1. Energy Release and Particle Acceleration in Flares Siming Liu University of Glasgow 9 th RHESSI Workshop, Genova, Italy, Sep. 2009

2. The Zoo of Flare HXRs Aschwanden et al.1995

3. Flares are Multi- Scale Phenomena Bai 1993; Lee et al. 1995; Veronig et al. 2002

4. So Are the Energy Release and Particle Acceleration Processes Aschwandon et al. 1995

5. Elementary Flare Bursts Have Variable Timescale De Jager & De Jonge 1978; Liu et al. 2003, 2006

6. How Many Elementary Bursts Are There? Aschwanden et al. 1996, 1998

7. The Zoo of Flare HXRs Aschwanden et al.1998

8. High Degree of Freedom Initial and Boundary Conditions Grigis & Benz2005; Chernov 2008

9. Physics is Scale-Dependent Chernov 2008

10. Injection and Trapping Aschwanden et al.1996, 1998

11. Injection is an Artificial Process Aschwanden et al.1998; Chernov 2008 Release of Large Scale Magnetic Field Energy Wave Generation, Transport, and Dissipation Plasma Heating: Energy gain of the low-energy background particles Particle Acceleration: Energization of high- energy particles. Particle Transport: Beam Chromospheric Evaporation driven by energy deposition through particle beam, conduction, and wave flux. Radiative Processes.

12. Possible Justification for an Injection Aschwanden et al.1998; Chernov 2008 Particle acceleration timescale is short and nonthermal particles decouple from the background plasma. But solar flare studies are more concerned with the energetics and artificial injection is only appropriate for test particles or energetically unimportant component.

13. The Key: Energetics Aschwanden et al. 1996

14. Flares are multi-scale phenomena with high degree of freedom The physical processes are likely different on different scales Energetics is the key for the study of energy release of particle acceleration.

15. Generic Particle Distribution of Stochastic Particle Acceleration Liu et al. 2009 Low DensityHigh Density High Temperature Low Temperature

16. Evolution of Elementary Events Appropriate for studies of the energetics Liu et al. 2009 Low DensityHigh Density High Temperature Low Temperature

17. Weakness of Stochastic Acceleration No spatial structure and the relevant physical processes have to be treated approximately Kontar et al. 2005; Liu 2006

18. Extended Bursts Warren & Doschek 2004

19. Conclusions Energy release and particle acceleration in solar flares are multi-scale processes with high degree of freedom and different physical processes dominate on different scales. Their studies should follow the energetics, i.e. modeling the energetically dominant component first and treating other phenomena as “perturbations”. Stochastic particle acceleration provides an appropriate frame work, which can be combined with small scale plasma processes and large scale MHD processes.

20. Sub-second scale features are likely caused by transport and plasma physics processes Liu et al. 2009 Multiple Loops

21. So Are the Energy Release and Particle Acceleration Processes Hoyng et al 1976; Liu et al. 2004, 2006; Lin et al. 2003 Impulsive HXR Bursts Extended HXR Bursts

22. General Constraints on the Particle Acceleration: Fast De Jager & De Jonge 1978

23. General Constraints on the Particle Acceleration: Selective Energetically (Stochastic Acc., Sub Dreicer Field, Shock Injection) Kontar et al. 2005; Eichler 1979

24. General Constraints on the Particle Acceleration: Selective Spatially (Intermittency, Super-Dreicer) Dauphin et al. 2007

25. Which one looks more like an HXR flare? Aschwandon et al.1998

26. Stochastic Acceleration Liu et al. 2009

27. flare ribbons chromosphere UV loops (10 K) 5 H-alpha loops (10 K) 4 X-ray loops (10 K) 7 magnetic field lines conduction front reconnection inflow current sheet Superhot hard X-ray region (>10 K) 8 (standard flare configuration adapted from Forbes & Acton, 1996) Extended HXR Bursts

28. Impulsive Well-Observed Bursts Liu et al. 2003, 2006

29. Asymmetry Kontar et al. 2005

30. Extended Bursts Lin et al. 2003

31. Statistical Studies Battaglia & Benz 2005 2.3-4.2 4.2-5.6 5.6-8.3

32. Statistical Studies Krucker et al. 2007

33. EUV at 171A (by TRACE) H-alpha 6563A (by BBSO) soft X-ray 1-8A (GOES) hard X-ray 20 keV (Yohkoh) hard X-ray 100 keV (Yohkoh) microwave 6.6 GHz (OVSA) Most flares have impulsive non-thermal and gradual thermal emission components

34. Measuring the electron acceleration efficiency?

35. Measuring the electron acceleration efficiency? Model the thermal X-ray emission and find the component correlated with the non-thermal X-ray emission. Challenging, if not impossible!

36. The particle transport, chromspheric evaporation, and radiative cooling processes are difficult to model for complex flares. If the particle acceleration process is universal for all flares, as we usually assume, we should study the particle acceleration efficiency with relatively simple flares.

37. Goals TheoryHow does the efficiency depend on properties of the background plasma: B, T, n, size of the flaring region, energy release rate, … ObservationsMeasure (constrain) the efficiency Explore its dependence on the emission characteristics: spectral features, flux density, size of the emission region, variation time scale, … Particle Acceleration Efficiency measures the energy partition between the emerging thermal and non-thermal particles.

38. Energy Release Event Turbulence Heating Acceleration Radiation (non-thermal) RADIATION (thermal) Radiation (non-thermal) Heating Turbulence Acceleration IMPULSIVE GRADUAL

39. Particle Distribution Function E E f(E)

40. fit ( th. +nth. )  = - 4.5 3-4 keV 6-9 keV 13-20 keV 20-30 keV GOES 1-8 A observed 7.8 GHz 9.4 GHz 11.8 GHz 14.8 GHz  = - 3.0  = - 3.1  = - 4.4 Hard X-ray microwave (Qiu et al. 2005) Measuring the electron acceleration efficiency?

41. Particle Distribution Function

42. Multi-Processes: reconnection geometry wave/turbulence generation and transport, particle generation and transport plasma evaporation and cooling

43. Theoretical Considerations Solar flares are multi-scale phenomena in terms of energy, duration, and spatial scale, so are likely the particle acceleration events.

44. Thermal vs. Non-thermal ThermalNon-thermal TheoryParticle DistributionMaxwellianPower-law EnergyLowHigh InteractionsCoulomb Collisions Particle-Field Collisionless Observ ations Emission Properties Temporal Spectral Spatial Energetics Gradual Narrow Extended? ? Impulsive Broad Compact? ? Particle Acceleration Efficiency measures the energy partition between the emerging thermal and non-thermal particles.