MHD simulations on eruption triggered by flux emergence

1. FEW2011 2011.8.22-25 Berkeley, USA MHD simulations on eruptiontriggered by flux emergence T. Yokoyama University of Tokyo Collaborators K. Nagashima (Stanford U.), S. Notoya, T. Kaneko (U. Tokyo) Based on the papers Notoya, 2006, master degree thesis, U. Tokyo Nagashima et al., 2007, ApJ, 668, 533 Kaneko & Yokoyama, 2011, in preparation

2. Introduction • Flare/CME trigger is one of the important issues that are still open. • It is suggested that there is a close relation between the plasma eruption and the flare/CME onset (e.g. Priest & Forbes 2002) because, at the onset of a large flare, an eruption of a plasma is frequently observed (e. g. Martin 1980; Ohyama & Shibata 1997). • So understanding the trigger of a plasma eruption is probably a key for understanding that of a flare/CME. • Martin et al. (1985) and Livi et al. (1989) suggested an importance of flux cancellations near a filament for the eruption. Feynman & Martin (1995) found a strong correlation of flux emergence and filament eruptions.

3. Chen & Shibata (2000) Based on the flux rope model, an emerging flux trigger mechanism is proposed for the onset of CMEs (and flares) using 2D MHD simulations. The basic idea comes from the catastrophe model (Hood & Priest 1980; Priest & Forbes 1990; Forbes & Priest 1995) Initially a flux rope is supported by the tension force of the overlying arcade. When magnetic flux emerges in the vicinity, it reconnects with the overlying field. Then the balance of the force is lost and leads to the eruption of the flux rope.

4. Introduction (continued) Today’s my talk • What happens to a filament long before its eruption ? • Observations of a filament eruption and its slow motion and mini-flares before the eruption (Nagashima et al. 2007) • How does an eruption is triggered by an emerging flux ? • MHD 3D & 2D simulations of an eruption induced by an emerging flux into the coronal arcade (Notoya 2006, Kaneko & TY 2011) Note that I am not saying all of the filament eruptions are due to the emerging flux. (Kink instability [Rust & Kumar 1996]; Torus instability [Kliem & Török (2006)] etc.)

5. Chen & Shibata (2000) In the initial condition of the simulation by Chen & Shibata, the flux rope was already in the equilibrium state close to the critical point. So once a magnetic flux emerges in the vicinity, then the flux rope immediately erupted. What happens to a filament long before its eruption ? How the flux rope’s equilibrium approaches to the critical point ?

6. Filament eruption on 2005 Sep. 13 Nagashima et al. (2007) X1.5flare accompanied by filament eruption and a halo CME Chifor et al. (2007); H. Wang et al. (2007); see also H. Li et al. (2007)

7. mini-flares on Sep. 12th and 13th filament Small flares and motion of the magnetic elements There were frequent occurrence of mini-flares around the filament. and magnetic patches moving in the vicinity of the neutral line. These are the signature of magnetic reconnection events around the filament magnetic field.

8. Slow and long-lasting motion of the filament Nagashima et al. (2007) Eruption filament magnetic neutral line The distance between the filament and the magnetic neutral line increases in time with speed of about 0.1 km/sec. This motion continues more than 40 hours. This motion indicates a change of the magnetic structure around the filament in this long time scale.

9. Evolution and triggering mechanism of the filament eruption Nagashima et al. (2007) Key observations are: (1) many mini flares (2) motions of magnetic elements and (3) relative motion of the filament away from the neutral line. Many small flares that occurredin the vicinity of the filament played a role in changing the topology of the magnetic field lines overlying the filament through small scale reconnection. Over two days, they changed its equilibrium state gradually and allowed the filament to ascend slowly. In the end, when the filaments were probably very close to the critical point for loss of equilibrium (Forbes 1990), a flare occurred and lead to the catastrophic filament eruption directly. filament

10. Chen & Shibata (2000) How does an eruption is triggered by an emerging flux ? → We performed MHD Simulations in 2D & 3D. The purpose of this study is to simulate the processes not only of the eruption but also of the approach to the stability/non-equilibrium critical point.

11. Initial condition Notoya (2006) sheared arcade field in the corona (24G @ btm; scale height 30000km) 60,000 km corona convection zone, photosphere, chromosphere twisted flux tube in the convection zone r=1200km, 2p/q=4000km, B=15kG, 7e20Mx @ -4300km

12. Results of 2D simulation 1/2plasmoid formation and eruption A plasmoid is formed and erupted by the reconnection in the RHS arcade which is deformed by the flux emergence. Eruption speed ~ 0.4 CA Temperature

13. Results of 2D simulation 2/2Structure of reconnection region Since the anomalous resistivity is assumed, the fast reconnection by the Petschek type is preferred. The "X"-letter structure of slow-mode MHD shocks is obtained. Jx

14. Chen & Shibata (2000) What happens in a 3D situation ?

15. Initial condition Notoya (2006) sheared arcade field in the corona (24G @ btm; scale height 30000km) 60,000 km corona convection zone, photosphere, chromosphere twisted flux tube in the convection zone r=1200km, 2p/q=4000km, B=15kG, 7e20Mx @ -4300km

16. Initial condition etc. (continued) Notoya et al. (2007) 10^5 km • resistive MHD equations with gravity • no heat conduction, no radiative cooling • anomalous resistivity (h is a function of j/r ) • Modified Lax-Wendroff scheme • 300^3 grid points with non-uniform spacing • constant-gradient condition for other boundaries plus wave damping zones

17. Emerging flux

18. 3D Simulation results Notoya et al. (2007)

19. 3D Simulation results reconnected lines emerging flux ‘unreconnected’ lines current sheet within the arcade A deformation of the arcade fields takes place by the pushing motion of the emerging flux. A current sheet is produced inside the arcade.

20. 3D Simulation results (continued) The magnetic reconnection takes place in the current sheet. A flux rope is formed by the reconnection and is erupted by the magnetic force of the reconnected field lines.

21. Height and Velocity of the flux rope （9*10^4 km） （3120sec） （5200sec） height (left axis） velocity (right axis) The flux rope is accelerated to 30% of the typical coronal Alfven velocity at the altitude of 1e5 km. Isosurface of T=0.4 MK

22. Conclusion • What happens to a filament long before its eruption ? • A long-term slow (maybe ascending) motion induced by many small flares were observed. It suggests a intermittent change of the field topology due to multiple reconnection events. • How does an eruption is triggered by an emerging flux ? • By 2D & 3D MHD simulations, we have shown processes of formation and eruption of a flux rope. It is formed by the reconnection in the pre-existing coronal field and is erupted through, again, by the reconnection.

24. Dependence on tube field strength 15kG The emerging flux with weaker field strength make the smaller deformation of the arcade field. Btube=9kG 12kG

25. Temporal behavior

26. Filament eruption on 2005 Sep. 13 9/13 19:27 X1.5flare accompanied by filament eruptions and a halo CME About 40 minutes before the flare peak (~18:50), small brightenings in EUV were observed at the foot point of the dark filament to erupt. (C2.9flare） 1 hour before the eruption eruption 2 dark filaments TRACE 195 Å movie （18:00-21:00) 500” ～400,000km square dark filament bright filament white-light image (SOHO MDI)

27. GOES SXR lightcurve (9-hour) 1-8 Å 0.5-4 Å ~18:50 (preflare) C2.9 flare ~20:00 2nd eruption (faint) SOHO/LASCO ~19:30 (main phase) filament eruption →~20:00 halo CME (LASCO/C2) CME Sep. 13th 0:00 - 14th 0:00 It was not so geoeffective (Wang et al. 2006) CME

28. Long-time evolution of the filaments in AR NOAA 10808 • To investigate the triggering mechanism of the eruption, we concentrate on the evolution of the filaments before the eruption. X1.5 9/13 19:27 another filament eruption M3.0 ９/11 13:12 TRACE 195Ådata 9/9 9/10 9/11 9/12 9/13 9/14 We focus on the data taken on 9/11 23:36- 9/11 16:00 and 9/11 23:30 – 9/13 21:00.

29. slow and long-lasting ascending motion of the filament filament Bright filament : 1.5×102km/s Dark filament : 2.5×102km/s more than 5 minutes (fast-rise?) magnetic neutral line Bright filament :1.3×102km/s Dark filament : 5.8×10 km/s more than 10 minutes（slow-rise?) speed：0.1km/s The filament displaced ~25arcsec(～18,000 km) from the neutral line during the period of 40hours. Eruption: 150-250km/s Eruption: 150-250km/s Such a slow and long-lasting ascending motion is probably different from so-called slow-rise phase of the erupting filaments.

30. Preflare brightenings on Sep. 12th and 13th During the slow rise of the filament, several M- and C-class flares and small brightenings in EUV occurred. Most of them occurred in the vicinity of the footpoints of the filament. At these sites, magnetic elements emerged and moved distinctively. Location of the flares on Sep. 12th and 13th (＋:C-class、◇：M-class) Red dashed line : magnetic neutral line

31. Long-time evolution of filaments ← We set a slit and made height-time profile along this slit. We investigate the evolution of the filament over 2 days before the eruption. bright filament dark filament

32. 2. Deformation of the arcade Green:unreconnected field lines of the arcade Orange:reconnected or will be reconnected field lines of the arcade Blue:emerging flux Gold:current sheet within the arcade Deformation of the arcade takes place since the emerging flux expands in the corona and current sheet is produced inside the arcade since the footpoints of it approach each other.

33. 3.Formation and Eruption of flux rope The formation of the flux rope occurs since the reconnection takes place in the produced current sheet. Current sheets are dissipated through the reconnection and produced structure is ejected into higher corona.

34. What is happening ? Notoya et al. (2007)

35. Simulation results Notoya et al. (2007)

36. Catastrophe model (Priest & Forbes 1990; Forbes & Priest 1995; Hood & Priest 1980)

37. MHD 3D simulations of an eruption induced by an emerging flux Notoya et al. (2007) The interaction between the coronal field and emerging flux has been studied numerically by many authors: (Forbes & Priest 1984; Shibata et al. 1992; Yokoyama & Shibata 1996; Isobe et al. 2006; Archontis et al. 2005; Galsgaard et al. 2005 etc.)