1. Characterizing Thermal and Non- Thermal Electron Populations in Solar Flares Using RHESSI Amir Caspi 1,2, Säm Krucker 2, Robert P. Lin 1,2 1 Department of Physics, University of California, Berkeley, CA 94720 2 Space Sciences Laboratory, University of California, Berkeley, CA 94720

2. June 3, 2004AAS/SPD Meeting2 Motivation

3. June 3, 2004AAS/SPD Meeting3 Questions Is the flare isothermal? –If not, what is the temperature distribution? –Where are thermal sources located? What is the low-energy cutoff for the nonthermal emission? –Critical for energy estimates  Want to determine flare energetics, heating and acceleration mechanisms

4. June 3, 2004AAS/SPD Meeting4 Fe & Fe/Ni line complexes Line(s) are visible in almost all RHESSI flare spectra Fluxes and equivalent width of lines are strongly temperature-dependent (Phillips 2004)

5. June 3, 2004AAS/SPD Meeting5 Fe & Fe/Ni line complexes Differing temperature profiles of line complexes suggests ratio is unique determination of isothermal temperature (Phillips 2004)

6. June 3, 2004AAS/SPD Meeting6 Fe & Fe/Ni line complexes Assume isothermal –Not necessarily the best fit! Single power law with low-energy cutoff 2 Gaussians to approximate Fe & Fe/Ni line complexes

7. June 3, 2004AAS/SPD Meeting7 Flux ratio vs. Temperature

8. June 3, 2004AAS/SPD Meeting8 Flux ratio vs. Temperature

9. June 3, 2004AAS/SPD Meeting9 Flux ratio vs. Temperature

10. June 3, 2004AAS/SPD Meeting10 Flux ratio vs. Temperature

11. June 3, 2004AAS/SPD Meeting11 Flux ratio vs. Temperature

12. June 3, 2004AAS/SPD Meeting12 More questions Well-defined correlation between Fe to Fe/Ni ratio and isothermal temperature for each flare, but… No agreement between observations and theory No agreement between individual flares! Why are observations so far from the theory? Why are the curves different between flares?

13. June 3, 2004AAS/SPD Meeting13 Possible answer Multi-thermal distribution –Differs between flares Imaging spectroscopy would be ideal –Obtain spectra based on source location Isolate and analyze multiple thermal plasmas at different temperatures within each flare Distinguish between thermal and non-thermal sources

14. June 3, 2004AAS/SPD Meeting14 Centroids of emission Clear displacement between centroids of lower energy and higher energy emission

15. June 3, 2004AAS/SPD Meeting15 Centroids of emission Higher energy emission from higher in the looptop –Strongly implies multi-thermal distribution Centroid of Fe line complex emission consistent with high- EM, lower-T plasma lower in looptop

16. June 3, 2004AAS/SPD Meeting16 Conclusions Observations do not agree with predictions –Multi-thermal distribution –Other variations Ongoing Work Obtain DEM to determine temperature distributions Imaging spectroscopy for spatially-separated sources, to separate thermal sources at different temperatures, and to distinguish between thermal and non-thermal sources  Determine flare energetics

17. June 3, 2004AAS/SPD Meeting17 Count Spectrum

18. June 3, 2004AAS/SPD Meeting18 Flux ratio vs. Temperature

19. June 3, 2004AAS/SPD Meeting19 Flux ratio vs. Temperature

20. June 3, 2004AAS/SPD Meeting20 Flare location/size