1. Abstract On 2004 November 10, TRACE observed an X2.5 flare in NOAA Active Region 10696. The observations were taken at very short cadence (~3.7 s) in the ultraviolet 1600 Å channel. The flare was accompanied by a filament eruption, whose initiation we investigate using both TRACE and SOHO images. We find signatures of tether weakening by both flux emergence and a lateral variation of “break-out”. However, the evolution of the erupting filament is consistent with that of a flux rope undergoing the MHD kink instability. This suggests that the kink instability is the main driver of the eruption. References Antiochos, de Vore & Klimchuk (1999) ApJ 510, 485 Aulanier et al. (2000) ApJ 540, 1126 Démoulin, Priest & Lonie (1996) JGR 101, 7631 Moore & Roumeliotis (1992) in ‘Eruptive Solar Flares’, Springer Verlag, p. 69 Török & Kliem (2005) ApJ 630, L97 Eruption of a Kink-unstable Filament in Region NOAA 10696 David R. Williams 1, Tibor Török 1, Pascal Démoulin 2, Lidia van Driel-Gesztelyi 1,2,3, Bernhard Kliem 4 1 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey, RH5 6NT, U.K. 2 Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, F-92195, France 3 Konkoly Observatory, Pf. 67, H-1525 Budapest, Hungary 4 Astrophysical Insititute Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany Figure 2 (above): Area-averaged light curves, before and during the impulsive stage of the flare, for the four boxed regions (A to D) shown in Figs. 1a,c, along with the full-Sun GOES soft X-ray flux. The hatched zone shows the time range covered by the data in Fig. 5. Figure 3 (above): (a) SOHO-EIT 195 Å image showing the filament f (in absorption) in its pre-eruption position. The black box shows the area covered by the adjacent inset. The inset shows the filament traced by a solid white line, with the apparent footpoints of the filament marked by black plus signs. (b,c) TRACE 1600 Å images showing the evolution of the shape of the kinked filament (now bright). The black line in the inset traces the kinked shape of the erupting structure. Note the reversed “J” shape of the ribbons between the filament’s apparent feet in (b), similar to the expected shape of quasi-separatrix layers (QSLs) intersecting the photosphere. (d–f) Magnetic field lines in the numerical simulation of Török & Kliem (2005), which outline the core of a kink-unstable flux rope at t = 0, 21, and 25 Alfvén times. The grey-scale levels in the plane indicate positive (white) and negative (black) concentrations of the magnetic field strength component normal to the plane (cf. photospheric magnetic field distribution in Fig. 1a). 1. Tether Weakening by Flux Emergence In the first SOHO-MDI magnetogram taken after the eruption, we see evidence of a newly emerged small bipole (Fig. 1a). A localised brightening in this region, some minutes before the filament erupts (Fig. 1b), indicates reconnection between this bipole and the pre­ existing coronal field. This reduces the stabilising tension in the field above the filament, so that the filament can begin to rise (Fig. 4a; see also Moore & Roumeliotis 1992). 2. Tether Weakening by “Lateral Break-out” Fig. 1c shows four bright ribbons observed with TRACE, shortly after the brightening caused by the emerging bipole (Fig. 1b). This indicates a quadrupolar reconnection taking place above the filament. Furthermore, we see a continual shearing motion of positive-polarity elements near the filament. These observations both implicate a “lateral break- out” mechanism (Fig. 4b; see also Aulanier et al. 2000), which further lowers the tension above the filament. This “lateral” break-out is distinct from the “central” break-out model of Antiochos et al. (1999) in that the overlying tension is not removed; instead, it is only weakened. Figure 5 (above): Upper panel shows the height of the filament’s apex as a function of time, on a logarithmic axis. We fit the data to several functions, the best result (seen above) being to an exponential function of the form: The lower panel gives the residuals of this fit. The plot also shows the evolution of the scaled apex height of the kink-unstable flux rope in the numerical simulations of Török & Kliem (2005). 3. The Kink Instability as Driver of the Eruption When the overlying tension is weak enough, the filament starts to erupt. During the eruption, its axis develops a strong writhe (Figs. 3b,c) and its apex appears to rise exponentially (Fig. 5). We compare these features with numerical simulations of the helical kink instability of a twisted coronal flux rope by Török & Kliem (2005), and find excellent qualitative agreement (Figs. 3 & 5). This strongly suggests that the filament eruption is driven by the kink instability, whereas the role of the tether weakening mechanisms is merely to release the previously confined flux rope. Figure 1 (below): (a) SOHO-MDI line-of-sight magnetogram showing the newly emerged bipole, whose negative polarity is circled in black (shown here after the filament eruption). (b) TRACE 1600 Å image showing a localised brightening at the position of the emerging bipole. (c) TRACE 1600 Å image showing four ribbons, shortly before the filament begins to brighten (Fig. 3b). Conclusions 1.The filament eruption is mainly driven by the kink instability 2.The instability sets in after the tension in the field above the filament is sufficiently reduced by one or more tether-weakening mechanisms. 3.We conclude that, rather than being mutually exclusive, several eruption mechanisms are at work in this single event. Figure 4 (above): 2D approximation of the magnetic topology. Dotted lines are separatrices. Dashed lines show connectivities transformed by reconnection. Thick orange lines are new connectivities after reconnection. Solid black lines indicate unchanged connectivities. (a) The emerging bipole E + F - reconnects with the pre-existing coronal field lines to lengthen (and thereby reduce tension in) the field between A + and B -. The reconnection is seen as the localised brightening in Fig. 1b, and as a bump in intensity at ~01:51 UT in Fig. 2. This reduction in tension allows the flux rope, which carries the filament f, to begin rising. (b) The rising flux rope, together with the shearing of the overlying field (indicated by the thick grey arrow, lower right; see Section 2), induces a current sheet at the intersection of the separatrices, which allows the transfer of connections from A + B - to A + D -, further weakening the tension above the filament. The four yellow rectangles indicate the positions of the four bright areas in Fig. 1c, which are signatures of this “lateral break-out” reconnection.