Non-equilibrium Flux Rope Formation by Confined Flares Preceding a Solar Coronal Mass Ejection |
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Bernhard Kliem Submitted: 2021-01-08 08:35
We present evidence that a magnetic flux rope was formed before a coronal mass ejection (CME) and its associated long-duration flare during a pair of preceding confined eruptions and associated impulsive flares in a compound event in NOAA Active Region 12371. Extreme-ultraviolet images and the extrapolated nonlinear force-free field show that the first two, impulsive flares, SOL2015-06-21T01:42, result from the confined eruption of highly sheared low-lying flux, presumably a seed flux rope. The eruption spawns a vertical current sheet, where magnetic reconnection creates flare ribbons and loops, a nonthermal microwave source, and a sigmoidal hot channel which can only be interpreted as a magnetic flux rope. Until the subsequent long-duration flare, SOL2015-06-21T02:36, the sigmoid's elbows expand, while its center remains stationary, suggesting non-equilibrium but not yet instability. The "flare reconnection" during the confined eruptions acts like "tether-cutting reconnection" whose flux feeding of the rope leads to instability. The subsequent full eruption is seen as an accelerated rise of the entire hot channel, seamlessly evolving into the fast halo CME. Both the confined and ejective eruptions are consistent with the onset of the torus instability in the dipped decay index profile which results from the region's two-scale magnetic structure. We suggest that the formation or enhancement of a non-equilibrium but stable flux rope by confined eruptions is a generic process occurring prior to many CMEs.
Authors: Bernhard Kliem, Jeongwoo Lee, Rui Liu, Stephen M. White, Chang Liu, Satoshi Masuda
Projects: GOES X-rays,Nobeyama Radioheliograph,RHESSI,SDO-AIA,SDO-HMI,SoHO-LASCO
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Publication Status: accepted by the ApJ
Last Modified: 2021-01-09 15:05
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Slow Rise and Partial Eruption of a Double-Decker Filament. II. A Double Flux Rope Model |
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Bernhard Kliem Submitted: 2014-07-13 05:34
Force-free equilibria containing two vertically arranged magnetic flux ropes of like chirality and current direction are considered as a model for split filaments/prominences and filament-sigmoid systems. Such equilibria are constructed analytically through an extension of the methods developed in Titov & Demoulin (1999) and numerically through an evolutionary sequence including shear flows, flux emergence, and flux cancellation in the photospheric boundary. It is demonstrated that the analytical equilibria are stable if an external toroidal (shear) field component exceeding a threshold value is included. If this component decreases sufficiently, then both flux ropes turn unstable for conditions typical of solar active regions, with the lower rope typically being unstable first. Either both flux ropes erupt upward, or only the upper rope erupts while the lower rope reconnects with the ambient flux low in the corona and is destroyed. However, for shear field strengths staying somewhat above the threshold value, the configuration also admits evolutions which lead to partial eruptions with only the upper flux rope becoming unstable and the lower one remaining in place. This can be triggered by a transfer of flux and current from the lower to the upper rope, as suggested by the observations of a split filament in Paper I (Liu et al. 2012). It can also result from tether-cutting reconnection with the ambient flux at the X-type structure between the flux ropes, which similarly influences their stability properties in opposite ways. This is demonstrated for the numerically constructed equilibrium.
Authors: B. Kliem, T. Toeroek, V. S. Titov, R. Lionello, J. A. Linker, R. Liu, C. Liu, & H. Wang
Projects: None
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Publication Status: ApJ, accepted
Last Modified: 2014-07-13 18:37
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Magnetohydrodynamic Modeling of the Solar Eruption on 2010 April 8 |
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Bernhard Kliem Submitted: 2014-07-11 09:00
The structure of the coronal magnetic field prior to eruptive processes and the conditions for the onset of eruption are important issues that can be addressed through studying the magnetohydrodynamic stability and evolution of nonlinear force-free field (NLFFF) models. This paper uses data-constrained NLFFF models of a solar active region that erupted on 2010 April 8 as initial condition in MHD simulations. These models, constructed with the techniques of flux rope insertion and magnetofrictional relaxation, include a stable, an approximately marginally stable, and an unstable configuration. The simulations confirm previous related results of magnetofrictional relaxation runs, in particular that stable flux rope equilibria represent key features of the observed pre-eruption coronal structure very well and that there is a limiting value of the axial flux in the rope for the existence of stable NLFFF equilibria. The specific limiting value is located within a tighter range, due to the sharper discrimination between stability and instability by the MHD description. The MHD treatment of the eruptive configuration yields very good agreement with a number of observed features like the strongly inclined initial rise path and the close temporal association between the coronal mass ejection and the onset of flare reconnection. Minor differences occur in the velocity of flare ribbon expansion and in the further evolution of the inclination; these can be eliminated through refined simulations. We suggest that the slingshot effect of horizontally bent flux in the source region of eruptions can contribute significantly to the inclination of the rise direction. Finally, we demonstrate that the onset criterion formulated in terms of a threshold value for the axial flux in the rope corresponds very well to the threshold of the torus instability in the considered active region.
Authors: B. Kliem, Y. N. Su, A. A. van Ballegooijen, & E. E. DeLuca
Projects: Hinode/XRT,SDO-AIA,SDO-HMI,STEREO
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Publication Status: Published in ApJ 779, 129, 2013
Last Modified: 2014-07-11 14:34
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A Parametric Study of Erupting Flux Rope Rotation. Modeling the ''Cartwheel CME'' on 9 April 2008 |
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Bernhard Kliem Submitted: 2011-12-16 08:03
The rotation of erupting filaments in the solar corona is addressed through aparametric simulation study of unstable, rotating flux ropes in bipolarforce-free initial equilibrium. The Lorentz force due to the external shearfield component and the relaxation of tension in the twisted field are themajor contributors to the rotation in this model, while reconnection with theambient field is of minor importance. Both major mechanisms writhe the fluxrope axis, converting part of the initial twist helicity, and produce rotationprofiles which, to a large part, are very similar in a range of shear-twistcombinations. A difference lies in the tendency of twist-driven rotation tosaturate at lower heights than shear-driven rotation. For parameterscharacteristic of the source regions of erupting filaments and coronal massejections, the shear field is found to be the dominant origin of rotations inthe corona and to be required if the rotation reaches angles of order 90degrees and higher; it dominates even if the twist exceeds the threshold of thehelical kink instability. The contributions by shear and twist to the totalrotation can be disentangled in the analysis of observations if the rotationand rise profiles are simultaneously compared with model calculations. Theresulting twist estimate allows one to judge whether the helical kinkinstability occurred. This is demonstrated for the erupting prominence in the'Cartwheel CME' on 9 April 2008, which has shown a rotation of approx 115degrees up to a height of 1.5 R☉ above the photosphere. Out of a range ofinitial equilibria which include strongly kink-unstable (twist Phi=5pi), weaklykink-unstable (Phi=3.5pi), and kink-stable (Phi=2.5pi) configurations, only theevolution of the weakly kink-unstable flux rope matches the observations intheir entirety.
Authors: B. Kliem, T. Toeroek, W. T. Thompson
Projects: STEREO
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Publication Status: Solar Physics, submitted
Last Modified: 2011-12-19 09:10
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3D Reconstruction of a Rotating Erupting Prominence |
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Bernhard Kliem Submitted: 2011-12-16 08:00
A bright prominence associated with a coronal mass ejection (CME) was seenerupting from the Sun on 9 April 2008. This prominence was tracked by both theSolar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, andwas seen to rotate about the line of sight as it erupted; therefore, the eventhas been nicknamed the 'Cartwheel CME'. The threads of the prominence in thecore of the CME quite clearly indicate the structure of a weakly to moderatelytwisted flux rope throughout the field of view, up to heliocentric heights of 4solar radii. Although the STEREO separation was 48 degrees, it was possible tomatch some sharp features in the later part of the eruption as seen in the 304{AA} line in EUVI and in the Hα -sensitive bandpass of COR1 by both STEREOAhead and Behind. These features could then be traced out in three-dimensionalspace, and reprojected into a view in which the eruption is directed towardsthe observer. The reconstructed view shows that the alignment of the prominenceto the vertical axis rotates as it rises up to a leading-edge height of approx2.5 solar radii, and then remains approximately constant. The alignment at 2.5solar radii differs by about 115 degrees from the original filament orientationinferred from Hα and EUV data, and the height profile of the rotation,obtained here for the first time, shows that two thirds of the total rotationis reached within approx 0.5 solar radii above the photosphere. These featuresare well reproduced by numerical simulations of an unstable moderately twistedflux rope embedded in external flux with a relatively strong shear fieldcomponent.
Authors: W. T. Thompson, B. Kliem, T. Toeroek
Projects: STEREO
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Publication Status: published in Solar Physics (Online First)
Last Modified: 2011-12-19 09:10
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Photospheric flux cancellation and associated flux rope formation and eruption |
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Bernhard Kliem Submitted: 2010-11-05 08:52
Aims: We study an evolving bipolar active region that exhibits flux cancellation at the internal polarity inversion line, the formation of a soft X-ray sigmoid along the inversion line and a coronal mass ejection. The aim is to investigate the quantity of flux cancellation that is involved in flux rope formation in the time period leading up to the eruption. Methods: The active region is studied using its extreme ultraviolet and soft X-ray emissions as it evolves from a sheared arcade to flux rope configuration. The evolution of the photospheric magnetic field is described and used to estimate how much flux is reconnected into the flux rope. Results: About one third of the active region flux cancels at the internal polarity inversion line in the 2.5 days leading up to the eruption. In this period, the coronal structure evolves from a weakly to a highly sheared arcade and then to a sigmoid that crosses the inversion line in the inverse direction. These properties suggest that a flux rope has formed prior to the eruption. The amount of cancellation implies that up to 60% of the active region flux could be in the body of the flux rope. We point out that only part of the cancellation contributes to the flux in the rope if the arcade is only weakly sheared, as in the first part of the evolution. This reduces the estimated flux in the rope to ~ 30% or less of the active region flux. We suggest that the remaining discrepancy between our estimate and the limiting value of ~ 10% of the active region flux, obtained previously by the flux rope insertion method, results from the incomplete coherence of the flux rope, due to nonuniform cancellation along the polarity inversion line. A hot linear feature is observed in the active region which rises as part of the eruption and then likely traces out field lines close to the axis of the flux rope. The flux cancellation and changing magnetic connections at one end of this feature suggest that the flux rope reaches coherence by reconnection shortly before and early in the impulsive phase of the associated flare. The sigmoid is destroyed in the eruption but reforms quickly, with the amount of cancellation involved being much smaller than in the course of its original formation.
Authors: L. M. Green, B. Kliem, A. J. Wallace
Projects: Hinode/EIS,Hinode/XRT,SoHO-MDI,SoHO-LASCO
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Publication Status: Astron. Astrophys., in press
Last Modified: 2010-11-08 07:34
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Reconnection of a kinking flux rope triggering the ejection of a microwave and hard X-ray source. II. Numerical modeling |
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Bernhard Kliem Submitted: 2010-08-09 03:16
Numerical simulations of the helical (m=1) kink instability of an arched, line-tied flux rope demonstrate that the helical deformation enforces reconnection between the legs of the rope if modes with two helical turns are dominant as a result of high initial twist in the range Phigtrsim6pi. Such reconnection is complex, involving also the ambient field. In addition to breaking up the original rope, it can form a new, low-lying, less twisted flux rope. The new flux rope is pushed downward by the reconnection outflow, which typically forces it to break as well by reconnecting with the ambient field. The top part of the original rope, largely rooted in the sources of the ambient flux after the break-up, can fully erupt or be halted at low heights, producing a ''failed eruption.'' The helical current sheet associated with the instability is squeezed between the approaching legs, temporarily forming a double current sheet. The leg-leg reconnection proceeds at a high rate, producing sufficiently strong electric fields that it would be able to accelerate particles. It may also form plasmoids, or plasmoid-like structures, which trap energetic particles and propagate out of the reconnection region up to the top of the erupting flux rope along the helical current sheet. The kinking of a highly twisted flux rope involving leg-leg reconnection can explain key features of an eruptive but partially occulted solar flare on 18 April 2001, which ejected a relatively compact hard X-ray and microwave source and was associated with a fast coronal mass ejection.
Authors: B. Kliem, M. G. Linton, T. Toeroek, M. Karlický
Projects:
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Publication Status: Solar Physics, in press
Last Modified: 2010-08-09 08:34
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Evidence for Mixed Helicity in Erupting Filaments |
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Bernhard Kliem Submitted: 2010-08-09 03:11
Erupting filaments are sometimes observed to undergo a rotation about the vertical direction as they rise. This rotation of the filament axis is generally interpreted as a conversion of twist into writhe in a kink-unstable magnetic flux rope. Consistent with this interpretation, the rotation is usually found to be clockwise (as viewed from above) if the post-eruption arcade has right-handed helicity, but counterclockwise if it has left-handed helicity. Here, we describe two non-active-region filament events recorded with the Extreme-Ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory in which the sense of rotation appears to be opposite to that expected from the helicity of the post-event arcade. Based on these observations, we suggest that the rotation of the filament axis is, in general, determined by the net helicity of the erupting system, and that the axially aligned core of the filament can have the opposite helicity sign to the surrounding field. In most cases, the surrounding field provides the main contribution to the net helicity. In the events reported here, however, the helicity associated with the filament ''barbs'' is opposite in sign to and dominates that of the overlying arcade.
Authors: K. Muglach, Y.-M. Wang, B. Kliem
Projects: SoHO-EIT,SoHO-MDI
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Publication Status: ApJ 703, 976 (2009)
Last Modified: 2010-08-09 07:49
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Endpoint Brightenings in Erupting Filaments |
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Bernhard Kliem Submitted: 2009-06-28 20:00
Two well known phenomena associated with erupting filaments are the transient coronal holes that form on each side of the filament channel and the bright post-event arcade with its expanding double row of footpoints. Here we focus on a frequently overlooked signature of filament eruptions: the spike- or fan-shaped brightenings that appear to mark the far endpoints of the filament. From a sample of non-active-region filament events observed with the Extreme-Ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory, we find that these brightenings usually occur near the outer edges of the transient holes, in contrast to the post-event arcades, which define their inner edges. The endpoints are often multiple and are rooted in and around strong network flux well outside the filament channel, a result that is consistent with the axial field of the filament being much stronger than the photospheric field inside the channel. The extreme ultraviolet brightenings, which are most intense at the time of maximum outward acceleration of the filament, can be used to determine unambiguously the direction of the axial field component from longitudinal magnetograms. Their location near the outer boundary of the transient holes suggests that we are observing the footprints of the current sheet formed at the leading edge of the erupting filament, as distinct from the vertical current sheet behind the filament which is the source of the post-event arcade.
Authors: Y.-M. Wang, K. Muglach, and B. Kliem
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO
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Publication Status: ApJ 699, 133 (2009)
Last Modified: 2009-06-29 09:40
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The evolution of twisting coronal magnetic flux tubes |
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Bernhard Kliem Submitted: 2003-11-06 17:14
We simulate the twisting of an initially potential coronal flux tube by
photospheric vortex motions, centred at two photospheric flux
concentrations, using the compressible zero-beta ideal MHD equations. A
twisted flux tube is formed, surrounded by much less twisted and sheared
outer flux. Under the action of continuous slow driving, the flux tube
starts to evolve quasi-statically along a sequence of force-free
equilibria, which rise slowly with increasing twist and possess
helical shape. The flux bundle that extends from the location of peak
photospheric current density (slightly displaced from the vortex centre)
shows a sigmoidal shape in agreement with observations of sigmoidal soft
X-ray loops. There exists a critical twist, above which no equilibrium can
be found in the simulation and the flux tube ascends rapidly. Then either
stable equilibrium ceases to exist or the character of the sequence
changes such that neighbouring stable equilibria rise by enormous amounts
for only modest additions of twist. A comparison with the scalings of the
rise of flux in axisymmetric geometry by Sturrock et al. (1995) suggests the
former. Both cases would be observed as an eruption. The critical
end-to-end twist, for a particular set of parameters describing the
initial potential field, is found to lie in the range
2.5pi < Phi_mathrm{c} < 2.75pi. There are some indications for the
growth of helical perturbations at supercritical twist. Depending on the
radial profiles of the photospheric flux concentration and vortex
velocity, the outer part or all of the twisted flux expands from the
central field line of the flux tube. This effect is particularly efficient
in the dynamic phase, provided the density is modeled realistically,
falling off sufficiently rapidly with height. It is expected to lead to
the formation of a cavity in which the twisted flux tube is embedded,
analogous to the typical structure of coronal mass ejections.
Authors: T. Toeroek and B. Kliem
Projects:
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Publication Status: Astron. Astrophys. 406, 1043 (2003)
Last Modified: 2003-11-21 10:28
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