explanation blue bibcodes open ADS page with paths to full text
Author name code: aly
ADS astronomy entries on 2022-09-14
author:"Aly, Jean-Jacques"
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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: 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: 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: 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.
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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.
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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.
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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).
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Title: Nonlinear Stability of a Class of Magnetostatic Equilibria
with an Application to Solar Prominences
Authors: Aly, J. -J.
2012ApJ...746...52A Altcode:
We consider a particular class of three-dimensional magnetostatic
equilibria in which the plasma is submitted to a vertical gravitational
field and the gradient of the total (thermal+magnetic) pressure
vanishes. We show analytically that an equilibrium in that class makes
the energy an absolute minimum in the set of all the configurations
accessible from it by an arbitrary finite deformation constrained by
ideal MHD and imposed to vanish on a rigid conducting wall (line-tying
condition). Along with energy conservation, this implies the nonlinear
ideal stability of that equilibrium in the following sense. Suppose
that a perturbation of energy w(0) is applied at time t = 0 and thus
evolves by obeying the nonlinear MHD equations. Then some measure of
the sizes of the plasma velocity and the deformation of the structure
can be made to stay at any t >= 0 below an arbitrarily prescribed
value by choosing w(0) small enough. Nonlinear stability also holds
true for a configuration obtained by superposing an equilibrium of the
previous type and a nonmagnetic equilibrium which is also an energy
minimizer—for instance an equilibrium with uniform specific entropy,
which is shown to have that property. Our result applies to a subset
of a family of equilibria, computed by B. C. Low, which includes in
particular the standard Kippenhahn-Schlüter model describing the
magnetic support of solar corona prominences.
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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.
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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.
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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.
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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.
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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.
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Title: Simple analytical examples of boundary driven evolution of
a two-dimensional magnetohydrostatic equilibrium
Authors: Aly, J. J.
2009A&A...507...29A Altcode:
Aims: We construct families of time-sequences of x-invariant
magnetostatic equilibria which describe ideal quasi-static evolutions
driven by stationary shearing motions imposed on a boundary. The change
in the thermal pressure of the plasma is determined by imposing either
an adiabatic, or an isothermal, or an isobaric, prescription. <BR
/>Methods: We start from a well known family of linear force-free
fields, on which we effect simple transforms. <BR />Results: In either
case, the magnetic field and the pressure are expressed analytically as
functions of space and time. The field is found to suffer an indefinite
expansion, with a decrease to zero of the pressure in the adiabatic and
isothermal cases, and to eventually open. Moreover, the configurations
forming any sequence are shown to be linearly stable with respect to
x-invariant perturbations.
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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.
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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.
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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.
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Title: Structure and evolution of the solar coronal magnetic field
Authors: Aly, J. J.; Amari, T.
2007GApFD.101..249A Altcode:
No abstract at ADS
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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.
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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: Conservation laws and theorems of confinement and stability
for a charged equatorial disk in a pulsar magnetosphere
Authors: Aly, J. J.
2005A&A...434..405A Altcode:
For studying the nonaxisymmetric stability of the bounded electrosphere
of an “aligned pulsar” (Michel's structure with polar domes
and equatorial belt), Pétri et al. (2002) recently introduced a
simplified but useful model in which all the charge-separated plasma
located outside the magnetized rotating star is concentrated into a
thin equatorial disk. In this paper, some aspects of this model are
investigated analytically. It is shown that the equations governing
the behaviour of the disk - in the case where there are no sources
of particles feeding it - imply a series of conservation laws (for
energy, angular momentum,...), and that there is a non-canonical
Hamiltonian structure hidden behind them. The conservation laws are
used to prove that: (i) for any initial conditions imposed on the disk,
its evolution cannot lead to charges escaping to infinity (confinement
theorem); (ii) a disk steady state with a possibly rotating pattern is
nonlinearly stable if the charge density per unit of magnetic flux is
a decreasing function of the electrostatic potential in the rotating
frame (stability theorem).
---------------------------------------------------------
Title: A stability property of a force-free surface bounding a
vacuum gap
Authors: Aly, J. J.
2005A&A...429..779A Altcode: 2010arXiv1005.4698A
A force-free surface (FFS) S is a sharp boundary separating a void
from a region occupied by a charge-separated force-free plasma. It
is proven here under very general assumptions that there is on S
a simple relation between the charge density μ on the plasma side
and the derivative of δ=E\cdotB along B on the vacuum side (with E
denoting the electric field and B the magnetic field). Combined with
the condition δ=0 on S, this relation implies that a FFS has a general
stability property, already conjectured by Michel (1979, ApJ 227, 579):
S turns out to attract charges placed on the vacuum side if they are of
the same sign as μ. In the particular case of a FFS existing in the
axisymmetric stationary magnetosphere of a “pulsar”, the relation
is given a most convenient form by using magnetic coordinates, and is
shown to imply an interesting property of a gap. Also, a simple proof
is given of the impossibility of a vacuum gap forming in a field B
which is either uniform or radial (monopolar).
---------------------------------------------------------
Title: A uniqueness result for a simple force-free magnetic field
submitted to a topological constraint
Authors: Aly, J. J.
2005A&A...429...15A Altcode:
A proof is given of the following statement: if B is a smooth force-free
magnetic field contained in a cylindrical domain of axis parallel
to z and of star-shaped cross-section, and if B is topologically
equivalent to the uniform field B<SUB>0</SUB>=B<SUB>0</SUB>z, then
B=B<SUB>0</SUB>. In addition to being a very first step in the general
study of the uniqueness of a magnetostatic equilibrium subject to
a topological constraint, this result has a direct relevance to
the approach recently developed by Ng & Bhattacharjee (1998,
Phys. Plasma, 5, 4028) to support Parker's theory of current sheets
formation in the solar corona.
---------------------------------------------------------
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: 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: 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: Thermodynamics of a two-dimensional unbounded self-gravitating
system
Authors: Aly, Jean-Jacques; Perez, Jérôme
1999PhRvE..60.5185A Altcode:
The thermodynamics of a two-dimensional self-gravitating
system occupying the whole plane is considered in the mean-field
approximation. First, it is proven that, if the number N of particles
and the total energy E are imposed as the only external constraints,
then the entropy admits the least upper bound S<SUP>+</SUP>(N,E)=2E/N+N
ln(eπ<SUP>2</SUP>) (in appropriate units). Moreover, there does exist a
unique state of maximum entropy, which is characterized by a Maxwellian
distribution function with a temperature T=N/2 independent of E. Next,
it is shown that, if the total angular momentum J is imposed as a
further constraint, the largest possible value of the entropy does
not change, and there is no admissible state of maximum entropy, but
in the case J=0. Finally, some inequalities satisfied by a class of
so-called H functions and related generalized entropies are given.
---------------------------------------------------------
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: 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: Stability of spherical stellar systems - I. Analytical results
Authors: Perez, Jerome; Aly, Jean-Jacques
1996MNRAS.280..689P Altcode: 1995astro.ph.11103P
The so-called `symplectic method' is used for studying the
linear stability of a self-gravitating collisionless stellar
system, in which the particles are also subjected to an external
potential. The system is steady and spherically symmetric, and
its distribution function f_0 thus depends only on the energy
E and the squared angular momentum L^2 of a particle. Assuming
that ∂f_0/∂<e1>E<</e1>0, it is first shown that
stability holds with respect to all the spherical perturbations -
a statement which turns out also to be valid for a rotating spherical
system. Thus it is proven that the energy of an arbitrary aspherical
perturbation associated with a `preserving generator' deltag_1 [i.e.,
one satisfying ∂f_0/∂L^2lcubdeltag_1,L^2rcub=0] is always positive
if ∂f_0/∂L^2<=0 and the external mass density is a decreasing
function of the distance r to the centre. This implies in particular
(under the latter condition) the stability of an isotropic system
with respect to all the perturbations. Some new remarks on the
relation between the symmetry of the system and the form of f_0
are also reported. It is argued, in particular, that a system with
a distribution function of the form f_0=f_0(E,L^2 ) is necessarily
spherically symmetric.
---------------------------------------------------------
Title: Stability of spherical stellar systems - II. Numerical results
Authors: Perez, Jerome; Alimi, Jean-Michel; Aly, Jean-Jacques;
Scholl, Hans
1996MNRAS.280..700P Altcode: 1995astro.ph.11090P
We have performed a series of high-resolution N-body experiments on a
connection machine CM-5 in order to study the stability of collisionless
self-gravitating spherical systems. We interpret our results in the
framework of symplectic mechanics, which provides the definition of a
new class of particular perturbations: the preserving perturbations,
which are a generalization of the radial ones. Using models defined
by the Ossipkov-Merritt algorithm, we show that the stability of a
spherical anisotropic system is directly related to the preserving or
non-preserving nature of the perturbations acting on the system. We then
generalize our results to all spherical systems. Since the `isotropic
component' of the linear variation of the distribution function cannot
be used to predict the stability or instability of a spherical system,
we propose a more useful stability parameter which is derived from the
`anisotropic' component of the linear variation.
---------------------------------------------------------
Title: Stability of an axisymmetric massive current sheet supported
by a potential magnetic field.
Authors: Lepeltier, T.; Aly, J. J.
1996A&A...306..645L Altcode:
The ideal linear stability of a class of axisymmetric magnetostatic
equilibria is investigated by using the classical energy principle of
Bernstein et al. (1958). The system under consideration is constituted
of an infinitely thin equatorial disk of cold dense matter and of a
corona filled up with a massless plasma. The disk is supported against
the radial gravity of a central object by a magnetic field which is
potential in the corona and has its footpoints firmly anchored in the
rigid boundary of that region (line-tying). Such a configuration is
proven to be always stable with respect to axisymmetric perturbations,
but to be stable against arbitrary ones if and only if two criteria
(in which stabilization by line-tying appears explicitly) are
satisfied. These criteria are applied to several particular equilibria
(constructed by a general superposition method), which may be considered
as crude models useful to understand some of the mechanisms at work in
solar prominences and in accretion disks around compact objects. Stable
configurations are shown to exist in each of the cases which have been
worked out.
---------------------------------------------------------
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: Nonequilibrium in Sheared Axisymmetric Force-free Magnetic
Fields
Authors: Aly, J. J.
1995ApJ...439L..63A Altcode:
The problem of the ideal evolution through a sequence of force-free
configurations of an axisymmetric magnetic field occupying the region
outside a sphere is considered. The field is initially a potential
arcade, and its footpoints on the boundary are slowly sheared by
azimuthal stationary motions. It is shown on general grounds that,
after an initial phase during which energy gets quietly stored, the
field starts expanding at an increasing rate, approaching very rapidly
an open field (possibly reached at finite time] -- which implies in
general a breakdown of the quasi-static approximation at some stage. he
possible importance of this result for our understanding of eruptive
processes in the solar corona is emphasized.
---------------------------------------------------------
Title: Computation of the magnetic field around a quiescent prominence
from observational data.
Authors: Lepeltier, T.; Aly, J. J.
1995A&A...293..906L Altcode:
A prominence is often modelized by a vertical infinitely thin massive
current sheet {SIGMA} embedded in the coronal half-space {z>0},
and supported against a uniform gravitational field -gz{hat} by an
x-invariant magnetic field which is potential outside {SIGMA}. In
the framework of this simple model, we reconsider the problem of
the determination of the magnetic field from the knowledge of its
normal components on both {SIGMA} and the photospheric plane {z=0},
the values of these quantities being assumed to be extracted from
actual observational data. It is shown that there is in general
no regular field satisfying these boundary conditions. A sensible
"regularization" procedure can however be applied to the problem,
which allows to determine a field in a unique way. Conditions on the
data for this field being actually able to support a prominence are
established, and an explicit example is computed.
---------------------------------------------------------
Title: Current sheet model of a normal prominence in which the
background magnetic lines show a `dip structure'
Authors: Lepeltier, T.; Aly, J. J.
1994SoPh..154..393L Altcode:
We propose a simple 2D current sheet model of a normal prominence,
in which the lines of the background magnetic field have the `dip
structure' which seems to be required for such an object to form and
to be stably supported.
---------------------------------------------------------
Title: Stability Criteria for Massive Current Sheets in
Two-dimensional Potential Magnetic Fields
Authors: Aly, J. J.; Colombi, S.; Lepeltier, T.
1994ApJ...432..793A Altcode:
We investigate the linear stability of a simple model describing a
solar prominence as a perfectly conducting vertical massive current
sheet located in the 'coronal half-space' (z greater than 0), and
supported against gravity by an x-invariant magnetic field. Assuming the
region outside the sheet to be current-free and to contain a low-beta
plasma having an infinite conductivity, and imposing the field lines
to be firmly tied to the 'photospheric plane' (z = 0): (1) We show
that the model is stable with respect to any perturbation which do
not depend on x. (2) We derive necessary and sufficient conditions
for three-dimensional stability to hold. As expected a priori, our
criteria are much less severe than those Anzer obtained by taking
the sheet to be embedded in a vacuum. They allow in particular --
contrary to Anzer's -- the stability of a sheet of low mass suspended
in a region where the lines of the background field would have their
concavity directed upward, were they unperturbed by the heavy plasma.
---------------------------------------------------------
Title: Asymptotic formation of a current sheet in an indefinitely
sheared force-free field: an analytical example
Authors: Aly, J. J.
1994A&A...288.1012A Altcode:
We construct an analytical sequence of x-invariant force-free magnetic
configurations occupying the half-space {Z>0} and having an arcade
topology. The field is initially current-free, and it evolves eventually
towards an open field, with all the currents concentrating into a
current sheet.
---------------------------------------------------------
Title: Thermodynamics of a two-dimensional self-gravitating system
Authors: Aly, J. J.
1994PhRvE..49.3771A Altcode:
The mean field thermodynamics of a system of N gravitationally
interacting particles confined in some bounded plane domain Ω
is considered in the four possible situations corresponding to the
following two pairs of alternatives: (a) Confinement is due either to a
rigid circular wall ∂Ω or to an imposed external pressure (in which
case ∂Ω is a free boundary). (b) The system is either in contact
with a thermal bath at temperature T, or it is thermally insulated. It
is shown in particular that (i) for a system at given temperature T,
a globally stable equilibrium (minimum free energy or minimum free
enthalpy state for ∂Ω rigid or free, respectively) exists and
is unique if and only if T exceeds a critical value T<SUB>c</SUB>,
and (ii) for a thermally insulated system, a unique globally stable
(maximum entropy) equilibrium exists for any value of the energy (rigid
∂Ω) or of the enthalpy (free ∂Ω). The case of a system confined
in a domain of arbitrary shape is also discussed. Bounds on the free
energy and the entropy are derived, and it is proven that no isothermal
equilibrium (stable or unstable) with a temperature T<=T<SUB>c</SUB>
can exist if the domain is “star shaped.”
---------------------------------------------------------
Title: Eruptive processes in the solar corona.
Authors: Aly, J. J.
1994cmcp.conf....7A Altcode:
The author discusses the following problem, which seems to be important
for our understanding of eruptive processes in the solar corona:
in which conditions does a sequence of evolving force-free magnetic
arcades lose equilibrium, thus entering a phase of fast dynamical
evolution in the course of which energy is released and mass and
magnetic flux are possibly ejected.
---------------------------------------------------------
Title: Recent Results on the Stability of Anisotropic Stellar Systems
Authors: Perez, J.; Aly, J. -J.
1994LNP...430..177P Altcode: 1994ecsd.conf..177P
We present a new approach to deal with the problem of stability of
collisionless stellar systems. This technique, based on the symplectic
structure of the Vlasov-Poisson system, allows us to derive a new
stability criterion for general systems. It is very useful in the
anisotropic spherical case.
---------------------------------------------------------
Title: Minimum Energy States of a Self-Gravitating System
Authors: Aly, J. -J.
1994LNP...430..226A Altcode: 1994ecsd.conf..226A
We prove the existence of a unique global minimum energy state for
a self-gravitating system whose mass and a "quasi-entropy" assume a
priori given values.
---------------------------------------------------------
Title: Stability of collisionless self-gravitating systems.
Authors: Alimi, J. -M.; Perez, J.; Aly, J. -J.; Scholl, H.
1994euoq.conf..403A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Source-Surface Model with Force-Free Fields
Authors: Seehafer, N.; Aly, J. J.; Schmidtmann, O.
1994emsp.conf..103S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Force-Free Magnetic Field - Source Surface Model
Authors: Aly, J. J.; Seehafer, N.
1993SoPh..144..243A Altcode:
Models of the magnetic field in the solar chromosphere and corona
are still mainly based on theoretical extrapolations of photospheric
measurements. For the practical calculation of the global field,
the so-called source-surface model has been introduced, in which the
influence of the solar wind is described by the requirement that the
field be radial at some exterior (source) surface. Then the assumption
that the field is current-free in the volume between the photosphere
and this surface allows for its determination from the photospheric
measurement. In the present paper a generalization of the source-surface
model to force-free fields is proposed. In the generalized model the
parameterα( = ∇ ×B.B/B<SUP>2</SUP>)must be non-constant (or vanish
identically) and currents are restricted to regions with closed field
lines. A mathematical algorithm for computing the field from boundary
data is devised.
---------------------------------------------------------
Title: Minimum Energy / Maximum Entropy States of a Self-gravitating
System
Authors: Aly, J. -J.
1993nbpg.conf...19A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A model for magnetic energy storage and Taylor's relaxation
in the solar corona. I: Helicity-constrained minimum energy state
in a half-cylinder
Authors: Aly, J. J.
1993PhFlB...5..151A Altcode:
The paper is concerned with the problem of the existence of a minimum
energy state in a set of all the magnetic fields B occupying a circular
half-cylinder D, with a normal component vanishing on the vertical part
of the boundary of D and taking prescribed axisymmetric values on its
lower horizontal part, and having a given relative helicity. It is shown
that the only possible solution is a constant-alpha force-free field
B-alpha. The main properties of this type of field (in particular,
its existence and uniqueness under the imposed conditions) are
determined. The implications of the results obtained for the problem
of the solar corona are discussed.
---------------------------------------------------------
Title: Stability of Spherical Stellar Systems by Symplectic Method:
Numerical Test
Authors: Perez, J.; Alimi, J. -M.; Aly, J. -J.; Scholl, H.
1993nbpg.conf...43P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reconnection in the solar atmosphere
Authors: Aly, J. J.
1992PPCF...34.1785A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the stability of a gaseous sphere against non-radial
perturbations
Authors: Aly, Jean-Jacques; Perez, Jerome
1992MNRAS.259...95A Altcode:
We present a simplified proof of the Antonov-Lebovitz theorem, asserting
that any spherical barotropic star having a mass density decreasing
monotonically outwards and vanishing at its surface is stable to all
non-radial perturbations. We also develop a simple argument showing in
a straightforward way a related but somewhat weaker result, according
to which any such star is stable if and only if it is stable to radial
perturbations. Extension of these results to a star with non-decreasing
specific entropy distribution is also briefly discussed.
---------------------------------------------------------
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: Some properties of finite energy constant-α force-free
magnetic fields in a half-space
Authors: Aly, J. J.
1992SoPh..138..133A Altcode:
Some useful properties of a finite energy, constant-α, force-free
magnetic field B<SUB>α</SUB> occupying a half-space D are presented. In
particular: Fourier and Green representations of B<SUB>α</SUB> are
obtained and used to derive conditions for the existence and uniqueness
of a B<SUB>α</SUB> having a given normal component B<SUB>z</SUB>
on the boundary ∂D.
---------------------------------------------------------
Title: How Much Energy Can Be Stored in a Three-dimensional Force-free
Magnetic Field?
Authors: Aly, J. J.
1991ApJ...375L..61A Altcode:
Simple physical considerations indicate that the energy of any
finite-energy, force-free magnetic field occupying a half-space, and
having all its lines unknotted and tied to the half-space boundary,
will not be larger than the energy of the 'open field' which has the
same half-space boundary flux distribution. Attention is given to the
consequences of this view for the solar corona's eruptive phenomena;
an upper bound on the energy which a flare can release is implied,
and any spontaneous transition of the coronal field to an open state
is precluded.
---------------------------------------------------------
Title: The stability of a line-tied force-free magnetic field in an
unbounded region of space
Authors: Aly, J. J.
1990PhFlB...2.1928A Altcode:
Sufficient criteria of linear ideal magnetohydrodynamic stability are
derived for a three-dimensional nonlinear force-free magnetic field B
occupying an infinite region Omega, which is either a half-space or an
'exterior domain'; all the field lines of B are assumed to be firmly
rooted in the line-typing condition, but their topological pattern may
be otherwise quite arbitrary. Roughly speaking, a field characterized
by a length scale Lambda is found to be stable if alpha sub 0 is less
than O(1), where alpha sub 0 is a number measuring the magnitude of
the force-free function. The stability criteria established here have
some important astrophysical applications (solar corona, magnetosphere
of a compact object, etc.), which are briefly discussed.
---------------------------------------------------------
Title: Quasi-static evolution of a force-free magnetic field
Authors: Aly, J. J.
1990CoPhC..59...13A Altcode:
We consider in the half-space {z > 0} a simple-topology force-free
magnetic field B embedded in a highly conducting plasma (resistivity σ)
and its quasi-static evolution driven by motions imposed to the feet
of its lines on the boundary {z = 0}. We first study the case when B
is an x-invariant arcade, discussing in particular: (i) for σ = 0,
the existence of stable equilibria corresponding to arbitrarily large
shearing, and the qualitative time behaviour of the field (in particular
in the limit t --> ∞) (ii) for σ ≠ 0, the possibility of a
fast transition by reconnection from an arcade to a complex topology
configuration having a lower energy, but the same “distribution of
magnetic fluxes”. We thus consider a fully three-dimensional field
(having an “arcade” or a “tube” topology), showing in particular
how it is possible to extend to that situation some of the results
obtained for an x-invariant arcade when σ = 0.
---------------------------------------------------------
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: Do Current Sheets Necessarily Form in 3D Sheared Magnetic
Force-Free Fields?
Authors: Aly, J. J.
1990PDHO....7..176A Altcode: 1990ESPM....6..176A; 1990dysu.conf..176A
No abstract at ADS
---------------------------------------------------------
Title: Stability of a Massive Current Sheet Supported by a
Two-Dimensional Potential Magnetic Field
Authors: Aly, J. J.; Colombi, S.
1990IAUS..142..341A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flaring interactions between accretion disk and neutron star
magnetosphere.
Authors: Aly, J. J.; Kuijpers, J.
1990A&A...227..473A Altcode:
The effect of magnetic reconnections between the magnetosphere of a
neutron star and the accretion disk is considered using a model of
a neutron-star/disk system, which assumes the existence of magnetic
'loops' anchored in the disk and extending into a corona on both
sides of the disk. It is shown that these magnetic links are efficient
transmitters of angular momentum. At the disk plane, the linked field
is forced to rotate with near-Keplerian velocity, while it corotates
with the star at the other end. The magnetic energy is stored in the
sheared and expanding field link, which is released upon a transition
to a lower energy state again caused by reconnection. This process
leads to a release of flaring energy in the magnetosphere and to
the transport of angular momentum between the disk and the star. It
is proposed that these magnetic flares might be an explanation for
quasi-periodic oscillations.
---------------------------------------------------------
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: Current Sheet Models for Solar Prominences.
Authors: Aly, J. J.; Amary, T.; Colombi, S.
1990ppsa.conf..181A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: How much Energy can be Stored in a Stable Force-Free Magnetic
Field?
Authors: Aly, J. J.
1990PDHO....7..179A Altcode: 1990dysu.conf..179A; 1990ESPM....6..179A
No abstract at ADS
---------------------------------------------------------
Title: Quasi-Static Evolution of a Force-Free Magnetic Field.
Authors: Aly, J. J.
1990ppsa.conf..167A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Storage and Release of Magnetic Energy in a Force-Free Field
Authors: Aly, J. J.
1990IAUS..142..313A Altcode:
Some of the analytical results obtained in a study by Aly (1989) of
the quasi-static evolution of a 2D force-free field are extended to
a 3D situation. For a 3D force-free field occupying a half-space D =
(z greater than 0), attention is given to: the storage of free magnetic
energy when the field evolves quasi-statically as a consequence
of motions imposed to its footpoints on the plane (z = 0); and (2)
the release of this energy during a reconnection process implying
a rearrangement of the lines which is either local or global. It is
shown that a 3D field may approach asymptotically an open field.
---------------------------------------------------------
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: On the lowest energy state of a collisionless self-gravitating
system under phase space volume constraints.
Authors: Aly, J. J.
1989MNRAS.241...15A Altcode:
The problem of minimizing the energy of a collisionless self-gravitating
system under the only constraint of phase space volume conservation is
reconsidered. With respect to the recent paper on the same subject by
Wiechen, Ziegler and Schindler, the author brings about essentially
three new results: 1. he establishes a lower bound on the energy by
using arguments which are both simpler and more general than those of
these authors; 2. he proves that the lowest energy state is necessarily
spherically symmetric (in the physical space), and 3. he shows that,
if one fixes also the total angular momentum J of the system, then there
is no lowest energy state satisfying this further constraint if J ≠ 0.
---------------------------------------------------------
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: Magnetospheres of magnetic stars surrounded by disks.
Authors: Zylstra, G. J.; Lamb, F. K.; Aly, J. J.
1989BAPS...34.1289Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Reconstruction of the Nonlinear Force-Free Coronal
Magnetic Field from Boundary Data
Authors: Aly, J. J.
1989SoPh..120...19A Altcode:
Using a simple model in which the corona is represented by the
half-space domain Ω = {z > 0} and the photosphere by the
boundary plane ∂Ω = {z = 0}, we discuss some important aspects
of the general problem of the reconstruction of the magnetic field
B in a small isolated coronal region from the values of the vector
B¦<SUB>∂Ω</SUB> measured by a magnetograph over its whole
basis. Assuming B to be force-free in Ω: (i) we derive a series
of relations which must be necessarily satisfied by the boundary
field B¦<SUB>∂Ω</SUB>, and then by the magnetograph data if
the force-free assumption is actually correct; (ii) we show how to
extract directly from the measured B¦<SUB>∂Ω</SUB> some useful
informations about the energy of B in Ω and the topological structure
of its field lines; (iii) we present a critical discussion of the two
methods which have been proposed so far for computing effectively B
in Ω from B¦<SUB>∂Ω</SUB>.
---------------------------------------------------------
Title: Quasi-static evolution of a three-dimensional force-free
magnetic field.
Authors: Aly, J. J.
1989sasf.confP.265A Altcode: 1989IAUCo.104P.265A; 1988sasf.conf..265A
No abstract at ADS
---------------------------------------------------------
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: Quasi-static evolution of a force-free magnetic field and
conditions for the onset of a stellar flare.
Authors: Aly, J. J.
1989sasf.confP.259A Altcode: 1988sasf.conf..259A; 1989IAUCo.104P.259A
No abstract at ADS
---------------------------------------------------------
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
---------------------------------------------------------
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}.
---------------------------------------------------------
Title: Some properties of the solutions of a non-linear boundary value
problem for a force-free field in an infinite region of space. I -
Energy estimates
Authors: Aly, J. J.
1988A&A...203..183A Altcode:
The author considers the boundary value problem which determines a
finite energy force-free field B in a domain of space Ω from the
values of its normal component B<SUB>n</SUB> on the boundary ∂Ω,
and of α = B^m∇×B^ on that part ∂Ω<SUP>+</SUP> of ∂Ω on
which B<SUB>n</SUB> > 0.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Quasi-static evolution of a force-free magnetic field -
Applications to the theory of stellar flares and coronal heating
Authors: Aly, J. J.
1988ASSL..143..153A Altcode: 1988acse.conf..153A
The author considers in the highly conducting half-space {z >
0} an x-invariant arcade force-free field which is evolving in a
quasi-static way as a consequence of a velocity field v = v(y)x^
imposed on the boundary plane {z = 0}. He shows in particular that,
when the shear exceeds a critical value, the configuration becomes
metastable with respect to non-linear perturbations which change the
topology of the field lines, but preserve the magnetic fluxes.
---------------------------------------------------------
Title: On the uniqueness of the determination of the coronal potential
magnetic field from line-of-sight boundary conditions.
Authors: Aly, J. J.
1987SoPh..111..287A Altcode:
We consider a simple model in which the coronal magnetic field B
is assumed to be potential in the region between the solar surface
Γ<SUB>o</SUB> and an exterior `source-surface' Γ<SUB>1</SUB>
of arbitrary shape. We prove that the boundary value problem
that determines B from the value B<SUB>l</SUB>of its component on
Γ<SUB>0</SUB> along either (orthoradial direction) or (fixed direction)
has at most one solution. On the other hand, we show that a solution
can exist only if B<SUB>l</SUB>satisfies some `solubility conditions'.
---------------------------------------------------------
Title: Evolving magnetostatic equilibria.
Authors: Aly, J. J.
1987imfo.work..240A Altcode:
The author reviews some of the work which has been done recently on
the problem of the externally driven quasi-static evolution of an ideal
magnetohydrostatic configuration. In particular, he pays much attention
to the possible development during such an evolution of current sheets.
---------------------------------------------------------
Title: Some topics in the magnetohydrodynamics of accreting magnetic
compact objects
Authors: Aly, J. J.
1986AIPC..144...45A Altcode: 1986mpa..book...45A
Magnetic compact objects (neutron stars or white dwarfs) are currently
thought to be present in many accreting systems that are releasing large
amounts of energy. The magnetic field of the compact star may interact
strongly with the accretion flow and play an essential role in the
physics of these systems. Some magnetohydrodynamic (MHD) problems that
are likely to be relevant in building up self-consistent models of the
interaction between the accreting plasma and the star's magnetosphere
are addressed in this series of lectures. The basic principles of MHD
are first introduced and some important MHD mechanisms (Rayleigh-Taylor
and Kelvin-Helmholtz instabilities; reconnection) are discussed, with
particular reference to their role in allowing the infalling matter to
penetrate the magnetosphere and mix with the field. The structurre of a
force-free magnetosphere and the possibility of quasi-static momentum
and energy transfer between regions linked by field-aligned currents
are then studied in some detail. Finally, the structure of axisymmetric
accretion flows onto magnetic compact objects is considered.
---------------------------------------------------------
Title: Magnetospheres of accreting compact objects in binary systems
Authors: Aly, J. J.
1986csms.coll...25A Altcode:
Bright pulsating X-ray sources (X-ray pulsars, AM Her stars, . . .) have
been identified as strongly magnetized compact objects accreting matter
from a binary companion. A summary of some of the work which has been
recently done to try to understand the interaction between the magnetic
field of the compact object and the matter around is given. The models
describing the interaction of the field with a spherically symmetric
accretion flow, a thin Keplerian accretion disk, and the companion
itself are examined. In all these cases, attention is paid to the
following problems: (1) how the external plasma interacting with the
magnetosphere can get mixed with the field; and (2) by which mechanism
the magnetic field controls the mass-momentum-energy exchanges between
the two stars. The magnetosphere of an accreting component object is
compared with that of a planet.
---------------------------------------------------------
Title: Structure of the Magnetospheres of Accreting Magnetic Compact
Objects
Authors: Aly, J.
1986ppm..conf..125A Altcode:
The author reviews some of the work which has been recently done to try
to understand the interaction between the magnetic field of a strongly
magnetized compact object and either a thin keplerian accretion disk,
or a spherically symmetric accretion flow. In each case, he pays
particular attention to the following problems: (1) how the external
plasma interacting with the magnetosphere can get mixed with the field;
(2) by which mechanism does the magnetic field control the transfer of
mass-momentum-energy to the compact object. In conclusion, the author
compares the magnetosphere of an accreting compact object with that
one of a planet.
---------------------------------------------------------
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
---------------------------------------------------------
Title: Quasi-static evolution of sheared force-free fields and the
solar flare problem.
Authors: Aly, J. J.
1985A&A...143...19A Altcode:
The author reports some new results showing the possible evolution
of a two-dimensional force-free field in the half-space {z > 0}
toward an open field. This evolution is driven by shearing motions
applied to the feet of the field lines on the boundary {z = 0}. The
author discusses the consequences of his results for a model of the
two-ribbon solar flare.
---------------------------------------------------------
Title: Synchronization of Magnetic White Dwarfs in Close Binary
Systems
Authors: Lamb, F. K.; Aly, J. -J.; Cook, M. C.; Lamb, D. Q.
1985ASSL..113..237L Altcode: 1985cvlm.proc..237L
Asynchronous rotation of strongly magnetic white dwarfs in close binary
systems drives substantial field-aligned electrical currents between
the magnetic star and its companion. The resulting magnetohydrodynamic
torque is able to account for the heretofore unexplained synchronous
rotation of the strongly magnetic degenerate dwarf component in systems
like AM Her, VV Pup, AN UMa, and EF Eri. The electric fields produced
by even a small asynchronism are large enough to accelerate electrons
to high energies, and may lead to radio emission. The total energy
dissipation rate in systems with white dwarf spin periods as short
as 1<SUP>m</SUP> may reach 10<SUP>33</SUP>ergs s<SUP>-1</SUP>. Total
luminosities of this order may be a characteristic feature of such
systems.
---------------------------------------------------------
Title: Opening of the magnetic field lines in a fast rotating
magnetosphere, with an application to Jupiter
Authors: Aly, J. J.
1985IAUS..107..217A Altcode:
A simple model of a magnetosphere rotating around a rotating
Jupiter-like object with a spin-aligned dipolar moment mu is
considered. A low-energy plasma is released by inner sources located
beyond the corotation radius and is diffused outward through closed
lines. The lines form a closed thin equatorial disk, maintaining a
quasi-static balance between the centrifugal force and the magnetic
tension. Solutions are obtained for the analytical equations
characterizing the plasma in the closed region; the structure of
the magnetic field outside the disk; and the self-consistent value
of the critical radius r<SUB>zero.</SUB> The analytical solutions
are compared to the observed properties of the Jovian magnetosphere,
and the results are discussed in detail.
---------------------------------------------------------
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}.
---------------------------------------------------------
Title: Quasi-static evolution of force-free magnetic fields and a
model for two-ribbon solar flares
Authors: Aly, J. J.
1985IAUS..107..221A Altcode:
It is shown that a two-dimensional force-free field in the solar corona
can evolve in a quasi-static manner toward an open configuration,
assuming the coronal field is invariant with respect to translations
parallel to the x-axis. The theoretical result is applied to the
quantitative theory of the evolution of two-ribbon solar flares of Kopp
and Pneuman (1976), and the results are discussed. It is concluded that
the two-dimensional force is the principal mechanism for the opening of
the coronal magnetic field prior to reconnection during a solar flare.
---------------------------------------------------------
Title: Disk Accretion by Magnetic Stars
Authors: Zylstra, G. J.; Lamb, F. K.; Aly, J. J.; Cohn, H.
1984BAAS...16..944Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On some properties of force-free magnetic fields in infinite
regions of space
Authors: Aly, J. J.
1984ApJ...283..349A Altcode:
Techniques for solving boundary value problems (BVP) for a force
free magnetic field (FFF) in infinite space are presented. A priori
inequalities are defined which must be satisfied by the force-free
equations. It is shown that upper bounds may be calculated for the
magnetic energy of the region provided the value of the magnetic
normal component at the boundary of the region can be shown to decay
sufficiently fast at infinity. The results are employed to prove a
nonexistence theorem for the BVP for the FFF in the spatial region. The
implications of the theory for modeling the origins of solar flares
are discussed.
---------------------------------------------------------
Title: Synchronization of magnetic stars in binary systems.
Authors: Lamb, F. K.; Aly, J. -J.; Cook, M. C.; Lamb, D. Q.
1983ApJ...274L..71L Altcode:
Asynchronous rotation of magnetic stars in close binary systems drives
substantial field-aligned electrical currents between the magnetic star
and its companion. The resulting magnetohydrodynamic torque is able
to account for the heretofore unexplained synchronous rotation of the
strongly magnetic degenerate dwarf component in systems like AM Her,
VV Pup, AN UMa, and EF Eri as well as the magnetic A type component
in systems like HD 98088 and 41 Tauri. The electric fields produced by
even a small asynchronism are large and may accelerate some electrons
to high energies, producing radio emission. The total energy dissipation
rate in systems with degenerate dwarf spin periods as short as 1 minute
may reach 10 to the 33rd ergs/s. Total luminosities of this order may
be a characteristic feature of such systems.
---------------------------------------------------------
Title: Creation of current sheets in sheared force-free fields and
two-ribbon solar flares
Authors: Aly, J. J.
1983ICRC....4....2A Altcode: 1983ICRC...18d...2A
It is shown that a two-dimensional force-free field in the half-space
(Z greater than 0) approaches asymptotically an open structure, with
all the current concentrated in an infinitesimally thin sheet, when
the feet of its lines on (Z = 0) are idenfinitely sheared and perfect
conductivity of the plasma is assumed. The result is used to suggest a
model for a two-ribbon flare. In this model, the coronal force-free
field is brought by the photospheric motions into a quasi-open
configuration, which then becomes unstable to the development of a
tearing mode. A large scale reconnection process is then initiated,
during which the stored magnetic free energy is released and converted
into other forms.
---------------------------------------------------------
Title: Electrodynamics of disk accretion onto magnetic neutron star.
Authors: Aly, J. J.
1980A&A....86..192A Altcode:
The problem of the structure of the magnetic field in a system
constituted of a magnetic neutron star and a disk of perfectly
conducting plasma orbiting around is considered. The location of the
magnetopause and the effect of the non-radial magnetic forces acting
upon the disk are discussed.
---------------------------------------------------------
Title: Has the Universe always been very regular?
Authors: Aly, J. J.
1979MNRAS.189..479A Altcode:
The argument of Barrow and Matzner (1977) that the finite value of
the entropy per baryon in the universe today is not compatible with an
arbitrarily high amount of initial chaos is considered. This conclusion
is shown to depend crucially on the implicit assumption that the
baryon number in a comoving volume is conserved during the expansion
of the universe. It is suggested that some matter-antimatter symmetry
breaking in the hot primordial radiation would render the finite value
of the entropy per baryon in the universe today very compatible with an
arbitrarily high amount of chaos at the time of dissipation of initial
anisotropy by particle-antiparticle pair creation. The possible nature
of the symmetry-breaking mechanism is briefly discussed.
---------------------------------------------------------
Title: Matter-antimatter hydrodynamics: computation of the
annihilation rate.
Authors: Aly, J. J.
1978A&A....67..199A Altcode:
This paper is devoted to the computation of the particle-antiparticle
annihilation rate at the boundary between two regions of space filled,
respectively, by matter and antimatter. A general analysis of the
problem is done, and a simple model useful for computing this important
quantity in all situations encountered in cosmological studies is
established. The annihilation rate is computed analytically in three
specific situations: (1) in a matter-antimatter emulsion filling
the universe in the radiative and plasma eras of the big bang; (2)
in a situation where there is a strong magnetic field; and (3) at the
boundary between a hot intergalactic medium and an antimedium.
---------------------------------------------------------
Title: Light element abundances in a matter-antimatter model of
the universe.
Authors: Aly, J. J.
1978A&A....64..273A Altcode:
A baryon symmetric big bang cosmology is considered in which at the
end of the leptonic era the universe is filled with a matter-antimatter
emulsion characterized by an entropy per baryon already of the order of
the presently observed value, so that no catastropic annihilation can
take place at a later time. Two mechanisms for deuterium production
are examined: (1) the nucleodisruption process, which can lead to
an appreciable amount of deuterium as soon as the temperature is
smaller than 0.07 MeV, and (2) disintegration by gamma rays produced
in proton-antiproton annihilation. These mechanisms are considered
in connection with an evolution according to the scenario of Omnes
(1972), and they are shown to lead to greater abundances in He-3 and
D than those computed in normal big-bang cosmology if He-4 has been
normally produced. The present calculations provide a test for a large
class of matter-antimatter models.
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Title: Contribution to the study of matter-antimatter cosmological
models
Authors: Aly, J. -J.
1977PhDT.........1A Altcode:
The hypothesis of matter-antimatter symmetry in the universe is
examined, and it is concluded that the mechanisms proposed so far
for the formation of such a universe are excluded by constraints
associated with limits on black hole distortion, deuterium abundance,
helium-3 abundance, and cosmic ray flux. The possibility is considered
that more efficient mechanisms such as coalescence might satisfy the
stringent conditions; the conditions might also be satisfied if the
annihilation energy were utilized in an efficient manner. The analysis
is concerned with mechanisms for separating nucleons and antinucleons
by thermal radiation and with the hydrodynamics of a matter-antimatter
emulsion. Other topics briefly discussed include radiative and plasma
eras, the quantity of matter in the universe, and the abundance of
light elements.
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Title: Matter-antimatter Separation
Authors: Aly, J. J.
1974A&A....35..311A Altcode:
Summary. In this note we give an estimate of the characteristic scale
of the separation of matter and antimatter in the first stage of the
symmetric cosmology proposed by Omnes. It is found that this scale is
determined by a hydrodynamical process due to the existence of a sur
face tension, rather than by a diffusion process. Its value at the
end of the separation period is of the order of 7.l0- cm. Key words:
cosmology antimatter.
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Title: The amount of matter in a matter-antimatter model of the
universe.
Authors: Aly, J. J.; Caser, S.; Omnes, R.; Puget, J. L.; Valladas, G.
1974A&A....35..271A Altcode:
The calculation of the amount of matter in a matter-antimatter symmetric
universe is reconsidered, assuming some symmetry breaking due to a
phase separation of matter from antimatter at high temperatures. The
original calculation by Omnes has been subjected to two main criticisms:
Zeldovich (1970) has called attention to the importance of initial
correlations, while Steigman (1973) has pointed out the determinant
role of neutron diffusion in annihilation. These two effects and
several others are included here and further analyzed. It turns out
that the final amount of matter that is found is lower than observed
in a perfectly quiet universe, as noticed by Zeldovich.
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Title: Generation of Magnetic Fields in a Matter-antimatter Universe
Authors: Aly, J. J.
1973A&A....26..457A Altcode:
Summary. In this paper, we study a mechanism capable of producing
magnetic fields in the early stages of Omnes' bary6n-symmetric
universe. We show that the Compton effect of high energy photons
produced during matter-antimatter annihilation generates electric
currents in the cosmic plasma and therefore magnetic fields. Plasma
effects, such as the excitation of a reverse current which tends
to cancel the original one and consequently to decrease the field,
are taken into account. The seed field generated by this process is
found to be of the order of 10 "G for a redshift z=103 (just before
the re- combination period); it can be subsequently amplified by
the annihilation generated turbulence. Key words: magnetic fields -
antimatter - cosmology
