Measuring the Temperature of Hot Solar Flare Plasma with RHESSI

1
Measuring the Temperature of Hot Solar Flare
Plasma with RHESSI

• Amir Caspi1,2, Sam Krucker2, Robert P. Lin1,2
• 1 Department of Physics, University of
  California, Berkeley, CA 94720
• 2 Space Sciences Laboratory, University of
  California, Berkeley, CA 94720

2
Typical flare characteristics

• Durations of 100-1000 seconds
• Plasma temperatures on order of a few times 107
  degrees
• Densities of 1010 to 1012 cm-3
• Energy content of 1032-1033 ergs
• Generally, loop structure with thermal emission
  from the looptop, non-thermal emission from
  footpoints

3
Basic flare model (cartoon and data)
(Tsuneta 1997)
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
SXR flare emission

• Electron bremsstrahlung (free-free continuum
  emission)
• Radiative recombination (free-bound continuum
  emission)
• Electron excitation decay (bound-bound line
  emission)

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 gt100MK
  plasma

(Crannell et al. 1978)
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
(No Transcript)
9
RHESSI - Instrument
10
RHESSI Spectra and Imaging
11
RHESSI Spectra and Imaging
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
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
Fe Fe/Ni line complexes

• Differing temperature profiles of line complexes
  suggests ratio is unique determination of
  isothermal temperature (Phillips 2004)

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
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
Flux ratio vs. Temperature
18
Flux ratio vs. Temperature
19
Flux ratio vs. Temperature
20
Flux ratio vs. Temperature
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
Emissivity vs. Temperature
23
Emissivity vs. Temperature
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
Flux ratio vs. Temperature
26
Flux ratio vs. Temperature
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
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