Physics
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009spd....40.2010s&link_type=abstract
American Astronomical Society, SPD meeting #40, #20.10; Bulletin of the American Astronomical Society, Vol. 41, p.854
Physics
Scientific paper
Solar hard X-ray (HXR) flare spectra often cannot be fitted by a single power-law, but rather require a downward break at energies between 50 to 100 keV. A possible explanation for this spectral break is nonuniform ionization in the emission region.
We have developed a computer code to calculate the photon spectrum from electrons with a power-law distribution injected into a thick-target in which the ionization decreases linearly from 100% to zero. We use the relativistic Bethe-Heitler bremsstrahlung cross-section, and compare photon spectra computed from this model with those obtained by Kontar, Brown and McArthur (2002), who used a step-function ionization model and the Kramers cross-section. We find that for spectra from a target with nonuniform ionization, the difference (Δγ) between the power-law indexes above and below the break has an upper limit that depends on the power-law index δ of the electron distribution. A broken power-law spectrum with a higher value of Δγ cannot result from nonuniform ionization alone.
The model is applied to spectra obtained around the peak times of 20 flares observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) from 2002 to 2004 to determine whether thick-target nonuniform ionization can explain the measured spectral breaks. A Monte Carlo method is used to determine the uncertainties of the best fit parameters, especially on Δγ. We find that 15 of the 20 flare spectra required a downward spectral break and that at least 6 of these could not be explained by nonuniform ionization alone because they had values of Δγ that were at least 2σ greater than the computed upper limits from the model.
Y. Su acknowledges NSF support through ATM-0725135. This work was supported in part by the RHESSI Project and a NASA HGI (Heliophysics Guest Investigator) Grant.
Dennis Brian R.
Holman Gordon D.
Schwartz Richard A.
Su Yang
Tolbert Kimberley A.
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