1. Reconnection in Large, High-Lundquist- Number Coronal Plasmas A.Bhattacharjee and T. Forbes University of New Hampshire Monday, August 3, Salon D, 2-5 pm

2. Speakers Invited: Lyndsay Fletcher, University of Glasgow William Daughton, LANL Contributions: Masaaki Yamada (PPPL), Raymond Fermo (University of Maryland), Paul Cassak (WVU), Alex Lazarian (University of Wisconsin, Madison), William Matthaeus (University of Delaware), Amitava Bhattacharjee (UNH)

3. Classical (2D) Steady-State Models of Reconnection Sweet-Parker [Sweet 1958, Parker 1957] Geometry of reconnection layer : Y-points [Syrovatskii 1971] Length of the reconnection layer is of the order of the system size >> width Reconnection time scale Solar flares: Too long to account for solar flares!

4. Onset of fast reconnection in large, high- Lundquist-number systems The scaling of reconnection in large systems is a problem of great interest where the dimensionless ratio of characteristic dissipation scale to system size is a very small number. What are the mechanisms for the onset of fast reconnection in such systems? Does the answer depend on the mechanism that breaks field lines? One criterion has emerged from Hall MHD (or two-fluid) models, and has been tested carefully in laboratory experiments (MRX at PPPL, VTF at MIT). The criterion is: (Ma and Bhattacharjee 1996, Cassak et al. 2005) or in the presence of a guide field (Aydemir 1992)

5. Neutral sheet Shape in MRX Changes from “Rectangular S-P” type to “Double edge X” shape as collisionality is reduced Rectangular shape Collisional regime: mfp <  Slow reconnection No Quadrupolar field Collisionless regime : mfp >  Fast reconnection Quadrupolar field present X-type shape <= Ma & Bhattacharjee 1996 M. Yamada

6. W. Daughton

8. Super-Alfvenic secondary tearing instabilities occur even in resistive MHD at high Lundquist number Loureiro et al. (2007) and Bhattacharjee et al. (2009) Harris sheet Thin sheets of aspect ratio Fastest growing tearing instability Sweet-Parker sheets,

9. L. Ni, poster presentation R. Fermo: Statistical model for island distribution

11. P. Cassak

12. L. Fletcher: Spatially extended reconnection? ‘Spatially extended’ could mean (1)Extended along current sheet length (i.e. long, drawn out CS, like Sweet-Parker) (1) Extended into 3 rd dimension (e.g. an X-line) (2) Both – i.e. the ‘monolithic current sheet’ Linker et al (2003)

13. Vertically-extended current sheets ? Departed CME, plus several plasmoids Extended post-CME current sheet? Ciaravella & Raymond 2008 Fe XVII emission -> hot, turbulent, narrow, bright structure. Closer to the Sun - HXR evidence for extended flare current sheet, but with multiple plasmoids – tearing instability? Sui et al (2005)

14. Flare Ribbons Particularly the flare late phase shows this ordered behaviour. Early impulsive phase ribbons tend to look more irregular. Bastille Day 2000 flare Red = UV emission (C IV) Blue = EUV Fe IX/X Green = EUV Fe XI/XII Image made in flare late phase 1.8 ×10 5 km Evidence for (2), (3)? Spatially extended ribbons in UV and Hα, and the arcade of loops joining them demonstrate coronal reconnection ‘orchestrated’ over scales of 10 5 km.

15. Turbulent Reconnection in 2D: W. Mattaheus

16. Lazarian & Vishniac (1999) L/  reconnection simultaneous events Reconnection of 3D weakly turbulent magnetic fields involves many simultaneous reconnection events B dissipates on a small scale || determined by turbulence statistics. Key element: Turbulent reconnection: 1. Outflow is determined by field wandering. 2. Reconnection is fast with Ohmic resistivity only.

17. Reconnection rate does not depend on anomalous resistivity Flat dependence on anomalous resistivity Reconnection does not require Hall MHD

18. Fast reconnection in large, high-Lundquist- number systems Questions: What controls the onset of fast reconnection in large coronal sheets? What is the role of secondary instabilities? Of collisionless mechanisms? What is the role of turbulence? How does reconnection scale with S, electron/ion skin depth, plasma beta, system size? What constraints do observations place on theories? How can observations be made to test theory?