Particle Acceleration in Solar Flares

1. Particle Acceleration in Solar Flares Siming Liu Purple Mountain Observatory liusm@pmo.ac.cn 025-83332173

2. Outline General Issues of Particle Acceleration Heliosphere Energetic Particles and Flares Conclusions

3. General Issues of Particle Acceleration 1: How high can the particle energy be? 2: How fast can particlesbe accelerated? 3: What causes the spectral breaks? 4: What determines the spectral index? 5: What determines the acceleration efficiency (flux)? 6: To what extent can we predict properties of accelerated particles?

4. 1: Hillas Diagram: Maximum Energy How can the particle energy be so high? ~qBL q: charge B: magnetic field L: size Confined by B: Gyro-radius Rg=E/qB<L Maximum potential difference: ~B L

5. 2: Acceleration Timescale How fast can particles be accelerated? Electric field seen by high energy test particles: E =V x B Momentum gain rate: ṗ = q E ~ qVB Acceleration time: p/ṗ ~ Rg/V

6. 3: What causes spectral breaks? 2.7 Galactic 2.0 Solar ExtraGalactic

7. 4: What determines the spectral index?

8. 4: Different Ion Spectra from Two Solar Flares

9. 4: Yearly Spectral Variation

10. 4: What determines the spectral index? Power law distribution may just indicate a lack of distinct scales in the Heliosphere. But why does the distribution converge to an index of 2? A highly dynamical process of particle acceleration, transport and energetic particle coupling with background plasma and waves!? Is the index of 2 universal? If not, what determines its value?

11. 5: Energy Partition between thermal and nonthermal components is observationally poorly constrained and theoretically not well defined

12. 6: Solar Flares Intermittency of Acceleration Hot Plasma EUV Image Energetic Particles X-ray Light curves Bremsstrahlung and Hadronic Processes Lin et al. 2003

13. 6: Observations of Intermittency

14. 6: Neupert Effect

15. Conclusions Power-law distribution of energetic particles origins from complex multi-scale systems. The broad smooth distribution in energy is intimately connected to the lack of distinct time and spatial scales in the relevant acceleration processes. Identification of prominent features from observations plays important roles in advancing our understanding of these complex systems. (Not easy, Keep trying!!!)

16. Low-Energy: Gradual Thermal Coronal Source High-Energy: Impulsive Non-thermal Chromospheric Footpoints Lin et al. 2003

17. A Simple Flare LIGHT CURVE: A simple classical flare with an impulsive hard X-ray pulse leading gradual evolution in soft X-rays

18. Count Spectral Evolution Rise Decay

19. Forward Fitting to Spectra Thermal Rise Rise

20. Generic Particle Distribution in Turbulent Plasmas Thermal Emission Balance between Collision and Acceleration Balance between Diffusive Acceleration and Frictional Energy Loss or Escape Samples of Particle Distribution Function Emissions by Energetic Particles Liu et al. 2009, 2010

21. A Case Study Thermal Decay Preheating Acceleration

22. Parameters Nonthermal Preheating Thermal Decay 3 X2 47 Log EM 44 5keV T 1keV Eb 20keV γ 4

23. Solar Flares Masuda et al.1994

24. 5: Heliospheric Energetic Particles

25. Energetically trivial intermittent HXRs

26. Injection and Trapping Model for High-Energy Emission Aschwanden et al.1996, 1998

27. Stochastic Acceleration of Protons and Electrons

28. Intermittency in Time Aschwanden et al.1996, 1998

29. 5: Heliospheric Energetic Particles 1AU 100AU

30. 4: What determines the Spectral index and flux?! 2.7 2.0

31. Fermi Acceleration 1984

34. Fermi Acceleration 1988 Stochastic Diffusive Shock

35. 5: Heliospheric Energetic Particles

36. Kappa Distribution

37. Levy Flights Impulsive Energy Gain Statistical properties of current sheet determine the particle distribution!