1. Super-Hot Thermal Plasmas in 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 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 seconds
Electrons and ions accelerated up to 100s of MeV and 10s of GeV (respectively)
Plasma temperatures up to MK
Densities of ~1010 to ~1012 cm-3
Energy content up to ~ ergs
Generally, loop structure with thermal emission from the looptop, non-thermal emission from footpoints

4. Open questions Evolution of the thermal plasma Energetics
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
Possible explanations:
Instrumental effects and coupled errors in multi-parameter fits
Ionization non-equilibrium
Incorrect assumptions about ionization fractions
Multi-thermal temperature distribution
… small contribution
… unlikely
… possible
… needs further investigation

32. Emissivity vs. Temperature

33. Emissivity vs. Temperature

34. Emissivity vs. Temperature
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
… small contribution
… 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

51. Emissivity vs. Temperature

52. Emissivity vs. Temperature

53. Flare location/size

54. 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