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FORMATION OF TORUS-UNSTABLE FLUX ROPES AND ELECTRIC CURRENTS IN ERUPTING SIGMOIDS
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Guillaume Aulanier Submitted: 2009-11-06 05:48
We analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause
its eruption. This is achieved by a zero-beta MHD simulation of an initially potential, asymmetric bipolar
field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvenic, line-tied
shearing motions in the photosphere. As in similar models, flux-cancellation driven photospheric
reconnection in a bald-patch separatrix transforms the sheared arcades into a slowly rising and stable
flux rope. A bifurcation from a bald-patch to a quasi-separatrix layer (QSL) topology occurs later on
in the evolution, while the flux rope keeps growing and slowly rising, now due to shear-driven coronal
slip-running reconnection, which is of tether-cutting type and takes place in the QSL. As the flux
rope reaches the altitude at which the decay index −d lnB/d ln z of the potential field exceeds ∼ 3/2,
it rapidly accelerates upward while the overlying arcade eventually develops an inverse tear-drop
shape, as observed in coronal mass ejections (CMEs). This transition to eruption is in accordance
with the onset criterion of the torus instability. Thus we find that photospheric flux-cancellation
and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are
key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the
photosphere at which the torus instability causes the eruption. In order to interpret recent Hinode X-
Ray Telescope observations of an erupting sigmoid, we produce simplified synthetic soft X-ray images
from the distribution of the electric currents in the simulation. We find that a bright sigmoidal
envelope is formed by pairs of J-shaped field lines in the pre-eruptive stage. These field lines form
through the bald-patch reconnection, and merge later on into S-shaped loops through the tethercutting
reconnection. During the eruption, the central part of the sigmoid brightens due to the
formation of a vertical current layer in the wake of the erupting flux rope. Slip-running reconnection
in this layer yields the formation of flare loops. A rapid decrease of currents due to field line expansion,
together with the increase of narrow currents in the reconnecting QSL, yields the sigmoid hooks to
thin in the early stages of the eruption. Finally, a slightly rotating erupting loop-like feature (ELLF)
detaches from the center of the sigmoid. Most of this ELLF is not associated with the erupting flux
rope, but with a current shell which develops within expanding field lines above the rope. Only the
short, curved end of the ELLF corresponds to a part of the flux rope. We argue that the features
found in the simulation are generic for the formation and eruption of soft X-ray sigmoids.
Authors: G. Aulanier, T. Torok, P. Demoulin, E.E. DeLuca
Projects: Hinode/XRT
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Publication Status: in press
Last Modified: 2009-11-06 09:59
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Subject will be restored when possible
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Guillaume Aulanier Submitted: 2007-09-26 02:32
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disc. EIT waves are all associated with a flare and a CME and flare commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed non-dimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development and the observability in EUV of narrow electric currents sheets which appear in the simulations. Both codes give similar results which we confront with two well-known SoHO/EIT observations of propagating EIT waves (April 7 and May 12, 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SoHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensities as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has a shape of an ellipse which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.
Authors: C. Delann?e, T. T?r?k, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT
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Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:56
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Subject will be restored when possible
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Guillaume Aulanier Submitted: 2007-09-26 02:32
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disc. EIT waves are all associated with a flare and a CME and flare commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed non-dimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development and the observability in EUV of narrow electric currents sheets which appear in the simulations. Both codes give similar results which we confront with two well-known SoHO/EIT observations of propagating EIT waves (April 7 and May 12, 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SoHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensities as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has a shape of an ellipse which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.
Authors: C. Delann?e, T. T?r?k, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT
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Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:56
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Subject will be restored when possible
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Guillaume Aulanier Submitted: 2007-09-26 02:32
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disc. EIT waves are all associated with a flare and a CME and flare commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed non-dimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development and the observability in EUV of narrow electric currents sheets which appear in the simulations. Both codes give similar results which we confront with two well-known SoHO/EIT observations of propagating EIT waves (April 7 and May 12, 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SoHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensities as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has a shape of an ellipse which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.
Authors: C. Delannee, T. Torok, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT
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Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:57
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Subject will be restored when possible
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Guillaume Aulanier Submitted: 2007-09-26 02:32
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disc. EIT waves are all associated with a flare and a CME and flare commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed non-dimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development and the observability in EUV of narrow electric currents sheets which appear in the simulations. Both codes give similar results which we confront with two well-known SoHO/EIT observations of propagating EIT waves (April 7 and May 12, 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SoHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensities as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has a shape of an ellipse which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.
Authors: C. Delannee, T. Torok, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT
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Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:59
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Subject will be restored when possible
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Guillaume Aulanier Submitted: 2007-09-26 02:32
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disc. EIT waves are all associated with a flare and a CME and flare commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed non-dimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development and the observability in EUV of narrow electric currents sheets which appear in the simulations. Both codes give similar results which we confront with two well-known SoHO/EIT observations of propagating EIT waves (April 7 and May 12, 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SoHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensities as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has a shape of an ellipse which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.
Authors: C. Delannée, T. Török, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT
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Publication Status: Solar Physics (in press)
Last Modified: 2007-09-26 06:39
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First observation of bald patches in a filament channel and at a barb endpoint
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Guillaume Aulanier Submitted: 2006-06-29 10:21
The 3D magnetic field topology of solar filaments/prominences is strongly
debated, because it is not directly measureable in the corona. Among
various prominence models, several are consistent with many observations,
but their related topologies are very different.
We conduct unprecedented observations to address this paradigm. We measure
the photospheric vector magnetic field in several small flux concentrations
surrounding a filament observed far from disc center. Our objective is to
test for the presence/absence of magnetic dips around/below the filament
body/barb, which is a strong constraint on prominence models, yet untested
by observations.
Our observations are performed with the THEMIS/MTR instrument. The four
Stokes parameters are extracted, from which the vector magnetic fields
are calculated using a PCA inversion. The resulting vector fields are
then deprojected onto the photospheric plane. The 180deg ambiguity is
then solved by selecting the only solution that matches filament chirality
rules. Considering the weakness of the resulting magnetic fields, a careful
analysis of the inversion procedure and its error bars was performed,
to avoid over-interpretation of noisy or ambiguous Stokes profiles.
Thanks to the simultaneous multi-wavelength THEMIS observations, the
vector field maps are coaligned with the Halpha image of the filament.
By definition, photospheric dips are identifiable where the horizontal
component of the magnetic field points from a negative toward a positive
polarity. Among six bipolar regions analyzed in the filament channel,
four at least display photospheric magnetic dips, i.e. bald patches.
Concerning a barb, the topology of the endpoint is that of a bald patch
located aside of a parasitic polarity, not of an arcade pointing within
the polarity.
The observed magnetic field topology in the photosphere tends to support
models of prominence based on magnetic dips located within weakly twisted
flux tubes. Their underlying and lateral extensions form photospheric
dips both within the channel and below barbs.
Authors: A. Lopez Ariste, G. Aulanier, B. Schmieder, A. Sainz Dalda
Projects: None
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Publication Status: A&A (in press)
Last Modified: 2006-06-29 10:36
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Solar prominence merging
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Guillaume Aulanier Submitted: 2006-06-01 03:03
In a recent paper, we described MHD simulations of
the interaction between a pair of distinct prominences
formed by the photospheric line-tied shearing of two
separated dipoles. One case was typical of solar
observations of prominence merging, in which the
prominences have the same axial-field direction and
sign of magnetic helicity. For that configuration,
we reported the formation of linkages between the
prominences due to magnetic reconnection of their
sheared fields. In this paper, we analyse the
evolution of the plasma-supporting magnetic dips
in this configuration. As the photospheric flux is
being progressively sheared, dip-related chromospheric
fibrils and high altitude threads form and develop
into the two prominences, which undergo internal
oscillations. As the prominences are stretched
farther along their axes, they come into contact
and their sheared fluxes pass each other, and new
dips form in the interaction region. The distribution
of these dips increasingly fills the volume between
the prominences, so that the two progenitors gradually
merge into a single prominence. Our model reproduces
typical observational properties reported from both
high-cadence and daily observations at various
wavelengths. We identify the multistep mechanism,
consisting of a complex coupling between photospheric
shear, coronal magnetic reconnection without null
points, and formation of quasi bald patches, that
is responsible for the prominence merging through
dip creation. The resulting magnetic topology differs
significantly from that of a twisted flux tube.
Authors: G. Aulanier, C. R. DeVore and S. K. Antiochos
Projects: None
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Publication Status: ApJ (in press)
Last Modified: 2006-06-01 07:45
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Current sheet formation in quasi-separatrix layers and hyperbolic flux tubes
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Guillaume Aulanier Submitted: 2005-07-22 07:49
In 3D magnetic field configurations, quasi-separatrix
layers (QSLs) are defined as volumes in which field lines
locally display strong gradients of connectivity. Considering
QSLs as the preferential locations for current sheet development
and magnetic reconnection in general, and as a natural model
for solar flares and coronal heating in particular, have been
strongly debated issues over the last decade.
In this paper, we perform zero-$eta$ resistive MHD simulations
of the development of electric currents in smooth magnetic
configurations, which are strictly speaking bipolar though they
are formed by four flux concentrations, and whose potential fields
contain QSLs. The configurations are driven by smooth and large-scale
sub-Alfv'enic footpoint motions.
Extended electric currents naturally form in the configurations,
which evolve through a sequence of quasi non-linear force-free
equilibria.
Narrow current layers also develop. They spontaneously form at small
scales, all around the QSLs, whatever the footpoint motions are.
For long enough motions, the strongest currents develop where the
QSLs are the thinnest, namely at the Hyperbolic Flux Tube (HFT) which
generalizes the concept of separator. These currents progressively take
the shape of an elongated sheet, whose formation is associated with
a gradual steepening of the magnetic field gradients over tens of
Alfv'en times, due to the different motions applied to the field
lines which pass on each side of the HFT.
Our model then self-consistently accounts for the long-duration energy
storage prior to a flare, followed by a switch-on of reconnection when
the currents reach the dissipative scale at the HFT.
In configurations whose potential fields contain broader QSLs, when
the magnetic field gradients reach the dissipative scale, the currents
at the HFT %quasi-separator
reach higher magnitudes. This implies that major
solar flares, that are not related with an early large-scale ideal instabilities,
must occur in regions whose corresponding potential fields have broader
QSLs.
Our results lead us to conjecture that physically, current layers
must always form on the scale of the QSLs. This implies that electric
currents around QSLs may be gradually amplified in time only if the
QSLs are broader than the dissipative length-scale.
We also discuss the potential role of QSLs in coronal heating in
bipolar configurations made of a continuous distribution of flux
concentrations.
Authors: G. Aulanier, E. Pariat and P. Demoulin
Projects: None
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Publication Status: A&A (accepted)
Last Modified: 2005-07-22 07:49
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Equilibrium and observational properties of line-tied twisted flux tubes
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Guillaume Aulanier Submitted: 2004-11-26 09:08
We describe a new explicit three-dimensional magnetohydrodymanic code, which solves the standard zero-beta MHD equations in cartesian geometry, with line-tied conditions at the lower boundary and open conditions at the other ones. Using this code in the frame of solar active regions, we simulate the evolution of an initially potential and concentrated bipolar magnetic field, subject to various sub-Alfvenic photospheric twisting motions which preserve the initial photospheric vertical magnetic field. Both continuously driven and relaxation runs are performed. Within the numerical domain, a steep equilibrium curve is found for the altitude of the apex of the field line rooted in the vortex centers as a function of the twist. Its steepness strongly depends on the degree of twist in outer field lines rooted in weak field regions. This curve fits the analytical expression for the asymptotic behaviour of force-free fields of spherical axisymmetric dipoles subject to azimuthal shearing motions, as well as the curve derived for other line-tied twisted flux tubes reported in previous works. This suggests that it is a generic property of line-tied sheared/twisted arcades. However, contrary to other studies we never find a transition toward a non-equilibrium within the numerical domain, even for twists corresponding to steep regions of the equilibrium curve. The calculated configurations are analyzed in the frame of solar observations. We discuss which specific conditions are required for the steepness of the generic equilibrium curve to result in dynamics which are typical of both fast and slow CMEs observed below 3 Ro. We provide natural interpretations for the existence of asymmetric and multiple concentrations of electric currents in homogeneoulsy twisted sunspots, due to the twisting of both short and long field lines. X-ray sigmoids are reproduced by integrating the Joule heating term along the line-of-sight. These sigmoids have inverse-S shapes associated with negative force-free parameters alpha, which is consistent with observed rules in the northern solar hemisphere. We show that our sigmoids are not formed in the main twisted flux tube, but rather in an ensemble of low-lying sheared and weakly twisted field lines, which individually never trace the whole sigmoid, and which barely show their distorded shapes when viewed in projection. We find that, for a given bipolar configuration and a given twist, neither the alpha nor the altitude of the lines whose envelope is a sigmoid depends on the vortex size.
Authors: Aulanier, G ; Démoulin, P. and Grappin; R.
Projects: None
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Publication Status: A&A (in press)
Last Modified: 2004-11-26 09:08
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