1. Measuring the Temperature of Hot Solar Flare Plasma with RHESSI Amir Caspi 1,2, Sam 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. 2 “Typical” flare characteristics Durations of 100-1000 seconds Plasma temperatures on order of a few times 10 7 degrees Densities of ~10 10 to ~10 12 cm -3 Energy content of ~10 32 -10 33 ergs Generally, loop structure with thermal emission from the looptop, non-thermal emission from footpoints

3. 3 Basic flare model (cartoon and data) (Tsuneta 1997)

4. 4 X-ray Flare Classification Photometers on board the GOES satellites monitor solar soft X-rays GOES class is determined by peak flux in the 1-8A channel Rough correlation between GOES class and temperature, energy

5. 5 SXR flare emission Electron bremsstrahlung (free-free continuum emission) Radiative recombination (free-bound continuum emission) Electron excitation & decay (bound- bound line emission)

6. 6 First X-Ray Observations Balloon and rocket flights 1959-1962 Orbiting Solar Observatory satellites Skylab Hinotori Solar Maximum Mission Yohkoh Poor energy resolution caused high uncertainties in interpretation of spectra –Initial fits interpreted HXR spectra as >100MK plasma (Crannell et al. 1978)

7. 7 Later X-Ray Observations Germanium detectors offered much higher spectral resolution –Allowed more accurate identification of thermal and non-thermal emission –Early balloon flight showed that HXR emission was most likely non- thermal, but plasma temperatures were still fairly high RHESSI offers the best spectral resolution in its energy range to date –Sensitive down to ~3 keV –~1 keV FWHM (Lin et al. 1981)

8. 8

9. 9 RHESSI - Instrument

10. 10 RHESSI Spectra and Imaging

11. 11 RHESSI Spectra and Imaging

12. 12 Open questions Evolution of the thermal plasma –What are the dominant heating and cooling mechanisms? –Is the looptop source primarily thermal? Non-thermal electrons –What happens at low energies (e.g. turnover, cutoff, etc.)? Energy content (thermal and non-thermal)  We can use X-ray spectral lines in addition to the continuum

13. 13 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)

14. 14 Fe & Fe/Ni line complexes Differing temperature profiles of line complexes suggests ratio is unique determination of isothermal temperature (Phillips 2004)

15. 15 Fe & Fe/Ni line complexes Lines are cospatial with the thermal source No appreciable emission from footpoints  The lines are a probe of the same thermal plasma that generates the continuum We can directly compare the continuum temperature to the line- ratio temperature

16. 16 Analytical method Fit spectra with thermal continuum, 3 Gaussians, and power law Calculate temperature from fit line ratio; may also calculate emission measure and equivalent widths from absolute line fluxes Compare to continuum temperature

17. 17 Flux ratio vs. Temperature

18. 18 Flux ratio vs. Temperature

19. 19 Flux ratio vs. Temperature

20. 20 Flux ratio vs. Temperature

21. 21 23 July 2002: Pre-impulsive phase Fit equally well with or without thermal continuum! –Iron lines indicate thermal plasma must be present, but much cooler than continuum fit implies

22. 22 Emissivity vs. Temperature

23. 23 Emissivity vs. Temperature

24. 24 Emissivity vs. Temperature Possible explanations: –Ionization lag –Low-temperature plasma w/o significant line emission –Multi-thermal temperature distribution –Instrumental effects and coupled errors in multi-parameter fits –Excitation by non-thermal electrons –Incorrect assumptions about abundances and/or ionization fractions –Abundance variations during the flare … small contribution

25. 25 Flux ratio vs. Temperature

26. 26 Flux ratio vs. Temperature

27. 27 Conclusions Fe & Fe/Ni features provide another measure of thermal plasma besides continuum emission –Help reject improper fits to thermal continuum –Provide thermal information even when continuum is difficult to analyze Line/continuum relationship appears to change during flare –Suggests theory may need corrections –Initial assumptions about abundances and/or ionization fractions may be incorrect Not all flares exhibit the same line/continuum relationship –Suggests different temperature distributions –Other differences (spectral hardness, abundances) may contribute

28. 28 Future Work Better instrumental calibration and modeling Differential Emission Measure (DEM) analysis –Determine the effects of a multi-temperature distribution on the relationship between the line ratio and the continuum temperature Imaging Spectroscopy –Obtain and analyze spectra for spatially-separated sources (e.g. footpoints and looptop) –Allows us to isolate presumed thermal and non-thermal sources to determine how thermal or non-thermal they are –Place limits on the extent of non-thermal excitation of the lines