Only a small fraction (1.3%) of the magnetic
elements reach the mid-chromosphere (z>750 km), while virtually
no unmagnetized elements in the sample rise above the level of the
reverse granulation (a few 100 km above the photosphere). We find that
the rise into the chromosphere occurs in the form of successive jumps
with intermediate stops rather than in a smooth continuous fashion
and propose a tentative explanation of this behavior. Finally, also
using Lagrange tracing, we document the creation of high-temperature
points in the chromosphere via rising shock fronts.
---------------------------------------------------------
Title: The emergence of toroidal flux tubes from beneath the solar
photosphere
Authors: Hood, A. W.; Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.
2009A&A...503..999H Altcode:
Context: Models of flux emergence frequently use a twisted cylindrical
loop as the initial starting configuration and ignore the coupling
between the radiation field and plasma. In these models, the axis of
the original tube never emerges through the photosphere. Without the
axis emerging, it is very difficult to form a realistic sunspot. <BR
/>Aims: The aim is to use a toroidal flux loop, placed beneath the solar
photosphere and study whether the axis of the system emerges fully into
the atmosphere. The toroidal curvature means that the plasma can drain
more effectively than in a straight cylindrical tube. <BR />Methods:
Three-dimensional magnetohydrodynamic numerical simulations of an
emerging magnetic flux tube are presented for an initial toroidal loop
model. The simulations use a Lagrangian-Remap code that is particularly
suited to dealing with shocks and strong current sheets. <BR />Results:
The evolution of the toroidal loop is followed and the characteristics
of the emergence process are compared with the traditional cylindrical
loops. The flux sources seen at the photosphere are more circular,
and there are less shearing motions in the upper photosphere. When the
initial magnetic field strength is relatively weak the evolution of the
system is similar to the cylindrical loop case, with the formation of
a new flux rope above the photosphere. A striking result is that for
large values of field strength the axial field of the toroidal loop
emerges fully into the corona. This is reported for the first time in
experiments of flux emergence in a highly stratified atmosphere that do
not solve self-consistently the radiation transfer problem. In addition,
the new flux rope forms below the original axis of the toroidal tube
when the field strength is sufficiently strong.
---------------------------------------------------------
Title: X-ray jets and magnetic flux emergence in the Sun
Authors: Moreno-Insertis, Fernando
2009IAUS..259..201M Altcode:
Magnetized plasma is emerging continually from the solar interior into
the atmosphere. Magnetic flux emergence events and their consequences
in the solar atmosphere are being observed with high space, time and
spectral resolution by a large number of space missions in operation
at present (e.g. SOHO, Hinode, Stereo, Rhessi). The collision of
an emerging and a preexisting magnetic flux system in the solar
atmosphere leads to the formation of current sheets and to field
line reconnection. Reconnection under solar coronal conditions is an
energetic event; for the field strengths, densities and speeds involved
in the collision of emerging flux systems, the reconnection outflows
lead to launching of high-speed (hundreds of km/s), high-temperature
(10<SUP>7</SUP> K) plasma jets. Such jets are being observed with the
X-Ray and EUV detectors of ongoing satellite missions. On the other
hand, the spectacular increase in computational power in recent years
permits to carry out three-dimensional numerical experiments of the
time evolution of flux emerging systems and the launching of jets with
a remarkable degree of detail. <P />In this review, observation and
modeling of the solar X-Ray jets are discussed. A two-decade long
computational effort to model the magnetic flux emergence events
by different teams has led to numerical experiments which explain,
even quantitatively, many of the observed features of the X-ray
jets. The review points out that, although alternative mechanisms
must be considered, flux emergence is a prime candidate to explain
the launching of the solar jets.
---------------------------------------------------------
Title: Magnetic Flux Emergence and Jet Formation in Coronal Holes
Authors: Galsgaard, K.; Moreno-Insertis, F.
2008ESPM...12.3.27G Altcode:
Recent observations of coronal holes with Hinode show with unprecedented
detail the launching of fast and hot jets. Many of these jets are found
to coincide with the emergence of new magnetic flux, and it is generally
assumed that the jets are initiated by magnetic reconnection between
the new emerging flux and the existing open magnetic field. Further
to this a comparison of a larger sample of jets show that about 70%
of these are followed by the formation of plumes within minutes
to an hour. <P />How do we understand these events from a physical
point of view? To investigate this we have carried out numerical 3D
MHD experiment modeling the emergence of magnetic flux from the upper
convection zone into an open magnetic flux region resembling a coronal
hole. The emergence process drives the formation of a strong and highly
localised current sheet. Time-dependent reconnection in the current
sheet gives rise to a high-velocity jet that eventually flows along the
previously open coronal field lines. Initially the jet has transition
region temperature, but as time progresses it eventually exceeds
the coronal temperature in the model. Investigating the development
of the structure of the magnetic field, it is found that it changes
in a very characteristic way, leading to a horizontal drift of the
jet. The experiment also shows how the reconnection speed influences the
dynamical properties of both the jet parameters and the evolution of the
underlying magnetic structure. Towards the end of the experiment the jet
speed decreases and leaves a large funnel-like region above the emerging
flux domain with an enhanced temperature and density distribution.
---------------------------------------------------------
Title: Magnetic Flux Emergence in the Solar Photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
2008ASPC..384..181C Altcode: 2008csss...14..181C
The most prominent magnetic structures on the surface of the Sun are
bipolar active regions. These magnetic complexes are comprised of
a hierarchy of magnetic structures of different sizes, the largest
of which are sunspots. Observations indicate that the appearance
of active regions on the solar surface result from the emergence of
bundles of magnetic flux from the underlying convection zone. We study
the emergence process by means of 3D radiation MHD simulations. In the
simulations, an initially buoyant magnetic flux tube is introduced into
the near-surface layers of the convection zone. Subject to the buoyancy
force, the flux tube rises towards the photosphere. Our simulations
highlight the importance of magneto-convection on the evolution of
the magnetic flux tube. The external convective flow field has an
important influence on the emergence morphology of the emerging magnetic
field. Depending on the initial properties of the magnetic flux tube
(e.g. field strength, twist, entropy etc.), flux emergence may lead
to a disturbance of the local granulation pattern. The observational
signatures associated with emerging magnetic flux in our simulations
are in qualitative and quantitative agreement with observational
studies of emerging flux regions on the Sun.
---------------------------------------------------------
Title: Jets in Coronal Holes: Hinode Observations and
Three-dimensional Computer Modeling
Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I.
2008ApJ...673L.211M Altcode: 2007arXiv0712.1059M
Recent observations of coronal hole areas with the XRT and
EIS instruments on board the Hinode satellite have shown with
unprecedented detail the launching of fast, hot jets away from the
solar surface. In some cases these events coincide with episodes
of flux emergence from beneath the photosphere. In this Letter we
show results of a three-dimensional numerical experiment of flux
emergence from the solar interior into a coronal hole and compare
them with simultaneous XRT and EIS observations of a jet-launching
event that accompanied the appearance of a bipolar region in MDI
magnetograms. The magnetic skeleton and topology that result in the
experiment bear a strong resemblance to linear force-free extrapolations
of the SOHO/MDI magnetograms. A thin current sheet is formed at the
boundary of the emerging plasma. A jet is launched upward along the
open reconnected field lines with values of temperature, density,
and velocity in agreement with the XRT and EIS observations. Below the
jet, a split-vault structure results with two chambers: a shrinking one
containing the emerged field loops and a growing one with loops produced
by the reconnection. The ongoing reconnection leads to a horizontal
drift of the vault-and-jet structure. The timescales, velocities, and
other plasma properties in the experiment are consistent with recent
statistical studies of this type of event made with Hinode data.
---------------------------------------------------------
Title: Photospheric Magnetic Flux Emergence: A comparative study
between Hinode/SOT Observations and MHD simulations
Authors: Cheung, M. C.; Schüssler, M.; Moreno-Insertis, F.; Tarbell,
T. D.
2007AGUFMSH53A1073C Altcode:
With high angular resolution, high temporal cadence and a stable
point spread function, the Solar Optical Telescope (SOT) onboard the
Hinode satellite is the ideal instrument for the study of magnetic
flux emergence and its manifestations on the solar surface. In this
presentation, we focus on the development of ephemeral regions and
small active regions. In many instances, SOT has been able to capture
the entire emergence process from beginning to end: i.e. from the
initial stages of flux appearance in granule interiors, through the
intermediate stages of G-band bright point formation, and finally
to the coalescence of small vertical flux elements to form pores. To
investigate the physics of the flux emergence process, we performed
3D numerical MHD simulations with the MURaM code. The models are able
to reproduce, and help us explain, various observational signatures
of magnetic flux emergence.
---------------------------------------------------------
Title: Three-dimensional numerical experiments of flux emergence
into the corona
Authors: Moreno-Insertis, F.
2007ASPC..369..335M Altcode:
Understanding the complicated non-linear thermal and magnetodynamic
processes observed to continuously reshape the solar atmosphere requires
the use of three-dimensional computer simulations and numerical
experimentation. In this paper, a number of results from the recent
generation of 3D numerical models of magnetic flux emergence from the
topmost layers of the solar interior to the corona are reviewed. Four
main areas are covered: current sheet formation, plasmoid generation and
ejection, 3D reconnection and jet launching. Multidimensional computer
simulation of MHD and radiative processes in the solar atmosphere is
particularly important at present given the high-quality data expected
from Solar B and other satellites in the coming years.
---------------------------------------------------------
Title: The Effect of the Relative Orientation between the Coronal
Field and New Emerging Flux. I. Global Properties
Authors: Galsgaard, K.; Archontis, V.; Moreno-Insertis, F.; Hood, A. W.
2007ApJ...666..516G Altcode: 2007arXiv0705.1097G
The emergence of magnetic flux from the convection zone into the
corona is an important process for the dynamical evolution of the
coronal magnetic field. In this paper we extend our previous numerical
investigations, by looking at the process of flux interaction as an
initially twisted flux tube emerges into a plane-parallel, coronal
magnetic field. Significant differences are found in the dynamical
appearance and evolution of the emergence process depending on the
relative orientation between the rising flux system and any preexisting
coronal field. When the flux systems are nearly antiparallel, the
experiments show substantial reconnection and demonstrate clear
signatures of a high-temperature plasma located in the high-velocity
outflow regions extending from the reconnection region. However, the
cases that have a more parallel orientation of the flux systems show
very limited reconnection and none of the associated features. Despite
the very different amount of reconnection between the two flux systems,
it is found that the emerging flux that is still connected to the
original tube reaches the same height as a function of time. As a
compensation for the loss of tube flux, a clear difference is found
in the extent of the emerging loop in the direction perpendicular to
the main axis of the initial flux tube. Increasing amounts of magnetic
reconnection decrease the volume, which confines the remaining tube
flux.
---------------------------------------------------------
Title: Magnetic Flux Emergence In Granular Convection: Radiative
MHD Simulations And Hinode SOT Observations
Authors: Cheung, Mark; Schüssler, M.; Moreno-Insertis, F.; Tarbell,
T.; SOT Team
2007AAS...210.9425C Altcode: 2007BAAS...39..221C
We model the emergence of buoyant magnetic flux from the convection
zone into the photosphere by means of 3D radiative MHD simulations
using the MURaM code. In a series of simulations, we study how
an initially buoyant magnetic flux tube rises in the presence of
granular convection. The simulations take into account the effects of
radiative energy exchange, ionization effects in the equation of state
and compressibility. An emphasis of this talk is the comparison of
observational diagnostics from the simulations with recent observations
from Hinode SOT.
---------------------------------------------------------
Title: Magnetic flux emergence in granular convection: radiative
MHD simulations and observational signatures
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
2007A&A...467..703C Altcode: 2007astro.ph..2666C
Aims:We study the emergence of magnetic flux from the near-surface
layers of the solar convection zone into the photosphere. <BR />Methods:
To model magnetic flux emergence, we carried out a set of numerical
radiative magnetohydrodynamics simulations. Our simulations take into
account the effects of compressibility, energy exchange via radiative
transfer, and partial ionization in the equation of state. All these
physical ingredients are essential for a proper treatment of the
problem. Furthermore, the inclusion of radiative transfer allows us
to directly compare the simulation results with actual observations
of emerging flux. <BR />Results: We find that the interaction between
the magnetic flux tube and the external flow field has an important
influence on the emergent morphology of the magnetic field. Depending
on the initial properties of the flux tube (e.g. field strength,
twist, entropy etc.), the emergence process can also modify the local
granulation pattern. The emergence of magnetic flux tubes with a
flux of 10<SUP>19</SUP> Mx disturbs the granulation and leads to the
transient appearance of a dark lane, which is coincident with upflowing
material. These results are consistent with observed properties of
emerging magnetic flux. <P />Movies are only available in electronic
form at http://www.aanda.org
---------------------------------------------------------
Title: The origin of the reversed granulation in the solar photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
2007A&A...461.1163C Altcode: 2006astro.ph.12464C
Aims:We study the structure and reveal the physical nature of the
reversed granulation pattern in the solar photosphere by means of
3-dimensional radiative hydrodynamics simulations. <BR />Methods: We
used the MURaM code to obtain a realistic model of the near-surface
layers of the convection zone and the photosphere. <BR />Results:
The pattern of horizontal temperature fluctuations at the base of
the photosphere consists of relatively hot granular cells bounded by
the cooler intergranular downflow network. With increasing height
in the photosphere, the amplitude of the temperature fluctuations
diminishes. At a height of z=130-140 km in the photosphere, the pattern
of horizontal temperature fluctuations reverses so that granular regions
become relatively cool compared to the intergranular network. Detailed
analysis of the trajectories of fluid elements through the photosphere
reveal that the motion of the fluid is non-adiabatic, owing to strong
radiative cooling when approaching the surface of optical depth
unity followed by reheating by the radiation field from below. The
temperature structure of the photosphere results from the competition
between expansion of rising fluid elements and radiative heating. The
former acts to lower the temperature of the fluid whereas the latter
acts to increase it towards the radiative equilibrium temperature with
a net entropy gain. After the fluid overturns and descends towards the
convection zone, radiative energy loss again decreases the entropy
of the fluid. Radiative heating and cooling of fluid elements that
penetrate into the photosphere and overturn do not occur in equal
amounts. The imbalance in the cumulative heating and cooling of
these fluid elements is responsible for the reversal of temperature
fluctuations with respect to height in the photosphere.
---------------------------------------------------------
Title: Flux Emergence at the Photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
2006ASPC..354...97C Altcode:
To model the emergence of magnetic fields at the photosphere, we
carried out 3D magneto-hydrodynamics (MHD) simulations using the MURaM
code. Our simulations take into account the effects of compressibility,
energy exchange via radiative transfer and partial ionization in the
equation of state. All these physical ingredients are essential for
a proper treatment of the problem. In the simulations, an initially
buoyant magnetic flux tube is embedded in the upper layers of the
convection zone. We find that the interaction between the flux tube
and the external flow field has an important influence on the emergent
morphology of the magnetic field. Depending on the initial properties of
the flux tube (e.g. field strength, twist, entropy etc.), the emergence
process can also modify the local granulation pattern. The inclusion
of radiative transfer allows us to directly compare the simulation
results with real observations of emerging flux.
---------------------------------------------------------
Title: 3D simulations identifying the effects of varying the twist
and field strength of an emerging flux tube
Authors: Murray, M. J.; Hood, A. W.; Moreno-Insertis, F.; Galsgaard,
K.; Archontis, V.
2006A&A...460..909M Altcode:
Aims.We investigate the effects of varying the magnetic field strength
and the twist of a flux tube as it rises through the solar interior
and emerges into the atmosphere.<BR /> Methods: .Using a 3D numerical
MHD code, we consider a simple stratified model, comprising of one
solar interior layer and three overlying atmospheric layers. We set
a horizontal, twisted flux tube in the lowest layer. The specific
balance of forces chosen results in the tube being fully buoyant and
the temperature is decreased in the ends of the tube to encourage the
formation of an Ω-shape along the tube's length. We vary the magnetic
field strength and twist independently of each other so as to give clear
results of the individual effects of each parameter.<BR /> Results:
.We find a self-similar evolution in the rise and emergence of the flux
tube when the magnetic field strength of the tube is modified. During
the rise through the solar interior, the height of the crest and
axis, the velocity of the crest and axis, and the decrease in the
magnetic field strength of the axis of the tube are directly dependent
upon the initial magnetic field strength given to the tube. No such
self-similarity is evident when the twist of the flux tube is changed,
due to the complex interaction of the tension force on the rise of the
tube. For low magnetic field strength and twist values, we find that
the tube cannot fully emerge into the atmosphere once it reaches the
top of the interior since the buoyancy instability criterion cannot
be fulfilled. For those tubes that do advance into the atmosphere,
when the magnetic field strength has been modified, we find further
self-similar behaviour in the amount of tube flux transported into
the atmosphere. For the tubes that do emerge, the variation in the
twist results in the buoyancy instability, and subsequent emergence,
occurring at different locations along the tube's length.<BR />
---------------------------------------------------------
Title: Flux Emergence from the Solar Interior to the Atmosphere:
the Passage through the Photosphere
Authors: Moreno-Insertis, F.
2006ASPC..354..183M Altcode:
The atmosphere of the Sun is being stirred and reshaped continuously
through the emergence of magnetized plasma from the convection
zone. Magnetic elements coming from the solar interior produce
structures in the photosphere, chromosphere and corona in a bewildering
range of length and time scales, from the smallest magnetic tubes
to the largest active regions. Although still at an early stage, the
computer simulation of these intrinsically three-dimensional processes
can already provide important physical insight and explain some of
the observational results obtained with ground-based detectors and in
space. This review summarizes the challenge posed by flux emergence
processes to numerical simulation, discusses why the magnetic plasma
can rise through the photosphere in spite of the latter's subadiabatic
stratification and presents a number of results on active region
formation at the photosphere obtained in recent years.
---------------------------------------------------------
Title: Three-dimensional Plasmoid Evolution in the Solar Atmosphere
Authors: Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.; Hood, A. W.
2006ApJ...645L.161A Altcode:
We present clear evidence of the formation of three-dimensional (3D)
plasmoids in the current sheet between two magnetic flux systems in a
3D numerical experiment of flux emergence into the solar atmosphere and
study their properties and time evolution. Plasmoids are most likely
the result of resistive tearing mode instabilities. They adopt the
shape of a solenoid contained within the current sheet: the solenoid
is tightly wound when the field in the two flux systems is close to
antiparallel. The plasmoids are expelled to the sides of the sheet as
a result of a reconnection imbalance between the two x-lines on their
sides. We show the complex, 3D field line geometry in various plasmoids:
individual plasmoid field lines have external linkages to the flux
system on either side of the current sheet; we also find field lines
that go through a few plasmoids in succession, probably indicating
that the field line has resulted from multiple reconnection events.
---------------------------------------------------------
Title: Moving magnetic tubes: fragmentation, vortex streets and the
limit of the approximation of thin flux tubes
Authors: Cheung, M. C. M.; Moreno-Insertis, F.; Schüssler, M.
2006A&A...451..303C Altcode:
Aims.We study the buoyant rise of magnetic flux tubes in a stratified
layer over a range of Reynolds numbers (25 ⪉ Re ⪉ 2600) by means
of numerical simulations. Special emphasis is placed on studying the
fragmentation of the rising tube, its trailing wake and the formation
of a vortex street in the high-Reynolds number regime. Furthermore,
we evaluate the relevance of the thin flux tube approximation
with regard to describing the evolution of magnetic flux tubes
in the simulations.<BR /> Methods: .We used the FLASH code, which
has an adaptive mesh refinement (AMR) algorithm, thus allowing the
simulations to be carried out at high Reynolds numbers.<BR /> Results:
.The evolution of the magnetic flux tube and its wake depends on the
Reynolds number. At Re up to a few hundred, the wake consists of two
counter-rotating vortex rolls. At higher Re, the vortex rolls break up
and the shedding of flux into the wake occurs in a more intermittent
fashion. The amount of flux retained by the central portion of the
tube increases with the field line twist (in agreement with previous
literature) and with Re. The time evolution of the twist is compatible
with a homologous expansion of the tube. The motion of the central
portion of the tube in the simulations is very well described by the
thin flux tube model whenever the effects of flux loss or vortex forces
can be neglected. If the flux tube has an initial net vorticity, it
undergoes asymmetric vortex shedding. In this case, the lift force
accelerates the tube in such a way that an oscillatory horizontal
motion is super-imposed on the vertical rise of the tube, which leaves
behind a vortex street. This last result is in accordance with previous
simulations reported in the literature, which were carried out at
lower Reynolds number.<BR />
---------------------------------------------------------
Title: Flux emergence and interaction with a coronal field: 3D
MHD simulations
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W.
2006IAUS..233...53A Altcode:
The dynamic process of magnetic flux emergence from the solar interior
to the outer atmosphere may well be related with eruptive phenomena
and intense events of the Solar activity. However, the physics of the
emergence is not still well understood. Thus, we have performed 3D MHD
simulations to study the rising motion of a twisted flux tube from the
convection zone of the Sun and its interaction with a preexisting
coronal magnetic field. The results show that the reconnection
process depends criticaly on the initial relative orientation between
the two magnetic flux systems into contact. On the other hand, the
overal process of emergence depends mostly on the dynamics of the
sub-photospheric plasma.
---------------------------------------------------------
Title: Magnetic flux emergence into the atmosphere
Authors: Moreno-Insertis, F.
2006IAUS..233...33M Altcode:
Understanding the mechanism that causes the emergence of magnetic flux
from the solar interior to the atmosphere, the drastic changes in the
properties of the matter and magnetic fields along the rise and the
interplay of dynamic and resistive phenomena that shape the emerged
regions is one of the major open tasks in solar physics. Important
advances are being made both in the theoretical modelling and in the
observation of the emergence events. This review concentrates on recent
advances through 3D numerical experiments carried out with massively
parallel MHD and radiative transfer codes.
---------------------------------------------------------
Title: The Three-dimensional Interaction between Emerging Magnetic
Flux and a Large-Scale Coronal Field: Reconnection, Current Sheets,
and Jets
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W.
2005ApJ...635.1299A Altcode:
Using MHD numerical experiments in three dimensions, we study the
emergence of a bipolar magnetic region from the solar interior
into a model corona containing a large-scale, horizontal magnetic
field. An arch-shaped concentrated current sheet is formed at the
interface between the rising magnetized plasma and the ambient
coronal field. Three-dimensional reconnection takes place along
the current sheet, so that the corona and the photosphere become
magnetically connected, a process repeatedly observed in recent
satellite missions. We show the structure and evolution of the
current sheet and how it changes in time from a simple tangential
discontinuity to a rotational discontinuity with no null surface. We
find clear indications that individual reconnection events in this
three-dimensional environment in the advanced stage are not one-off
events, but instead take place in a continuous fashion, with each
field line changing connectivity during a finite time interval. We
also show that many individual field lines of the rising tube undergo
multiple processes of reconnection at different points in the corona,
thus creating photospheric pockets for the coronal field. We calculate
global measures for the amount of subphotospheric flux that becomes
linked to the corona during the experiment and find that most of
the original subphotospheric flux becomes connected to coronal field
lines. The ejection of plasma from the reconnection site gives rise to
high-speed and high-temperature jets. The acceleration mechanism for
those jets is akin to that found in previous two-dimensional models,
but the geometry of the jets bears a clear three-dimensional imprint,
having a curved-sheet appearance with a sharp interface to the overlying
coronal magnetic field system. Temperatures and velocities of the jets
in the simulations are commensurate with those measured in soft X-rays
by the Yohkoh satellite.
---------------------------------------------------------
Title: D Magneto-Convection and Flux Emergence in the Photosphere
Authors: Cheung, M.; Schüssler, M.; Moreno-Insertis, F.
2005ESASP.596E..54C Altcode: 2005ccmf.confE..54C
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Flux Emergence and its Interaction with AN Existing
Coronal Field
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ESASP.596E..27G Altcode: 2005ccmf.confE..27G
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Flux Emergence and its Interaction with AN Existing
Coronal Field
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ESASP.596E..55G Altcode: 2005ccmf.confE..55G
No abstract at ADS
---------------------------------------------------------
Title: A Three-dimensional Study of Reconnection, Current Sheets,
and Jets Resulting from Magnetic Flux Emergence in the Sun
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ApJ...618L.153G Altcode: 2004astro.ph.10057G
We present the results of a set of three-dimensional numerical
simulations of magnetic flux emergence from below the photosphere
and into the corona. The corona includes a uniform and horizontal
magnetic field as a model for a preexisting large-scale coronal
magnetic system. Cases with different relative orientations of the
upcoming and coronal fields are studied. Upon contact, a concentrated
current sheet with the shape of an arch is formed at the interface
that marks the positions of maximum jump in the field vector between
the two systems. Relative angles above 90° yield abundant magnetic
reconnection and plasma heating. The reconnection is seen to be
intrinsically three-dimensional in nature and to be accompanied by
marked local heating. It generates collimated high-speed outflows
only a short distance from the reconnection site, and these propagate
along the ambient magnetic field lines as jets. As a result of the
reconnection, magnetic field lines from the magnetized plasma below
the surface end up connecting to coronal field lines, thus causing a
profound change in the connectivity of the magnetic regions in the
corona. The experiments presented here yield a number of features
repeatedly observed with the TRACE and Yohkoh satellites, such as the
establishment of connectivity between emergent and preexisting active
regions, local heating, and high-velocity outflows.
---------------------------------------------------------
Title: Flux Emergence from the Solar Interior Into a Uniformly
Magnetized Corona
Authors: Moreno-Insertis, F.; Galsgaard, K.; Archontis, V.; Hood, A.
2004ESASP.575..216M Altcode: 2004soho...15..216M
No abstract at ADS
---------------------------------------------------------
Title: 3D MHD Simulations on Magnetic Flux Emergence
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A.
2004ESASP.575..342A Altcode: 2004soho...15..342A
No abstract at ADS
---------------------------------------------------------
Title: Emergence of magnetic flux from the convection zone into
the corona
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood,
A.; O'Shea, E.
2004A&A...426.1047A Altcode:
Numerical experiments of the emergence of magnetic flux from the
uppermost layers of the solar interior to the photosphere and its
further eruption into the low atmosphere and corona are carried out. We
use idealized models for the initial stratification and magnetic
field distribution below the photosphere similar to those used for
multidimensional flux emergence experiments in the literature. The
energy equation is adiabatic except for the inclusion of ohmic and
viscous dissipation terms, which, however, become important only at
interfaces and reconnection sites. Three-dimensional experiments for the
eruption of magnetic flux both into an unmagnetized corona and into a
corona with a preexisting ambient horizontal field are presented. The
shocks preceding the rising plasma present the classical structure of
nonlinear Lamb waves. The expansion of the matter when rising into the
atmosphere takes place preferentially in the horizontal directions: a
flattened (or oval) low plasma-β ball ensues, in which the field lines
describe loops in the corona with increasing inclination away from the
vertical as one goes toward the sides of the structure. Magnetograms
and velocity field distributions on horizontal planes are presented
simultaneously for the solar interior and various levels in the
atmosphere. Since the background pressure and density drop over many
orders of magnitude with increasing height, the adiabatic expansion
of the rising plasma yields very low temperatures. To avoid this, the
entropy of the rising fluid elements should be increased to the high
values of the original atmosphere via heating mechanisms not included in
the present numerical experiments. The eruption of magnetic flux into
a corona with a preexisting magnetic field pointing in the horizontal
direction yields a clear case of essentially three-dimensional
reconnection when the upcoming and ambient field systems come into
contact. The coronal ambient field is chosen at time t=0 perpendicular
to the direction of the tube axis and thus, given the twist of the
magnetic tube, almost anti-parallel to the field lines at the upper
boundary of the rising plasma ball. A thin, dome-shaped current layer
is formed at the interface between the two flux systems, in which ohmic
dissipation and heating are taking place. The reconnection proceeds
by merging successive layers on both sides of the reconnection site;
however, this occurs not only at the cusp of the interface, but, also,
gradually along its sides in the direction transverse to the ambient
magnetic field. The topology of the magnetic field in the atmosphere
is thereby modified: the reconnected field lines typically are part of
the flanks of the tube below the photosphere but then join the ambient
field system in the corona and reach the boundaries of the domain as
horizontal field lines.
---------------------------------------------------------
Title: The Emergence of Magnetic Field into Stellar Atmospheres
Authors: Moreno-Insertis, F.
2004Ap&SS.292..587M Altcode:
The magnetic activity in the atmosphere of the Sun, possibly of most
other magnetically active late-type stars, is a consequence of the
emergence of magnetic flux from the stellar interior. Research in
this field has to consider the rise of magnetized plasma from the
depths of the convective envelope through the stellar surface and out
across the corona. In the present review, a number of key questions,
difficulties and results are discussed.
---------------------------------------------------------
Title: Galaxies at High Redshift
Authors: Pérez-Fournon, Ismael; Balcells, Marc; Moreno-Insertis,
Fernando; Sánchez, Francisco
2003ghr..conf.....P Altcode:
This book covers normal galaxies, distant galaxies, studies based on
far-infrared diagnostics, quasar absorption lines, and the properties
of nearby galaxies. The timely volume provides an essential reference
for astronomers working in the field of high-redshift galaxies. It
includes the lectures delivered at the XI Canary Islands Winter
School of Astrophysics, and reviews scientific results as well as main
questions in the field.
---------------------------------------------------------
Title: Galaxies at High Redshift
Authors: Pérez-Fournon, I.; Balcells, M.; Moreno-Insertis, F.;
Sánchez, F.
2003gahr.book.....P Altcode:
Participants; Group photograph; Preface; Acknowledgements; 1. Galaxy
formation and evolution: recent progress R. Ellis; 2. Galaxies at high
redshift M. Dickinson; 3. High-redshift galaxies: the far-infrared
and sub-millimeter view A. Franceschini; 4. Quasar absorption lines
J. Bechtold; 5. Stellar population synthesis models at low and high
redshift G. Bruzual A.; 6. Elliptical galaxies K. C. Freeman; 7. Disk
galaxies K. C. Freeman; 8. Dark matter in disk galaxies K. C. Freeman.
---------------------------------------------------------
Title: Preface (Galaxies at high redshift)
Authors: Pérez-Fournon, Ismael; Balcells, Marc; Moreno-Insertis,
Fernando; Sánchez, Francisco
2003ghr..confD..13P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal properties of magnetic flux tubes. I. Solution of
the diffusion problem
Authors: Moreno-Insertis, F.; Schüssler, M.; Glampedakis, K.
2002A&A...388.1022M Altcode:
The heat flow and temperature structure within and surrounding a
magnetic flux tube stored in mechanical equilibrium in a stellar
convection zone are considered. The stationary thermal equilibrium
state is determined through the analytical solution of a two-dimensional
heat diffusion problem for an infinitely long cylinder with different
thermal conductivities inside and outside the cylinder, both spatially
variable. In the exterior of the cylinder, convective heat transport
is approximated in terms of a linear diffusive process, while in its
interior convection is assumed to be suppressed and only the much
smaller radiative conductivity remains. The results show that, under
the conditions prevailing near the bottom of the solar convection zone
and in the limit of small cylinder radius, the temperature disturbance
(thermal shadow) in the exterior of the insulating cylinder is
almost negligible due to the large effiency of convective energy
transport. The spatial dependence of the conductivities and the
curvature of the external temperature profile lead to a temperature
excess in the interior with respect to the undisturbed temperature
profile far away from the cylinder. We show that, within the framework
of the thin magnetic flux tube approximation, this temperature excess
is due to a heating term equal to the negative divergence of the
undisturbed radiative heat flow, as suggested earlier by Fan &
Fisher (\cite{Fan:Fisher:1996}). These results are independent of the
treatment of the convective transport in the exterior as long as the
stratification is almost adiabatic. The consequences for the storage
of magnetic flux in the solar convection zone, brought about by the
enhanced buoyancy and caused by the heating effect, are discussed.
---------------------------------------------------------
Title: Thermal Instability in a Cooling and Expanding Medium Including
Self-Gravity and Conduction
Authors: Gomez-Pelaez, A. J.; Moreno-Insertis, F.
2002ApJ...569..766G Altcode: 2002astro.ph..1150G
A systematic study of the linear thermal stability of a medium subject
to cooling, self-gravity, and thermal conduction is carried out for
the case when the unperturbed state is subject to global cooling
and expansion. A general, recursive WKB solution for the perturbation
problem is obtained that can be applied to a large variety of situations
in which there is a separation of timescales for different physical
processes. Solutions are explicitly given and discussed for the case in
which sound propagation and/or self-gravity are the fastest processes,
with cooling, expansion, and thermal conduction operating on slower
timescales. A brief discussion is also added for the solutions in
the cases in which cooling or conduction operate on the fastest
timescale. The general WKB solution obtained in this paper permits
solving the problem of the effect of thermal conduction and self-gravity
on the thermal stability of a globally cooling and expanding medium. As
a result of the analysis, the critical wavelength (often called the
“Field length”) above which cooling makes the perturbations unstable
against the action of thermal conduction is generalized to the case
of an unperturbed background with net cooling. As an astrophysical
application, the “generalized Field length” is calculated for a hot
(10<SUP>4</SUP>-10<SUP>8</SUP> K), optically thin medium (as pertains,
for instance, for the hot interstellar medium of supernova remnants or
superbubbles) using a realistic cooling function and including a weak
magnetic field. The stability domains are compared with the predictions
made on the basis of models for which the background is in thermal
equilibrium. The instability domain of the sound waves in particular
is seen to be much larger in the case with net global cooling.
---------------------------------------------------------
Title: Astrophysical spectropolarimetry
Authors: Trujillo-Bueno, J.; Moreno-Insertis, F.; Sánchez, F.
2002apsp.conf.....T Altcode:
The polarization of light is the key to obtaining a wealth of essential
information that lies encoded in the electromagnetic radiation from
cosmic objects. Spectropolarimetry and imaging polarimetry provide
powerful diagnostics of the physical conditions in astrophysical
plasmas, which cannot be obtained via conventional spectroscopy. Whilst
its application to other fields of astrophysics is still at an early
stage of development, spectropolarimetry is being used with great
success in solar physics. The book contains the lectures delivered
at the XII Canary Islands Winter School of Astrophysics. Written by
eight prestigious astrophysics researchers, it covers the physics of
polarization, polarized radiation diagnostics of solar magnetic fields,
stellar magnetic fields, polarization insights for active galactic
nuclei, compact objects and accretion disks, astronomical masers and
their polarization, infrared-submillimeter spectropolarimetry, and
instrumentation for astrophysical spectropolarimetry. This timely volume
will provide graduate students and researchers with an unprecedented
introduction to the field of astrophysical spectropolarimetry.
---------------------------------------------------------
Title: Preface (Astrophysical spectropolarimetry)
Authors: Trujillo-Bueno, Javier; Moreno-Insertis, Fernando
2002apsp.confD..11T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Astrophysical Spectropolarimetry
Authors: Trujillo-Bueno, Javier; Moreno-Insertis, Fernando; Sanchez
Martinez, Francisco
2002assp.book.....T Altcode:
This book contains the lectures delivered at the XII Canary Islands
Winter School of Astrophysics on Astrophysical Spectropolarimetry. It
highlights how recent developments in theoretical astrophysics and
astronomical instrumentation are leading an ever-growing number of
astrophysicists to appreciate the enormous diagnostic potential offered
by spectropolarimetry.
---------------------------------------------------------
Title: The Zigzag Path of Buoyant Magnetic Tubes and the Generation
of Vorticity along Their Periphery
Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P.
2001ApJ...549.1212E Altcode:
We study the generation of vorticity in the magnetic boundary layer
of buoyant magnetic tubes and its consequences for the trajectory of
magnetic structures rising in the solar convection zone. When the
Reynolds number is well above 1, the wake trailing the tube sheds
vortex rolls, producing a von Kármán vortex street, similar to the
case of flows around rigid cylinders. The shedding of a vortex roll
causes an imbalance of vorticity in the tube. The ensuing vortex force
excites a transverse oscillation of the flux tube as a whole so that
it follows a zigzag upward path instead of rising along a straight
vertical line. In this paper, the physics of vorticity generation in
the boundary layer is discussed and scaling laws for the relevant terms
are presented. We then solve the two-dimensional magnetohydrodynamic
equations numerically, measure the vorticity production, and show the
formation of a vortex street and the consequent sinusoidal path of the
magnetic flux tube. For high values of the plasma beta, the trajectory
of the tubes is found to be independent of β but varying with the
Reynolds number. The Strouhal number, which measures the frequency
of vortex shedding, shows in our rising tubes only a weak dependence
with the Reynolds numbers, a result also obtained in the rigid-tube
laboratory experiments. In fact, the actual values measured in the
latter are also close to those of our numerical calculations. As
the Reynolds numbers are increased, the amplitude of the lift force
grows and the trajectory becomes increasingly complicated. It is
shown how a simple analytical equation (which includes buoyancy,
drag, and vortex forces) can satisfactorily reproduce the computed
trajectories. The different regimes of rise can be best understood in
terms of a dimensionless parameter, χ, which measures the importance
of the vortex force as compared with the buoyancy and drag forces. For
χ<SUP>2</SUP><<1, the rise is drag dominated and the trajectory
is mainly vertical with a small lateral oscillation superposed. When
χ becomes larger than 1, there is a transition toward a drag-free
regime and epicycles are added to the trajectory.
---------------------------------------------------------
Title: Magnetic Fields in Cool Stars (CD-ROM Directory:
contribs/moreno)
Authors: Moreno Insertis, F.; Saar, S. H.; Solanki, S. K.
2001ASPC..223..435M Altcode: 2001csss...11..435M
No abstract at ADS
---------------------------------------------------------
Title: Storage of a Strong Magnetic Field Below the Solar Convection
Zone (CD-ROM Directory: contribs/rempel)
Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F.; Tóth, G.
2001ASPC..223..738R Altcode: 2001csss...11..738R
No abstract at ADS
---------------------------------------------------------
Title: Solar Interior: Emerging Magnetic Flux Tubes
Authors: Moreno-Insertis, F.
2000eaa..bookE2521M Altcode:
The magnetic field observed in the photosphere and at higher levels
of the solar atmosphere is generated in the convective envelope that
extends along the first 200 000 km below the solar surface (see DYNAMOS:
SOLAR AND STELLAR and SOLAR INTERIOR). To properly understand the solar
magnetic cycle, it is necessary to explain the processes whereby the
magnetic field, once generated, emerges across ...
---------------------------------------------------------
Title: The Dynamics of Buoyant Magnetic Ropes and the Generation of
Vorticity in their Periphery
Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P.
2000SPD....31.0133E Altcode: 2000BAAS...32..807E
When the Reynolds number is not small, the wake trailing a buoyant
magnetic flux tube sheds vortex rolls therefore producing a Von Karman
vortex street and an imbalance of vorticity in the tube which results
in a transverse oscillation of the tube as a whole. The actual path
followed by the magnetic structure is therefore directly affected by
the amount of vorticity being produced in its boundary. Analytical
expressions for the magnetic generation and viscous dissipation of
vorticity in the boundary layer of buoyant magnetic flux tubes are
obtained. Corresponding scaling laws are deduced and checked using a
full compressible 2D MHD code. Interestingly, the observed trajectories
can be satisfactorily reproduced by a simple analytical equation (which
includes buoyancy, drag and vortex forces). I will conclude with some
comparisons with classical results from the hydrodynamical literature
(Strouhal number), and some comments about the rise time of buoyant
magnetic structures through the solar convection zone.
---------------------------------------------------------
Title: Effects of Non-Uniform Thermal Conduction on Solar
Convection-Zone Flux Tubes
Authors: Lenz, D. D.; Moreno-Insertis, F.
2000SPD....31.0137L Altcode: 2000BAAS...32..807L
The dynamo model of solar magnetic field generation assumes that
magnetic flux is retained at the dynamo site for times of the order of
the solar-cycle period. However, flux tubes in the solar convection
zone are expected to be buoyant, rising to the surface on timescales
much shorter than the solar cycle. Since the initial 1955 paper by
Parker on this puzzle, there have been numerous investigations into the
detailed physics of buoyant flux tubes, but no definitive conclusions
have yet been reached. We investigate the role of thermal conduction
in flux tube dynamics using MHD simulations. We expect that the thermal
conductivity can vary with conditions inside the tube relative to those
in the ambient fluid and that such variation in turn affects the tube's
energetics and evolution. Preliminary results suggest that suppressed
thermal conductivity inside the tube can significantly affect tube
morphology and evolution, depending on the characteristics of the tube
and its surroundings. We discuss possible implications of our results
for solar surface magnetic fields. D.D.L. acknowledges support from an
NSF-NATO Postdoctoral Fellowship in Science and Engineering. We thank
Thierry Emonet for helpful discussions. We are grateful to Mark Rast
for the use of his MHD code. The numerical simulations were performed
on the Cray T3E at CIEMAT in Madrid, Spain.
---------------------------------------------------------
Title: Distribution of magnetic flux on the solar surface and
low-degree p-modes
Authors: Moreno-Insertis, F.; Solanki, S. K.
2000MNRAS.313..411M Altcode:
The frequencies of solar p-modes are known to change over the solar
cycle. There is also recent evidence that the relation between frequency
shift of low-degree modes and magnetic flux or other activity indicators
differs between the rising and falling phases of the solar cycle,
leading to a hysteresis in such diagrams. We consider the influence
of the changing large-scale surface distribution of the magnetic
flux on low-degree (l<=3) p-mode frequencies. To that end, we use
time-dependent models of the magnetic flux distribution and study the
ensuing frequency shifts of modes with different order and degree as a
function of time. The resulting curves are periodic functions (in simple
cases just sine curves) shifted in time by different amounts for the
different modes. We show how this may easily lead to hysteresis cycles
comparable to those observed. Our models suggest that high-latitude
fields are necessary to produce a significant difference in hysteresis
between odd- and even-degree modes. Only magnetic field distributions
within a small parameter range are consistent with the observations
by Jiménez-Reyes et al. Observations of p-mode frequency shifts
are therefore capable of providing an additional diagnostic of the
magnetic field near the solar poles. The magnetic distribution that
is consistent with the p-mode observations also appears reasonable
compared with direct measurements of the magnetic field.
---------------------------------------------------------
Title: Storage of toroidal magnetic field below the solar convection
zone
Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F.
1999AGAb...15R..74R Altcode: 1999AGM....15..J15R
Simulations of erupting flux tubes in the thin flux tube approximation
show that essential properties of sunspots can only be explained if the
initial field strength of the flux tube at the base of the convection
zone is about 10 T. Such strong magnetic field can only be stored below
the solar convection zone in a subadiabatic stratification. We consider
mechanical equilibria in form of magnetic flux tubes and magnetic sheets
and discuss the influence of radiative and convective energy transport
on these configurations. In the case of magnetic flux tubes, radiative
inflow of heat leads to enhanced buoyancy which causes the flux tube
to move upwards and leave the storage region. In the case of magnetic
sheets, the compensation of the poleward directed magnetic tension
force requires a deviation of the temperature from the hydrostatic
background stratification. Convective energy transport disturbs the
equilibrium and leads to thermal circulations.
---------------------------------------------------------
Title: Emerging Flux Tubes in the Solar Convection Zone. II. The
Influence of Initial Conditions
Authors: Caligari, P.; Schüssler, M.; Moreno-Insertis, F.
1998ApJ...502..481C Altcode:
Numerical simulations of rising magnetic flux tubes in the solar
convection zone have contributed significantly to our understanding
of the basic properties of sunspot groups. They have provided an
important clue to the operation of the solar dynamo by predicting
strong (super-equipartition) magnetic fields near the bottom of the
convection zone. We have investigated to what extent the simulation
results (obtained on the basis of the thin flux tube approximation)
depend on the assumptions made about the initial state of a magnetic
flux tube at the start of the simulation. Two initial conditions used in
the literature have been considered in detail: mechanical equilibrium
(MEQ) and temperature balance (TBL). It turns out that the requirement
of super-equipartition field strength is a robust feature of the
simulations, largely independent of the choice of initial conditions:
emergence of active regions at low latitudes and the correct dependence
of their tilt angle (with respect to the east-west direction) as a
function of heliographic latitude require an initial magnetic field
strength on the order of 10<SUP>5</SUP> G. Other properties of rising
flux tubes, such as the asymmetries of shape and field strength between
the leading and following wings (with respect to the direction of
rotation) of a rising loop, or the anchoring of part of the flux tube
in the overshoot region, depend on the initial condition. Observed
asymmetries in the magnetic flux distribution and of proper motions
in emerging active regions favor MEQ over TBL as the proper initial
condition. MEQ should also be preferred for other theoretical reasons:
it allows for fewer free parameters, it requires no fine tuning for the
tube geometry and background stratification in the overshoot region,
and it can be easily made compatible with an encompassing model of the
generation, storage, and eruption of the magnetic flux. We have also
studied whether an external upflow (convective updraft) can trigger
the eruption of an otherwise stably stored flux tube in the overshoot
region. We find that a significant deformation and destabilization of
a flux tube with equipartition field strength requires coherent upflow
velocities of 20-50 m s<SUP>-1</SUP> in the overshoot layer, which is
an order of magnitude larger than current estimates for such velocities.
---------------------------------------------------------
Title: A crossroads for European solar and heliospheric
physics. Recent achievements and future mission
possibilities. Proceedings.
Authors: Priest, E. R.; Moreno-Insertis, F.; Harris, R. A.
1998cesh.conf.....P Altcode:
The following topics were dealt with: the solar interior, photosphere,
solar corona, the solar wind, SOHO spacecraft mission results, the
Ulysses mission, the Yohkoh mission, scientific objectives and future
missions, the RAMSES proposal, INTERHELIOS, SOLARNET, the solar STEREO
mission, HIREX, Solar-B mission, SOLAR LITE, UV coronagraphs.
---------------------------------------------------------
Title: The Physics of Twisted Magnetic Tubes Rising in a Stratified
Medium: Two-dimensional Results
Authors: Emonet, T.; Moreno-Insertis, F.
1998ApJ...492..804E Altcode: 1997astro.ph.11043E
The physics of a twisted magnetic flux tube rising in a stratified
medium is studied using a numerical magnetohydrodynamic (MHD)
code. The problem considered is fully compressible (has no Boussinesq
approximation), includes ohmic resistivity, and is two-dimensional,
i.e., there is no variation of the variables in the direction of
the tube axis. We study a high-plasma β-case with a small ratio
of radius to external pressure scale height. The results obtained
will therefore be of relevance to understanding the transport of
magnetic flux across the solar convection zone. <P />We confirm
that a sufficient twist of the field lines around the tube axis can
suppress the conversion of the tube into two vortex rolls. For a tube
with a relative density deficit on the order of 1/β (the classical
Parker buoyancy) and a radius smaller than the pressure scale height
(R<SUP>2</SUP><<H<SUP>2</SUP><SUB>p</SUB>), the minimum amount
of twist necessary corresponds to an average pitch angle on the order
of sin<SUP>-1</SUP> [(R/H<SUB>p</SUB>)<SUP>1/2</SUP>]. The evolution
of a tube with this degree of twist is studied in detail, including
the initial transient phase, the internal torsional oscillations,
and the asymptotic, quasi-stationary phase. During the initial phase,
the outermost, weakly magnetized layers of the tube are torn off its
main body and endowed with vorticity. They yield a trailing magnetized
wake with two vortex rolls. The fraction of the total magnetic flux
that is brought to the wake is a function of the initial degree of
twist. In the weakly twisted case, most of the initial tube is turned
into vortex rolls. With a strong initial twist, the tube rises with
only a small deformation and no substantial loss of magnetic flux. The
formation of the wake and the loss of flux from the main body of the
tube are basically complete after the initial transient phase. <P />A
sharp interface between the tube interior and the external flows is
formed at the tube front and sides; this area has the characteristic
features of a magnetic boundary layer. Its structure is determined as
an equilibrium between ohmic diffusion and field advection through the
external flows. It is the site of vorticity generation via the magnetic
field during the whole tube evolution. <P />From the hydrodynamical
point of view, this problem constitutes an intermediate case between
the rise of air bubbles in water and the motion of a rigid cylinder
in an external medium. As with bubbles, the tube is deformable and the
outcome of the experiment (the shape of the rising object and the wake)
depends on the value of the Weber number. Several structural features
obtained in the present simulation are also observed in rising air
bubbles, such as a central tail, and a skirt enveloping the wake. As
in rigid cylinders, the boundary layer satisfies a no-slip condition
(provided for in the tube by the magnetic field), and secondary rolls
are formed at the lateral edges of the moving object.
---------------------------------------------------------
Title: Turbulent Erosion of Magnetic Flux Tubes
Authors: Petrovay, K.; Moreno-Insertis, F.
1997ApJ...485..398P Altcode: 1997astro.ph..3152P
Results from a numerical and analytical investigation of the solution of
a nonlinear axisymmetric diffusion equation for the magnetic field are
presented for the case when the nonlinear dependence of the diffusivity
ν(B) on the magnetic field satisfies basic physical requirements. We
find that for sufficiently strong nonlinearity (i.e., for sufficiently
strong reduction of ν inside the tube) a current sheet is spontaneously
formed around the tube within one diffusion timescale. This sheet
propagates inward with a velocity inversely proportional to the
ratio of the field strength just inside the current sheet to the
equipartition field strength B<SUB>0</SUB>/B<SUB>e</SUB>, so the
lifetime of a tube with constant internal flux density is increased
approximately by a factor not exceeding B<SUB>0</SUB>/B<SUB>e</SUB>,
even for infinitely effective inhibition of turbulence inside the
tube. Among the applications of these results, we point out that
toroidal flux tubes in the solar convective zone are subject to
significant flux loss owing to turbulent erosion on a timescale of ~1
month and that turbulent erosion may be responsible for the formation
of a current sheet around a sunspot. It is further proposed that,
despite the simplifying assumptions involved, our solutions correctly
reflect the essential features of the sunspot decay process.
---------------------------------------------------------
Title: Emergence of magnetic flux in the convection zone. The limits
of the thin flux tube approximation.
Authors: Moreno-Insertis, F.
1997smf..conf....3M Altcode:
The thin flux tube approximation cannot describe some important
phenomena occurring during the journey of the tubes across the
convection zone. This review summarizes five of those processes. One
of them is related to the dramatic off-axis expansion suffered by
the top of some magnetic tubes of moderate field strength in the
middle of the convection zone. Another one concerns the expansion
that all rising tubes experience in the final few 10000 km below the
photosphere. The thin flux tube approximation does not account for
the development of vorticity and twist in the magnetic tubes (or only
under very restrictive circumstances). However, vorticity and twist
are fundamental ingredients that have to be considered at various
stages of the rise. All this is explained on the basis of the recent
results of two-dimensional MHD simulations of the initial stages of
the rise. In the final section, the validity of the local approach
for a rectilinear and non-rotating tube as well as its failure in more
general cases are explained.
---------------------------------------------------------
Title: The Rise of Magnetic Flux Tubes across a Stratified Medium:
Effects of the Twist
Authors: Emonet, T.; Moreno-Insertis, F.
1997ASPC..118...71E Altcode: 1997fasp.conf...71E
The results of a 2D numerical simulation of the rise of twisted
magnetic flux tubes are sketched. The theoretical criterion for the
minimum twist necessary to prevent the conversion of the tube into
a pair of strong vortices is shown to be correct. The transition
from a low-twist to a high-twist regime is exemplified. There is a
sharp transition between the tube interior and the outside medium. A
well-developed wake is formed, which however contains only a fraction
of the original magnetic flux of the tube.
---------------------------------------------------------
Title: Emergence of magnetThe limits of the thin flux tube
approximation
Authors: Moreno-Insertis, F.
1997smf..proc....3M Altcode:
The studies of the emergence of magnetic flux across the convection
zone show how the individual magnetic tubes rise toward the surface
at the same time remaining anchored at the interface between the
convection zone and the radiative interior. A number of recent numerical
simulations, in particular, reproduce several observed features of
the resulting active regions (e.g., tilt angles, preceding-following
asymmetries, etc). These simulations are based on the thin flux tube
approximation, which simplifies the magnetic ropes as a one-dimensional
continuum. The thin flux tube approximation has provided valuable
insights concerning the rise of the magnetic tubes. Yet, it cannot
describe some important phenomena occurring during the journey of the
tubes across the convection zone. This review summarizes five of those
processes. One of them is related to the dramatic off-axis expansion
suffered by the top of some magnetic tubes of moderate field strength
in the middle of the convection zone (a phenomenon dubbed explosion)
as a consequence of the adiabatic character of their evolution. Another
one concerns the expansion that all rising tubes experience in the
final few $10,000$ km below the photosphere. Further, the thin flux
tube approximation does not account for the development of vorticity
and twist in the magnetic tubes (or only under very restrictive
circumstances). However, vorticity and twist are fundamental ingredients
that have to be considered at various stages of the rise. Without twist,
the buoyant tubes tend to split and yield vortex filament pairs which
separate horizontally instead of rising. The transverse field component
of a twisted tube helps maintain the unity of the tube, but it yields
an evolutionary pattern far more complicated than as described by the
thin flux tube approximation. All this is explained on the basis of
the recent results of two-dimensional MHD simulations of the initial
stages of the rise. Finally, the back reaction of the external medium
to the advance of the magnetic region is difficult to treat in the
thin tube approximation. The simple local prescription commonly used
is inconsistent in that it is based on the assumption of a potential
(i.e., essentially non-local) flow around the tube. As a result, the
expression for the enhanced inertia it yields may violate the condition
of global momentum conservation. In the final section of the paper,
the validity of the local approach for a rectilinear and non-rotating
tube as well as its failure in more general cases are explained.
---------------------------------------------------------
Title: Multidimensional simulation of the rise of magnetic flux tubes
Authors: Moreno-Insertis, F.
1997ASPC..118...45M Altcode: 1997fasp.conf...45M
Recent results obtained through the 2D or 3D numerical simulation
of the rise of magnetic flux tubes in the solar convection zone are
discussed. The one--dimensional approximation known as the slender
flux tube approximation permits calculation of the time evolution of
a rising magnetic tube through most of the phases before it erupts at
the surface. However, the 1D model disregards the off-axis structure
of the magnetic tube, which turns out to play a non-negligible role
in the tube evolution from the very beginning of its rise. This is
discussed on the basis of a few examples: if the tube is untwisted,
the hydrodynamical forces of the surrounding flow may easily convert
it into a vortex tube pair whose components, asymptotically, stop to
rise. If the tube is sufficiently twisted, then the development of
vorticity is prevented in most of the tube interior, and the tube rises
in a way reminiscent of air bubbles in a liquid. This suggests that the
magnetic flux may be transported to the photosphere by means of twisted
magnetic tubes. The physical conditions under which the magnetic flux is
stored may be decisive for this issue: if the magnetic tubes are stored
in mechanical equilibrium, then the minimum degree of twist required to
prevent the generation of vorticity in the tube may be reduced. This
review centers on the evolution of the magnetic flux in a tube-like
geometry. However, recent 2D and 3D simulations of the time evolution
of a Rayleigh-Taylor unstable magnetic slab are briefly discussed.
---------------------------------------------------------
Title: Origin of the Proper Motions of Emerging Bipolar Magnetic
Regions
Authors: Caligari, P.; Schussler, M.; Moreno-Insertis, F.
1997ASPC..118...76C Altcode: 1997fasp.conf...76C
We have performed numerical simulations of the rise of magnetic
flux tubes through the convection zone. We find that the observed
proper motions of pores and sunspots in young active regions can
be understood as a consequence of the Coriolis force: conservation
of angular momentum leads to a retardation of the rising flux loop
with respect to those parts of the flux tube that remain anchored in
the overshoot layer below the convection zone proper. The result is
an asymmetric shape with the following flank of the loop being more
vertical than the leading part. When emerging at the solar surface,
the asymmetric shape of the tube leads to proper motions which are
in qualitative agreement with the observations. By studying the
dependence of the asymmetry on the initial state of the flux tube we
find that the observed proper motions favor a mechanical equilibrium
of the magnetic field in the overshoot layer. We also find that small
active regions (emerging from flux tubes with little magnetic flux)
are less asymmetric and should show weaker proper motions than large
bipolar regions. This prediction can be put to an observational test.
---------------------------------------------------------
Title: Emergence of magnetic flux from the solar interior
Authors: Moreno-Insertis, F.
1997MmSAI..68..429M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Rise of Twisted Magnetic Tubes in a Stratified Medium
Authors: Moreno-Insertis, F.; Emonet, T.
1996ApJ...472L..53M Altcode:
First results from a two-dimensional numerical study of the buoyant
rise of twisted magnetic flux tubes in the solar convection zone
are presented. We show in detail the process by which the transverse
component of the field can suppress the splitting of the rising tube
into two vortex filaments. For the suppression to be effective, the
pitch angle of the twisted field lines has to be above a threshold
given by the condition that the magnetic equivalent of the Weber number
(see § 2.2) be below 1. The shape obtained for the tube and wake is
strongly reminiscent of laboratory experiments with air bubbles rising
in liquids. The magnetized region outside an equipartition boundary is
peeled away from the tube: two sidelobes are formed, which lag behind
the tube and contain only a fraction of the initial magnetic flux. This
is similar to the formation of a skirt in the fluid dynamical case. The
velocities of rise predicted by the thin flux tube approximation are
compared with those obtained here.
---------------------------------------------------------
Title: Enhanced inertia of thin magnetic flux tubes.
Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A.
1996A&A...312..317M Altcode:
Accelerated bodies immersed in a fluid experience enhanced inertia
due to the associated co-acceleration of a certain volume of fluid in
their environment. We discuss the concept of enhanced inertia in the
framework of the approximation of thin flux tubes, which is widely
used to describe the dynamics of concentrated magnetic structures in
astrophysical objects. Previous attempts to incorporate this effect
have used a local approach, in which the reaction force of the external
medium on a given tube mass element solely depends on the relative
acceleration of tube and environment at that element. We show that
those previous formulations are inconsistent (either on physical
or geometrical grounds). We present here an alternative derivation
of the enhanced inertia term by geometrical means, still within a
local treatment of the problem but avoiding the pitfalls of previous
works. Our formulation, on the other hand, reveals a basic problem: all
local approaches are bound to give incorrect answers for the reaction
force in as far as they disregard the variation of the external flow in
the direction parallel to the flux tube: in doing so, they generally
fail to provide for global momentum conservation. An exact solution
and detailed analysis for an instance of this failure is given. The
discussion of this paper may be of use also in the hydrodynamical
framework of vortex tube dynamics.
---------------------------------------------------------
Title: Equilibrium of Twisted Horizontal Magnetic Flux Tubes
Authors: Emonet, T.; Moreno-Insertis, F.
1996ApJ...458..783E Altcode:
The equilibrium of non-force-free twisted horizontal magnetic flux tubes
is studied including gravity and an arbitrary pressure perturbation on
the tube boundary. To solve this free-boundary problem, we use general
nonorthogonal flux coordinates and consider the two-dimensional case
in which there is no variation of the physical quantities along the
tube axis. For the applications in the convection zone and corona,
we consider the case of weak external stratification by assuming that
the radius of the tube is smaller than the external pressure scale
height. This allows us to introduce a perturbation scheme which is much
less restrictive than the customary slender flux-tube approximation. In
particular, it has the advantage of not imposing any limitation on
the strength of the azimuthal field as compared to the longitudinal
field. Within this scheme, one retains to zero order all the functional
degrees of freedom of a general axisymmetric magnetostatic equilibrium;
the geometry of the perturbed azimuthal field lines is then obtained
from the equilibrium equations as a consequence of the zero-order
density (or rather buoyancy) distribution in the tube and of the
circular wavenumber of the external pressure perturbation. We show
that, as a result of the presence of gravity, the field lines are no
longer concentric, although they continue being circular. The resulting
changes in magnetic pressure and tension of the azimuthal field exactly
counteract the differences in buoyancy in the tube cross section. On
the other hand, external pressure fluctuations of circular wavenumber
higher than one can only be countered by bending the azimuthal field
lines. In general terms, the present scheme allows one to study in
detail the mutual dependence of the (differential) buoyancy in the tube,
the intensity and field line geometry of the azimuthal magnetic field,
and the gas pressure and longitudinal magnetic field distributions. <P
/>The main application of the equations and results of this paper is
to study the transverse structure of magnetic flux rings embedded in
a stratified medium with a flow around the tube that causes pressure
fluctuations on its surface. This includes tubes in the deep convection
zone, e.g., in its subadiabatic lower part, or those kept in place by a
meridional flow. It also applies to flux rings moving in a quasi-static
regime in which the drag force of the relative motion with respect
to the external medium exactly compensates the total buoyancy of the
tube. In this way, this study can complement the numerical simulations
of the rise of magnetized tubes and bubbles toward the surface.
---------------------------------------------------------
Title: Asymmetries in solar active regions and flux emergence models
Authors: Cauzzi, G.; Moreno-Insertis, F.; van Driel-Gesztelyi, L.
1996ASPC..109..121C Altcode: 1996csss....9..121C
No abstract at ADS
---------------------------------------------------------
Title: MHS-Equilibrium of Twisted Magnetic Tubes
Authors: Emonet, T.; Moreno-Insertis, F.
1996ApL&C..34....9E Altcode:
No abstract at ADS
---------------------------------------------------------
Title: “Explosion” and Intensification of Magnetic Flux Tubes
Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M.
1995ApJ...452..894M Altcode:
A magnetic flux tube anchored at the bottom of the solar convection
zone and rising toward the surface as a result of an undulatory
instability can be affected by a sudden catastrophic expansion and
weakening of the magnetic field at its apex if the original field
strength is below a few times 10<SUP>4</SUP> G. Such an "explosion"
occurs if the flux tube evolves close enough to (adiabatic) hydrostatic
equilibrium along the magnetic field lines in a super-adiabatically
stratified environment. This condition is satisfied if the diameter of
the tube is small enough for the drag force to dominate the dynamical
evolution. For example, rising flux tubes with equipartition field
(10<SUP>4</SUP> G) at their basis explode in the middle of the
convection zone if their magnetic flux is below ≌10<SUP>21</SUP>
Mx. <P />Apart from preventing flux tubes with equipartition field
from reaching the surface, the explosion process may have other
consequences for the evolution of magnetic fields in the convection
zone: (a) it provides a source of weak field to be acted upon by
the convective flows in the course of a turbulent dynamo process;
(b) upflow of matter into the inflated top of a loop intensifies the
magnetic field in the submerged part of the flux tube at the bottom of
the convection zone. This might constitute a mechanism to produce the
strong azimuthal fields in the overshoot region suggested by recent
studies of the undular instability of magnetic flux tubes.
---------------------------------------------------------
Title: Emerging Flux Tubes in the Solar Convection Zone. I. Asymmetry,
Tilt, and Emergence Latitude
Authors: Caligari, P.; Moreno-Insertis, F.; Schussler, M.
1995ApJ...441..886C Altcode:
The process of emergence of magnetic flux from the depths of the
convection zone to the surface is presented in the framework of
self-consistent model for the storage of field in the lower overshoot
regions and as the mechanism responsible for some of the regularities
observed in active regions. We have performed numerical simulations
of the emergence of flux tubes in the solar convection zone including
the effects of spherical geometry and rotation. The magnetic flux
tubes can be stored in mechanical equilibrium in the overshoot region,
which is the natural equilibrium of the flux rings in a subadiabatic
layer. An undular instability leads to the formation of loops once
a critical magnetic field strength of the order of 10<SUP>5</SUP>
G is exceeded. In the nonlinear phase of their unstable evolution,
the tubes move across the convection zone on a very fast time-scale,
typically about one month. The geometry and dynamics of the flux tubes
studied in these simulations permit prediction of some of the observed
properties of the active regions. First, the wings of the tube show a
marked asymmetry of inclination and velocity, which is compatible with
the observed asymmetric proper motions of sunspots and with the position
of the neutral line in emerging active regions. Second, upon emergence
the flux tubes show a tilt angle with respect to the equator which
fits reasonably well with the observed values. Third, the flux tubes
rise roughly within a cone of radial directions in the Sun so that no
outbreak at high latitudes takes place. The calculations lend further
support to the possibility of superequipartition field strengths in the
overshoot region. The implications of the present results for the dynamo
mechanism are discussed and hints for observational work are also given.
---------------------------------------------------------
Title: Active Region Asymmetry as a Result of the Rise of Magnetic
Flux Tubes
Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M.
1994SoPh..153..449M Altcode:
The magnetic flux tubes that rise across the convection zone to produce
active regions are shown to develop a difference in inclination between
their preceding and follower sides. This asymmetry is such that the
follower wing is more vertical (i.e., closer to the radial direction)
than the preceding side. An asymmetry of this kind can be obtained as a
natural consequence of the conservation of angular momentum along the
rise. This process may explain a number of the observed asymmetries
in morphology and behavior of the preceding and follower parts of the
active regions. We present results of numerical simulations showing
this effect and discuss possible observational consequences.
---------------------------------------------------------
Title: Forces on Magnetic Flux Tubes Moving in Inhomogeneous Flows
Authors: Moreno-Insertis, F.; Ferriz-Mas, A.; Schussler, M.
1994ApJ...422..652M Altcode:
The back-reaction of an incompressible two-dimensional flow of constant
strain to the accelerated motion of a straight cylinder is calculated
in the general case that the flow is inhomogeneous, nonstationary, and
with nonvanishing (although constant) vorticity. The resulting enhanced
inertia of the cylinder is basically given by its relative acceleration
with respect to the background flow. Further force terms that appear
because of the nonstationary and inhomogeneity of the background flow
are the following: a force identical to that experienced by the mass
elements of the unperturbed flow at the position of the axis of the
cylinder; the customary lift force because of the circulation around
the body; finally, a force term that appears only with nonvanishing
relative speed between the body and the background flow and which
is related to the energy which has to be imparted by the body to the
surrounding fluid in order to adapt the perturbation to the new local
velocity. The results have application to the dynamics of magnetic
flux tubes in the convection zone and atmosphere of the Sun as well
as to other astrophysical problems.
---------------------------------------------------------
Title: Dynamics of erupting magnetic flux tubes
Authors: Moreno-Insertis, F.; Schüssler, M.; Caligari, P.
1994ASIC..433..407M Altcode: 1994ssm..work..407M
The eruption of magnetic flux tubes from the overshoot layer due to
instability and the dynamics of their subsequent rise through the solar
convection zone are followed by numerical simulation. Special emphasis
is put on the possibility of explaining observed regularities of the
active regions at the surface (tilt angles, latitude of emergence,
asymmetry between preceding and following parts, etc). Instability
sets in with non-axisymmetric (undular) modes at azimuthal wavenumbers
$m=1$ and $m=2$ if the field strength exceeds values of the order of
$10^5$ G. At the same time, such strong initial fields are required
to reproduce the observable properties of sunspots and active
regions. Consequently, a consistent picture of storage, instability
and eruption of solar magnetic fields emerges.
---------------------------------------------------------
Title: Instability and eruption of magnetic flux tubes
Authors: Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F.;
Schüssler, M.
1994smf..conf..139C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The magnetic field in the convection zone as a link between
the active regions on the surface and the field in the solar interior
Authors: Moreno-Insertis, F.
1994smf..conf..117M Altcode:
Recent developments in the theory of the rise of magnetic flux from the
bottom of the convection zone to the surface to produce active regions
are reviewed. These include the explanation for the observed tilt angle
of the main magnetic axis of the active region as well as for some
of the asymmetries between the preceding and follower polarities. It
is stressed how this kind of studies may provide a way to gain insight
about the magnetic field in the dynamo layer at the same time explaining
some of the regularities observed in the active regions at the surface.
---------------------------------------------------------
Title: Instability and eruption of magnetic flux tubes in the solar
convection zone.
Authors: Schussler, M.; Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis,
F.
1994A&A...281L..69S Altcode:
We present a consistent model of storage, instability and dynamical
eruption of magnetic flux tubes in the solar convection zone
and underlying overshoot region. Using a convection zone model
with self-consistent overshoot layer, we calculate equilibrium
configurations of magnetic flux tubes and determine their linear
stability properties, taking into consideration the effects of
stratification and rotation. Instability of flux tubes stored in
the overshoot layer with growth times below one year requires field
strengths of the order of 10<SUP>5</SUP> G; in many cases, the dominant
mode has an azimuthal wave number of m = 2. Numerical simulations are
used to follow the nonlinear evolution of such unstable flux tubes and
their rise through the convection zone, from which they emerge to form
active regions. The results are in accordance with the following two
requirements, based on observational facts: (a) the upward motion of
the tubes is not significantly deflected by the Coriolis force so that
they can emerge at low latitudes, (b) their inclination with respect
to the East-West direction (tilt angle) as a function of the latitude
of emergence is consistent with observations of acitve regions.
---------------------------------------------------------
Title: The distribution of sunspot decay rates.
Authors: Martinez Pillet, V.; Moreno-Insertis, F.; Vazquez, M.
1993A&A...274..521M Altcode:
The distribution of sunspot decay rates is studied using the Greenwich
Photoheliographic Results (GPR) for a total of approximately hundred
years between 1874 and 1976. <P />The decay rates are seen to be
lognormally distributed. The discrepancies between the decay rates
given in the past by different authors are shown to originate as
a consequence of this asymmetric distribution. It is pointed out
that the extended tails shown by the lognormal distributions are
associated to spots decaying much faster than suggested by Bumba's
(1963) work. A cycle by cycle analysis of the lognormal distributions
associated with each sunspot group type and for single spots is
presented. The differences between the nine solar cycles involved are
studied. <P />As a remarkable property of the decay process, we show
that it happens at a nearly constant total to umbral area ratio. This
property holds for decaying spots which are still large enough to
show a penumbra. <P />We have studied the suitability of a decay law
with the instantaneous decay rate proportional to the length of the
spot boundary. This law predicts a parabolic decay pattern with some
specific characteristics. No definite conclusion in favour of this
law is reached, but it is suggested that a linear decay is as weakly
supported by the GPR data as a peripheral one. On the other hand, weak
non-linearities are seen in the decay of isolated spots with a clear
tendency to produce a convex pattern in the area vs. time diagram. The
implication is that sunspot decay is braked as time proceeds.
---------------------------------------------------------
Title: Storage of Magnetic Flux in the Overshoot Region
Authors: Moreno-Insertis, F.; Schussler, M.; Ferriz-Mas, A.
1993IAUS..157...41M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Distribution of Sunspot Decay Rates
Authors: Martinez Pillet, V.; Moreno-Insertis, F.; Vazquez, M.
1993ASPC...46...67M Altcode: 1993mvfs.conf...67M; 1993IAUCo.141...67M
No abstract at ADS
---------------------------------------------------------
Title: The Motion of Magnetic Flux Tubes in the Convection Zone and
the Subsurface Origin of Active Regions
Authors: Moreno-Insertis, F.
1993sto..work..385M Altcode:
The rise of kink-unstable magnetic flux tubes through the solar
convection zone is discussed with a view to explaining the subsurface
origin of active regions. In spite of the advances and insights obtained
by analytical and numerical means in the past decade(s), there are
still important uncertainties as to the viability of a mechanism of
this kind to explain the origin of sunspots and active regions. In
this paper, the main features of the rise of kink unstable tubes and
some open questions regarding this process are presented.
---------------------------------------------------------
Title: Shock wave propagation in a magnetic flux tube
Authors: Ferriz-Mas, A.; Moreno-Insertis, F.
1992PhFlA...4.2700F Altcode: 1992PhFl....4.2700F
The propagation of a shock wave in a magnetic flux tube is studied
within the framework of the Brinkley-Kirkwood theory adapted to
a radiating gas. Simplified thermodynamic paths along which the
compressed plasma returns to its initial state are considered. It is
assumed that the undisturbed medium is uniform and that the flux tube
is optically thin. The shock waves investigated, which are described
with the aid of the thin flux-tube approximation, are essentially
slow magnetohydrodynamic shocks modified by the constraint of lateral
pressure balance between the flux tube and the surrounding field-free
fluid; the confining external pressure must be balanced by the internal
gas plus magnetic pressures. Exact analytical solutions giving the
evolution of the shock wave are obtained for the case of weak shocks.
---------------------------------------------------------
Title: Storage of magnetic flux tubes in a convective overshoot region
Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A.
1992A&A...264..686M Altcode:
Consideration is given to the suppression of the radial and polar escape
of magnetic flux in the form of toroidal flux tubes (flux rings) from
low latitudes in the overshoot region below the solar convection zone
through the combined action of the subadiabatic ambient stratification
and the rotationally induced forces. It is shown that a flux ring which
is initially in thermal equilibrium with its environment and rotates
with the ambient angular velocity moves radially and latitudinally
towards an equilibrium configuration of lower internal temperature
and larger internal rotation rate with respect to the surrounding
nonmagnetic gas. Flux rings perform superposed buoyancy and inertial
oscillations around their equilibrium positions. From a study of the
frequencies and amplitudes of these oscillations, it is concluded that
flux rings with B of less than about 100,000 G can be kept within the
overshoot region if the superadiabaticity is sufficiently negative,
i.e., less than about -0.00004.
---------------------------------------------------------
Title: Instability of magnetic flux tubes in the solar convection
zone.
Authors: Caligari, P.; Moreno-Insertis, F.; Schüssler, M.
1992AGAb....7..152C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Motion of Magnetic Flux Tubes in the Convection Zone and
the Subsurface Origin of Active Regions
Authors: Moreno-Insertis, F.
1992ASIC..375..385M Altcode: 1992sto..work..385M
The rise of kink-unstable magnetic flux tubes through the solar
convection zone is discussed with a view to explaining the subsurface
origin of active regions. In spite of the advances and insights obtained
by analytical and numerical means in the past decade(s), there are
still important uncertainties as to the viability of a mechanism of
this kind to explain the origin of sunspots and active regions. In
this paper, the main features of the rise of kink unstable tubes and
some open questions regarding this process are presented.
---------------------------------------------------------
Title: Damping of Shocks in Magnetic Flux Tubes
Authors: Ferriz Mas, A.; Moreno Insertis, F.
1991mcch.conf..417F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Decay rates of sunspot groups from 1874 to 1976
Authors: Martinez Pillet, V.; Moreno-Insertis, F.; Vazquez, M.
1990Ap&SS.170....3M Altcode:
The global behaviour and fine structure of the distribution of sunspot
decay rates from activity cycle 13 to 20 are presented. It is shown
that the distribution of this parameter is lognormal. Statistically
significantly lower values of decay rates are found in cycles 13,
14, and 18 for isolated spots. The complex groups had no appreciable
changes.
---------------------------------------------------------
Title: Stability of Sunspots to Convective Motions. I. Adiabatic
Instability
Authors: Moreno-Insertis, F.; Spruit, H. C.
1989ApJ...342.1158M Altcode:
For determining the adiabatic stability of a uniform vertical field
in an arbitrary stratification it is sufficient to consider the
limit of infinitesimal horizontal wavelength. It is shown how the
behavior of the instability can be estimated qualitatively from the
dependence of the equipartition field strength on depth. Modes are
calculated numerically for analytic stratification models and for a
detailed sunspot stratification, including the effects of partial
ionization. It is concluded that for the observed field strengths
of umbrae the stratification is indeed unstable, with a growth time
of about 18 minutes. The unstable eigenfunctions have a maximum at
about 2300 km below the surface of the umbra and are about 3900 km
deep. Deeper layers may also be unstable depending on unknown details
of the stratification. A connection between fluting instability and
convective instability is noted.
---------------------------------------------------------
Title: A statistical study of the decay phase of sunspot groups from
1874 to 1939
Authors: Moreno-Insertis, F.; Vazquez, M.
1988A&A...205..289M Altcode:
A statistical study of the decay of sunspots and sunspot groups is
carried out using data contained in the Greenwich Photoheliographic
Results. The period studied spans more than six decades, namely from
1874 to 1939. The adequacy of exponential, quadratic and linear
decay laws to the area decay data is considered. The dependence,
or otherwise, of the mean area decay rate for each single spot, D,
on its maximum area, A<SUB>0</SUB>, is studied; no relationship of the
form D(A<SUB>0</SUB>) can be ascertained. In fact, when plotted on a
D vs. A<SUB>0</SUB> diagram, the couples (A<SUB>0</SUB>, D) cluster
in two pencils issuing from the origin of coordinates. These two
clusters can be seen to correspond with the families of the recurrent
and non-recurrent spot groups. The adequacy of a quadratic law to
the decay data is considered. A most probable parabolic decay law is
obtained; this law corresponds to the case in which the decay rate
is proportional to the spot perimeter. Finally, the variation of the
decay rates in successive activity cycles is also discussed.
---------------------------------------------------------
Title: An analytical study of shock waves in thin magnetic flux tubes
Authors: Ferriz-Mas, A.; Moreno-Insertis, F.
1987A&A...179..268F Altcode:
The jump conditions across a shock front in a thin magnetic flux
tube are studied by purely analytical means. Some properties of
magnetohydrodynamic shock waves in extended media are shown also to
hold in the more complicated case of thin magnetic flux tubes. It is
shown that flux tube shock waves are always compressive, thus being
accompanied by a weakening of the magnetic field strength and increase
of the tube radius. Some consequences of this are examined, such as
the sub- or supercritical character of the flow velocity with respect
to the Alfvén, sound and "tube" speeds. The range of variation of the
ratios of the different variables across the shock front is determined
along with the equivalent of the Hugoniot curve and further properties
of the shocks in magnetic flux tubes. The analogies and differences
with HD and MHD shocks in extended media are pointed out. Finally,
some implications of the shock wave structure for the solar atmosphere
are briefly discussed.
---------------------------------------------------------
Title: The Subsurface Structure of Sunspots and the Origin of Solar
Active Regions
Authors: Moreno-Insertis, F.
1987rfsm.conf..167M Altcode:
Several topics concerning the subsurface structure and evolution of
sunspots are discussed. These include the morphology and stability of
the magnetized plasma below a sunspot as well as the heat blockage in
it. Current ideas on the origin and physical processes preceding the
formation of an active region are reviewed.
---------------------------------------------------------
Title: The subsurface structure of sunspots and the origin of solar
active regions
Authors: Moreno-Insertis, F.
1987fsmf.conf..167M Altcode:
Several topics concerning the subsurface structure and evolution of
sunspots are discussed. These include the morphology and stability
of the magnetized plasma below a sunspot as well as the heat blockage
in it. In the second part of this paper, current ideas on the origin
and physical processes preceding the formation of an active region
are reviewed.
---------------------------------------------------------
Title: Decay Rates of Sunspot Groups from 1874 to 1939
Authors: Moreno-Insertis, F.; Vázquez, M.
1987rfsm.conf..196M Altcode:
Various possible decay laws (exponential, linear, parabolic) are tried
to fit the observational material. The dependence of the decay rates on
the spot's maximum area as well as their variation along the activity
cycle are studied.
---------------------------------------------------------
Title: Nonlinear time-evolution of kink-unstable magnetic flux tubes
in the convective zone of the sun
Authors: Moreno-Insertis, F.
1986A&A...166..291M Altcode:
The nonlinear development of the kink instability in a horizontal
magnetic flux tube with a field strength exceeding the equipartition
value lying initially in the deep convective zone is calculated. A
Lagrangian numerical code is developed to obtain solutions for the
system of equations dictating the evolution of a thin magnetic flux
tube in a stratified medium. The relevance of this calculation to
the understanding of active region formation and solar activity cycle
is considered.
---------------------------------------------------------
Title: Non linear time-evolution of kink-unstable magnetic flux
tubes in theconvection zone of the Sun.
Authors: Moreno-Insertis, F.
1986MPARp.226.....M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of the Birth and Fine Structure of Sunspot
Penumbrae
Authors: Collados, M.; Garcia de La Rosa, J. I.; Moreno-Insertis,
F.; Vazquez, M.
1985LNP...233..133C Altcode: 1985hrsp.proc..133C
High resolution white-light pictures of sunspot penumbrae are
presented. These include pictures showing details of their filamentary
structure and some instances of the birth of a penumbra. The
observations are discussed in the framework of current penumbra
theories.
---------------------------------------------------------
Title: Nonlinear time-evolution of kink-unstable magnetic flux tubes
in the convection zone of the Sun.
Authors: Moreno-Insertis, F.
1984ESASP.220...81M Altcode: 1984ESPM....4...81M
The nonlinear development of the kink instability of a magnetic flux
tube initially lying in mechanical equilibrium in the deep convection
zone is presented. A numerical code was developed; for the convection
zone stratification the model by Spruit (1977) was used.
---------------------------------------------------------
Title: Rise times of horizontal magnetic flux tubes in the convection
zone of the sun
Authors: Moreno-Insertis, F.
1983A&A...122..241M Altcode:
Solutions of the equations describing the motion of a straight
horizontal magnetic flux tube in the convection zone of the sun
including inertial term and radiative exchange with the surroundings
are presented. Cases where the tube is initially both in thermal or
in mechanical equilibrium with the surroundings are considered. It is
shown that the superadiabatic stratification of the convection zone
affects greatly the time scale for the rise of tubes with weak field
strength (less than the equipartition value), reducing it by a large
factor compared with tubes in permanent thermal equilibrium with the
surroundings. The various possible regimes for the rise starting from
a state of initial mechanical equilibrium are discussed. It is shown
that all tubes with magnetic fluxes approximately equal to or greater
than 10 to the 20th Mx will cross the convection zone in time scales
of less than two years.
