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

2. Michael Stevens: magnetic reconnection in the heliosphere
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
Each of these goals might constitute a thesis chapter
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Michael Stevens: magnetic reconnection in the heliosphere

3. The central theorem of ideal MHD:
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”
A flux tube is a waveguide, conduit, rubber band, etc
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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
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Michael Stevens: magnetic reconnection in the heliosphere

5. MHD obviously breaks down at flux tube interfaces
Resistive breakdown is microscopic (kinetic scale)
Topology is macroscopic (fluid scale)
nonlinear kinetic phenomena have impact on global scales
This is a recipe for many unsolved problems in plasma physics
Mention RESISTIVITY
Mention that kinetics is HARD
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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
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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
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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 low ÑxB, and low plasma b
Occasionally Petschek-like, though not shocked
evidence to support scales up to a fraction of an AU!
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Michael Stevens: magnetic reconnection in the heliosphere
Image cred. T. Phan et al (2006)

9. Michael Stevens: magnetic reconnection in the heliosphere
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
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Michael Stevens: magnetic reconnection in the heliosphere

10. Michael Stevens: magnetic reconnection in the heliosphere
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
Site your own work. Show q-plot, qdef, sw pwr spec
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Michael Stevens: magnetic reconnection in the heliosphere

11. Michael Stevens: magnetic reconnection in the heliosphere
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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.
- agrees with Wind and ACE studies at 1 AU.
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Michael Stevens: magnetic reconnection in the heliosphere

13. Michael Stevens: magnetic reconnection in the heliosphere
~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
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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
Emphasize step in VT
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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 at a = 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
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Michael Stevens: magnetic reconnection in the heliosphere

17. Michael Stevens: magnetic reconnection in the heliosphere
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?
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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
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Michael Stevens: magnetic reconnection in the heliosphere

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

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

22. Michael Stevens: magnetic reconnection in the heliosphere
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Michael Stevens: magnetic reconnection in the heliosphere

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

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

25. Michael Stevens: magnetic reconnection in the heliosphere
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?
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Michael Stevens: magnetic reconnection in the heliosphere