Current Sheet Formation and Reconnection in CMEs/Flares

1. Current Sheet Formation and Reconnection in CMEs/Flares

2. Current Sheet Formation and Reconnection in CMEs/Flares S. K. Antiochos, J. T. Karpen (GSFC), C. R. DeVore (NRL), (from Karpen et al 2011) • Understand CME onset, acceleration, flare impulsive phase • Reconnection is primary driver in many models: esp., breakout, tether-cutting • Must understand reconnection dynamics: current sheet formation, switch-on? stay-on? … • Sensitive to flux-breaking process

3. Current Sheet Formation and Reconnection in CMEs/Flares • Flux breaking in corona occurs at kinetic scales • Must couple kinetic and global models • Goal of LWS TR&T Team: GSFC, UNH, PPPL, UCB, UMD, APL • Report on progress in understanding MHD case • Use ultra-high res. AMR runs of 2.5D breakout model • Allows for clear separation of eruption phases • Start with potential field and use straightforward, physical driver – footpoint shear

4. Breakout Model for CME Initiation • Requires only three simple ingredients: • Multi-polar topology • Sheared filament channel forms slowly • Flux-breaking mechanism (effective “resistivity”) scale dependent • Model has two very different reconnection sites • Magnetopause-like and magnetotail-like

5. Numerical Model • Solve standard ideal MHD on AMR grid • Refine grid on a measure of J/B • Developed analysis tools for finding null points • e.g., Greene 1992; Haynes & Parnell 2007 • Also measure degree of null point, i.e., X, O, or sheet type • e.g., Arnold 1992; Parnell et al 1996

6. Results: Breakout Reconnection • Formation/reconnection of breakout current sheet • Sweet-Parker like in that grows to system size • Reconnection dominated by formation of magnetic islands • Islands O – type nulls appear before fast reconnection dynamics (jets)

7. Results: CME Onset • Height of cavity front • Clear break at start of fast breakout reconnection • CME onset

8. Results: Flare Reconnection • Global view • CME fluxrope formation • Form classic 3-part structure

9. Results: Flare Reconnection • Local view • Flare loop formation • Lots of islands, but also shocks & turbulence

10. Flare Reconnection • Time history of O-points in flare current sheet • Downward moving islands well before significant dynamics

11. CME Acceleration • CME “take-off” due to flare reconnection transition • Accelerate up to Alfven speed

12. CME Acceleration • Flare reconnection both accelerates fluxrope directly and removes tethers

13. Results: Energy Evolution • Time history of magnetic and kinetic energies • CME onset corresponds to start of breakout reconnection • “Take-off” corresponds to start of fast flare reconnection

14. Results: Magnetic Islands • Numbers of “O” nulls in breakout and flare current sheets • Clear increase during “take-off ” phase • Necessary for fast reconnection ~ .09 VA

15. Results: Eruption Mechanism • Magnetic energy vs energy of corresponding “open” state • No evidence for ideal instability/loss of equilibrium • Resistive instability

16. Conclusions • Takeoff due to start of fast flare reconnection • Primary energy release process, both for flare heating and CME acceleration • Onset due to start of fast breakout reconnection • Produces expansion necessary to form flare sheet • Fast reconnection dominated by island formation • Vrec ~ 10% VA • Large aspect ratios for curent sheets > 200 • Expect similar results to carry over to 3D • BUT!! Need to include kinetic resistivity