Articles d'astrophysique de Thomas Lepeltier et al.
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Résumé :
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>0}
and supported against gravity by an x-invariant potential
magnetic field. Assuming that the region outside the sheet
contains a low-beta plasma having an infinite conductivity and
that the field lines are firmly tied to the "photospheric plane"
{z=0}: (i) We show that the model is stable with respect to
any perturbation which do not depend on x. (ii) 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 (1969) obtained under the assumption
that the sheet is 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 upwards, were they unperturbed
by the heavy plasma.
Résumé :
We propose a simple 2D current sheet model of 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.
Résumé :
A prominence is often modelized by a vertical infinitely thin
massive current sheet Σ embedded in the coronal half-space
{z>0}, and supported against the uniform gravitational
field -gz by an x-invariant magnetic field
which is potential outside Σ. In the framework of this simple
model, we reconsider the problem of the determination of the
magnetic field from the values of its normal component on both Σ
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.
Résumé :
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 appleid 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 configuration
are shown to exist in each of the cases which have been worked
out.
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