Magnetic Reconnection & Particle Acceleration in Solar Flares Markus Aschwanden Lockheed Martin Solar and Astrophysi

1. Magnetic Reconnection & Particle Acceleration in Solar Flares Markus Aschwanden Lockheed Martin Solar and Astrophysics Laboratory Magnetic Reconnection in Relativistic Wind Workshop Stanford Linear Accelerator Center (SLAC), Menlo Park, April 28-29, 2011

2. Laboratory: Accelerator known – but target products unknown Solar Flares: Accelerator unknown – but target emission known

3. Contents: Magnetic Reconnection & Particle Acceleration in solar flares • Magnetic Topology in Solar Flares • Localization of Acceleration Region • Physics of Particle Acceleration • Particle Kinematics and Propagation • Largest flares (SEP and GLE events) • Self-organized criticality Refs : (1) Aschwanden M.J. 2002, Space Science Reviews, Vol. 101, p.1-227 “Particle Acceleration and Kinematics in Solar Flares” (2) Aschwanden M.J. 2004, Springer/Praxis, Berlin, New York “Physics of the Solar Corona. An Introduction” http://www.lmsal.com/~aschwand/eprints/2004_book/ (3) Aschwanden M.J. 2011, Springer/Praxis, Berlin, New York “Self-Organized Criticality in Astrophysics”

4. MACROSCOPIC SCALES : Magnetic Topology in Solar Flares The basic configurations of X-type magentic reconnection topologies in solar flares entail combinations between open and closed field lines : bipolar, tripolar, and quadrupolar cases, in 2D as well as in 3D.

5. Energy argument for location of particle accelerator in solar flares: Free magnetic energy is available in a reconnection region where the magnetic field lines shrink:

6. Bipolar reconnection (examples from Yohkoh)

7. Tripolar topologies Bipolar topologies

8. Bastille-Day flare • 2000 Jul 14: • The footpoints • In two-ribbon • flares separate • Consequence of magnetic reconnecion to progress in altitude Aschwanden & Alexander (2001)

9. Discovery of symmetrc dual coronal hard X-ray sources confirm the X-point geometry with upward/downward acceleration of hard X-ray producing electrons. 6-8 keV 10-12 keV 6-20 keV TRACE 1600 A on 2002 Apr 15, 23:07 UT, overlaid with RHESSI 10-15 keV contours. The energies of the symmetric coronal sources decrease progressively with distance from X-point, as expected from the temperature drop of conductive cooling (Sui & Holman 2003).

10. Sui & Holman (2003) Coronal hard X-ray source is found to initally drop in altitude before it rises in the later flare phase. This discovery is still unexplained: - relaxation of newly-reconnected field line ? - implosion after CME launch ? --> check EUV dimming !

11. The hard X-ray flux F(t) is correlated with the acceleration d2h/dt2(t) of the CME (Temmer et al. 2008). The upward-directed CME acceleration could push the X-point initially down, while it sucks it in upward direction after the pressure drops when the CME expands into interplanetary space.

12. Does a simple HXR double footpoint source mean a single flare loop? Time profiles indicate multiple loops with different timing Krucker & Lin (2002)

13. Propagation of • reconnection sites • 2002 July 23 flare: • Motions of footpoints • are observed to • increase in size and • to move systematically • along the ribbons. • see simulations of zipper effect by Mark Linton Krucker, Hurford & Lin (2003)

14. Grigis & Benz (2005) • The footpoints of conjugate hard X-ray footpoints are observed • to systematically propagate along the flare ribbons (in 2002 Nov 9 flare), • rather than apart as predicted in the Kopp-Pneuman model • reconnection propagates along neutral line

15. Where are the hard X-ray double ribbons ? (Chang) Liu et al. (2007) During the 2005 May 13 flare (color=RHESSI, white contour= TRACE 1600 A) extended hard X-ray ribbons are seen, interpreted in terms of a particular sigmoid-to-arcade evolution.

16. 3D nullpoint spine reconnection Krucker et al. (2004)

17. Magnetic flux transfer (Melrose) 3D flare geometry: Hanaoka, Nishio, Aschwanden Quadrupolar topologies

18. MACROSCOPIC SCALES : Localization of Acceleration Region • Volume of field-line • shrinking (relaxation) • after magn. reconnection • defines geometry of • acceleration region : • cusps • double cusps • jets • curved hyperboloids • spines

19. Anatomy of hard X-ray sources in a solar flare: How do we localize the particle acceleration sources from this ?

20. Force on accelerated particles : Energy gain from shortened (relaxed) force-free field line: Acceleration regions are expected in locations where newly reconnected field lines relax into a force-free configuration.

21. Measurements of ratio of electron time-of-flight distance L to flare loop half length s L/s = 1.43 +/- 0.30 (Aschwanden et al. 1996) L/s = 1.6 +/- 0.6 (Aschwanden et al. 1998, 1999)

22. Reconstruction of height of electron acceleration region in Masuda flare: L/s ~ 1.5-2.0 (Aschwanden et al. 1996)

23. Measurement of electron time-of-flight distance : • velocity dispersion from hard X-ray energy-time delay t=L/v • pitch angle correction (vparallel/v = cos ) • magnetic field line twist correction (Lprojected/LTOF)

25. Altitudes averaged from northern and southern footpoints : (error bars correspond to difference between N and S)

26. The height distribution of HXR emission dI/dz(z) is shown for 5 different HXR photon energies e=5, 10, 20, 30, 40 keV

27. Hurford et al. (2006) Acceleration sources of electrons versus ions: The standard flare scenarios predict identical sources but the observations reveal different locations for hard X-ray electrons and 2.2 MeV producing neutrons !

28. The gamma-ray source of the 2.2 MeV neutron-capture line • was found to be displaced by 20”+/- 6” from the 25 keV hard • X-ray footpoints during the 2002 Jul 23 flare. A similar • result was found for the Oct/Nov 2003 flares. • Energized electrons and ions show displaced energy loss sites (1) different acceleration sites for electrons and ions ? - different path lengths for stochastic accleration (Emslie 2004) - charge separation in super-Dreicer electric field (Zharkova & Gordovskyy 2004) (2) different propagation paths for electrons and ions ?

29. MICROSCOPIC SCALES : Physics of Particle Acceleration Fast (subsecond) time structures of hard X-ray and radio pulses in solar flares suggest small-scale, fragmented, bursty magnetic reconnection mode.

30. Basic Particle Motion Particle orbits in magnetic fields: Electrons and ions experience Lorentz force that makes them to gyrate around the guiding magnetic field. A force perpendicular to the magnetic field (e.g., electric force, polarization drift force, magnetic field gradient force, curvature force) produces a drift of the charged particle, while a force parallel to the magnetic field accelerates the particle.

31. Magnetic island formation by tearing mode instability (Furth, Killeen, & Rosenbouth et al. 1963) Magnetic X-point and O-points form  coalescence instability (Pritchett & Wu 1979) Magnetic island formation + coalescence instability  regime of impulsive bursty reconnection (Leboef et al. 1982; Tajima et al. 1987; Kliem 1998, 1995)

32. Karpen et al. 1995, 1998 Schumacher & Kliem 1997 Kliem et al. 1995, 1998 Kliem, Karlicky, & Benz 2000

33. Electric field at X-point in impulsive bursty reconnection mode (Kliem et al. 2000) Hard X-ray pulses resulting from accelerated electrons dt ~ 0.1-0.3 s (Aschwanden et al. 1996)

34. MICROSCOPIC & MACROSCOPIC SCALES : • Observations show a scaling law between Hard X-ray pulse • durations and flare loop size : Tpulse ~ 0.5 s [rloop/10 Mm] • scale invariance of magnetic reconnection region (Aschwanden et al. 1998) Lower limit of pulse durations: collisional deflection time

35. a) Electric DC field acceleration : • Sub-Dreicer field needs too large current sheets • (Holman 1985; Tsuneta 1985) • Super-Dreicer field applicable in magnetic islands • (Litvinenko 1996) • Generalization to dynamics of filamentary current sheets • (Tajima et al. 1987; Kliem 994)

36. Sub-Dreicer DC electric field: mv=change of momentum tse,i=collisional slowing-down time Under the action of an electric DC field, the bulk of the electron distribution drifts with a velocity vd, but is not accelerated because of the frictional drag force is stronger than the electric field. Above the critical velocity vr defined by the Dreicer field, the electric force overcomes the frictional force and electrons can be accelerated freely out of the thermal distribution (runaway accleration regime).

37. Super-Dreicer fields require much smaller spatial scales but higher electric fields. Energy gain in an electric field perpendicular to the guiding magnetic field: Sub-Dreicer: particle acclerated along full length of current sheet Super-Dreicer: particle drifts perpendicularly out of current sheet

38. (Kliem 1994) Convective electric field : Econv = - u/c  B (convective flow speed u ~ (0.01-0.1) vA Particle orbit near magnetic O-point in magnetic island shows largest acceleration kick due to B-drift next X-point

39. Particle acceleration near X-point (chaotic orbits) (Hannah et al. 2002)

40. Stochastic acceleration • Wave turbulence spectrum (Kolmogorov, Kraichnan) • Particle randomly gains energy by wave-particle interactions • (Doppler gyroresonance condition)

41. Gyroresonant wave-particle interactions are described by coupled equation system for changes of photon wave spectrum N(k,t) and particle distribution f(p). Γ(k,f[p])=wave amplification growth rate ΓColl(k)=wave damping rate due to collision Dij(N[k])=quasi-linear diffusion tensor of particles

42. Miller et al. (1996) • -Electron acceleration by whistler waves • Ion acceleration by Alfven waves • Enhanced ion abundances in flares reproduced by stochastic acc. • (C, O, Ne, Mg, Si, Fe, but some problems with He3/He4)

43. c) Shock acceleration • First-order Fermi acceleration • (electric field E=-(vshock/c)xB • in deHoffman-Teller frame) • -Diffusive (second-order Fermi) • (multiple shock crossings)

44. Shock-Drift (First-Order Fermi) Acceleration Adiabatic particle orbity theory can be applied to collisionless shocks. Particle gains perpendicular momentum due to the conservation of magnetic moment across shock front: De Hoffman-Teller frame: ratio of reflected to incident kinetic energy:

45. Diffusive shock acceleration Particles encounter multiple transversals of shock fronts and pick up each time an increment of momentum that is proportional to its momentum. Momentum after N shock crossings is: leading to a powerlaw spectrum for the particle momentum General treatment: diffusion convection equation:

46. Particle orbit undergoes diffusive shock acceleration in a quasi-perpendicular shock (60 deg) by multiple crossings of the shock front with magnetic mirroring upstream the shock front (x<0) Decker & Vlahos (1986)

47. Somov & Kosugi (1997) Tsuneta & Naito (1998) • Applications of shock acceleration to solar flares : • First-order Fermi in mirror trap in flare loop cusp • Fast shock in reconnection outflow above flare loop • -Type II as shock front signature in interplanetary space

48. Classification of acceleration mechanisms :

49. MICROSCOPIC & MACROSCOPIC SCALES : Particle Kinematics and Propagation • Each particle transport • process has its characteristic • energy-dependent timing • that can be used for diagnostic • acceleration dE/dt > 0 • injection [pitch angle, (t)] • time-of-flight t(E) ~ t/v(E) • trapping: collisional deflection • time t(E) ~ E 3/2 / ne • -energy loss: • tloss << tTOF

50. Electron velocity Dispersion: Pitch angle: Magnetic twist: Electron energy: Photon energy: (Bremsstrahlung cross-section)