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Flare-CME models: an observational perspective  

Guillaume Aulanier   Submitted: 2015-06-10 09:06

Eruptions, flares, and coronal mass ejection (CMEs) are due to physical phenomena mainly driven by an initially force-free current-carrying magnetic field.We review some key observations relevant to the current theoretical trigger mechanisms of the eruption and to the energy release via reconnection. Sigmoids observed in X-rays and UV, as well as the pattern (double J-shaped) of electric currents in the photosphere show clear evidence of the existence of currents parallel to the magnetic field and can be the signature of a flux rope that is detectable in CMEs. The magnetic helicity of filaments and active regions is an interesting indirectly measurable parameter because it can quantify the twist of the flux rope. On the other hand, the magnetic helicity of the solar structures allows us to associate solar eruptions and magnetic clouds in the heliosphere. The magnetic topology analysis based on the 3D magnetic field extrapolated from vector magnetograms is a good tool for identifying the reconnection locations (null points and/or the 3D large volumes ? hyperbolic flux tube, HFT). Flares are associated more with quasi-separatrix layers (QSLs) and HFTs than with a single null point, which is a relatively rare case. We review various mechanisms that have been proposed to trigger CMEs and their observable signatures: by "breaking" the field lines overlying the flux rope or by reconnection below the flux rope to reduce the magnetic tension, or by letting the flux rope to expand until it reaches a minimum threshold height (loss of equilibrium or torus instability). Additional mechanisms are commonly operating in the solar atmosphere. Examples of observations are presented throughout the article and are discussed in this framework.

Authors: Schmieder B., Aulanier G., Vrsnak B.
Projects: None

Publication Status: published "online first"
Last Modified: 2015-06-10 13:22
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The physical mechanisms that initiate and drive solar eruptions  

Guillaume Aulanier   Submitted: 2013-09-27 02:25

Solar eruptions are due to a sudden destabilization of force-free coronal magnetic fields. But the detailed mechanisms which can bring the corona towards an eruptive stage, then trigger and drive the eruption, and finally make it explosive, are not fully understood. A large variety of storage-and-release models have been developed and opposed to each other since 40 years. For example, photospheric flux emergence vs. flux cancellation, localized coronal reconnection vs. large-scale ideal instabilities and loss of equilibria, tether-cutting vs. breakout reconnection, and so on. The competition between all these approaches has led to a tremendous drive in developing and testing all these concepts, by coupling state-of-the-art models and observations. Thanks to these developments, it now becomes possible to compare all these models with one another, and to revisit their interpretation in light of their common and their different behaviors. This approach leads me to argue that no more than two distinct physical mechanisms can actually initiate and drive prominence eruptions: the magnetic breakout and the torus instability. In this view, all other processes (including flux emergence, flux cancellation, flare reconnection and long-range couplings) should be considered as various ways that lead to, or than strengthen, one of the aforementioned driving mechanisms.

Authors: Aulanier G.
Projects: None

Publication Status: Proceedings of the IAU S300, in press
Last Modified: 2013-09-30 10:00
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Twisting solar coronal jet launched at the boundary of an active region  

Guillaume Aulanier   Submitted: 2013-09-20 02:41

A broad jet was observed in a weak magnetic field area at the edge of active region NOAA 11106 that also produced other nearby recurring and narrow jets. The peculiar shape and magnetic environment of the broad jet raised the question of whether it was created by the same physical processes of previously studied jets with reconnection occurring high in the corona. We carried out a multi-wavelength analysis using the EUV images from the Atmospheric Imaging Assembly (AIA) and magnetic fields from the Helioseismic and Magnetic Imager (HMI) both on-board the SDO satellite, which we coupled to a high-resolution, nonlinear force-free field extrapolation. Local correlation tracking was used to identify the photospheric motions that triggered the jet, and time-slices were extracted along and across the jet to unveil its complex nature. A topological analysis of the extrapolated field was performed and was related to the observed features. The jet consisted of many di fferent threads that expanded in around 10 minutes to about 100 Mm in length, with the bright features in later threads moving faster than in the early ones, reaching a maximum speed of about 200 km s-1. Time-slice analysis revealed a striped pattern of dark and bright strands propagating along the jet, along with apparent damped oscillations across the jet. This is suggestive of a (un)twisting motion in the jet, possibly an Alfvén wave. Bald patches in field lines, low-altitude flux ropes, diverging flow patterns, and a null point were identified at the basis of the jet. Unlike classical  or Ei ffel-tower shaped jets that appear to be caused by reconnection in current sheets containing null points, reconnection in regions containing bald patches seems to be crucial in triggering the present jet. There is no observational evidence that the flux ropes detected in the topological analysis were actually being ejected themselves, as occurs in the violent phase of blowout jets; instead, the jet itself may have gained the twist of the flux rope(s) through reconnection. This event may represent a class of jets diff erent from the classical quiescent or blowout jets, but to reach that conclusion, more observational and theoretical work is necessary.

Authors: Schmieder B., Guo Y., Moreno-Insertis F., Aulanier G., Yelles Chaouche L., Nishizuka N., Harra L.K., Thalmann J.K., Vargas Dominguez S., Liu Y.
Projects: SDO-AIA,SDO-HMI

Publication Status: A&A, in press
Last Modified: 2013-09-22 08:03
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The standard flare model in three dimensions, II. Upper limit on solar flare energy  

Guillaume Aulanier   Submitted: 2012-11-15 02:39

Solar flares strongly affect the Sun's atmosphere as well as the Earth's environment. Quantifying the maximum possible energy of solar flares of the present-day Sun, if any, is thus a key issue in heliophysics. The largest solar flares observed over the past few decades have reached energies of a few times 1032 ergs, possibly up to 1033 ergs. Flares in active Sun-like stars reach up to about 1036 ergs. In the absence of direct observations of solar flares within this range, complementary methods of investigation are needed to assess the probability of solar flares beyond those in the observational record. Using historical reports for sunspot and solar active region properties in the photosphere we scale to observed solar values a realistic dimensionless 3D MHD simulation for eruptive flares, which originate from a highly sheared bipole. This enables us to calculate the magnetic fluxes and flare energies in the model in a wide paramater space. Firstly, commonly observed solar conditions lead to modeled magnetic fluxes and flare energies that are comparable to those estimated from observations. Secondly, we evaluate from observations that 30% of the area of sunspot groups are typically involved in flares. This is related to the strong fragmentation of such groups, which naturally results from sub-photospheric convection. When the model is scaled to 30% of the area of the largest sunspot group ever reported, with its peak magnetic field being set to the strongest value ever measured in a sunspot, it produces a flare with a maximum energy of ~ 6x1033 ergs. The results of the model suggest that the Sun is able to produce flares up to about six times as energetic in TSI fluence as the strongest directly-observed flare from Nov 4, 2003. Sunspot groups larger than historically reported would yield superflares for spot pairs that would exceed tens of degrees in extent. We thus conjecture that superflare-productive Sun-like stars should have a much stronger dynamo than in the Sun.

Authors: G. Aulanier, P. Démoulin, C.J. Schrijver, M. Janvier, E. Pariat, B. Schmieder
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2012-11-20 20:25
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The standard flare model in three dimensions I. Strong-to-weak shear transition in post-flare loops  

Guillaume Aulanier   Submitted: 2012-05-20 02:24

The standard CSHKP model for eruptive flares is two-dimensional. Yet observational interpretations of photospheric currents in pre-eruptive sigmoids, shear in post-flare loops, and relative positioning and shapes flare ribbons,all together require three-dimensional extensions to the model. The paper focuses on the strong-to-weak shear transition in post-flare loop, and on the time-evolution of the geometry of photospheric electric currents, which occur during the development of eruptive flares. The objective is to understand the three-dimensional physical processes which cause them, and to know how much the post-flare and the pre-eruptive distributions of shear depend on each other. The strong-to-weak shear transition in post-flare loops is identified and quantified in a flare observed by STEREO, as well as in a magnetohydrodynamic simulation of CME initiation performed with the OHM code. In both approaches, the magnetic shear is evaluated with field line footpoints. In the simulation, the shear is also estimated from ratios between magnetic field components. The modeled strong-to-weak shear transition in post-flare loops comes from two effects. Firstly, a reconnection-driven transfer of the differential magnetic shear, from the pre- to the post-eruptive configuration. Secondly, a vertical straightening of the inner legs of the CME, which induces an outer shear weakening. The model also predicts the occurrence of narrow electric current layers inside J-shaped flare ribbons, which are dominated by direct currents. Finally, the simulation naturally accounts for energetics and time-scales for weak and strong flares, when typical scalings for young and decaying solar active regions are applied. The results provide three-dimensional extensions to the standard flare model. These extensions involve MHD processes that should be tested with observations.

Authors: G. Aulanier, M. Janvier, and B. Schmieder
Projects: SDO-HMI,STEREO

Publication Status: A&A (accepted)
Last Modified: 2012-05-20 08:16
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The problematic of large field-of-view spectropolarimetric observations with a large aperture telescope  

Guillaume Aulanier   Submitted: 2011-09-26 01:05

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Molodij, G., Aulanier, G.
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2011-09-26 09:18
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A single picture for solar coronal outflows and radio noise storms  

Guillaume Aulanier   Submitted: 2010-09-28 03:22

We propose a unified interpretation for persistent coronal outflows and metric radio noise storms, two phenomena typically observed in association with quiescent solar active regions. Our interpretation is based on multi-wavelength observations of two such regions as they crossed the meridian in May and July 2007. For both regions, we observe a persistent pattern of blue-shifted coronal emission in high-temperature lines with Hinode/EIS, and a radio noise storm with the Nanc?ay Radioheliograph. The observations are supplemented by potential and linear force-free extrapolations of the photospheric magnetic field over large computational boxes, and by a detailed analysis of the coronal magnetic field topology. We find true separatrices in the coronal field and null points high in the corona, which are preferential locations for magnetic reconnection and electron acceleration.We suggest that the continuous growth of active regions maintains a steady reconnection across the separatrices at the null point. This interchange reconnection occurs between closed, high-density loops in the core of the active region and neighbouring open, low-density flux tubes. Thus, the reconnection creates strong pressure imbalances which are the main drivers of plasma upflows. Furthermore, the acceleration of low-energy electrons in the interchange reconnection region sustains the radio noise storm in the closed loop areas, as well as weak type III emission along the open field lines. For both active regions studied, we find a remarkable agreement between the observed places of persistent coronal outflows and radio noise storms with their locations as predicted by our interpretation.

Authors: Del Zanna, G., Aulanier, G., Klein, K.-L., Torok, T.
Projects: Hinode/EIS,Hinode/SOT,Hinode/XRT,SoHO-MDI

Publication Status: A&A (accepted)
Last Modified: 2010-09-28 09:31
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FORMATION OF TORUS-UNSTABLE FLUX ROPES AND ELECTRIC CURRENTS IN ERUPTING SIGMOIDS  

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-Alfvénic, 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

Publication Status: in press
Last Modified: 2009-11-06 09:59
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Subject will be restored when possible  

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

Publication Status: Solar Physics (in press)
Last Modified: 2007-09-26 06:39
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Subject will be restored when possible  

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. Török, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT

Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:56
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Subject will be restored when possible  

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. Török, G. Aulanier, J.-F. Hochedez
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT

Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:56
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Subject will be restored when possible  

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

Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:57
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Subject will be restored when possible  

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

Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:59
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First observation of bald patches in a filament channel and at a barb endpoint  

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 Hα 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

Publication Status: A&A (in press)
Last Modified: 2006-06-29 10:36
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Solar prominence merging  

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

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  

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-Alfvénic 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 Alfvén 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

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  

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-Alfvénic 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 α 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 α 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

Publication Status: A&A (in press)
Last Modified: 2004-11-26 09:08
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