Electron-Scale Dissipations During Magnetic Reconnection

1. Electron-Scale Dissipations During Magnetic Reconnection Hantao Ji Contributors: W. Daughton*, S. Dorfman, E. Oz, Y. Ren, V. Roytershteyn*, M. Yamada, and J. Yoo Princeton Plasma Physics Laboratory * Los Alamos National Laboratory The 17th Cluster Workshop May 12-15, 2009 at Uppsala, Sweden

2. Why Do We Need Experiments ? • Verify/confront theory • often motivated by theory • Benchmark/challenge simulation • Compare with observations • Discover new physics

3. Two Types of Experiments • All-in-one: many competing processes coexist • e.g. tokamaks • Problem-specific: one process dominates • e.g. MRX Controllability is the key: specify conditions, when, and where to observe how

4. Outline • Introduction • The reconnection problem(s) • Fundamental physics questions • Recent results from Magnetic Reconnection Experiment (MRX) • Electron layer dissipation and comparisons with PIC • Electromagnetic waves and global 3D structure • Summary

5. Many Reconnection Problems • Vastly different environments; multiple reconnection processes • Part of magnetic, multiple-scale, self-organization processes • Seek common fundamental physics

6. Common Fundamental Physics Questions • How does reconnection start? (The trigger problem) • Why reconnection is fast compared to classical theory? (The rate problem) • How ions and electrons are heated or accelerated? (The heating problem) • How does reconnection take place in three dimensions? (The 3D problem) • How do boundary conditions affect reconnection process? (The boundary condition problem) • How to apply local reconnection physics to a large system? (The scaling problem)

7. Samples of Reconnection Experiments

8. Magnetic Reconnection Experiment (MRX)

9. Experimental Setup in MRX

10. Realization of Stable Current Sheet and Quasi-steady Reconnection Detailed diagnostics: quantitative studies possible

11. The Rate Problem:Why Reconnection Is Fast?(compared to the predictions by classical theories) Focusing on electron layers and waves

12. In 2D collisionless reconnection, laminar electron non-gyrotropic pressure dominates the dissipation. Vasyliuna (‘75), Sonnerup (‘88), Dungey (‘88), Lyons & Pridmore-Brown (‘90) Cai & Lee (‘97), Hesse et al. (‘99), Pritchett (‘01), Kuznetsova et al. (‘01) Electron Diffusion Layers Crucial in Collisionless Reconnection • Hall effects separate electron layer from ion layer • Ion demagnetized in ion layers with thickness on order of c/pi • Electron demagnetized in electron layers with thickness on order of c/pe • Manifest as quadrupole out-of-the-plane magnetic field [Ren et al. (‘05), Yamada et al. (‘06)] • Ion layers allow fast mass flows • Magnetic field reconnects in electron layer to change its topology while electrons are energized ion electron (e.g. Drake et al. ‘98)

13. Limited Observations of Electron Layer in Space • Scudder et al. (‘02) by Polar spacecraft • Reported electron-layer like events including first signatures of electron nongyrotropic pressure • Mozer (‘05) by Polar spacecraft • Documented many electron-scale layers but most of them are magnetized • Wygant et al. (05) by Cluster spacecraft • A demagnetized electron-scale layer at δ = 3-5 c/pe • Phan et al. (07) by Cluster spacecraft • A demagnetized electron-scale layer downstream at δ = 4.5 c/pe • Chen et al. (08) by Cluster spacecraft • An demagnetized electron layer between islands at δ = 4 c/pe

14. Cluster Observations: Electron-scale Layers Embedded in Ion-scale Layers Wygant et al. (‘05) Phan et al. (‘07) δ = 3-5 c/pe δ = 4.5 c/pe

15. Limited Observations of Electron Layer in Space • Scudder et al. (‘02) by Polar spacecraft • Reported electron-layer like events including first signatures of electron nongyrotropic pressure • Mozer (‘05) by Polar spacecraft • Documented many electron-scale layers but most of them are magnetized • Wygant et al. (05) by Cluster spacecraft • A demagnetized electron-scale layer at δ = 3-5 c/pe • Phan et al. (07) by Cluster spacecraft • A demagnetized electron-scale layer downstream at δ = 4.5 c/pe • Chen et al. (08) by Cluster spacecraft • An demagnetized electron layer between islands at δ = 4 c/pe Dissipation processes and relative location to the X-line difficult to determine

16. First Detection of Electron Layer in Laboratory Electron layer Ren et al. PRL (‘08)

17. Sizes of Electron Layer Are Independent of Ion Mass Length: Width:

18. 2D PIC Simulations in Geometry Similar to MRX • Driven by currents in PF coils • Flux core surface either absorbing or reflecting • No toroidal effects • Box boundary either conducting or insulating • Small numbers of (macro) particles • pe/ ce = a few, compared to ~100 in MRX • Artificially heavy electrons • mi/me=10-400, compared to large mass ratios in H, D, and He plasmas Dorfman et al. PoP (‘08)

19. All Features in Ion Scales Are Reproduced by 2D PIC Simulations 2D PIC MRX Ji et al. GRL (‘08)

20. … But NOT in Electron Scales:δ = 8c/ωpe vs δ = 1.5-2 c/ωpe Independent of ion mass

21. 135 135 140 140 145 145 150 150 155 155 160 160 Residual Collisions Can Broaden Layer, But Still Not Enough • Width from 2D PIC increases by about (50-70)% to 2.5-3.5 c/ωpe • Width from MRX: 5.5-7.5 c/ωpe with probe corrections • A factor of 2-3 difference: non-gyrotropic pressure unimportant conducting field, reflecting particle B.C. absorbing particle B.C.  3D effects? collisionless with collisions 5 0 -5 Roytershteyn et al. (‘09)

22. Electrostatic Fluctuations Observed at CS Edge: Not Directly Important To Reconnection MRX Polar Cluster EM ES Identified as LHDW Carter et al. PRL (’02) Bale et al. GRL (’02) Vaivads et al. GRL (’04)

23. Electromagnetic Fluctuations Observed at Current Sheet Center in MRX Ji et al. PRL (’04)

24. Electromagnetic Fluctuations Also Observed At High-β Areas By Cluster Phan et al. (’03) Zhou et al. JGR (’09)

25. Fluctuations Correlated with Large Reconnection Electric Field and Large Current Density (Preliminary) Dorfman et al. (’09) • Suggestive of anomalous resistivity due to waves? b:z, g:r, r:t, c:z2

26. Fluctuations Correlated with Large Reconnection Electric Field and Large Current Density (Preliminary) Dorfman et al. (’09) Larger E, J Smaller E, J

27. Fluctuations Correlated with 3D Global Structures (Preliminary) Dorfman et al. (’09) Larger E, J Smaller E, J

28. Summary (I) • Laboratory experiments can be, should be, and is already part of plasma space and astrophysics. • A growing field of laboratory plasma space/astrophysics • First one-to-one comparisons attempted between experiments and PIC simulations • All ion scale features reproduced by 2D PIC simulations • However, the electron layers are 2-3 times thicker than simulations: something is missing in 2D PIC models

29. Summary (II) • Electrostatic waves at current sheet edge identified as LHDW in MRX, simulations and space observations: not directly important. • Electromagnetic waves in LH frequency range are less well understood • Propagate perpendicularly to magnetic field (MRX, PIC, Cluster) • Consistent with a theory on EM LHDW by Wang et al, PoP (‘08) • Preliminary evidence of correlations between EM waves and locally fast reconnection associated with globally 3D structures • Current focuses: 4-way close collaborations • (1) MRX: global and local 3D effects • (2) Simulations: 2D with real mass ratios and collisions, and 3D • (3) Space: electron layer structures, and EM waves • (4) Theory: EM wave linear and nonlinear dynamics