Multiscale issues in modeling magnetic reconnection

1. J. F. Drake University of Maryland IPAM Meeting on Multiscale Problems in Fusion Plasmas January 10, 2005 Multiscale issues in modeling magnetic reconnection

2. Magnetic energy dissipation in the universe • The conversion of magnetic energy to heat and high speed flows underlies many important phenomena in nature • solar and stellar flares • magnetospheric substorms • disruptions in laboratory fusion experiments • More generally understanding how magnetic energy is dissipated is essential to model the generation and dissipation of magnetic field energy in astrophysical systems • accretion disks • stellar dynamos • supernova shocks • Known systems are characterized by a slow buildup of magnetic energy and fast release • trigger? • mechanism for fast release? • Mechanism for the production of energetic particles?

3. B x x x x x x x x x x x x x x x x x x x x x x x x x x J Magnetic Free Energy • A reversed magnetic field is a source of free energy • Can imagine B simply self-annihilating • What happens in a plasma? • How does magnetic reconnection work?

4. Frozen-in Condition • In an ideal plasma (=0), the fluid moves so that the magnetic flux through any fluid element is preserved.

5. Energy Release from Squashed Bubble • Magnetic field lines want to become round magnetic tension

6. Energy Release (cont.) w • Evaluate initial and final magnetic energies • use conservation law for ideal motion • magnetic flux conserved • area for nearly incompressible motion L R Wf ~ (w2/L2) Wi << Wi • Most of the magnetic energy is released

7. Flow Generation • Released magnetic energy is converted into plasma flow • Alfven time A is much shorter than observed energy release time

8. Magnetic Reconnection • Strong observational support for this general picture

9. Resistivity and the multiscale problem • The frozen-in condition implies that in an ideal plasma (=0) no topological change in the magnetic field is possible • tubes of magnetic flux are preserved • Breaking of magnetic field lines requires resistivity or some other dissipation process • As in fluid systems, dissipation can only be important at small spatial scales • Breaking of field lines occurs at very small spatial scales where the magnetic field reverses  dissipation region • Release of energy in a macroscopic system depends on the complex dynamics of a boundary layer • Typically kinetic and turbulent • Reconnection is inherently a multiscale problem whose description is a computational challenge

10. Expulsion of the core temperature during sawteeth in tokamaks • Reconnection is broadly important in fusion experiments • The “sawtooth crash” is an important example • Periodic expulsion of the plasma from the core of tokamaks Yamada, et al, 1994

11. resistive time Characteristic Times Resistive Time Alfven Time Release Time Laboratory Tokamaks 1 - 10 sec ~ 1 sec 50 sec Solar Flares ~ 104 years ~ 0.1 sec ~ 20 min Magnetosphere  100 sec ~ 30 min

12. Resistive Magnetohydrodynamic (MHD) Theory • Formation of macroscopicSweet-Parker layer V ~ ( /L) CA ~ (A/r)1/2 CA << CA • Slow reconnection • sensitive to resistivity • macroscopic nozzle

13. Failure of the MHD model • Resistive MHD reconnection rates are too slow to explain observations • solar flares • sawtooth crash • magnetospheric substorms • Some form of anomalous resistivity is often invoked to explain discrepancies • strong electron-ion streaming near x-line drives turbulence and associated enhanced electron-ion drag • Non-MHD physics at small spatial scales produces fast reconnection • coupling to dispersive waves critical • Mechanism for strong particle heating during reconnection?

14. Role of dispersive waves • Coupling to dispersive waves at small scale is key to understanding magnetic reconnection • rate of reconnection insensitive to the mechanism that breaks the frozen-in condition • fast reconnection even for large systems • no macroscopic nozzle

15. s c/pe c/pi scales kinetic Alfven waves Electron inertia whistler waves Generalized Ohm’s Law • Electron equation of motion • MHD valid at large scales • Below c/pi electron and ion motion decouple • electrons frozen-in • Whistler and kinetic Alfven waves are dispersive • Electronfrozen-incondition broken below c/pe

16. Kinetic Reconnection • Ion motion decouples from that of the electrons at a distance from the x-line • ion outflow width • electron current layer and outflow width • Whistler and kinetic Alfven waves control the dynamics in the inner region c/pi c/pi c/pe

17. GEM Reconnection Challenge • National collaboration to explore reconnection with a variety of codes • MHD, two-fluid, hybrid, full-particle • nonlinear tearing mode in a 1-D Harris current sheet Bx = B0 tanh(z/w) w = 0.5 c/pi Birn, et al., 2001

18. Rates of Magnetic Reconnection • Rate of reconnection is the slope of the  versus t curve • All models that include the Hall term in Ohm’s law yield essentially identical rates of reconnection • Consequence of dispersive waves • MHD reconnection is too slow by orders of magnitude

19. Why is wave dispersion important? • Quadratic dispersion character  ~ k2 Vp ~ k • smaller scales have higher velocities • weaker dissipation leads to higher outflow speeds • flux from x-line~vw • insensitive to dissipation

20. T= 160 -1 T= 220 -1 Fast reconnection in large systems • Large scale hybrid simulation (Shay, et al., 1999) • Rate of reconnection insensitive to system size vi ~ 0.1 CA • No large scale nozzle in kinetic reconnection

21. 3-D Magnetic Reconnection • Turbulence and anomalous resistivity • 2-D models produce strong electron streaming around the magnetic x-line • Can such streams drive turbulence? • Electron-ion streaming instability (Buneman) evolves into nonlinear state with strong wave turbulence • Electron scattering produces enhanced electron-ion drag, (anomalous resistivity) that is sufficient to break magnetic field lines even without classical resistivity

22. Observational evidence for turbulence • There is strong observational support that the dissipation region becomes strongly turbulent during reconnection • Earth’s magnetopause • broad spectrum of E and B fluctuations • Sawtooth crash in laboratory tokamaks • strong fluctuations peaked at the x-line • Magnetic fluctuations in Magnetic Reconnection eXperiment (MRX)

23. 3-D Magnetic Reconnection: with guide field • Particle simulations (PIC) with up to 1.4 billion particles • Development of strong current layer • Current layer becomes turbulent • Electron-ion streaming instability (Buneman) evolves into electron holes y x

24. B Turbulence and the formation of electron holes • Intense electron beam generates Buneman instability • nonlinear evolution into “electron holes” • localized regions of depleted electron density • Seen in satellite observations in the magnetosphere Ez z x

25. Anomalous drag on electrons • Parallel electric field scatter electrons producing effective drag • Average over fluctuations along z direction to produce a mean field electron momentum equation • correlation between density and electric field fluctuations yields drag • Normalized electron drag

26. Electron drag due to scattering by parallel electric fields • Drag Dz has complex spatial and temporal structure with positive and negative values • Sufficient to break magnetic field lines during reconnection y x

27. The computational challenge • Modeling reconnection in plasma systems (solar corona, fusion plasmas, the Earth’s magnetosphere) requires the description of the dynamics of the largest spatial scales • describes the buildup and storage of magnetic energy • MHD description adequate • At the same time must include the dynamics of a microscale boundary layer • This dissipation region is both kinetic and turbulent • Modeling the dissipation region • Including the coupling to dispersive waves to model fast reconnection requires a two-fluid or kinetic (PIC, gyrokinetic) description • Modeling turbulence and anomalous resistivity • Kinetic (PIC) description down to Debye scales • Modeling the production of energetic particles • Kinetic (PIC) description

28. Range of spatial scales • Modeling kinetic turbulence requires even smaller spatial scales!! • Even AMR codes will not be able to treat such disparate scales • The development of innovative multiscale algorithms for handling • such problems is an imperative

29. Conclusions • Magnetic reconnection causes an explosive release of energy in plasma systems • similar to other types of explosions • sonic flows • a difference is that the explosion is non-isotropic • Fast reconnection depends critically on the coupling to dispersive waves at small scales • rate independent of the mechanism which breaks the frozen-in condition • rate independent of all kinetic scales~ 0.1 CA • rate consistent with observations • Modeling magnetic reconnection in a macroscale system requires the simultaneous treatment of a microscale boundary layer that is both collisionless and therefore inherently kinetic and turbulent • Describing the dynamics is a multiscale challenge

30. Outstanding Issues • Onset • Structure of slow shocks • Electron heating • Role of turbulence and anomalous resistivity