Super-Hot Thermal Plasmas in Solar Flares

1. Super-Hot Thermal Plasmasin Solar Flares Amir Caspi Research advisor: R.P. Lin

2. Why study solar flares? • The most powerful explosions in the solar system - energies of up to 109-1010 H-bombs! • Provide a “local” laboratory to explore the physics that govern other astrophysical phenomena (stellar flares, accretion disks, etc.) • Allow us to explore plasma physics in regimes not (easily) re-creatable in the lab

3. “Typical” flare characteristics • Durations of 100-1000 seconds • Electrons and ions accelerated up to 100s of MeV and 10s of GeV (respectively) • Plasma temperatures up to 10-50 MK • Densities of ~1010 to ~1012 cm-3 • Energy content up to ~1032-1033 ergs • Generally, loop structure with thermal emission from the looptop, non-thermal emission from footpoints

4. Open questions • Evolution of the thermal plasma • What are the dominant heating mechanisms, especially for super-hot (T > 30 MK) plasmas? • Where does heating occur? • Is there a fundamental limit on the plasma temperature? • What is the relationship between the thermal plasma and accelerated particles? • Energetics • How much energy contained in thermal electrons? • Compared to the energy in accelerated electrons (and ions)?

5. Basic flare model

6. Basic flare model

7. X-ray emission mechanisms • Electron bremsstrahlung (free-free continuum emission) • Radiative recombination (free-bound continuum emission) • Electron excitation & decay (bound-bound line emission)

8. Free-free (bremsstrahlung) • Thermal: Maxwellian electron distribution yields • Nonthermal: “injected” electron spectrum yields

9. Free-bound & bound-bound • Free-bound continuum: free (thermal) electrons recombine and emit a photon of energy • Bound-bound lines: bound electron excited (primarily through collisions with ambient free electrons) and de-excites via emission of a photon of energy • Line profile (peak energy, FWHM, amplitude, shape) depends on T, v, n • In X-rays, primary solar lines are from ions of O, Si, Ca, Fe, and Ni

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

11. X-ray Flare Phases • Impulsive (rise) phase - bursty HXR, fast but smoothly rising SXR • Gradual (decay) phase - little to no HXR, gradual decline in SXR • Pre-impulsive gradual rise observed in some flares

12. Early X-Ray Observations • Balloon, rocket, satellite • Broadband spectrometers • Bragg crystal (narrowband) spectrometers • Broadband imagers • Instrumental limitations • BBS: coarse energy resolution allowed interpretation of HXR spectra as thermal w/ T > 100 MK • BCS: lines suggested T ~ 20 MK • No “complete” picture of flare emission (Crannell et al. 1978)

13. X-Ray Observations: TNG • Germanium detectors: much higher broadband spectral resolution • Allow more accurate ID of thermal vs. non-thermal emission • First results • HXR emission likely non-thermal • Emission from “super-hot” (T > 30 MK) thermal component • RHESSI offers the first “complete” picture of flare emission: SXR/HXR continuum and line emission, plus imaging in arbitrary energy bands (Lin et al. 1981)

15. RHESSI Spectra and Imaging

16. Benefits of RHESSI • Good spectral resolution - can distinguish between thermal/non-thermal emission • Good temporal resolution - can observe evolution of spectra over short times • Good angular resolution - can distinguish spatially-separate sources (and do spectroscopy) • First broadband instrument with simultaneous spectral and imaging observations of continuum (thermal + nonthermal) and lines • Now have multiple measurements of thermal emission

17. 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)

18. Fe & Fe/Ni line complexes • Differing temperature profiles of line complexes suggests ratio is unique determination of isothermal temperature (Phillips 2004) • Only weakly dependent on abundances

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

20. Analytical method • Fit spectra with isothermal continuum, 3 Gaussians, and power law • Calculate temperature from fit line ratio; may also calculate emission measure & equiv. widths from line fluxes • Compare to continuum temperature

21. Two flares: 23/Jul/02 & 02/Nov/03

22. Flux ratio vs. Temperature

23. Flux ratio vs. Temperature

24. Flux ratio vs. Temperature

25. Fe Equivalent Width vs. Temperature • Method of Phillips et al. (2005) • Defined as integrated line flux divided by continuum flux (at peak energy) • Compared to predictions, trend is opposite from ratio temperatures • Not independent of abundances

26. Flux ratio vs. Temperature

27. 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

28. 24 Aug 2002: Pre-impulsive phase

29. Flux ratio vs. Temperature

30. Flux ratio vs. Temperature

31. Flux Ratio vs. Temperature … small contribution • Possible explanations: • Instrumental effects and coupled errors in multi-parameter fits • Ionization non-equilibrium • Incorrect assumptions about ionization fractions • Multi-thermal temperature distribution … unlikely … possible … needs further investigation

32. Emissivity vs. Temperature

33. Emissivity vs. Temperature

34. Emissivity vs. Temperature … small contribution • Possible explanations: • Instrumental effects and coupled errors in multi-parameter fits • Ionization non-equilibrium • Multi-thermal temperature distribution • Incorrect assumptions about ionization fractions • Line excitation by non-thermal electrons • Abundance variations during the flare … unlikely … needs further investigation

35. Conclusions • Fe & Fe/Ni line complexes provide a probe of the thermal plasma in addition to continuum emission • Help constrain fits to thermal continuum • Provide thermal information even when continuum is difficult to analyze • Not all flares exhibit the same line/continuum relationship • May suggest different temperature distributions • Other differences (e.g. spectral hardness) may contribute • Ratio & equivalent width results are not self-consistent • Suggests theoretical predications may need corrections • Assumptions about ionization fractions may be incorrect

36. Future Work • Statistical survey of Fe & Fe/Ni emission in M/X flares • Differential Emission Measure (DEM) analysis • Determine effects of multi-temperature distribution on relationship between line ratio and isothermal approx. • Use line emission to constrain DEM models • Imaging Spectroscopy • Obtain and analyze spectra for spatially-separated sources (e.g. footpoints and looptop) • Isolate presumed thermal and non-thermal sources to determine individual thermal/non-thermal properties • Place limits on the extent of non-thermal excitation of the lines

37. EXTRA SLIDES

38. Basic flare model (cartoon and data) (Aschwanden & Benz 1997)

39. (Krucker)

40. RHESSI Spectra and Imaging

41. Flux ratio vs. Temperature

42. Flux ratio vs. Temperature

43. Flux ratio vs. Temperature

44. Flux ratio vs. Temperature

45. Flux ratio vs. Temperature

46. Emissivity vs. Temperature

47. Emissivity vs. Temperature

48. Emissivity vs. Temperature

49. Emissivity vs. Temperature

50. Emissivity vs. Temperature