The Solar Wind as a Magnetic Reconnection Laboratory

1. The Solar Wind as a Magnetic Reconnection Laboratory J. T. Gosling LASP / University of Colorado Boulder, Colorado, USA “He who controls magnetism controls the universe.” - Diet Smith in Dick Tracy, circa late 1940s and early 1950s. February 15, 2008, Boston University

2. Reconnection in the solar wind: Commonly is quasi-stationary and commonly occurs at extended X-lines. Produces Petschek-type exhausts of jetting plasma bounded by back-to-back rotational discontinuities. Occurs at thin current sheets and commonly bifurcates them. Is quite common in low-speed wind (40-70 events/month at 1 AU) and within ICMEs, but occurs less frequently in the turbulent, high-speed wind. Commonly occurs at field shear angles considerably < 90˚ (strong guide fields).

3. Reconnection in the solar wind: Often is undriven (spontaneous). Commonly occurs in low beta plasma. Usually is not associated with slow-mode shocks. Usually does not (never?) produce energetic particles. Is “fast” but not “explosive” - magnetic energy release occurs over a long interval following reconnection.

4. Reconnection at the Leading Edge of an ICME Np Tp V Na/Np B Bn Reconnection is observed only occasionally at ambient wind/ICME interfaces, but relatively frequently in ICME interiors.

5. An Expanded View of the 2/18/99 Event Np Tp Vn B Bn VA  Vn Roughly Alfvenic accelerated plasma flow filling field reversal. Field shear angle = 120˚. Bifurcated (double step) current sheet.

6. Reconnection Exhausts in the Solar Wind 2D projection Br Vr Bt Vt Bn Vn A reconnection jet or exhaust is identified as roughly Alfvenic accelerated plasma confined to a field reversal region. Current sheet is typically bifurcated (double step). Changes in V and B are anti-correlated at one edge and correlated at the other, indicates Alfven waves propagating in opposite directions along reconnected field lines.

7. Interpenetrating Proton Beams Within Reconnection Exhausts Protons - Plasma Rest Frame 11/23/1997 Before Exhaust After Exhaust Interpenetrating proton beams demonstrate magnetic connection across an exhaust and, although not always well resolved, are characteristic of these events.

8. Do the reconnection exhausts contain slow mode shocks? Plasma Rest Frame Within Exhaust Real solar wind case is intermediate between these 2 extremes. Many exhaust boundary crossings are not slow-mode-like.

9. An Extended Reconnecting Current Sheet Observed by 5 Spacecraft Time-shifted Data STEREO A and B were separated by 1215 Re.

10. Oppositely Directed Exhausts from a Common X-Line B V Exhaust’s signature at ACE and Geotail (not shown) was similar to that at Wind. Bx Vx By Vy Bz Note: 3D velocity data are not yet available for STEREO B. Vz

11. Observational Summary X-line tilt:6.7˚ to xy plane. Delays relative to STEREO A: Calculated:(96, 115, 182, 309) minutes Observed:(105, 126, 184, 320) minutes X-line length:At least 668 Re = 4.26 x 106 km = 0.0284 AU = 6.12 solar radii. Reconnection duration: At least 320 minutes. Exhaust intersection and X-line orientation were determined from MVAB of STEREO A field data.

12. A Grazing Encounter by 5 Spacecraft with a Large Reconnection Exhaust Within the Heliospheric Current Sheet Exhaust intersection with xy plane This is the largest exhaust yet identified in the solar wind and demonstrates continuous reconnection at an extended and continuous X-line.

13. The Exhaust Encounter in Minimum Variance L,M,N Coordinates S/C trajectories across the exhaust Energy extracted from the current sheet was at least 1.2 x 1024 ergs - comparable to energy released in a large geomagnetic storm, but small compared to energy released in a large flare or CME (1032 ergs).

14. Some Details of the Prolonged Reconnection Event Exhaust occurred at the heliospheric current sheet. Note the lack of electron heating within the exhaust. Tecore Tp Suprathermal Electron Strahl

15. Strahl Disappearance and An Asymmetric Halo : Evidence for Magnetic Disconnection From the Sun Demonstrates magnetic connection across the exhaust and magnetic disconnection from Sun.

16. Counterstreaming Strahls within a Sunward-Directed Reconnection Exhaust at the Heliospheric Current Sheet: Evidence for Formation of Closed Field Lines

17. A Set of 7 Reconnection Exhausts Within and Trailing an ICME Reconnection frequently occurs within ICMEs, often at edges of filaments of folded field.

18. A Very Narrow Reconnection Exhaust 3-s resolution data 92-ms resolution data Bt  = 62˚ Br Bn BN BL BM Exhaust was 25.4 c/pi wide. Clear bifurcated current sheet. Strong guide field (BM). |BM| increased in exhaust. No Hall field rotations obvious.

19. Reconnection Exhausts Observed by Wind in March 2006 V 1-hr avgs. B   B shear angle: 30˚ 74˚

20. Some Statistics for the March 2006 Reconnection Exhausts V Beta VA Reconnection: Is quite common in the low-speed wind. Commonly occurs at small field shear angles. Commonly produces relatively narrow exhausts. Occurs preferentially in low beta plasma.

21. A Motivation “…most of the energy in a nearly incompressible MHD cascade is expected to dissipate through small scale reconnection processes.” Matthaeus et al. [2003] (see also Leamon et al. [2000] and Retino et al. [2007])

22. Alfvenic Fluctuations Associated with MHD Turbulence in the High-Speed Solar Wind 64-s data Alfvenic fluctuations propagating away from Sun along B in solar wind frame permeate high-speed wind. Turbulent cascade produces many thin current sheets. Is reconnection important mechanism by which turbulence is dissipated and solar wind heated far from the Sun?

23. A Reconnection Exhaust Embedded in an Alfvenic Fluctuation in the Turbulent, High-Speed Wind 3-s data

24. Results of Survey of 1358 Hours (56.6 Days) of Turbulent, High-Speed (> 600 km/s) Wind Data Only a very small fraction of resolved current sheets contain reconnection exhausts. 34 reconnection exhausts identified (~0.6 events/day). All but 3 of the exhausts were embedded within outward propagating Alfvenic fluctuations. Current sheets and exhausts in turbulent wind have relatively small spatial extents.

25. Turbulence Conclusions Reconnection is one way by which the turbulence is dissipated and the solar wind is heated far from the Sun. It is questionable if reconnection is the major way by which turbulence is dissipated in the high-speed wind - but the jury is still out on that.

26. Energetic Electron and Proton Measurements in 3-Hr Intervals Encompassing 7 Solar Wind Reconnection Exhausts Reconnection Exhaust Various energetic particle background events prevailed during the 3-hr intervals. There were no substantial energetic electron or proton enhancements one would associate with the reconnection exhausts in any of these intervals. Does reconnection necessarily accelerate particles to high energies?

27. Questions, Problems, Topics for Future Work Sweet-Parker vs Petschek-type reconnection What initiates reconnection in solar wind? Why is low beta preferred in solar wind reconnection? Tearing mode? Thin current sheets? How long does/can reconnection persist? What turns it off? Is reconnection driven or spontaneous or both? X-line extents : How long? How are long X-lines formed? Signatures of patchy (in space and time) reconnection? Evidence for magnetic islands / flux ropes within exhausts? Reconnection exhaust evolution and interaction with ambient wind Slow mode shocks or back-to-back Alfven waves?

28. Questions, Problems, Topics for Future Work (continued) Particle acceleration necessarily associated with reconnection? Reconnection rates and exhaust wedge angles Physics of coupling of interpenetrating ion beams and electrons in reconnection exhausts Overall role in producing dissipation in the solar wind? Important effects associated with guide field? Comparison of observations with fluid and kinetic (hybrid) simulations of reconnection exhausts Heliospheric signatures of reconnection in corona? *Comparative reconnection: corona, solar wind, magnetopause, magnetotail, laboratory. How does reconnection in the solar wind relate to reconnection in other plasma environments?

29. The End Reprints and preprints on reconnection in solar wind (15 available): jack.gosling@lasp.colorado.edu