Flare Particle Acceleration in Large-scale Magnetic Fields

1. Flare Particle Acceleration in Large-scale Magnetic Fields Peter Cargill Imperial College With thanks to Rim Turkmani, Loukas Vlahos, Heinz Isliker and Klaus Galsgaard, ISSI October 5, 2006

2. The context: solar flares • Release of 1032 ergs in 1000 sec. Evidence from SMM, RHESSI is that: • Large fraction of energy (at least 20%, perhaps more) is in energetic particles: • Electrons > 10 keV. Appears to be solid result. • Ions? Controversial in past (see 1996 EOS “debate”). Less so now, but still measurement difficulties. To put in context……. • Acceleration in solar wind, substorm etc. low-efficiency processes • Even SNR shocks few percent into particles WHY ARE FLARES SPECIAL? A magnetic complexity not present elsewhere.

3. Particle acceleration and reconnection Assume that reconnection process responsible for flare energy release. 3 possible processes: • Shock acceleration (fast and slow) • MHD turbulence • Direct electric fields (sub- and super-Dreicer) Need to accelerate: • Particles quickly (usually < 1 sec) • To relativistic energies • A lot of particles Basic theory of acceleration processes in isolation quite well established.

4. Shock Acceleration (single shock) Needs: Prompt shock formation Drift accn (quasi-perp shocks) • Energy limited by escape from shock of particle. • Number of particles limited by short interaction time. Diffusive accn (quasi-par shocks) • Can give very high energies through multiple interactions with shock • Limited energy gain due to loss of particles. • Needs injection (ok for ions, problem for electrons) Decker and Vlahos (1986)

5. Direct Electric Fields Split into sub-Dreicer (V/m) or super-Dreicer (reconnection: 103 V/m) Sub-Dreicer • long potential drops to get interesting energies • Serious problems with current and charge neutrality (many oppositely-directed currents). Super-Dreicer • Can readily attain high energies. • No issues of neutrality • Readily attainable in reconnection geometry Holman and Benka (1992)

6. MHD Turbulence Alfvén and fast mode waves can interact strongly with ions and electrons respectively. • Can produce very high particle energies quickly • Injection required cascade to short wavelengths. • Theory largely based on quasi-linear models. Miller (1997)

7. Efficiency question at single site Consider turbulent acceleration: • Fraction of coronal EM energy available for dissipation (10 – 20%). Rest in potential field. • Fraction of energy in reconnection goes to jets (50%) • Fraction of jet energy goes to turbulence (50%) • Fraction of turbulence goes to particles (20%) End up with requirement for massive coronal field (Cargill, 1996), Miller et al. 1997) WRONG! Flare involved a transition in physics. • Somehow (and rarely), things conspire for massively efficient acceleration of particles.

8. Cargill and Priest (1983) Shibata, (199X) Flare Energy Release: the Monolithic Current Sheet • Originally due to Carmichael, Sturrock etc (1964, 1966). • Kopp and Pneuman proposed for “post”-flare loops (1976). • Numerous manifestations (see H. Hudson “cartoon” collection)

9. Perhaps a reasonable model for eruptive and long-lived flares (+ post-flare loops). • Extension to “all” flares in Yohkoh era • “Cartoon” physics. • No effort to assess viability on basis of physics (particle acceleration, global electrodynamics) • Still needs a proper physics-based assessment…… What is an alternative?

10. Distributed energy release. • Flare comprised of many distributed energy sites. • Each one is current sheet (as in previous model) Questions: • How does dissipation initiate? • How does it spread? • How are particles accelerated? • Do particles interact with multiple sites? Approach. MHD or PIC codes? Vlahos (1994)

11. Distributed Energy Release: SOC models Start from power law distribution of flares: N(E) = (E0/E)1.8 • Lu and Hamilton (1991) applied ideas of Self Organised Criticality (SOC: Bak et al., 1987) to coronal field. • Used “rules” to link magnetic field at neighbouring points. • Drive magnetic field locally. Stressed fields then relaxed. Energy release. • Obtained power law of N(E). • Very rarely, field relaxed at ALL points at same time…… this could be said to be a flare. Controversy!!! • What do the relaxation rules mean? • Is the forcing reasonable? • What about real coronal geometry?......... and • What about Maxwell (and Faraday!)? Much argument….

12. Synchronised Energy Release: SOC/CA models Isliker et al., (2001) • Distribution of current in CA model: zoom on right • Get many dissipation regions

13. Acceleration in CA model Example: Super-Dreicer E-field. (Arzner and Vlahos, 2004) • Parameterise acceleration process • Calculate distributed sites with CA model • Ballistic particle motion between accelerators. • Assume particle loss from system

14. Particles gain (and lose) energy in jumps at multiple sites • Systematic gain in time. Fast (<< 1 sec) • Proof of principle of acceleration at many sites. BUT…. What is relation to MHD description?

15. Acceleration in MHD models 3 kinds • Simple current sheet, x-line etc. • Spectral models of MHD turbulence • 3-D models of coronal current sheet formation.

16. Turbulent E and B fields Acceleration in MHD Turbulence Early work, Matthaeus, Goldstein, 1986. X- and o-points + superposed turbulence. • Energisation and trapping (o-points) Dmitruk et al (2003, 2004) • Full 3-D turbulence model • Generate multiple current sheets • Track test particles in model

17. Energy vs. E (aE) Particle trajectories Distributions

18. Acceleration in Stressed Magnetic Fields(Turkmani et al, ApJ Lett, 2005) Step 1: 3-D Coronal MHD model (Galsgaard) Drive corona by stressing footpoints Develop a turbulent corona with multiple dissipation sites.

19. Step 2: Freeze fields at an “interesting” time Get global E-field: E = -(u x B) + J Inductive field plays no role……

20. Step 3 • Fully relativistic test particle code • Inject distribution (T ~ 5 x 106) • Track electrons and ions through multiple current sheets.

21. Energy Sample Particles Fast acceleration • 0.1 (1) sec for e-(p+) Encounters multiple acceleration sites. • 74 % are accelerated (> 1 keV) • 40 % Leave from either footpoint • 13 % Leave from the sides • 21 % Trapped • 26 % Not accelerated E-field Sample electron

22. end start Several phases. Time evolution and distribution functions

23. Particles accelerated in multiple dissipation site MHD model • Fast • Plentiful • At many places Effect of resistivity model? Relation to CA models Computed Bremsstrahlung signature

24. Some questions Can MHD ever describe a flare? • Not if acceleration efficiency is as high as we believe. • Transition from MHD to different plasma state • In MHD sense particles require an “equation of state”. What is it? What is influence of accelerated particles on reconnection? • Can rapid acceleration change reconnection rates to very fast? • Can these particles change character of diffusion regions? Can production of large particle flux destabilise field over large area? • “Dissipation spreading” (Papadopoulos, 1977) Some may be answered by PIC codes. Most require…….. conceptual thinking, not simulations (yet).

25. The observational future “Son of RHESSI” • Need for comprehensive payload • Ions! • Especially context measurements (plasma flows) Solar-B/EIS • Expect to get measurements of mass flows and turbulence in flares (synchronise with RHESSI)