1. The Structure of Thin Current Sheets Associated with Reconnection X-lines Marc Swisdak The Second Workshop on Thin Current Sheets April 20, 2004

2. Collaborators J. Drake M. Shay J. McIlhargey B. Rogers A. Zeiler U. of Maryland Dartmouth College MPP-Garching UMBC

3. B guide J B reconn x y z Simulation: Reconnecting field:x Inflow velocity: y Guide field/Current:z

4. p3d Details Relativistic PIC code Boris algorithm for particles Trapezoidal leapfrog for fields Multigrid for Poisson’s equation MPI parallelization Biggest runs: 512x256x256 2048 processors ~10 9 particles How we cheat: m e /m i large c/c A small Also: Double Harris sheet Periodic BCs

5. The Point Q: At what strength does the guide field become important? A: B g  0.1 B 0

6. No Guide Field: Overview

7. Development of Bifurcation

8. Temperature

9. Velocity Distributions @ x-line: Beams are due to Speiser figure-8 orbits @ bifurcation: Multiple peaks from two beams

10. Balancing the Reconnection Electric Field Ideal MHD Pressure tensor Electron Inertia

11. Balancing the Reconnection Electric Field

12. Guide Field: B g =1B 0 Current sheet not bifurcated Electrons magnetized at the x-line Canted separtrices E || interacting with B g

13. Temperature, B g =1

14. Balancing the Reconnection Electric Field

15. Guide Field Criterion What is the minimum B g so that the e - excursions are less than d e ? Reconnection Rate:

16. X-line Structure: B g = 0, 0.2, 1

17. Temperature, B g =0.2

18. Off-Diagonal Pressure Tensor, P yz

19. Why is this important? Development of x-line turbulence. Why does it happen? B g means longer acceleration times. X-line Distribution Functions

20. Conclusions B g ~ 0.1B 0 is enough to influence the structure of x-lines. –Affects: Flow geometries, separatrices, particle orbits (temperatures), particle energization, development of turbulence (?) –Doesn’t affect: Reconnection rate, breaking of frozen-in condition Implication: Anti-parallel reconnection is rare in real systems. Most reconnection is component reconnection

21. Cut Through the X-line

22. Reconnection Rate & Guide Field Reconnected Flux Time

24. Anti-parallel reconnection Guide field reconnection Why the difference? Within the diffusion region electrons are unmagnetized & execute wandering orbits. Electrons are always magnetized and are not heated. T final T init

26. Generalized Ohm’s Law The final three terms become important at different scales:  i  c/  pi  s,  e  i  e What terms does MHD neglect? Ideal MHD Pressure tensorResistive MHD Hall termElectron Inertia

27. 3D Reconnection with Guide Field

28. Buneman Instability Electron-ion two-stream instability. If the distribution functions do not (roughly) overlap then the system is unstable. Ions Electrons ~J

29. 3D Reconnection w/o Guide Field Initial turbulence (LHDI) disappears as reconnection strengthens. X-line shows no sign of instability at late times. early late

30. Temperature

31. Temperature, B g =0.2

32. Temperature, B g =1

33. Dissipation mechanism What balances E p during guide field reconnection? Scaling with electron Larmor scale suggests the non- gyrotropic pressure can balance E p (Hesse, et al, 2002). B z =0B z =1.0 yy

34. Transition from anti-parallel to guide field reconnection Structure of non-gyrotropic part of the pressure tensor, P yz –Remove gyrotropic portion –Significant changes for B z0 =0.1 B z0 =0B z0 =1.0B z0 =0.1