Reconnection Physics in the Solar Wind with Voyager 2

1. Reconnection Physics in the Solar Wind with Voyager 2 Michael L. Stevens April 13, 2009

2. Overview • Magnetic Reconnection • Solar wind studies • Examples • Results • New goals to pursue • Voyager 2 study • Procedures • Overview of Voyager 2 events, comparison • Petschek shock-bound event at 31 AU • CME event and connectivity • Corotating Interaction Regions Michael Stevens: magnetic reconnection in the heliosphere

3. Flux tubes are preserved Equivalent Statements: Plasma is a good (perfect) conductor Plasma is “collisionless” Plasma is “frozen in” Plasma is “non-diffusive” Plasma is “low b” Plasma is “topologically invariant” The central theorem of ideal MHD: Michael Stevens: magnetic reconnection in the heliosphere

4. The Magnetic Reconnection Problem • Astro plasmas are very close to ideal • Flux tubes tangle and interweave • MHD won’t let plasma slip past field lines… system cannot untangle Michael Stevens: magnetic reconnection in the heliosphere

5. Resistive breakdown is microscopic (kinetic scale) Topology is macroscopic (fluid scale) MHD obviously breaks down at flux tube interfaces nonlinear kinetic phenomena have impact on global scales This is a recipe for many unsolved problems in plasma physics Michael Stevens: magnetic reconnection in the heliosphere

6. Sweet-Parker Petschek • Steady-state MHD solutions exist • Reconnection rate scales with L-1, h1/2 • Fast reconnection strategies • reduce L • Figure out why h might increase Michael Stevens: magnetic reconnection in the heliosphere

7. 2.1 How reconnection is observed in situ • jet ~ vA confined to region of large ÑxB • bifurcated current sheet • dV and dB anti-correlated, correlated at steps Michael Stevens: magnetic reconnection in the heliosphere

8. 2.2 The solar wind as a reconnection laboratory • MR first recognized in SW in 2005 • Fast • Scales up to ~0.1 AU • “quasi” steady-state • Actually prefers lowÑxB, and low plasma b • Occasionally Petschek-like, though not shocked Michael Stevens: magnetic reconnection in the heliosphere Image cred. T. Phan et al (2006)

9. 2.3 Where to move forward • Surveys are primarily near 1 AU • Slow shocks have only been hypothesized further out • Surveys are anecdotal • Design “experiments” to glean physical insight • What about current sheets that do not reconnect? • How to approach symmetries/boundary conditions without multiple spacecraft? • Uniqueness of the Walen (+-) signature has been challenged Michael Stevens: magnetic reconnection in the heliosphere

10. 3.1 Survey Method • Target dB, dv: • transient rotations measured relative to local variance, boundaries checked for Walen orientations • Where W+-= {0, 1} • Algorithm optimized for free scaling Michael Stevens: magnetic reconnection in the heliosphere

11. Michael Stevens: magnetic reconnection in the heliosphere

12. 3.2 Distant reconnection events and Voyager 2 data coverage • 138 MR signatures identified, 58 beyond 5 AU. • initial rate of ~0.6 events/day at scales ³12 s near solar max • rate is highest near solar maximum- correlates to variable magnetic and stream structure, ICMEs • Top panel: data coverage and resolution as a function of heliocentric distance at various sampling rates (high-resolution field data was unavailable after 1990) • Middle panel: reconnection exhaust signatures observed per AU. The frequent detection of short-timescale events early in the mission is attributable in large part to the availability of fast PLS measurements. • Lower panel: smoothed sunspot number rescaled to the Voyager 2 location. Note the dearth of MR events at the cycle 21/22 minimum. Michael Stevens: magnetic reconnection in the heliosphere

13. ~40% of events occur at sector boundaries, i.e. isolated long-term reversals of BR, BT • Events are typically driven by ICMEs or CIR/MIR structures. These structures usually exhibit enhanced plasma b • All are compressive structures where Michael Stevens: magnetic reconnection in the heliosphere

14. 3.3 An exceptional event at 31 AU • Most distant example of MR to date (31.3 AU) • Reversal of q indicates possible HCS crossing • MR is forced within a merged stream interaction region Michael Stevens: magnetic reconnection in the heliosphere

15. Consistency with the Rankine-Hugoniot Relations Trial orientations n=(f, q) and frame velocities are used to test the continuity of conserved quantities across the exhaust boundaries. conservation of mass electrostatic equilibrium B admits no divergence dynamic equilibrium conservation of momentum conservation of energy In the above, [ ] denotes changes across the discontinuity. E is derived from the ideal Ohm’s law.

16. 3.3 Petchek model consistency at 31 AU • Best-fit boundary planes intersect ata = 9.46 ± 1.2° • Slow wave mach number M ~2 outside, M~0.15 in exhaust • ninvin/nexvex =tan a, individual slow-mode shock accelerations and orientations predict flow conservation for whole structure • vex = vAcos a ~ 24 ± 3 km/s, exhaust jet is Alfvénic as in switch-off limit Michael Stevens: magnetic reconnection in the heliosphere

17. 3.4 Other curiosities • Have observed Walen (++) signatures that are qualitatively similar (left) • Have observed strongly chaotic inflows w/ordered outflows (right) • Geometrical complexity beyond X-line model? Michael Stevens: magnetic reconnection in the heliosphere

18. 3.5 Connectivity in an ICME at 34 AU • Well-documented flare, CME- ICME observed at Voyager 2 in February, 1991 • Proton double streaming (along B) on day 160 precedes dramatic MR event Michael Stevens: magnetic reconnection in the heliosphere

19. Reconnection exhaust traversal 1-day upstream of double-streaming site • One explanation: Reconnection in the sheath is diverting compressed plasma onto CME field lines Michael Stevens: magnetic reconnection in the heliosphere

20. 4.1 Current Sheet “Experiments” • Study current sheets in CIRs • Use Corotating stream model to constrain symmetry, boundary conditions • Corotating interaction regions (CIRs) are some of the best-studied structures in the SW • High MR rate in CIRs is convenient! Michael Stevens: magnetic reconnection in the heliosphere

21. 4.2 Searching for Symmetrical driving conditions • Find SIRs consistent with Parker Field • Can find many current sheets with loading/shear that mimics global stream interaction Michael Stevens: magnetic reconnection in the heliosphere

22. Michael Stevens: magnetic reconnection in the heliosphere

23. 4.3 Patchy, intermittent, or absent? Michael Stevens: magnetic reconnection in the heliosphere

24. 4.3 Fractionation • Strong turbulence creates current sheet substructure • Lack of observable jets in CIR sheets g most MR is not global scale Michael Stevens: magnetic reconnection in the heliosphere

25. Summary • Automated method with Voyager 2 reproduces observation rates near 1 AU • Large exhausts and large distances correspond to: • Solar activity- HCS complexity, SIRs, CMEs • Enhanced plasma b • MR exhausts have been observed that are consistent with the Petschek model • Ongoing reconnection has been demonstrated in an ICME at 34 AU • CIRs probably provide the simplest driving conditions for 2D reconnection • Forced current sheets in CIRs are associated with [large-scale] MR <20% of the time • Current sheets with high field shear are more likely to contain large-scale MR exhausts • Reconnecting fraction is loosely correlated to compression ratio and inflow rate • Questions for the future: • Are different conclusions a question of scale? • Can RDs create false positives? Michael Stevens: magnetic reconnection in the heliosphere