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Author name code: amari
ADS astronomy entries on 2022-09-14
author:"Amari, Tahar"
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Title: MOVES - V. Modelling star-planet magnetic interactions of
HD 189733
Authors: Strugarek, A.; Fares, R.; Bourrier, V.; Brun, A. S.; Réville,
V.; Amari, T.; Helling, Ch; Jardine, M.; Llama, J.; Moutou, C.;
Vidotto, A. A.; Wheatley, P. J.; Zarka, P.
2022MNRAS.512.4556S Altcode: 2022arXiv220310956S; 2022MNRAS.tmp..872S
Magnetic interactions between stars and close-in planets may lead to
a detectable signal on the stellar disc. HD 189733 is one of the key
exosystems thought to harbour magnetic interactions, which may have
been detected in 2013 August. We present a set of 12 wind models at that
period, covering the possible coronal states and coronal topologies of
HD 189733 at that time. We assess the power available for the magnetic
interaction and predict its temporal modulation. By comparing the
predicted signal with the observed signal, we find that some models
could be compatible with an interpretation based on star-planet
magnetic interactions. We also find that the observed signal can be
explained only with a stretch-and-break interaction mechanism, while
that the Alfvén wings scenario cannot deliver enough power. We finally
demonstrate that the past observational cadence of HD 189733 leads
to a detection rate of only between 12 and 23 per cent, which could
explain why star-planet interactions have been hard to detect in past
campaigns. We conclude that the firm confirmation of their detection
will require dedicated spectroscopic observations covering densely the
orbital and rotation period, combined with scarcer spectropolarimetric
observations to assess the concomitant large-scale magnetic topology
of the star.
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Title: HiRISE - High-Resolution Imaging and Spectroscopy Explorer
- Ultrahigh resolution, interferometric and external occulting
coronagraphic science
Authors: Erdélyi, Robertus; Damé, Luc; Fludra, Andrzej; Mathioudakis,
Mihalis; Amari, T.; Belucz, B.; Berrilli, F.; Bogachev, S.; Bolsée,
D.; Bothmer, V.; Brun, S.; Dewitte, S.; de Wit, T. Dudok; Faurobert,
M.; Gizon, L.; Gyenge, N.; Korsós, M. B.; Labrosse, N.; Matthews,
S.; Meftah, M.; Morgan, H.; Pallé, P.; Rochus, P.; Rozanov, E.;
Schmieder, B.; Tsinganos, K.; Verwichte, E.; Zharkov, S.; Zuccarello,
F.; Wimmer-Schweingruber, R.
2022ExA...tmp...21E Altcode:
Recent solar physics missions have shown the definite role of waves and
magnetic fields deep in the inner corona, at the chromosphere-corona
interface, where dramatic and physically dominant changes occur. HiRISE
(High Resolution Imaging and Spectroscopy Explorer), the ambitious new
generation ultra-high resolution, interferometric, and coronagraphic,
solar physics mission, proposed in response to the ESA Voyage 2050
Call, would address these issues and provide the best-ever and most
complete solar observatory, capable of ultra-high spatial, spectral,
and temporal resolution observations of the solar atmosphere, from the
photosphere to the corona, and of new insights of the solar interior
from the core to the photosphere. HiRISE, at the L1 Lagrangian
point, would provide meter class FUV imaging and spectro-imaging,
EUV and XUV imaging and spectroscopy, magnetic fields measurements,
and ambitious and comprehensive coronagraphy by a remote external
occulter (two satellites formation flying 375 m apart, with a
coronagraph on a chaser satellite). This major and state-of-the-art
payload would allow us to characterize temperatures, densities, and
velocities in the solar upper chromosphere, transition zone, and inner
corona with, in particular, 2D very high resolution multi-spectral
imaging-spectroscopy, and, direct coronal magnetic field measurement,
thus providing a unique set of tools to understand the structure and
onset of coronal heating. HiRISE's objectives are natural complements
to the Parker Solar Probe and Solar Orbiter-type missions. We present
the science case for HiRISE which will address: i) the fine structure
of the chromosphere-corona interface by 2D spectroscopy in FUV at
very high resolution; ii) coronal heating roots in the inner corona by
ambitious externally-occulted coronagraphy; iii) resolved and global
helioseismology thanks to continuity and stability of observing at the
L1 Lagrange point; and iv) solar variability and space climate with,
in addition, a global comprehensive view of UV variability.
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Title: Necessary Conditions for a Hot Quiet Sun Atmosphere:
Chromospheric Flares and Low Corona Twisted Flux Rope Eruptions
Authors: Amari, Tahar; Luciani, Jean-Francois; Aly, Jean-Jacques;
Canou, Aurelien; Mikic, Zoran; Velli, Marco
2021AGUFMSH12B..05A Altcode:
The issue of relevant scales involved in the heating of the solar
atmosphere is an important one. Since the temperature already reaches 1
MK a few megameters above the photosphere, observations made by Parker
Solar Probe will be able to explore those at larger heights but only
indirectly at those lower heights, where small scale coupling between
sub-photospheric, chromospheric and coronal structure and dynamics
occurs. While Solar Orbiter will be able to bring such observations,
modeling appears a complementary interesting approach to interpret
those observations Taking a sub-surface dynamo and a sharp realistic
VAL- like scale profile from photosphere to corona, with a fixed
temperature profile in time, we investigate the necessary conditions
implied on the structures and dynamics of the atmosphere to keep this
thermal structuration, as well as their implication in the energy
budget of the atmosphere. Under those hypothesis we show that :i)
the transverse photospheric field below 100km plays a major role;
ii) an associated scale of one megameter activity naturally results
to produce a zone above the photosphere with high confined electric
currents, which then expands into the chromosphere and releases energy(4
500 W/m2) through small-scale eruptions driving sonic motions; iii)
meso scale structuration, leads to the formation of larger coherent
twisted flux ropes, and associated eruptive like activity in a way
similar to large scale eruptive phenomena, as result of cancellation,
emergence, and convergence motions. Finally a wave dynamics is also
naturally driven in core corona associated to above 300 W/m2.
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Title: Models and data analysis tools for the Solar Orbiter mission
Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.;
Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.;
Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.;
Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi,
N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla,
T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.;
Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.;
Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.;
Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.;
Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot,
V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.;
Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.;
Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp,
E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud,
B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.;
Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.;
Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti,
S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.;
Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez,
L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca,
L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.;
Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov,
A. N.
2020A&A...642A...2R Altcode:
Context. The Solar Orbiter spacecraft will be equipped with a wide
range of remote-sensing (RS) and in situ (IS) instruments to record
novel and unprecedented measurements of the solar atmosphere and
the inner heliosphere. To take full advantage of these new datasets,
tools and techniques must be developed to ease multi-instrument and
multi-spacecraft studies. In particular the currently inaccessible
low solar corona below two solar radii can only be observed
remotely. Furthermore techniques must be used to retrieve coronal
plasma properties in time and in three dimensional (3D) space. Solar
Orbiter will run complex observation campaigns that provide interesting
opportunities to maximise the likelihood of linking IS data to their
source region near the Sun. Several RS instruments can be directed
to specific targets situated on the solar disk just days before
data acquisition. To compare IS and RS, data we must improve our
understanding of how heliospheric probes magnetically connect to the
solar disk. <BR /> Aims: The aim of the present paper is to briefly
review how the current modelling of the Sun and its atmosphere
can support Solar Orbiter science. We describe the results of a
community-led effort by European Space Agency's Modelling and Data
Analysis Working Group (MADAWG) to develop different models, tools,
and techniques deemed necessary to test different theories for the
physical processes that may occur in the solar plasma. The focus here
is on the large scales and little is described with regards to kinetic
processes. To exploit future IS and RS data fully, many techniques have
been adapted to model the evolving 3D solar magneto-plasma from the
solar interior to the solar wind. A particular focus in the paper is
placed on techniques that can estimate how Solar Orbiter will connect
magnetically through the complex coronal magnetic fields to various
photospheric and coronal features in support of spacecraft operations
and future scientific studies. <BR /> Methods: Recent missions such as
STEREO, provided great opportunities for RS, IS, and multi-spacecraft
studies. We summarise the achievements and highlight the challenges
faced during these investigations, many of which motivated the Solar
Orbiter mission. We present the new tools and techniques developed
by the MADAWG to support the science operations and the analysis of
the data from the many instruments on Solar Orbiter. <BR /> Results:
This article reviews current modelling and tool developments that ease
the comparison of model results with RS and IS data made available
by current and upcoming missions. It also describes the modelling
strategy to support the science operations and subsequent exploitation
of Solar Orbiter data in order to maximise the scientific output
of the mission. <BR /> Conclusions: The on-going community effort
presented in this paper has provided new models and tools necessary
to support mission operations as well as the science exploitation of
the Solar Orbiter data. The tools and techniques will no doubt evolve
significantly as we refine our procedure and methodology during the
first year of operations of this highly promising mission.
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Title: Bounding the Energy of Solar Eruptions
Authors: Linker, Jon A.; Downs, Cooper; Caplan, Ronald M.; Torok,
Tibor; Riley, Pete; Titov, Viacheslav; Lionello, Roberto; Mikic,
Zoran; Amari, Tahar
2019AAS...23431704L Altcode:
Major solar eruptions such as X-class flares and coronal mass ejections
(CMEs) are the fundamental source of solar energetic particles and
geomagnetic storms, and are thus key drivers of space weather at
Earth. The energy for solar eruptions is recognized to originate in
the solar magnetic field, and is believed to be stored as free magnetic
energy (energy above the potential field state) prior to eruption. Solar
active regions are the site of the most violent activity. Solar active
regions can store widely varying amounts of energy, so knowledge of
the free energy alone does not necessarily tell us when an eruption
is imminent. For estimates of the free energy to provide predictive
power, we must know how much energy a region can store - what is the
energy bound? <P />In recent work, we have found that the energy of a
particular field, the partially open field (POF), can place a useful
bound on the energy of an eruption from real active regions, a much
tighter constraint than the energy of the fully open field. However,
in general, it is difficult to solve for the POF. In this presentation,
we discuss methods for approximating the energy of this field, and
show a comparison of the approximation for a case where the solution
is known. We discuss the implications for understanding and predicting
major solar eruptions. <P />Research supported by NASA and AFOSR
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Title: Global Non-Potential Magnetic Models of the Solar Corona
During the March 2015 Eclipse
Authors: Yeates, Anthony R.; Amari, Tahar; Contopoulos, Ioannis; Feng,
Xueshang; Mackay, Duncan H.; Mikić, Zoran; Wiegelmann, Thomas; Hutton,
Joseph; Lowder, Christopher A.; Morgan, Huw; Petrie, Gordon; Rachmeler,
Laurel A.; Upton, Lisa A.; Canou, Aurelien; Chopin, Pierre; Downs,
Cooper; Druckmüller, Miloslav; Linker, Jon A.; Seaton, Daniel B.;
Török, Tibor
2018SSRv..214...99Y Altcode: 2018arXiv180800785Y
Seven different models are applied to the same problem of simulating
the Sun's coronal magnetic field during the solar eclipse on 2015
March 20. All of the models are non-potential, allowing for free
magnetic energy, but the associated electric currents are developed
in significantly different ways. This is not a direct comparison
of the coronal modelling techniques, in that the different models
also use different photospheric boundary conditions, reflecting
the range of approaches currently used in the community. Despite
the significant differences, the results show broad agreement in the
overall magnetic topology. Among those models with significant volume
currents in much of the corona, there is general agreement that the
ratio of total to potential magnetic energy should be approximately
1.4. However, there are significant differences in the electric current
distributions; while static extrapolations are best able to reproduce
active regions, they are unable to recover sheared magnetic fields in
filament channels using currently available vector magnetogram data. By
contrast, time-evolving simulations can recover the filament channel
fields at the expense of not matching the observed vector magnetic
fields within active regions. We suggest that, at present, the best
approach may be a hybrid model using static extrapolations but with
additional energization informed by simplified evolution models. This
is demonstrated by one of the models.
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Title: Partially Open Fields and Solar Eruptions
Authors: Linker, Jon; Mikic, Zoran; Downs, Cooper; Caplan, Ronald M.;
Riley, Pete; Torok, Tibor; Titov, Viacheslav S.; Lionello, Roberto;
Amari, Tahar
2018tess.conf10905L Altcode:
Partially Open Fields and Solar Eruptions* <P />Major solar eruptions
such as X-class flares and coronal mass ejections (CMEs) are the
progenitors of solar energetic particles and geomagnetic storms, and are
thus key drivers of space weather at Earth. The solar magnetic field
is the ultimate source of these massive events, the energy of which
is believed to be stored as free magnetic energy (energy above the
potential field state) prior to eruption. The amount of free magnetic
energy available in a given region is therefore a crucial indicator
of its propensity for eruption. However, solar active regions,
from which the largest events originate, can store widely varying
amounts of energy. Therefore, estimates of the free energy alone are
likely to be insufficient for knowing when a region will erupt; we
must also estimate the bounds on how much energy can be stored in a
given region. <P />The Aly-Sturrock theorem (Aly, ApJ 1991; Sturrock,
ApJ 1991) shows that the energy of a fully force-free field cannot
exceed the energy of the so-called open field. If the theorem holds,
this places an upper limit on the amount of free energy that can
be stored. In this paper, we describe how a closely related field,
the partially open field (Wolfson & Low ApJ 1992; Hu, ApJ 2004;
Aly & Amari, GAFD 2007), may place a much tighter bound on energy
storage and yield insights as to when major eruptions from an active
region are imminent (Amari et al., Nature, 2014). We demonstrate
the idea for AR9077, the source of the July 14, 2000 "Bastille Day"
flare/CME. <P />*Research supported by NASA and AFOSR
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Title: Magnetic cage and rope as the key for solar eruptions
Authors: Amari, Tahar; Canou, Aurélien; Aly, Jean-Jacques; Delyon,
Francois; Alauzet, Fréderic
2018Natur.554..211A Altcode:
Solar flares are spectacular coronal events that release large amounts
of energy. They are classified as either eruptive or confined, depending
on whether they are associated with a coronal mass ejection. Two types
of model have been developed to identify the mechanism that triggers
confined flares, although it has hitherto not been possible to decide
between them because the magnetic field at the origin of the flares
could not be determined with the required accuracy. In the first type
of model, the triggering is related to the topological complexity
of the flaring structure, which implies the presence of magnetically
singular surfaces. This picture is observationally supported by the
fact that radiative emission occurs near these features in many flaring
regions. The second type of model attributes a key role to the formation
of a twisted flux rope, which becomes unstable. Its plausibility is
supported by simulations, by interpretations of some observations and
by laboratory experiments. Here we report modelling of a confined event
that uses the measured photospheric magnetic field as input. We first
use a static model to compute the slowly evolving magnetic state of the
corona before the eruption, and then use a dynamical model to determine
the evolution during the eruption itself. We find that a magnetic flux
rope must be present throughout the entire event to match the field
measurements. This rope evolves slowly before saturating and suddenly
erupting. Its energy is insufficient to break through the overlying
field, whose lines form a confining cage, but its twist is large
enough to trigger a kink instability, leading to the confined flare,
as previously suggested. Topology is not the main cause of the flare,
but it traces out the locations of the X-ray emission. We show that
a weaker magnetic cage would have produced a more energetic eruption
with a coronal mass ejection, associated with a predicted energy upper
bound for a given region.
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Title: Validating coronal magnetic field reconstruction methods
using solar wind simulations and synthetic imagery
Authors: Pinto, Rui; Rouillard, Alexis; Génot, Vincent; Amari, Tahar;
Buchlin, Eric; Arge, Nick; Sasso, Clementina; Andretta, Vincenzo;
Bemporad, Alessandro
2017EGUGA..1913650P Altcode:
We present an ongoing effort within the ESA Modeling and Data Analysis
Working Group (MADAWG) to determine automatically the magnetic
connectivity between the solar surface and any point in interplanetary
space. The goal is to produce predictions of the paths and propagation
delays of plasma and energetic particle propagation. This is a key
point for the data exploitation of the Solar Orbiter and Solar Probe
Plus missions, and for establishing connections between remote and
in-situ data. The background coronal magnetic field is currently
determined via existing surface magnetograms and PFSS extrapolations,
but the interface is ready to include different combinations of coronal
field reconstruction methods (NLFFF, Solar Models), wind models (WSA,
MULTI-VP), heliospheric models (Parker spiral, ENLIL, EUHFORIA). Some
model realisations are also based on advanced magnetograms based on
data assimilation techniques (ADAPT) and the HELCATS catalogue of
simulations. The results from the different models will be combined in
order to better assess the modelling uncertainties. The wind models
provide synthetic white-light and EUV images which are compared to
coronographic imagery, and the heliospheric models provide estimations
of synthetic in-situ data wich are compared to spacecraft data. A part
of this is work (wind modelling) is supported by the FP7 project #606692
(HELCATS).
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Title: Space-weather assets developed by the French space-physics
community
Authors: Rouillard, A. P.; Pinto, R. F.; Brun, A. S.; Briand, C.;
Bourdarie, S.; Dudok De Wit, T.; Amari, T.; Blelly, P. -L.; Buchlin,
E.; Chambodut, A.; Claret, A.; Corbard, T.; Génot, V.; Guennou, C.;
Klein, K. L.; Koechlin, L.; Lavarra, M.; Lavraud, B.; Leblanc, F.;
Lemorton, J.; Lilensten, J.; Lopez-Ariste, A.; Marchaudon, A.; Masson,
S.; Pariat, E.; Reville, V.; Turc, L.; Vilmer, N.; Zucarello, F. P.
2016sf2a.conf..297R Altcode:
We present a short review of space-weather tools and services developed
and maintained by the French space-physics community. They include
unique data from ground-based observatories, advanced numerical
models, automated identification and tracking tools, a range of space
instrumentation and interconnected virtual observatories. The aim of
the article is to highlight some advances achieved in this field of
research at the national level over the last decade and how certain
assets could be combined to produce better space-weather tools
exploitable by space-weather centres and customers worldwide. This
review illustrates the wide range of expertise developed nationally
but is not a systematic review of all assets developed in France.
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Title: Small Scale Dynamo Magnetism And the Heating of the Quiet
Sun Solar Atmosphere.
Authors: Amari, T.
2015AGUFMSH31B2412A Altcode:
The longstanding problem of the solar atmosphere heating has been
addressed by many theoretical studies. Two specific mechanisms have been
shown to play a key role in those : magnetic reconnection and waves. On
the other hand the necessity of treating together chromosphere and
corona has also been been stressed, with debates going on about the
possibility of heating coronal plasma by energetic phenomena observed
in the chromosphere,based on many key observations such as spicules,
tornadoes…. We present some recent results about the modeling
of quiet Sun heating in which magnetic fields are generated by a
subphotospheric fluid dynamo which is connected to granulation. The
model shows a topologically complex magnetic field of 160 G on the Sun's
surface, agreeing with inferences obtained from spectropolarimetric
observations.Those generated magnetic fields emerge into the
chromosphere, providing the required energy flux and then small-scale
eruptions releasing magnetic energy and driving sonic motions. Some of
the more energetic eruptions can affect the very low corona only.It is
also found that taking into account a vertical weak network magnetic
field then allows to provide energy higher in the corona, while leaving
unchanged the physics of chromospheric eruptions. The coronal heating
mechanism rests on the eventual dissipation of Alfven waves generated
inside the chromosphere and carrying upwards an adequate energy flux,
while more energetic phenomena contribute only weakly to the heating
of the corona.
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Title: The Influence of Spatial resolution on Nonlinear Force-free
Modeling
Authors: DeRosa, M. L.; Wheatland, M. S.; Leka, K. D.; Barnes, G.;
Amari, T.; Canou, A.; Gilchrist, S. A.; Thalmann, J. K.; Valori,
G.; Wiegelmann, T.; Schrijver, C. J.; Malanushenko, A.; Sun, X.;
Régnier, S.
2015ApJ...811..107D Altcode: 2015arXiv150805455D
The nonlinear force-free field (NLFFF) model is often used to
describe the solar coronal magnetic field, however a series of
earlier studies revealed difficulties in the numerical solution of the
model in application to photospheric boundary data. We investigate
the sensitivity of the modeling to the spatial resolution of the
boundary data, by applying multiple codes that numerically solve the
NLFFF model to a sequence of vector magnetogram data at different
resolutions, prepared from a single Hinode/Solar Optical Telescope
Spectro-Polarimeter scan of NOAA Active Region 10978 on 2007 December
13. We analyze the resulting energies and relative magnetic helicities,
employ a Helmholtz decomposition to characterize divergence errors, and
quantify changes made by the codes to the vector magnetogram boundary
data in order to be compatible with the force-free model. This study
shows that NLFFF modeling results depend quantitatively on the spatial
resolution of the input boundary data, and that using more highly
resolved boundary data yields more self-consistent results. The
free energies of the resulting solutions generally trend higher
with increasing resolution, while relative magnetic helicity values
vary significantly between resolutions for all methods. All methods
require changing the horizontal components, and for some methods also
the vertical components, of the vector magnetogram boundary field in
excess of nominal uncertainties in the data. The solutions produced
by the various methods are significantly different at each resolution
level. We continue to recommend verifying agreement between the modeled
field lines and corresponding coronal loop images before any NLFFF
model is used in a scientific setting.
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Title: Small-scale dynamo magnetism as the driver for heating the
solar atmosphere
Authors: Amari, Tahar; Luciani, Jean-François; Aly, Jean-Jacques
2015Natur.522..188A Altcode:
The long-standing problem of how the solar atmosphere is heated has been
addressed by many theoretical studies, which have stressed the relevance
of two specific mechanisms, involving magnetic reconnection and waves,
as well as the necessity of treating the chromosphere and corona
together. But a fully consistent model has not yet been constructed and
debate continues, in particular about the possibility of coronal plasma
being heated by energetic phenomena observed in the chromosphere. Here
we report modelling of the heating of the quiet Sun, in which magnetic
fields are generated by a subphotospheric fluid dynamo intrinsically
connected to granulation. We find that the fields expand into the
chromosphere, where plasma is heated at the rate required to match
observations (4,500 watts per square metre) by small-scale eruptions
that release magnetic energy and drive sonic motions. Some energetic
eruptions can even reach heights of 10 million metres above the surface
of the Sun, thereby affecting the very low corona. Extending the model
by also taking into account the vertical weak network magnetic field
allows for the existence of a mechanism able to heat the corona above,
while leaving unchanged the physics of chromospheric eruptions. Such a
mechanism rests on the eventual dissipation of Alfvén waves generated
inside the chromosphere and that carry upwards the required energy
flux of 300 watts per square metre. The model shows a topologically
complex magnetic field of 160 gauss on the Sun's surface, agreeing with
inferences obtained from spectropolarimetric observations, chromospheric
features (contributing only weakly to the coronal heating) that can
be identified with observed spicules and blinkers, and vortices that
may be possibly associated with observed solar tornadoes.
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Title: Characterizing and predicting the magnetic environment leading
to solar eruptions
Authors: Amari, Tahar; Canou, Aurélien; Aly, Jean-Jacques
2014Natur.514..465A Altcode:
The physical mechanism responsible for coronal mass ejections has been
uncertain for many years, in large part because of the difficulty of
knowing the three-dimensional magnetic field in the low corona. Two
possible models have emerged. In the first, a twisted flux rope moves
out of equilibrium or becomes unstable, and the subsequent reconnection
then powers the ejection. In the second, a new flux rope forms as a
result of the reconnection of the magnetic lines of an arcade (a group
of arches of field lines) during the eruption itself. Observational
support for both mechanisms has been claimed. Here we report modelling
which demonstrates that twisted flux ropes lead to the ejection,
in support of the first model. After seeing a coronal mass ejection,
we use the observed photospheric magnetic field in that region from
four days earlier as a boundary condition to determine the magnetic
field configuration. The field evolves slowly before the eruption,
such that it can be treated effectively as a static solution. We find
that on the fourth day a flux rope forms and grows (increasing its free
energy). This solution then becomes the initial condition as we let
the model evolve dynamically under conditions driven by photospheric
changes (such as flux cancellation). When the magnetic energy stored
in the configuration is too high, no equilibrium is possible and the
flux rope is `squeezed' upwards. The subsequent reconnection drives
a mass ejection.
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Title: Reconstruction of the solar coronal magnetic field in
spherical geometry
Authors: Amari, T.; Aly, J. -J.; Canou, A.; Mikic, Z.
2013A&A...553A..43A Altcode:
Context. High-resolution vector magnetographs either onboard spacecrafts
or satellites (HMI/SDO, etc.) or ground based (SOLIS, etc.) now
gives access to vector synoptic maps, composite magnetograms made of
multiple interactive active regions, and full disk magnetograms. It
thus become possible to reconstruct the coronal magnetic field on the
full Sun scale. <BR /> Aims: We present a method for reconstructing
the global solar coronal magnetic field as a nonlinear force-free
field. It is based on a well-posed Grad-Rubin iterative scheme adapted
to spherical coordinates <BR /> Methods: This method is a natural
extension to spherical geometry of the one we previously developed in
Cartesian geometry. It is implemented in the code XTRAPOLS, which
is a massively parallel code. It allows dealing with the strong
constraints put on the computational methods by having to handle the
very large amounts of data contained in high-resolution large-scale
magnetograms. The method exploits the mixed elliptic-hyperbolic nature
of the Grad-Rubin boundary value problem. It uses a finite-difference
method for the elliptic part and a method of characteristics for the
hyperbolic part. The computed field guarantees to be divergence free
up to round-off errors, by introducing a representation in terms of a
vector potential satisfying specific gauge conditions. The construction
of the latter - called here the restricted DeVore gauge - is described
in detail in an appendix. <BR /> Results: We show that XTRAPOLS performs
well by applying it to the reconstruction of a particular semi-analytic
force-free field that has already been considered by various authors.
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Title: Progress on Reconstructing the Solar Coronal Magnetic Field
above Active region at different scales
Authors: Canou, A.; Amari, T.
2013enss.confE.112C Altcode:
he low solar corona is dominated by the magnetic field which is created
in the Sun's interior by a dynamo process and which then emerges into
the atmosphere. This magnetic field plays an important role in most
structures and phenomena observed at various wavelengths such as
prominences, small and large scale eruptive events, and continuous
heating of the plasma. It is therefore important to understand
its three-dimensional properties in order to elaborate efficient
theoretical models. Unfortunately, the magnetic field is difficult
to measure locally in the hot and tenuous corona. But this can be
done at the level of the cooler and denser photosphere, and several
instruments with high resolution vector magnetographs are currently
available (e.g. THEMIS/MTR, SOLIS/VSM, HINODE/SOT/SP or SDO/HMI) or
will be available on future programmed missions (e.g. Solar Orbiter,
ATST and EST). This has lead solar physicists to develop an approach
which consists in reconstructing the coronal magnetic field from
boundary data given on the photosphere. <P />We will present our recent
progress and results to solve this problem at the active region scale
or the larger one such as the full disk or synoptic one, for which
the large amount of data as well as their sparsity on the solar disk,
require to develop particular strategies. We will also show how this
can be helpful to characterize the many aspects of active regions
during their static or pre-eruptive evolution phases.
---------------------------------------------------------
Title: On Some Algorithm for Modeling the Solar Coronal Magnetic
Field as MHD Equilibrium on Unstructured Mesh
Authors: Amari, T.; Delyon, F.; Alauzet, F.; Frey, P.; Olivier, G.;
Aly, J. J.; SDO/HMI Team
2012ASPC..459..189A Altcode:
The low solar corona is dominated by the magnetic field which is
created inside the sun by a dynamo process and then emerges into
the atmosphere. This magnetic field plays an important role in most
structures and phenomena observed at various wavelengths such as
prominences, small and large scale eruptive events, and continuous
heating of the plasma, and therefore it is important to understand
its three-dimensional properties in order to elaborate efficient
theoretical models. Unfortunately, the magnetic field is difficult
to measure locally in the hot and tenuous corona. But this can be
done at the level of the cooler and denser photosphere, and several
instruments with high resolution vector magnetographs are currently
available (THEMIS, Imaging Vector Magnetograph (IVM), the Advanced
Stokes Polarimeter (ASP), SOLIS, HINODE , Solar Dynamics Observatory
(SDO), or will be shortly availableby future telescopes such as EST
and solar missions as SOLAR-ORBITER. This has lead solar physicists
to develop an approach which consists in "Reconstructing" the coronal
magnetic field from boundary data given on the photosphere. We will
present some of the issues we encountered in solving this problem
numerically as well our recent progress and results.
---------------------------------------------------------
Title: Quasi-static evolution of the reconstructed magnetic field
of an emerging and eruptive active region
Authors: Canou, A.; Amari, T.
2012EAS....55..125C Altcode:
We study the evolution of the emerging and eruptive Active Region
(AR) 10930 for which the coronal magnetic field is reconstructed as
a nonlinear force-free field along with vector magnetograms obtained
from Hinode/SOT/SP data. We show that the AR evolves quasi-statically
during several days through a sequence of equilibria until the time
of the eruption. In addition this sequence emphasizes the formation
and expansion of a coronal Twisted Flux Rope (TFR) which is related to
other observed coronal structures. On the other hand, the pre-eruptive
and post-eruptive magnetic configurations show that the pre-eruptive
TFR plays a crucial role during the eruption.
---------------------------------------------------------
Title: Coronal relative magnetic helicities and subsurface kinetic
helicities of active regions
Authors: Petrie, Gordon; Komm, Rudolf; Amari, Tahar
2012shin.confE.117P Altcode:
Excess of helicity in coronal magnetic structures has often been
linked to their instability and eruption. Moreover, active regions
associated with subphotospheric patterns of strong subsurface kinetic
helicity have been found to be more flare productive. How are these
atmospheric magnetic and subsurface fluid helicities related? Using
SOLIS vector magnetic magnetograms, the XTRAPOL nonlinear force-free
field extrapolation code and GONG helioseismic data we study the coronal
relative magnetic helicities of a set of active regions in combination
with their subsurface kinetic helicities in order to better understand
the transport processes of helicity in solar activity.
---------------------------------------------------------
Title: Self-gravitating Body with an Internal Magnetic Field. I. New
Analytical Equilibria
Authors: Aly, J. -J.; Amari, T.
2012ApJ...750....4A Altcode:
We construct exact analytical solutions of the equations describing
the equilibrium of a self-gravitating magnetized fluid body,
possibly rigidly rotating, by superposing two solutions of finite
energy defined in the whole space, one describing a non-magnetized
gravitating equilibrium (ST1) and the other describing a magnetized
non-gravitating equilibrium (ST2). A large number of ST1s can be
found in the literature and directly used for our constructions, and
we thus concentrate on ST2s, which are difficult to obtain. We derive
some of their general properties and exhibit two explicit classes of
axisymmetric "elementary" such equilibria. The first one is extracted
from the stellar models proposed by Prendergast and by Kutvitskii &
Solov'ev, respectively. The second one is constructed by using Palumbo's
theory of isodynamic equilibria, for which the magnetic pressure is
constant on each flux surface. Both types of ST2s have their magnetic
field confined within a bounded region, respectively, of spherical and
toroidal shapes. A much more general ST2 can be obtained by juxtaposing
n+q elementary ST2s, with n of the first type and q of the second type,
in such a way that the magnetic regions do not pairwise overlap. The
specific equilibria we obtain by superposition thus have no external
field extending to infinity, and may be three dimensional (3D), which
invalidates a recent nonexistence conjecture. Moreover, they may be
arranged to contain force-free regions. Our superposition method can
be considered as a 3D generalization of the axisymmetric splitting
method previously developed by Kutvitskii & Solov'ev.
---------------------------------------------------------
Title: Connecting the photosphere to the corona : Reconstructing
the Solar Coronal Magnetic Field
Authors: Amari, T.; Delyon, F.; Alauzet, F.; Canou, A.; Mikic, Z.;
Aly, J. J.; Solis Team; Stanford Sdo/Hmi Team
2012decs.confE..50A Altcode:
The low solar corona is dominated by the magnetic field which is
created inside the sun by a dynamo process and then emerges into
the atmosphere. This magnetic field plays an important role in most
structures and phenomena observed at various wavelengths such as
prominences, small and large scale eruptive events, and continuous
heating of the plasma, and therefore it is important to understand
its three-dimensional properties in order to elaborate efficient
theoretical models. Unfortunately, the magnetic field is difficult
to measure locally in the hot and tenuous corona. But this can be
done at the level of the cooler and denser photosphere, and several
instruments with high resolution vector magnetographs are currently
available (THEMIS, Imaging Vector Magnetograph (IVM), the Advanced
Stokes Polarimeter (ASP)), SOLIS, HINODE , Solar Dynamics Observatory
(SDO), or will be shortly available and future programmed missions
such as , SOLAR-ORBITER. This has lead solar physicists to develop
an approach which consists in reconstructing the coronal magnetic
field from boundary data given on the photosphere. We will present
our recent progress and results to solve this problem at the scale of
active regions or larger ones such as full disk or synoptic scales,
for which the large amount of data as well as their sparsity on the
solar disk, require to develop particular strategies. We will also
illustrate the interest of the reconstruction for characterizing
the magnetic environments of prominences, emerging sub-photospheric
structures and the pre-eruptive ones.
---------------------------------------------------------
Title: Magnetic bubbles and magnetic towers - I. General properties
and simple analytical models
Authors: Aly, J. -J.; Amari, T.
2012MNRAS.420..237A Altcode: 2011MNRAS.tmp.2086A
We consider magnetostatic equilibria in which a bounded region D
containing a magnetized plasma is either fully confined by a field-free
external medium - magnetic bubble equilibria (MBEqs) - or is confined
by both such a medium and line-tying in a dense plasma region -
magnetic tower equilibria (MTEqs). We first establish some of their
general properties. In particular, we derive a series of useful integral
equalities relating the magnetic field and the thermal pressures inside
and outside D, respectively. We use them to prove the non-existence
of an axisymmetric MBEq with a purely poloidal field, and to discuss
some recent results of Braithwaite on MBEq formation by relaxation
from an initial non-equilibrium state. We next present two families
of exact analytical axisymmetric MBEqs with, respectively, spherical
and toroidal shapes. The first family is extracted from Prendergast's
model of a self-gravitating magnetized body, while the second one
is constructed by using Palumbo's theory of isodynamic equilibria,
for which both magnetic and thermal pressures take constant values on
any flux surface. MTEqs with a large variety of structures are thus
obtained in a simple way: we start from an arbitrary MBEq and just
consider the part of it above a given plane cutting the bubble D. For
MBEqs and MTEqs in either family, we compute in closed form most of the
interesting physical quantities (such as energy, magnetic helicity and
twist). Our results are expected to be useful for building up simple
models of several astrophysical objects (such as X-ray cavities in the
intracluster medium, jets emitted by disc accreting compact objects,
eruptive events in stellar coronae and their ejecta).
---------------------------------------------------------
Title: Coronal Mass Ejection Initiation by Converging Photospheric
Flows: Toward a Realistic Model
Authors: Amari, T.; Aly, J. -J.; Luciani, J. -F.; Mikic, Z.; Linker, J.
2011ApJ...742L..27A Altcode:
In the context of coronal mass ejections triggering, we reconsider
the class of models in which the evolution of an active region
(AR) is driven by imposed boundary motions converging toward the
polarity inversion line (PIL). We introduce a new model problem in
which there is a large-scale flow with a diverging structure on the
photosphere. This flow is reminiscent of that of the well-known moat
flow around each of the two spots of a bipolar AR and transports only
part of the magnetic flux toward the PIL. It is thus more compatible
with observations than the one used in our previous study, which forced
the whole positive and negative polarity parts of the AR approaching
each other. We also include a diffusion term associated with small-scale
turbulent photospheric motions, but keep the associated diffusivity at
a low value in the particular study described here. We show that the
evolution of an initial sheared force-free field first leads to the
formation of a twisted flux rope which stays in equilibrium for some
time. Eventually, however, the configuration suffers a global disruption
whose underlying mechanism is found by energetic considerations to
be nonequilibrium. It begins indeed when the magnetic energy becomes
of the order of the energy of an accessible partially open field. For
triggering an eruption by converging flows, it is thus not necessary
to advect the whole AR toward the PIL, but only its central part.
---------------------------------------------------------
Title: Nonlinear Force-Free and Potential-Field Models of
Active-Region and Global Coronal Fields during the Whole Heliosphere
Interval
Authors: Petrie, G. J. D.; Canou, A.; Amari, T.
2011SoPh..274..163P Altcode: 2010arXiv1010.6283P
Between 24 March 2008 and 2 April 2008, the three active regions (ARs)
NOAA 10987, 10988 and 10989 were observed daily by the Synoptic Optical
Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph
(VSM) while they traversed the solar disk. We use these measurements and
the nonlinear force-free magnetic field code XTRAPOL to reconstruct the
coronal magnetic field for each active region and compare model field
lines with images from the Solar Terrestrial RElations Observatory
(STEREO) and Hinode X-ray Telescope (XRT) telescopes. Synoptic maps
made from continuous, round-the-clock Global Oscillations Network Group
(GONG) magnetograms provide information on the global photospheric field
and potential-field source-surface models based on these maps describe
the global coronal field during the Whole Heliosphere Interval (WHI)
and its neighboring rotations. Features of the modeled global field,
such as the coronal holes and streamer-belt locations, are discussed
in comparison with extreme ultra-violet and coronagraph observations
from STEREO. The global field is found to be far from a minimum, dipolar
state. From the nonlinear models we compute physical quantities for the
active regions such as the photospheric magnetic and electric current
fluxes, the free magnetic energy and the relative helicity for each
region each day where observations permit. The interconnectivity of
the three regions is addressed in the context of the potential-field
source-surface model. Using local and global quantities derived from
the models, we briefly discuss the different observed activity levels
of the regions.
---------------------------------------------------------
Title: Reconstruction of the solar coronal magnetic field, from
active region to large scale
Authors: Amari, T.; Canou, A.; Delyon, F.; Aly, J. J.; Frey, P.;
Alauzet, F.
2011sf2a.conf..389A Altcode:
The low solar corona is dominated by the magnetic field which is
created inside the sun by a dynamo process and then emerges into
the atmosphere. This magnetic field plays an important role in most
structures and phenomena observed at various wavelengths such as
prominences, small and large scale eruptive events, and continuous
heating of the plasma, and therefore it is important to understand
its three-dimensional properties in order to elaborate efficient
theoretical models. Unfortunately, the magnetic field is difficult
to measure locally in the hot and tenuous corona. But this can be
done at the level of the cooler and denser photosphere, and several
instruments with high resolution vector magnetographs are currently
available (THEMIS, Imaging Vector Magnetograph (IVM), the Advanced
Stokes Polarimeter (ASP), SOLIS, HINODE, Solar Dynamics Observatory
(SDO), or will be shortly available by future telescopes such as EST
and solar missions as SOLAR-ORBITER. This has lead solar physicists to
develop an approach which consists in " reconstructing" the coronal
magnetic field from boundary data given on the photosphere. We will
discuss some of the issues encountered in solving this problem as well
our recent progress and results at the scale of active region scales
or the larger one such as full sun scale.
---------------------------------------------------------
Title: Nonlinear Force-Free and Potential-Field Models of Active
Region and Global Coronal Fields
Authors: Petrie, Gordon; Canou, Aurelien; Amari, Tahar
2011shin.confE..48P Altcode:
We use daily observations by the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM)
and the nonlinear force-free magnetic field code XTRAPOL to reconstruct
the coronal magnetic field for selected active regions. We compare model
field lines with images from the Solar Terrestrial RElations Observatory
(STEREO) and Hinode X-ray Telescope (XRT) telescopes. Synoptic maps
made from continuous, round-the-clock Global Oscillations Network Group
(GONG) magnetograms provide information on the global photospheric
field and potential-field source-surface models based on these maps
describe the global coronal field. <P />Features of the modeled
global field, such as the coronal holes and streamer-belt locations,
are presented in comparison with extreme ultra-violet and coronagraph
observations from STEREO. From the nonlinear models we compute physical
quantities for the active regions such as the photospheric magnetic
and electric current fluxes, the free magnetic energy and the relative
helicity for each region each day where observations permit. The
potential-field source-surface models also provide information on the
connectivity of the active region fields to their surroundings and to
the heliosphere. Using local and global quantities derived from the
models, we briefly discuss the different observed activity levels of
the regions.
---------------------------------------------------------
Title: Observational constraints on well-posed reconstruction methods
and the optimization-Grad-Rubin method
Authors: Amari, T.; Aly, J. -J.
2010A&A...522A..52A Altcode:
Context. Grad-Rubin type methods are interesting candidates for
reconstructing the force-free magnetic field of a solar coronal
region. As input these methods, however, require the normal component
B<SUB>n</SUB> of the field on the whole boundary of the numerical
box and the force-free function α on the part of the boundary where
B<SUB>n</SUB> > 0 (or B<SUB>n</SUB> < 0), while observations
provide data only on its lower photospheric part. Moreover, they
introduce an unpleasing asymmetry between the opposite polarity
parts of the boundary, and certainly do not take full advantage
of the available data on α. <BR /> Aims: We address these issues
resulting from observations. We present a possible way to supply the
missing information about B<SUB>n</SUB> and α on the non-photospheric
sides of the box, and to use more effectively the data provided by the
measurements. <BR /> Methods: We introduce the optimization-Grad-Rubin
method (OGRM), which is in some sense midway between optimization
methods and the standard Grad-Rubin methods. It is based on an iterative
scheme in which the α used as a boundary condition is imposed to
take identical values at both footpoints of any field line and to be
as close as possible to the α provided by the measurements on the
photosphere. The degree of “closeness” is measured by an “error
functional” containing a weight function reflecting the confidence
that can be placed on the observational data. <BR /> Results: The new
method is implemented in our code XTRAPOL, along with some technical
improvements. It is thus tested for two specific choices of the weight
function by reconstructing a force-free field from data obtained by
perturbing in either a random or a non-random way boundary values
provided by an exact solution.
---------------------------------------------------------
Title: Coronal Mass Ejection Initiation: On the Nature of the Flux
Cancellation Model
Authors: Amari, T.; Aly, J. -J.; Mikic, Z.; Linker, J.
2010ApJ...717L..26A Altcode: 2010arXiv1005.4669A
We consider a three-dimensional bipolar force-free magnetic field with a
nonzero magnetic helicity, occupying a half-space, and study the problem
of its evolution driven by an imposed photospheric flux decrease. For
this specific setting of the Flux Cancellation Model describing
coronal mass ejections occurring in active regions, we address the
issues of the physical meaning of flux decrease, of the influence on
field evolution of the size of the domain over which this decrease is
imposed, and of the existence of an energetic criterion characterizing
the possible onset of disruption of the configuration. We show that
(1) the imposed flux disappearance can be interpreted in terms of
transport of positive and negative fluxes toward the inversion line,
where they get annihilated. (2) For the particular case actually
computed, in which the initial state is quite sheared, the formation
of a twisted flux rope and the subsequent global disruption of the
configuration are obtained when the flux has decreased by only a
modest amount over a limited part of the whole active region. (3)
The disruption is produced when the magnetic energy becomes of the
order of the decreasing energy of a semi-open field, and then before
reaching the energy of the associated fully open field. This suggests
that the mechanism leading to the disruption is nonequilibrium as in
the case where flux is imposed to decrease over the whole region.
---------------------------------------------------------
Title: A Twisted Flux Rope as the Magnetic Structure of a Filament
in Active Region 10953 Observed by Hinode
Authors: Canou, A.; Amari, T.
2010ApJ...715.1566C Altcode:
The presence of twisted flux ropes (TFRs) in pre-eruptive/flaring
magnetic configurations is of main interest for our understanding
of the structure and dynamics of the solar corona. On the one hand,
their presence is a key ingredient in several theoretical models for the
magnetic support of material in filaments, or triggering of coronal mass
ejections as well as the emergence of structures from the convection
zone into the corona. On the other hand, several observations have shown
the presence of twist and shear during eruptive and flaring phases of
eruptive phenomena. In this paper, we consider the determination of
the magnetic structure of active region (AR) 10953 observed by Hinode
and reconstructed using our two nonlinear force-free models. We show
that the reconstructed magnetic configurations exhibit a TFR along
the southern part of the neutral line. Moreover, the location of the
magnetic dips within the TFR agrees within a good level of accuracy with
the Hα images taken by SMART and the vertically integrated current
density recovers the main structure present in Hinode/XRT images. The
free magnetic energy is also found to be large enough to power the two
C-class flares of the following days. We finally compare our results
with those of Su et al. who proposed an interesting model of the same
AR in which a TFR is inserted at the same location using the flux rope
insertion method.
---------------------------------------------------------
Title: Does the Compression or the Expansion of a Simple Topology
Potential Magnetic Field Lead to the Development of Current Sheets?
Authors: Aly, J. J.; Amari, T.
2010ApJ...709L..99A Altcode:
Janse & Low have most recently addressed the following
question. Consider a cylindrical domain containing a simple topology
potential magnetic field threading its lower and upper horizontal
faces, and a perfectly conducting plasma. Suppose that this domain
is made to slowly contract or expand in the vertical direction, so
driving the field into a quasi-static evolution through a series of
force-free configurations. Then are these configurations smooth, or do
they contain current sheets (CSs)? We reexamine here their three-step
argument leading to the conclusion that CSs form most generally. We
prove analytically that the field has to evolve through "topologically
untwisted" and "nonpotential" configurations, thus confirming the first
two steps. However, we find the third step—leading to the conclusion
that a smooth untwisted force-free field is necessarily potential—to
be very disputable.
---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953:
A Critical Assessment
Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan,
J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.;
Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.;
Inhester, B.; Tadesse, T.
2009SPD....40.3102D Altcode:
Nonlinear force-free field (NLFFF) modeling seeks to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have failed to arrive at consistent solutions when
applied to (thus far, two) cases using the highest-available-resolution
vector magnetogram data from Hinode/SOT-SP (in the region of the
modeling area of interest) and line-of-sight magnetograms from
SOHO/MDI (where vector data were not available). One issue is that
NLFFF models require consistent, force-free vector magnetic boundary
data, and vector magnetogram data sampling the photosphere do not
satisfy this requirement. Consequently, several problems have arisen
that are believed to affect such modeling efforts. We use AR 10953
to illustrate these problems, namely: (1) some of the far-reaching,
current-carrying connections are exterior to the observational field
of view, (2) the solution algorithms do not (yet) incorporate the
measurement uncertainties in the vector magnetogram data, and/or (3)
a better way is needed to account for the Lorentz forces within the
layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
---------------------------------------------------------
Title: A Critical Assessment of Nonlinear Force-Free Field Modeling
of the Solar Corona for Active Region 10953
Authors: De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham;
Leka, K. D.; Lites, Bruce W.; Aschwanden, Markus J.; Amari, Tahar;
Canou, Aurélien; McTiernan, James M.; Régnier, Stéphane; Thalmann,
Julia K.; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas;
Cheung, Mark C. M.; Conlon, Paul A.; Fuhrmann, Marcel; Inhester,
Bernd; Tadesse, Tilaye
2009ApJ...696.1780D Altcode: 2009arXiv0902.1007D
Nonlinear force-free field (NLFFF) models are thought to be viable
tools for investigating the structure, dynamics, and evolution of
the coronae of solar active regions. In a series of NLFFF modeling
studies, we have found that NLFFF models are successful in application
to analytic test cases, and relatively successful when applied
to numerically constructed Sun-like test cases, but they are less
successful in application to real solar data. Different NLFFF models
have been found to have markedly different field line configurations
and to provide widely varying estimates of the magnetic free energy in
the coronal volume, when applied to solar data. NLFFF models require
consistent, force-free vector magnetic boundary data. However,
vector magnetogram observations sampling the photosphere, which is
dynamic and contains significant Lorentz and buoyancy forces, do not
satisfy this requirement, thus creating several major problems for
force-free coronal modeling efforts. In this paper, we discuss NLFFF
modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT,
STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process
illustrate three such issues we judge to be critical to the success of
NLFFF modeling: (1) vector magnetic field data covering larger areas
are needed so that more electric currents associated with the full
active regions of interest are measured, (2) the modeling algorithms
need a way to accommodate the various uncertainties in the boundary
data, and (3) a more realistic physical model is needed to approximate
the photosphere-to-corona interface in order to better transform the
forced photospheric magnetograms into adequate approximations of nearly
force-free fields at the base of the corona. We make recommendations
for future modeling efforts to overcome these as yet unsolved problems.
---------------------------------------------------------
Title: Evidence for a Pre-Eruptive Twisted Flux Rope Using the Themis
Vector Magnetograph
Authors: Canou, A.; Amari, T.; Bommier, V.; Schmieder, B.; Aulanier,
G.; Li, H.
2009ApJ...693L..27C Altcode:
Although there is evidence that twisted structures form during
large-scale eruptive events, it is not yet clear whether these exist in
the pre-eruptive phase as twisted flux ropes (TFRs) in equilibrium. This
question has become a major issue since several theoretical mechanisms
can lead to the formation of TFRs. These models consider either the
evolution of a coronal configuration driven by photospheric changes or
the emergence of TFR from the convection zone. We consider as a target
for addressing this issue the active region NOAA AR 10808 known at
the origin of several large-scale eruptive phenomena, and associated
with the emergence of a δ-spot. Using the THEMIS vector magnetogram
as photospheric boundary conditions for our nonlinear force-free
reconstruction model of the low corona and without any other assumption,
we show that the resulting pre-eruptive configuration exhibits a TFR
above the neutral line of the emerging δ-spot. In addition, the free
magnetic energy of this configuration could even be large enough to
explain such resulting large-scale eruptive events.
---------------------------------------------------------
Title: Magnetic flux ropes: Fundamental structures for eruptive
phenomena
Authors: Amari, Tahar; Aly, Jean-Jacques
2009IAUS..257..211A Altcode:
We consider some general aspects of twisted magnetic flux ropes
(TFR), which are thought to play a fundamental role in the structure
and dynamics of large scale eruptive events. We first discuss
the possibility to show the presence of a TFR in a pre-eruptive
configuration by using a model along with observational informations
provided by a vector magnetograph. Then we present, in the framework of
a generic model in which the coronal field is driven into an evolution
by changes imposed at the photospheric level, several mechanisms which
may lead to the formation and the disruption of a TFR, including the
development of a MHD instability, and we discuss the issues of the
energy and helicity contents of an erupting configuration. Finally we
report some results of a recent and more ambitious approach to the
physics of TFRs in which one tries to describe in a consistent way
their rising through the convection zone, their emergence through the
photosphere, and their subsequent evolution in the corona.
---------------------------------------------------------
Title: Coronal Closure of Subphotospheric MHD Convection for the
Quiet Sun
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.
2008ApJ...681L..45A Altcode:
We use our resistive layer model (RLM), which stresses the importance of
the resistivity at the photospheric interface, to study the evolution of
a solar coronal quiet region driven by subphotospheric convection. The
initial version of the RLM is improved by introducing a new Boussinesq
MHD model for the upper part of the convection zone (CZ), while the
low-beta corona is still described by a MHD model. We compute the
evolution of a weak magnetic field introduced initially in the CZ. We
observe its amplification by the turbulence, the concentration of
the photospheric flux at the boundaries of the convection cells, the
coalescence and the cancellation of flux elements, and the transfer
of about 10% of the magnetic energy into the corona. The currents
associated with the nonpotential coronal field are found to be organized
in filament-like localized structures due to the photospheric vortices
and the complexity of the magnetic topology. Their resistive dissipation
contributes to the heating of the quiet corona.
---------------------------------------------------------
Title: Non-Linear Force-Free Field Modeling of a Solar Active Region
Around the Time of a Major Flare and Coronal Mass Ejection
Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes,
G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann,
T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J.
2008AGUSMSP31A..06D Altcode:
Solar flares and coronal mass ejections are associated with rapid
changes in coronal magnetic field connectivity and are powered by
the partial dissipation of electrical currents that run through
the solar corona. A critical unanswered question is whether the
currents involved are induced by the advection along the photosphere
of pre-existing atmospheric magnetic flux, or whether these currents
are associated with newly emergent flux. We address this problem by
applying nonlinear force-free field (NLFFF) modeling to the highest
resolution and quality vector-magnetographic data observed by the
recently launched Hinode satellite on NOAA Active Region 10930 around
the time of a powerful X3.4 flare in December 2006. We compute 14
NLFFF models using 4 different codes having a variety of boundary
conditions. We find that the model fields differ markedly in geometry,
energy content, and force-freeness. We do find agreement of the best-fit
model field with the observed coronal configuration, and argue (1)
that strong electrical currents emerge together with magnetic flux
preceding the flare, (2) that these currents are carried in an ensemble
of thin strands, (3) that the global pattern of these currents and
of field lines are compatible with a large-scale twisted flux rope
topology, and (4) that the ~1032~erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection. We discuss the relative merits of
these models in a general critique of our present abilities to model
the coronal magnetic field based on surface vector field measurements.
---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
around the Time of a Major Flare and Coronal Mass Ejection
Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.;
Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.;
Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J. K.
2008ApJ...675.1637S Altcode: 2007arXiv0712.0023S
Solar flares and coronal mass ejections are associated with rapid
changes in field connectivity and are powered by the partial dissipation
of electrical currents in the solar atmosphere. A critical unanswered
question is whether the currents involved are induced by the motion of
preexisting atmospheric magnetic flux subject to surface plasma flows or
whether these currents are associated with the emergence of flux from
within the solar convective zone. We address this problem by applying
state-of-the-art nonlinear force-free field (NLFFF) modeling to the
highest resolution and quality vector-magnetographic data observed
by the recently launched Hinode satellite on NOAA AR 10930 around
the time of a powerful X3.4 flare. We compute 14 NLFFF models with
four different codes and a variety of boundary conditions. We find
that the model fields differ markedly in geometry, energy content,
and force-freeness. We discuss the relative merits of these models in
a general critique of present abilities to model the coronal magnetic
field based on surface vector field measurements. For our application
in particular, we find a fair agreement of the best-fit model field
with the observed coronal configuration, and argue (1) that strong
electrical currents emerge together with magnetic flux preceding the
flare, (2) that these currents are carried in an ensemble of thin
strands, (3) that the global pattern of these currents and of field
lines are compatible with a large-scale twisted flux rope topology,
and (4) that the ~10<SUP>32</SUP> erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection.
---------------------------------------------------------
Title: Coronal Mass Ejection Initiation and Complex Topology
Configurations in the Flux Cancellation and Breakout Models
Authors: Amari, T.; Aly, J. J.; Mikic, Z.; Linker, J.
2007ApJ...671L.189A Altcode:
We present some new results showing that the flux cancellation
model for coronal mass ejections (CMEs) works well also in a
complex-topology magnetic field. We consider as a model problem
the case of the flux-cancellation-driven evolution of a quadrupolar
configuration. We find that (1) during the first phase, the field
evolves slowly, with a twisted flux rope in equilibrium being
created at some time; (2) nonequilibrium sets in at a critical time
and the configuration experiences a major global disruption. These
features are similar to those previously obtained for a bipolar
configuration. Some differences between the two cases are however
observed: (1) the presence of an X-point above the twisted flux
rope makes the expulsion of the latter much easier due to the weaker
confinement near this point; this difference may be at the origin of
the existence of two classes of CMEs-fast and slow; (2) the energy W(t)
of the configuration remains smaller than the energy W<SUB>σ</SUB>(t)
of the associated totally open field, and then the disruption does not
occur when W(t)~W<SUB>σ</SUB>(t), as in the bipolar case. Rather we
get nonequilibrium when W(t)~W<SUB>SO</SUB>(t), where W<SUB>SO</SUB>(t)
is the energy of a semiopen field which has its open lines connected
to the two central spots on which flux cancellation is imposed. A
consequence of our results is that the topological complexity of a
preerupting configuration cannot be taken as a criterion for eliminating
the flux cancellation model in favor of the well-known breakout model.
---------------------------------------------------------
Title: Structure and evolution of the solar coronal magnetic field
Authors: Aly, J. J.; Amari, T.
2007GApFD.101..249A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Resolution of the 180° Ambiguity for Inverse Horizontal
Magnetic Field Configurations
Authors: Li, Jing; Amari, Tahar; Fan, Yuhong
2007ApJ...654..675L Altcode:
A well-known problem in solar physics is that solutions for the
transverse magnetic field direction are ambiguous with respect to
a 180° reversal in the field direction. In this paper we focus on
three methods for the removal of the 180° ambiguity applied to three
MHD models. These methods are (1) the reference field method, (2)
the method of magnetic pressure gradient, and (3) the magnetic field
divergence-free method. All three methods are noniterative, and methods
2 and 3 are analytical and fast. We apply these methods to three MHD
equilibrium model fields: (1) an analytical solution of a nonlinear
force-free magnetic field equilibrium from Low, (2) a simulation of an
emerging twisted flux tube from Fan & Gibson, and (3) a pre-eruptive
twisted magnetic flux rope equilibrium reached by relaxation from Amari
et al. We measure the success of methods within “inverse horizontal
field” regions in the boundary, which are mathematically defined by
B<SUB>⊥</SUB>˙∇<SUB>⊥</SUB>B<SUB>z</SUB>>0. When such regions
overlap with the magnetic field neutral lines, they are known as “bald
patches” (BPs) or inverse topology. Our most important conclusion is
that the magnetic divergence-free method is far more successful than
the other two methods within BPs. This method requires a second level of
measurements of the vertical magnetic field. As high-quality multilevel
magnetograms will come online in the near future, our work shows that
multilayer magnetic field measurements will be highly desirable to
objectively and successfully tackle the 180° ambiguity problem.
---------------------------------------------------------
Title: Well posed reconstruction of the solar coronal magnetic field
Authors: Amari, T.; Boulmezaoud, T. Z.; Aly, J. J.
2006A&A...446..691A Altcode:
We present and compare two methods for the reconstruction of the solar
coronal magnetic field, assumed to be force-free, from photospheric
boundary data. Both methods rely on a well posed mathematical boundary
value problem and are of the Grad-Rubin type, i.e., the couple ({B},α)
is computed iteratively. They do differ from each other on the one
hand by the way they address the zero-divergence of {B} issue, and
on the other hand by the scheme they use for computing α at each
iteration. The comparison of the two methods is done by numerically
computing two examples of nonlinear force-free fields associated
to large scale strong electric current distributions, whose exact
forms can be otherwise determined semi-analytically. In particular,
the second solution has a large nonlinearity even in the weak field
region - a feature which is not present in the actual magnetograms, but
is interesting to consider as it does allow to push the methods to the
limits of their range of validity. The best results obtained with those
methods give a relative vector error smaller than 0.01. For the latter
extreme case, our results show that higher resolution reconstructions
with bounded convergence improve the approximated solution, which may be
of some interest for the treatment of the data of future magnetographs.
---------------------------------------------------------
Title: Self and mutual magnetic helicities in coronal magnetic
configurations
Authors: Régnier, S.; Amari, T.; Canfield, R. C.
2005A&A...442..345R Altcode:
Together with the magnetic energy, the magnetic helicity is an
important quantity used to describe the nature of a magnetic field
configuration. In the following, we propose a new technique to
evaluate various components of the total magnetic helicity in the
corona for an equilibrium reconstructed magnetic field. The most
meaningful value of helicity is the total relative magnetic helicity
which describes the linkage of the field lines even if the volume
of interest is not bounded by a magnetic surface. In addition if the
magnetic field can be decomposed into the sum of a closed field and
a reference field (following , Berger 1999 in Magnetic Helicity in
Space and Laboratory Plasmas, ed. M. R. Brown, R. C. Canfield, &
A. A. Pevtsov, 1), we can introduce three other helicity components:
the self helicity of the closed field, the mutual helicity between
the closed field and the reference field, and the vacuum helicity
(self helicity of the reference field). To understand the meaning of
those quantities, we derive them from the potential field (reference)
and the force-free field computed with the same boundary conditions
for three different cases: a single twisted flux tube derived from the
extended Gold-Hoyle solutions, a simple magnetic configuration with
three balanced sources and a constant distribution of the force-free
parameter, and the AR 8210 magnetic field observed from 17:13 UT to
21:16 UT on May 1, 1998. We analyse the meaning of the self and mutual
helicities: the self and mutual helicities correspond to the twist and
writhe of confined flux bundles, and the crossing of field lines in
the magnetic configuration respectively. The main result is that the
magnetic configuration of AR 8210 is dominated by the mutual helicity
and not by the self helicity (twist and writhe). Our results also show
that although not gauge invariant the vacuum helicity is sensitive to
the topological complexity of the reference field.
---------------------------------------------------------
Title: Non-Current-free Coronal Closure of Subphotospheric MHD Models
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.
2005ApJ...629L..37A Altcode:
We propose a method that allows the matching of two classes of models
that have been well developed so far, but largely independently
from each other: (1) convection zone (CZ) models, which generally
either end up below the photosphere or are matched with an external
potential field, and (2) coronal models of eruptive processes and
heating, which usually consider the evolution of current-carrying
magnetic fields driven by given photospheric changes. In our approach,
the thin turbulent photospheric layer between the two large regions is
modeled as a resistive layer across which the physical quantities suffer
stiff variations. We show that this layer enables the transport of an
electric current into the corona through the tangential component of the
electric field (continuous across the various interfaces), as well as
good conservation of the global magnetic helicity. To illustrate our
general approach, we present in detail a model problem in which the
rising of an initially twisted flux rope through the CZ is described
kinematically while the physics inside the corona is described by a
full magnetohydrodynamic model. We show that the evolution leads to
the emergence of magnetic flux and electric current into the corona,
with the creation of a flux rope that eventually suffers a dynamical
transition toward fast expansion.
---------------------------------------------------------
Title: Coronal Magnetohydrodynamic Evolution Driven by Subphotospheric
Conditions
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.
2004ApJ...615L.165A Altcode:
We consider the approach to the theory of formation, evolution, and
major disruption of coronal twisted flux ropes, in which subphotospheric
structures play a crucial role. We set a boundary value problem in the
corona in which the boundary conditions at the photospheric level are
determined by a simple kinematic model describing the rising of a tube
throughout the convection zone. In addition to peculiar features like
the existence of areas of flux concentration on the lower boundary and
the bending of the polarity inversion line, we find that the coronal
configuration suffers a transition from arcade to rope topology and
(later) a transition from a slow quasi-static evolution to a dynamic
nonequilibrium one, both these critical phenomena occurring during the
phase of decrease of the net photospheric flux. There is a continuous
injection of magnetic helicity into the corona, and the magnetic energy
remains smaller than that of the corresponding open field. Contrary to
what has been observed in some other simulations, the formation of the
equilibrium flux rope prior to the disruption is not associated with
some reconnection on the “photospheric” surface. This may possibly
suggest the utility of different observational diagnostics.
---------------------------------------------------------
Title: 3D magnetic configuration of the Hα filament and X-ray
sigmoid in NOAA AR 8151
Authors: Régnier, S.; Amari, T.
2004A&A...425..345R Altcode:
We investigate the structure and relationship of an Hα filament and an
X-ray sigmoid observed in active region NOAA 8151. We first examine
the presence of such structures in the reconstructed 3D coronal
magnetic field obtained from the non-constant-α force-free field
hypothesis using a photospheric vector magnetogram (IVM, Mees Solar
Observatory). This method allows us to identify several flux systems:
a filament (height 30 Mm, aligned with the polarity inversion line
(PIL), magnetic field strength at the apex 49 G, number of turns
0.5-0.6), a sigmoid (height 45 Mm, aligned with the PIL, magnetic
field strength at the apex 56 G, number of turns 0.5-0.6) and a highly
twisted flux tube (height 60 Mm, magnetic field strength at the apex
36 G, number of turns 1.1-1.2). By searching for magnetic dips in
the configuration, we identify a filament structure which is in good
agreement with the Hα observations. We find that both filament and
sigmoidal structures can be described by a long twisted flux tube
with a number of turns less than 1 which means that these structures
are stable against kinking. The filament and the sigmoid have similar
absolute values of α and J<SUB>z</SUB> in the photosphere. However,
the electric current density is positive in the filament and negative
in the sigmoid: the filament is right-handed whereas the sigmoid
is left-handed. This fact can explain the discrepancies between the
handedness of magnetic clouds (twisted flux tubes ejected from the Sun)
and the handedness of their solar progenitors (twisted flux bundles in
the low corona). The mechanism of eruption in AR 8151 is more likely
not related to the development of instability in the filament and/or
the sigmoid but is associated with the existence of the highly twisted
flux tube (∼1.1-1.2 turns).
---------------------------------------------------------
Title: Eruptive Behavior Originating in Active Regions
Authors: Linker, J. A.; Mikic, Z.; Lionello, R.; Riley, P.; Amari, T.
2003AGUFMSH41A..04L Altcode:
Coronal mass ejections (CMEs) are spectacular manifestations of
solar acitivy. These immense eruptions of plasma and magnetic field
are propelled outward from the sun with velocities as high as 2000
km/s. The fastest CMEs typically originate from active regions on the
Sun. MHD models of the eruption of large scale coronal fields have
demonstrated significant energy release in idealized 2D (Antiochos
et al., ApJ 512, 985, 1999) and 3D (Linker et al., Phys. Plasmas 10,
1971, 2003) geometry. Eruptive behavior has also been shown for a 3D
localized arcade (Amari et al., ApJ 529, L49, 2000). In active regions
on the Sun, both the localized field due to the active region and the
overlying fields in the large scale corona are important. We describe
MHD computations of the eruptive behavior of a localized active region
field (modeled as a localized bipole) within a large-scale dipolar
configuration. We discuss the differences between this more realistic
configuration and the idealized configurations that have been considered
previously. Work supported by NASA and the Center for Integrated Space
Weather Modeling (an NSF Science and Technology Center).
---------------------------------------------------------
Title: Coronal Mass Ejection: Initiation, Magnetic Helicity, and
Flux Ropes. II. Turbulent Diffusion-driven Evolution
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.; Mikic, Z.; Linker, J.
2003ApJ...595.1231A Altcode:
We consider a three-dimensional bipolar magnetic field B, occupying
a half-space, which is driven into evolution by the slow turbulent
diffusion of its normal component on the boundary. The latter is imposed
by fixing the tangential component of the electric field and leads to
flux cancellation. We first present general analytical considerations on
this problem and then construct a class of explicit solutions in which
B keeps evolving quasi-statically through a sequence of force-free
configurations without exhibiting any catastrophic behavior. Thus,
we report the results of a series of numerical simulations in which B
evolves from different force-free states, the electric field on the
boundary being imposed to have a vanishing electrostatic part (the
latter condition is not enforced in the analytical model, and thus it is
possible a priori for the results of the two types of calculations to
be different). In all the cases, we find that the evolution conserves
the magnetic helicity and exhibits two qualitatively different
phases. The first one, during which a twisted flux rope is created,
is slow and almost quasi-static, while the second one is associated
with a disruption, which is confined for a small initial helicity and
global for a large initial helicity. Our calculations may be relevant
for modeling the coronal mass ejections that have been observed to
occur in the late dispersion phase of an active region. In particular,
they may allow us to understand the role played by a twisted flux rope
in these events.
---------------------------------------------------------
Title: Force-free Fields in Active Regions: Magnetic configurations
and Energetics
Authors: Regnier, S.; Canfield, R. C.; Amari, T.
2003SPD....34.0103R Altcode: 2003BAAS...35..805R
To determine the 3D configuration of solar active regions and to define
boundary conditions for MHD codes, we reconstruct the coronal magnetic
field from photospheric measurements. For that, we consider the active
region in an equilibrium state. Three hypotheses are then often used:
the potential field (no electric current), the linear force-free field
(the current density is α times the magnetic field, α is a constant),
the nonlinear force-free field (α is a function of space). We here
compare those three models in terms of connectivity changes and
energetics for two different active regions: AR 8151 and AR 8210. AR
8151 shows dramatical changes between these models whereas AR 8210
configuration stays close to the potential field. The structure of the
filament and of the sigmoid observed in AR 8151 can only be determined
in the nonlinear force-free field: both structures are identified with
twisted flux tubes with ∼ 0.5-0.6 turns. Interestingly, the filament
is a right-handed structure whereas the sigmoid is left-handed! This
work is a part of the Solar MURI project.
---------------------------------------------------------
Title: Coronal Mass Ejection: Initiation, Magnetic Helicity, and
Flux Ropes. I. Boundary Motion-driven Evolution
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.; Mikic, Z.; Linker, J.
2003ApJ...585.1073A Altcode:
In this paper we study a class of three-dimensional magnetohydrodynamic
model problems that may be useful to understand the role of twisted flux
ropes in coronal mass ejections. We construct in a half-space a series
of force-free bipolar configurations with different helicity contents
and bring them into an evolution by imposing to their footpoints on
the boundary slow motions converging toward the inversion line. For
all the cases that have been computed, this process leads, after a
phase of quasi-static evolution, to the formation of a twisted flux
rope by a reconnection process and to the global disruption of the
configuration. In contrast with the results of some previous studies,
however, the rope is never in equilibrium. It thus appears that
the presence of a rope in the preeruptive phase is not a necessary
condition for the disruption but may be the product of the disruption
itself. Moreover, the helicity keeps an almost constant value during the
evolution, and the problem of the origin of the helicity content of an
eruptive configuration appears to be that of the initial force-free
state. In addition to these numerical simulations, we report some
new relations for the time variations of the energy and the magnetic
helicity and develop a simple analytical model in which the magnetic
field evolution exhibits essential features quite similar to those
observed during the quasi-static phase in the numerics.
---------------------------------------------------------
Title: Magnetic Field Topology in Prominences
Authors: Lionello, Roberto; Mikić, Zoran; Linker, Jon A.; Amari, Tahar
2002ApJ...581..718L Altcode:
We present a study of the magnetic field lines of a prominence using
MHD and thermodynamic/hydrodynamic (TH) models. Previous modeling of
prominences has tended to emphasize either magnetic field modeling or
TH modeling in isolation. In this paper, we combine these approaches
to model a long-lived filament observed in 1996 August-September. In
our new approach, we (1) use magnetograms to prescribe the boundary
conditions for the magnetic flux in three-dimensional MHD simulations,
(2) show that observed magnetic flux changes can produce a fluxrope
and that the dipped (concave upward) portion of the field lines form
in the approximate location of the observed prominence, and (3) show
that TH computations, using the computed geometry of magnetic field
lines that are in three-dimensional MHD equilibrium, have condensations
forming in the dipped portions of the field lines.
---------------------------------------------------------
Title: Global budget for an eruptive active region . I. Equilibrium
reconstruction approach
Authors: Bleybel, A.; Amari, T.; van Driel-Gesztelyi, L.; Leka, K. D.
2002A&A...395..685B Altcode:
We present results on the magnetic structure of NOAA Active Region
#7912 which was involved in a long duration flare on 14 October 1995,
and was the source region for a magnetic cloud observed by the WIND
spacecraft from October 18-20. Using vector magnetograms from the
Imaging Vector Magnetograph (“IVM”), we reconstruct the magnetic
field above this active region, assuming it is in a non-linear
force-free state. This reconstruction is used to determine global
properties of the active region magnetic field including topology,
magnetic energy, and relative magnetic helicity. A comparison of some
global quantities before and after the eruptive event is discussed. We
show that the magnetic energy and relative helicity of the active
region decreased after the eruption, consistent with the ejection
of a large amount of helicity (in the magnetic cloud). We also show
that the relaxed post-flare state still contains nonlinearities and is
not consistent with a linear force-free state as predicted by Taylor's
theory of relaxation. These results agree with those of recent numerical
simulations concerning plasmoid ejection and helicity redistribution in
the disruption of magnetic configurations. We propose as an explanation
that the anchoring of field lines in the photosphere prevents a full
cascade to the Taylor state, and that a variational formulation in which
the action functional would describe this constraint should be derived.
---------------------------------------------------------
Title: Non-constant-α force-free field of active region NOAA 8210
Authors: Régnier, S.; Amari, T.; Canfield, R. C.
2002ESASP.505...65R Altcode: 2002IAUCo.188...65R; 2002solm.conf...65R
We investigate the 3D coronal magnetic configuration of the active
region NOAA 8210 (AR8210). This active region observed on May 1, 1998
is the site of numerous flares. Using the non-constant-α force-free
hypothesis, we determine the coronal magnetic field of AR8210. The
EIT/SOHO observations and the reconstructed magnetic configuration
suggest that the initiation of the eruptive events is related to the
existence of a complex topology (e.g. separatrix surfaces). From some
characteristic parameters before and after the eruptions, we note
that the magnetic energy (the free magnetic budget) decreases by 28%
(55%, respectively).
---------------------------------------------------------
Title: 3D Coronal magnetic field from vector magnetograms:
non-constant-alpha force-free configuration of the active region
NOAA 8151
Authors: Régnier, S.; Amari, T.; Kersalé, E.
2002A&A...392.1119R Altcode:
The Active Region 8151 (AR 8151) observed in February 1998 is the
site of an eruptive event associated with a filament and a S-shaped
structure, and producing a slow Coronal Mass Ejection (CME). In order
to determine how the CME occurs, we compute the 3D coronal magnetic
field and we derive some relevant parameters such as the free magnetic
energy and the relative magnetic helicity. The 3D magnetic configuration
is reconstructed from photospheric magnetic magnetograms (IVM, Mees
Solar Observatory) in the case of a non-constant-alpha force-free
(nlff) field model. The reconstruction method is divided into three
main steps: the analysis of vector magnetograms (transverse fields,
vertical density of electric current, ambiguity of 180<SUP>deg</SUP>),
the numerical scheme for the nlff magnetic field, the interpretation
of the computed magnetic field with respect to the observations. For
AR 8151, the nlff field matches the coronal observations from EIT/SOHO
and from SXT/Yohkoh. In particular, three characteristic flux tubes
are shown: a highly twisted flux tube, a long twisted flux tube and
a quasi-potential flux tube. The maximum energy budget is estimated
to 2.6*E<SUP>31</SUP> erg and the relative magnetic helicity to
4.7*E<SUP>34</SUP> G<SUP>2</SUP> cm<SUP>4</SUP>. From the simple
photospheric magnetic distribution and the evidence of highly twisted
flux tubes, we argue that the flux rope model is the most likely to
describe the initiation mechanism of the eruptive event associated
with AR 8151.
---------------------------------------------------------
Title: From the SunA~éÂ’s atmosphere to the EarthA~éÂ’s
atmosphere: an overview of scientific models available for space
weather developments
Authors: Lathuillère, C.; Menvielle, M.; Lilensten, J.; Amari, T.;
Radicella, S. M.
2002AnGeo..20.1081L Altcode:
Space weather aims at setting operational numerical tools in
order to nowcast, forecast and quantify the solar activity events,
the magnetosphere, ionosphere and thermosphere responses and the
consequences on our technological societies. These tools can be divided
in two parts. The first has a geophysical base (Sun, interplanetary
medium, magnetosphere, atmosphere). The second concerns technological
applications (telecommunications, spacecraft orbits, power plants
...). In this paper, we aim at giving an overview of the models that
belong to the first class (geophysics) that might serve in the future
as a basis for building global operational codes. For each model, we
consider the physics underneath, the input and output parameters, and
whether it is already operational, whether it may become operational
in the near future, or if it is an academic research tool. Relevant
references are given in order to serve as a starting point for further
readings.
---------------------------------------------------------
Title: Helicity injection and redistribution during relaxation of
solar/astrophysical plasmas
Authors: Amari, T.; Luciani, J.
2002cosp...34E3036A Altcode: 2002cosp.meetE3036A
We will discuss various MHD processes occurring in the context of
evolution of solar coronal magnetic configurations, due to some
boundary (photospheric) constraints. Several model problems will be
presented and their implication in the injection of magnetic helicity
as well triggering of confined and large scale disruptions will be
dis- cussed. The nature of the relaxation state reached after the
disruption will be discussed in the context of Taylor's theory of plasma
relaxation. We will show some recent nu- merical results (relevant to
astrophysical/solar disruptions), indicating that although this theory
seems to be in agreement with experiments in Reversed Field pinch De-
vices (but not in Tokamaks), it exhibits strong discrepancies with
these astrophysical disruptions.
---------------------------------------------------------
Title: Non-constant-alpha force-free reconstruction of the coronal
magnetic field. Example of the active region NOAA 8151
Authors: Regnier, S.; AMARI, T.; Kersale, E.
2001AGUFMSH11C0729R Altcode:
We present the method and results of a non-constant-alpha force-free
reconstruction of the 3D coronal magnetic field of an active
region. AR 8151 observed in February 1998 exhibits a filament,
a system of coronal loops, and a sigmoid. In order to determine
the magnetic configuration of such structures, a non-constant-alpha
force-free method (Amari et al 1997, Solar Physics 174, 129) is used to
reconstruct the coronal magnetic field using the photospheric vector
magnetograms as boundary condition. We compare the geometric and the
topological changes in the magnetic configurations given by the three
following methods: current-free field, constant-alpha force-free field
and non-constant-alpha force-free field. The comparison between the
observations (SXT/Yohkoh, EIT/SOHO) and the three models confirms that
the non-constant-alpha force-free field matches the observations. In
particular, the sigmoid can be reproduced by twisted flux tubes with
a height of 40,000 or 60,000 km, and the system of coronal loops is
well described by an untwisted flux tube (40,000 km). We also look for
the magnetic dips which are combined with the filament material. The
possible magnetic configurations for supporting the filament are either
a quadrupolar magnetic field or a long twisted flux tube. From the
chromospheric and magnetic observations, we conclude that the most
likely configuration is the long twisted flux tube with a height of
30,000 km. In summary, with the non-constant-alpha force-free field
we are able to reconstruct the coronal magnetic structures such as
the system of coronal loops and the sigmoid, and to determine the
structure of the magnetic field lines supporting the filament material.
---------------------------------------------------------
Title: Magnetohydrodynamic modeling of prominence formation within
a helmet streamer
Authors: Linker, J. A.; Lionello, R.; Mikić, Z.; Amari, T.
2001JGR...10625165L Altcode:
We present a 2.5-D axisymmetric MHD model to self-consistently describe
the formation of a stable prominence that supports cool, dense material
in the lower corona. The upper chromosphere and transition region
are included in the calculation. Reducing the magnetic flux along
the neutral line of a sheared coronal arcade forms a magnetic field
configuration with a flux rope topology. The prominence forms when dense
chromospheric material is brought up and condenses in the corona. The
prominence sits at the base of a helmet streamer structure. The dense
material is supported against gravity in the dips of the magnetic field
lines in the flux rope. Further reduction in magnetic flux leads to
an eruption of the prominence, ejecting material into the solar wind.
---------------------------------------------------------
Title: Reconstruction of the Coronal Magnetic Field for Active Region
NOAA 8151
Authors: Régnier, S.; Amari, T.
2001IAUS..203..441R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Are the EIT waves really waves?
Authors: Delannee, C.; Amari, T.
2000SPD....31.0603D Altcode: 2000BAAS...32R.838D
One sample of EIT wave is presented and discussed. The bright
front of the wave present two parts: one stationnary and one moving
arch. The stationnary part of the wave is compared to the magnetic
field lines extrapolated in spherical coordinates with the potential
assumptions. The stationnary part is located where the footpoints
of the separatrixes of the magnetic field are. Another case of a
moving arch of an EIT wave is analyzed in conjonction with a numerical
simulation of the ejection of a magnetic flux rope. The phenomelogy and
the morphology of the both the moving arch and the electric currents
created by the ejection of the flux rope are similar. We conclude
that the EIT wave phenomenon is possibly related to the generation of
electric currents while the magnetic field lines are openning during
a coronal mass ejection.
---------------------------------------------------------
Title: Are the EIT waves really waves?
Authors: Delannee, C.; Amari, T.
2000BAAS...32S.837D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Photospheric Magnetic Flux Changes as a Trigger for Coronal
Mass Ejections
Authors: Linker, J. A.; Lionello, R.; Mikic, Z.; Amari, T.
2000SPD....31.0281L Altcode: 2000BAAS...32..825L
Eruptive solar phenomena, such as coronal mass ejections, are believed
to be initiated by the release of energy stored in the coronal magnetic
field. Nonpotential magnetic field structures with significant amounts
of free magnetic energy are known to exist in the corona. However,
the mechanism(s) by which this energy is released are not well
understood. Previously, we have shown that the emergence of new magnetic
flux can lead to disruption of a helmet streamer, with liberation of
a significant fraction of the stored magnetic energy. Specifically,
eruption can be triggered when magnetic flux of opposite polarity
emerges near the neutral line, canceling some of the ambient flux. Prior
to eruption, a stable flux rope forms, which is a candidate magnetic
structure for prominence support. In this paper, we will describe how
other changes to the photospheric magnetic flux can lead to essentialy
the same eruptive process, and we will discuss the relationship of these
changes to flux reduction. Using full thermodynamic MHD simulations of
global coronal structure, we will also show that chromospheric material
can indeed be trapped in flux rope structures and lifted against the
solar gravity. Research supported by NASA.
---------------------------------------------------------
Title: Helicity Redistribution during Relaxation of Astrophysical
Plasmas
Authors: Amari, T.; Luciani, J. F.
2000PhRvL..84.1196A Altcode:
We present the first 3D numerical MHD simulations that show that
Taylor's relaxation conjecture is not satisfied in some MHD evolution
of magnetic configurations encountered in solar physics. We show that
magnetic helicity can be slowly injected through the boundary into
a magnetic configuration which then evolves into a MHD disruption,
with the formation in finite time of a current sheet through which
reconnection occurs, leading to a release of magnetic energy. While
helicity is well conserved during the process, it is shown that the
relaxed state is far from the constant- α linear force-free field
that would be predicted by Taylor's conjecture.
---------------------------------------------------------
Title: A Twisted Flux Rope Model for Coronal Mass Ejections and
Two-Ribbon Flares
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.; Linker, J.
2000ApJ...529L..49A Altcode:
We present a new approach to the theory of large-scale solar
eruptive phenomena such as coronal mass ejections and two-ribbon
flares, in which twisted flux tubes play a crucial role. We show
that it is possible to create a highly nonlinear three-dimensional
force-free configuration consisting of a twisted magnetic flux rope
representing the magnetic structure of a prominence (surrounded by
an overlaying, almost potential, arcade) and exhibiting an S-shaped
structure, as observed in soft X-ray sigmoid structures. We also
show that this magnetic configuration cannot stay in equilibrium and
that a considerable amount of magnetic energy is released during its
disruption. Unlike most previous models, the amount of magnetic energy
stored in the configuration prior to its disruption is so large that
it may become comparable to the energy of the open field.
---------------------------------------------------------
Title: Non Linear Force-Free Reconstruction of a Flaring Active Region
Authors: Bleybel, A.; Amari, T.; van Driel-Gesztelyi, L.; Leka, K. D.
1999ESASP.448..709B Altcode: 1999ESPM....9..709B; 1999mfsp.conf..709B
No abstract at ADS
---------------------------------------------------------
Title: Magnetic and Dynamic Evolution of an Active Region
Authors: Régnier, S.; Solomon, J.; Vial, J. C.; Amari, T.; Mickey, D.
1999ESASP.448..519R Altcode: 1999mfsp.conf..519R; 1999ESPM....9..519R
No abstract at ADS
---------------------------------------------------------
Title: Multi-Instrument Study Of The Evolution Of An Active Region
Authors: Regnier, S.; Amari, T.; Solomon, J.; Vial, J. C.; Mickey, D.
1999ESASP.446..571R Altcode: 1999soho....8..571R
We follow, for about half a solar rotation, the dynamic evolution of
an acive region (AR 8151) which exhibited a single heading sunspot
and a more diffuse following magnetic flux and a filament eruption
on 12 February 1998 at 13:51. For longitudinal magnetic observations,
we use the MDI (SoHO) magnetograph and for vector magnetic fields the
IVM (Mees Solar Observatory, Hawaii). The dynamic evolution is derived
from time-sequence images in the Fe XII line (195 Angstrom) from EIT
(SoHO). Density and abundance diagnostic of the filament are obtained
from SoHO spectrometers (CDS, SUMER).
---------------------------------------------------------
Title: An iterative method for the reconstructionbreak of the solar
coronal magnetic field. I. Method for regular solutions
Authors: Amari, T.; Boulmezaoud, T. Z.; Mikic, Z.
1999A&A...350.1051A Altcode:
We present a method for reconstructing the coronal magnetic field,
assumed to be in a non-linear force-free state, from its values given
in the photosphere by vector magnetograph measurements. In this paper,
that is the first of a series, we propose a method that solves the
boundary value problem set in the functional space of regular solutions
(i.e., that do not contain current sheets). This is an iterative method
introduced by Grad and Rubin. It is associated with a well-posed
boundary-value problem. We present some results obtained with this
method on two exact solutions of the magnetostatic equations, used as
theoretical magnetograms. Unlike some other extrapolations methods,
that are associated with ill-posed boundary value problems, our method
allows extrapolation to arbitrarily large heights, with no blowing up
due to the presence in these methods of an intrinsic instability that
makes errors growing up exponentially.
---------------------------------------------------------
Title: On the Role of Magnetic Flux Ropes for Helicity Injection
and Ejection in the Solar Corona
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.
1999ASPC..184...70A Altcode:
We present some work on the role of magnetic flux ropes for the
structure and dynamics of the corona in term of its magnetic helicity
content and redistribution for solar eruptive and non eruptive
events. We show that is possible to construct 3D MHD solutions that
represents the first type of models for prominence magnetic support
as highly twisted non linear force-free type configuration. We also
present some results for the disruption of twisted magnetic flux rope
in non eruptive and eruptive confined events.
---------------------------------------------------------
Title: On the removal of the sign ambiguitybreak in the photospheric
transverse magnetic field
Authors: Boulmezaoud, T. Z.; Amari, T.
1999A&A...347.1005B Altcode:
We present a method for removing the ambiguity in the transverse
component of the photospheric magnetic field measured by
vector magnetographs. The method is based upon the use of the
divergence-free condition satisfied by the magnetic field in the
sense of least squares. The method requires the measurement of the
longitudinal component of the magnetic field at two depth levels in
the photosphere. The method is is shown to be efficient when compared
to other existing methods on some particular analytical force-free
magnetic fields.
---------------------------------------------------------
Title: Three-dimensional Solutions of Magnetohydrodynamic Equationsfor
Prominence Magnetic Support: Twisted Magnetic Flux Rope
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.; Linker, J.
1999ApJ...518L..57A Altcode:
The search for a background magnetic configuration favorable for
prominence support has been given a great deal of attention for several
decades. The most recent theoretical studies seem to agree that a
promising candidate for the support of the dense and cooler prominence
material, which fulfills several of the theoretical and observational
requirements such as twist, shear along the neutral line, and dips,
is a magnetic flux rope. The most convincing models take an infinitely
long periodic configuration that consists of a linear constant-α
force-free magnetic field. These models, however, assume values of α
that are close to its maximum possible value. In this Letter, we report
our recent results, which show that it is indeed possible to produce a
configuration that consists of a twisted magnetic flux tube embedded in
an overlaying, almost potential, arcade such that high electric currents
(and therefore values of α) are confined to the inner twisted magnetic
flux rope. We present two MHD processes--corresponding to two different
types of boundary conditions--that produce such a configuration. Our
results show that the process associated variations of B<SUB>z</SUB>
at the photospheric level by applying an electric field involving
diffusion is much more efficient for creating a structure with more
twist and dips.
---------------------------------------------------------
Title: Confined Disruption of a Three-dimensional Twisted Magnetic
Flux Tube
Authors: Amari, T.; Luciani, J. F.
1999ApJ...515L..81A Altcode:
In this Letter, we study the evolution of a magnetic flux tube confined
by an overlaying arcade, considering the gradual build up of the twist
in the flux tube. During a first stage, the magnetic field evolves
quasi-statically through a sequence of force-free equilibria, building
up self-helicity (twist) in the flux rope. When a sufficient amount
of twist is injected in the system, it cannot reach a neighboring
equilibrium and experiences a major disruption in which the magnetic
field of the flux tube reconnects with the overlaying arcade. After the
relaxation is completed, the configuration remains globally closed and
consists of two almost untwisted flux tubes and a closed overlaying
arcade. The model might represent a basis for a unified theory of
noneruptive and eruptive flares, in which twisted magnetic flux tubes
play a crucial role.
---------------------------------------------------------
Title: Magnetohydrodynamic models of solar coronal magnetic fields.
Authors: Amari, T.; Luciani, J. F.; Mikic, Z.
1999PPCF...41..779A Altcode:
The authors present some results concerning the possibility of
determining the structure of solar active regions using measurements of
the vector magnetic field on the Sun's surface as boundary conditions
for the new numerical extrapolation codes. From these computations
the main features of these configurations, shear and twist (which are
particular forms of magnetic helicity), are then used as ingredients
to define model problems and solved for the magnetohydrodynamic
(MHD) analysis of solar eruptive phenomena, in which ejection (or
redistribution) of helicity occurs.
---------------------------------------------------------
Title: A regularization method for the extrapolationbreak of the
photospheric solar magnetic field. I. Linear force-free field
Authors: Amari, T.; Boulmezaoud, T. Z.; Maday, Y.
1998A&A...339..252A Altcode:
We present a method for reconstructing the magnetic field B above the
photosphere \{z=0\} as the solution of the boundary value problem (BVP)
for a bounded regular force-free magnetic field in Omega =\{z>0\}
from its boundary values supposed to be given on \{z=0\}. We propose
a way for regularizing the class of standard extrapolation methods
which turns out to diverge quickly with height, because of the ill
posedness of the BVP that prevents extrapolation at a reasonable
height. Our method, which is a Projection Method, allows us to improve
considerably the possible height of extrapolation that can be reached by
non regularized or even approximate filtering method. In this paper,
because of its linear nature the method is applied to the case of
linear force-free field.
---------------------------------------------------------
Title: Reconstructing the Solar Coronal Magnetic Field as a Force-Free
Magnetic Field
Authors: Amari, T.; Aly, J. J.; Luciani, J. F.; Boulmezaoud, T. Z.;
Mikic, Z.
1997SoPh..174..129A Altcode:
We present some preliminary results on different mathematical
problems encountered in attempts to reconstruct the coronal magnetic
field, assumed to be in a force-free state, from its values in the
photosphere. We discuss the formulations associated with these problems,
and some new numerical methods that can be used to get their approximate
solutions. Both the linear constant-α and the nonlinear cases are
considered. We also discuss the possible use of dynamical 3D MHD
codes to construct approximate solutions of the equilibrium force-free
equations, which are needed for testing numerical extrapolation schemes.
---------------------------------------------------------
Title: Current sheets in two-dimensional potential magnetic
fields. III. Formation in complex topology configurations and
application to coronal heating.
Authors: Aly, J. J.; Amari, T.
1997A&A...319..699A Altcode:
We study the spontaneous formation of a current sheet (CS) in
an x-invariant y-symmetric magnetic field B(y,z,t) occupying the
half-space {z>0}, and embedded in a pressureless perfectly conducting
plasma. At the initial time t=0, B(y,z,0) is potential and quadrupolar,
and therefore its lines in a poloidal plane have a complex topology:
there is either one separatrix, which contains a neutral X-point or
is tangent to the y-axis (X- and U-topology, respectively), or two
separatrices extending to infinity (I-topology). For t>=0, the field
is made to evolve quasi-statically by imposing its footpoints on the
boundary {z=0} to move parallel to the y-axis at the slow velocity
v(y,t). It thus passes through a sequence of configurations which are
either potential equilibria or quasi-potential singular equilibria,
the latter containing a CS, assumed a priori to be vertical. We
compute analytically B(y,z,t) and its free-energy contents δ W
(t) as functionals of B_z_(y,0,t) (this boundary value depending on
B_z_(y,0,0) and v(y,t)), and also, when there is a CS, of the unknown
heights z_1_(t) and z_2_(t) of its bottom and top, respectively. We
derive equations satisfied by the latter quantities, and use them
to show that: (i) When the initial field is of the U- or I-type,
a CS - and a vertical one indeed - is actually present at time t if
and only if the potential field B^p^(y,z,t) associated to B_z_(y,0,t)
has a X-topology. (ii) When the initial field is of the X-type, a CS
exists in general at each time t>0, but it is vertical if and only
if a quite specific condition is satisfied - which may not be the case
for arbitrarily chosen data and puts a limit on the generality of our
model. Finally, we derive for z_1_(t), z_2_(t), B(y,z,t) and δW(t)
useful approximate explicit expressions, which are valid just after
the CS has started forming at some time t_c_>=0. As an application,
we consider a plasma heating process in which a field evolving through
a sequence of singular equilibria as described above, relaxes at each
time t_k_ = k τ_D_ (k=1,2, ...,N) to a new potential equilibrium,
the vertical CS being destroyed by some reconnection process. We
present an estimate of the resulting heating rate, which is found
to depend on the ratio τ_D_/τ_ev_ (assumed to be <<1) of a
given phenomenological dissipation time τ_D_ to the ideal evolution
time τ_ev_ of the system. The relevance of this process for heating
a stellar corona is briefly discussed.
---------------------------------------------------------
Title: Two efficient methods for reconstructing the coronal magnetic
field as a linear force-free field.
Authors: Boulmezaoud, T. Z.; Amari, T.; Maday, Y.
1997joso.proc...53B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Very Fast Opening of a Three-dimensional Twisted Magnetic
Flux Tube
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.; Tagger, M.
1996ApJ...466L..39A Altcode:
This Letter is devoted to the still open problem of the evolution
of a three-dimensional coronal flux tube embedded in a low-beta
ideal plasma and having its footpoints twisted by slow photospheric
motions. Such a process has been simulated with a recently developed
magnetohydrodynamic code. In the particular calculation reported
here, the system occupies a large cubic box. The field is initially
potential, being generated by an underlying horizontal dipole, and it
is twisted by two vortices located on the lower face {z = 0} of the
box, on both sides of the neutral line. In a first phase, the field
roughly evolves quasi-statically through a sequence of force-free
configurations. Thus, it enters a dynamical phase during which it
suffers a very fast expansion, closely approaching after some finite
time a semi-open configuration. The energy increases monotonically
during all the evolution, and it tends to a limit, which is equal to
about 80% of the energy of the totally open field associated with Bz.
---------------------------------------------------------
Title: Plasmoid formation in a single sheared arcade and application
to coronal mass ejections.
Authors: Amari, T.; Luciani, J. F.; Aly, J. J.; Tagger, M.
1996A&A...306..913A Altcode:
We address the question whether a plasmoid can be produced and ejected
by an isolated x-invariant arcade located in a half-space, when its
footpoints are sheared parallel to the x-axis, but no converging
y-motions are applied. We use a recently developed MHD numerical code
based upon a new efficient semi-implicit method, and well adapted for
treating long time evolution problems in which small spatial scales
develop spontaneously. Starting from a configuration created by a dipole
located under the photosphere, we follow numerically the evolution
of the sheared arcade. In the ideal case, and in contrast to previous
attempts effected in such a geometry, our simulations show that, after
a long phase of slow evolution, the poloidal magnetic configuration
strongly inflates, while the electric current concentrates into a thin
layer extending vertically in the central region, as predicted by
analytical studies. Adding a small amount of resistivity then leads
to the formation of a rising plasmoid, with stronger flows appearing
along the separatrices.
---------------------------------------------------------
Title: Champs magnetiques : le cosmos EN filigrane.
Authors: Bel, N.; Amari, T.
1995C&E...307...58B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar corona's magnetism.
Authors: Amari, T.; Démoulin, P.
1993Rech...24..258A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Flare High-Energy Electron Spectra
Authors: Heristchi, D.; Amari, T.
1992SoPh..142..209H Altcode:
It is shown that the solar flare electron spectra in a large energy
range fit well with a power law in energy. This is used as a basis
to discuss the possible division of solar flares into various groups,
and also to compare the spectra of electrons in interplanetary space
and at the flare site.
---------------------------------------------------------
Title: Extended massive current sheet in a two-dimensional
constant-alpha force-free field - A model for quiescent prominences
Authors: Amari, T.; Aly, J. J.
1992A&A...265..791A Altcode:
We consider a simple 2D analytical model of a solar prominence
represented by a massive current sheet supported in a uniform
gravitational field by an external constant-alpha force-free magnetic
field. We show that it is possible to construct so-called inverse
configurations above either bipolar or quadrupolar photospheric regions
by a suitable adjustment of the free parameters of the model, namely
the boundary flux distribution, the current in the sheet, and the
value of alpha. It turns out that such configurations may exist only
for large value of alpha, and thus in highly sheared magnetic fields,
a result which should prove important for our understanding of the
formation of the prominences of the Kuperus-Raadu type.
---------------------------------------------------------
Title: Prominence Sheets Supported by Constant-Current Force-free
Fields. II. Imposition of Normal Photospheric Field Component and
Prominence Surface Current
Authors: Ridgway, C.; Amari, T.; Priest, E. R.
1992ApJ...385..718R Altcode:
Attention is given to a method of constructing longitudinally invariant
magnetic field configurations in which a symmetric finite vertical
current sheet is in magnetohydrostatic equilibrium between the
combined forces exerted by a background constant-current force-free
field and a uniform gravitational field. Both the normal magnetic
field component along the photosphere and the current density along
the prominence sheet are imposed as functions of position. The method
is used to generate both N- and I-type configurations by selecting a
convenient form for the imposed functions. Consideration is given to
the evolution of these configurations as the strength of the current
(and hence the mass) is increased while all other parameters are held
fixed. It is shown that in general the sheet loses equilibrium near
its upper extremity as I is increased beyond a certain value.
---------------------------------------------------------
Title: Prominence Sheets Supported by Constant-Current Force-free
Fields. I. Imposition of Normal Magnetic Field Components at the
Current Sheet and the Photosphere
Authors: Ridgway, C.; Amari, T.; Priest, E. R.
1991ApJ...378..773R Altcode:
We present an analytical model for the support of a prominence
(represented by a sheet of mass and current) in a constant-current
force-free field. The model allows us to produce both normal-type and
inverse-type configurations which are in equilibrium everywhere along
the sheet and for which the field is locally bounded.
---------------------------------------------------------
Title: Formation of current sheets in force-free magnetic fields.
Authors: Vekstein, G.; Priest, E. R.; Amari, T.
1991A&A...243..492V Altcode:
It is shown that the evolution of coronal magnetic fields in response
to slow photospheric footpoint motions in general produces states
with current sheets rather than smooth force-free equilibria. When a
separatrix surface is present, shearing motions of a two-dimensional
double arcade in general produce a current sheet all along the
separatrix, whether the motions are infinitesimal or of finite
amplitude; local singularities in the horizontal field component
occur at the footpoints of the separatrix where it becomes tangential
to the solar surface although the flux function is regular there. The
particular case of equal lobes and an antisymmetric shear gives a smooth
field without current sheets. When there is an X-type neutral line (or
separator) present, converging or divering motions create a current
sheet with Y-points at its ends; shearing motions create a current
sheet all along the separattrices, which coalesce at two cusp-points
to give a single current sheet near the original separator. In general
these current sheets could heat the corona as they dissipate, but when
new flux emerges into a highly sheared field they could also trigger
a solar flare.
---------------------------------------------------------
Title: A Twisted Flux-Tube Model for Solar Prominences. III. Magnetic
Support
Authors: Ridgway, C.; Priest, E. R.; Amari, T.
1991ApJ...367..321R Altcode:
The model for the support of a solar prominence in a twisted flux tube
by Priest et al. (1989) is here extended to consider large deviations
of the tube from cylindrical symmetry. The prominence is modeled as a
finite current sheet with mass which is locally free from singularities
in the magnetic field. Analytical solutions are found to the nonlinear
force-free equations for the structure around the prominence for two
functional forms of the field component parallel to the prominence
axis. An associated mixed boundary-value problem is also solved in
which the prominence is connected at its base to the photosphere,
and the normal magnetic field components along the prominence and the
photosphere are imposed.
---------------------------------------------------------
Title: Weak solutions of magnetostatics: an aspect of current
sheet theory.
Authors: Amari, T.
1991gamp.conf..173A Altcode:
This paper is only concerned with the processes of formation of a
current sheet (CS) during the quasi-static evolution towards weak
equilibrium of classical magnetostatic equilibria embedded in a
perfectly conducting plasma. The author is mainly interested in the
spontaneous process of classical magnetostatic equilibria embedded in
a perfectly conducting plasma. Contents: 1. Introduction. 2. 2D quasi
potential singular equilibria (QPSE): spontaneous formation of a CS,
asymptotic formation of QPSE. 3. 2D QFFSE: evolution towards a 2D
force-free field, T- and X-like topology. 4. 3D force-free solutions.
---------------------------------------------------------
Title: Weak Solutions of Magnetostatics
Authors: Amari, T.
1991assm.conf..173A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The creation of the magnetic environment for prominence
formation in a coronal arcade
Authors: Amari, T.; Démoulin, P.; Browning, P.; Hood, A.; Priest, E.
1991A&A...241..604A Altcode:
The possibility of prominence formation in sheared coronal arcades is
investigated. The creation of a dip at the summit of field lines is
a likely requirement before a prominence can form; then dense plasma
can be supported against gravity by the Lorentz force. It is proved
that, in fact, no shear profile can create a dip in a two-dimensional
force-free arcade if the photospheric field is bipolar. However,
numerical investigations show that shearing an arcade can induce very
flat field lines. It is investigated, in order of magnitude, how this
flattening of the field can increase the free fall time of a dense
plasma. Also, the interaction between shear and twist is analyzed; the
critical twist needed to have a dip is a decreasing function of shear.
---------------------------------------------------------
Title: Extended massive current sheet in a two-dimensional
constant-alpha force-free field - A model for quiescent prominences. I
- Theory
Authors: Amari, T.; Aly, J. J.
1990A&A...231..213A Altcode:
We present an analytical model of extended massive current sheet Σ
in equilibrium in a 2 D x-invariant constant-α force-free field, and
a vertical gravitational field. We show in particular that there is a
maximal mass that can be supported by the magnetic field and that the
topology of the field lines is always of the "Kippenhahn-Schlüter"
type. This model can be used to describe the magnetic support of solar
quiescent prominences. This work is an extension of Amari and Aly
(1989) in which the simpler approximation of a prominence by a filament
was considered.
---------------------------------------------------------
Title: Current Sheets in Two-Dimensional Potential Magnetic Fields -
Part Two - Asymptotic Limits of Indefinitely Sheared Force-Free Fields
Authors: Amari, T.; Aly, J. J.
1990A&A...227..628A Altcode:
We compute analytically the singular state B<SUB>∞</SUB> which is
approached asymptotically by an arcade-like x-invariant force- free
field in {z > 0} when indefinitely sheared. B<SUB>∞</SUB> is a
field which is potential everywhere in {z > 0} but on a current
sheet in equilibrium extending up to infinity; it is determined from
the values of B<SUB>z</SUB> on the boundary plane {z = 0} and of a
number A<SUP>1</SUP> characterizing the magnetic surface from which
the sheet is starting.
---------------------------------------------------------
Title: Structure of two-dimensional magnetostatic equilibria in the
presence of gravity
Authors: Amari, T.; Aly, J. J.
1990GMS....58..327A Altcode:
Results are presented from an analytical study of three different
2D, x-invariant equilibrium models for a plasma that occupies the
half-space and is subject to both a magnetic field and a constant
vertical gravitational field. The models are differentiated by their
assumptions concerning the spatial distribution of matter; while models
1 and 2, respectively, take the plasma to be concentrated in a vertical
sheet and in the filament, model 3's plasma occupies all of half-space
and possesses a uniform temperature. In all three cases, attention
is given to the existence and uniqueness of solutions. Nonequilibrium
phenomena are noted.
---------------------------------------------------------
Title: Support of a Prominence by a Constant-Current Force-Free Field
Authors: Ridgway, C.; Amari, T.; Priest, E. R.
1990PDHO....7..166R Altcode: 1990dysu.conf..166R; 1990ESPM....6..166R
No abstract at ADS
---------------------------------------------------------
Title: Two Dimensional Quasi-static Boundary Value Problems of
Solar Magnetostatics.
Authors: Amari, T.
1990ppsa.conf..193A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasi-potential-singular-equilibria and evolution of the
coronal magnetic field due to photospheric boundary motions
Authors: Amari, T.; Aly, J. J.
1990GMS....58..245A Altcode:
Novel general properties are presented of configurations in a class
of 2D, x-invariant magnetostatic equilibria, showing that the general
condition of equilibrium at the extremities of the current sheet implies
a heretofore unsuspected constraint on the magnetic field. Attention is
given to situations in which singular states are obtained asymptotically
by means of an arcadelike, x-invariant force-free field that is
indefinitely sheared. A method is given for the analytical computation
of such asymptotic states, as the solutions of boundary-value problems.
---------------------------------------------------------
Title: Current sheets in two-dimensional potential magnetic fields. I
- General properties
Authors: Aly, J. J.; Amari, T.
1989A&A...221..287A Altcode:
The equilibrium of a set of current sheets embedded in a potential
magnetic field is studied as well as some general properties of
quasi-potential singular equilibrium (QPSE). Representations were
established for an x-invariant magnetic field which is potential
everywhere but on a set of singular surfaces. It is shown that, in a
QPSE, the current sheets are analytical curves. An analytical curve
may always be interpreted as a current sheet embedded in a QPSE.
---------------------------------------------------------
Title: Non-equilibrium of a cylindrical magnetic arcade
Authors: Steele, C. D. C.; Hood, A. W.; Priest, E. R.; Amari, T.
1989SoPh..123..127S Altcode:
A cylindrically-symmetric magnetic arcade with its axis on the
photosphere is perturbed by means of an alteration in the pressure
along the base. The perturbation is examined with a view to finding
equilibrium configurations close to the original equilibrium. It
is found that equilibria can only be found when the integral of the
excess pressure along the base is zero. In other cases no equilibria
can be found and the arcade is likely either to collapse or, in the
case of a coronal mass ejection, to erupt. For an initial arcade
whose field increases linearly with radial distance from the axis,
the neighbouring equilibria have been found.
---------------------------------------------------------
Title: Two-dimensional isothermal magnetostatic equilibria in a
gravitational field. I - Unsheared equilibria
Authors: Amari, T.; Aly, J. J.
1989A&A...208..361A Altcode:
Some new results concerning the structure and the quasi-static
evolution of two-dimensional x-invariant magnetostatic equilibria in
the half-space (z greater than 0) are presented. The plasma pressure p
and the gravity are taken into account, but the field is assumed to be
shearless (Bx = 0). The values of Bz and of p are given on the boundary
(z =0). The problem of the existence of solutions is discussed, and
some of their general properties are determined.
---------------------------------------------------------
Title: Interaction between a line current and a two-dimensional
constant-alpha force-free field - an analytical model for quiescent
prominences
Authors: Amari, T.; Aly, J. J.
1989A&A...208..261A Altcode:
A simple analytical model describing the equilibrium of a massive
line current submitted to the action of a vertical gravitational field
and of a two-dimensional constant-alpha force-free magnetic field is
described. This model is then applied to the problem of the magnetic
support of quiescent prominences. It is shown in particular that the
introduction of a background force-free field instead of a potential
one may be a clue for solving difficulties which appear when some
previous models are confronted with the observations.
---------------------------------------------------------
Title: An analytical study of the structure of two-dimensional
magnetostatic equilibria in the presence of gravity.
Authors: Amari, T.; Aly, J. J.
1989sasf.confP.271A Altcode: 1989IAUCo.104P.271A; 1988sasf.conf..271A
No abstract at ADS
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Title: Two-dimensional non-symmetric models of quiescent prominences
in potential magnetic fields
Authors: Aly, J. J.; Amari, T.
1988A&A...207..154A Altcode:
The authors present a general method which allows to construct in the
half-space {z > 0} non-y-symmetric x invariant models describing
the equilibrium of a massive current sheet submitted to the action of
a vertical gravitational field and of a potential magnetic field. They
also derive general relations which determine the current and the mass
of the sheet and the total energy of the configuration from the only
value of the magnetic field on the plane {z = 0}.
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Title: Some integral properties of two-dimensional force-free fields
supporting massive current sheets
Authors: Amari, T.; Aly, J. J.
1988A&A...193..291A Altcode:
A set of useful integral relations satisfied by any x-invariant
nonlinear force-free field occupying the half-space z greater than zero
and supporting a massive current sheet against the action of a vertical
gravitational field is derived. These relations are used to prove a
nonexistence theorem for the solutions of a boundary value problem which
determines the equilibrium of a massive sheet when the amount of mass
per unit of flux of the background nonlinear force-free field is given.
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Title: Some New Results in the Theory of Two-Dimensional Magnetostatic
Equilibria
Authors: Aly, J. J.; Amari, T.
1985tphr.conf..319A Altcode:
No abstract at ADS
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Title: Some new results in the theory of two-dimensional magnetostatic
equilibria.
Authors: Aly, J. J.; Amari, T.
1985MPARp.212..319A Altcode:
X-invariant magnetostatic equilibria in the half-space {z > 0} have
been very often considered in theoretical models for prominences or
two-ribbon flares. In this paper, the authors report some new results
on the structure and quasi-static evolution of such equilibria. These
results concern the two following limiting situations: (1) the field
is force-free and the value of B<SUB>z</SUB> as well as the positions
of the feet of the lines are given on {z = 0}; (2) plasma pressure
p and gravity are taken into account, but the field is shearless
(B<SUB>x</SUB> = 0); the values of B<SUB>z</SUB> and of p are given
on {z = 0}.
