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

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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! 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere Image cred. T. Phan et al (2006)

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

Michael Stevens: magnetic reconnection in the heliosphere 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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. 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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.

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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? 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

Michael Stevens: magnetic reconnection in the heliosphere 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

4.3 Patchy, intermittent, or absent? 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere

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? 1/17/2019 Michael Stevens: magnetic reconnection in the heliosphere