FASR Presentation

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Interpreting RHESSI solar flare spectra produced
by non-thermal electron bremsstrahlung. Pascal
Saint-Hilaire ETHZ/PSI PSI, September 18th, 2003
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Deducing injected electron spectrum F0(E0) e-
s-1 keV-1 from photon spectrum ?(?) ph s-1
cm-2 keV-1 observed at Earth
D 1 A.U. QB bremsstrahlung differential
cross-section QC Coulomb energy-loss
cross-section
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If using
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?4
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Non-thermal energies
Typ. 1028-1032 ergs, above 20 keV
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(2002/05/17)
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Effect that harden the photon spectrum
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Non-uniform ionization (E25 keV, approx. 1020
cm-2) (Brown, 1972 Brown et al., 1998 Kontar,
Brown McArthur, 2003)
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Albedo effect (photospheric Compton
back-scattering) (Bai Ramaty, 1978 R. C.
Alexander Brown, 2003)
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Low-energy cutoff and turnover (20 keV)
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Effects that soften the photon spectrum
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Applying a high-energy cutoff (50 keV) to the
electron spectrum
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Applying a high-energy cutoff (300 keV) to the
electron spectrum
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Hot target (e.g. 3 keV) (Emslie, 2003)
As E nears kT, QC decreases
Actually, non-thermal energies may now be
computed without ECO! (Emslie, 2003)
low-energy cutoff problem may be circumvented
or only displaced to determining a superhot kBT
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Using the quantum relativistic Bethe-Bloch
formula for collisional energy losses
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Brems. cross-section (Haug,1997)
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Conclusions

• Biggest error source on non-thermal energies is
  the
• low-energy cutoff/turnover... It is apparently
  lost in the
• thermal part of the spectrum.

- Why should ECO/TO always be lt kT ?

• RHESSI imaging spectroscopy could answer
• that question...

- Some effects can be corrected for
(cross-sections)

• For some others, preliminary first order
  corrections
• are already available (albedo).

-For the rest (non-uniform ionization, hot
target, cutoffs, etc), forward fitting method
are implemented (or will be), but can be very
computationally intensive!