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A Test of Thick-Target Nonuniform Ionization as an Explanation for Breaks in Solar Flare Hard X-ray Spectra View all abstracts by submitter

Yang Su   Submitted: 2009-09-24 13:21

Solar nonthermal hard X-ray (HXR) flare spectra often cannot be fitted by a single power law, but rather require a downward break in the photon spectrum. 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 bremsstrahlung cross-section from Haug (1997), which closely approximates the full relativistic Bethe-Heitler 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 approximation to the cross-section. We find that for HXR spectra from a target with nonuniform ionization, the difference (Deltagamma) between the power-law indexes above and below the break has an upper limit between ~0.2 and 0.7 that depends on the power-law index delta of the injected electron distribution. A broken power-law spectrum with a higher value of Deltagamma 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 Deltagamma. We find that 15 of the 20 flare spectra require a downward spectral break and that at least 6 of these could not be explained by nonuniform ionization alone because they had values of Deltagamma with less than a 2.5% probability of being consistent with the computed upper limits from the model. The remaining 9 flare spectra, based on this criterion, are consistent with the nonuniform ionization model.

Authors: Yang Su, Gordon D. Holman, Brian R. Dennis, Anne K. Tolbert, Richard A. Schwartz
Projects: RHESSI

Publication Status: ApJ, accepted
Last Modified: 2009-09-25 08:08
Go to main E-Print page  Magnetic Reconnection along Quasi-Separatrix Layers as a Driver of Ubiquitous Active Region Outflows  High Spectral Resolution Observation of Decimetric Radio Spikes Emitted by Solar Flares - First Results of the Phoenix-3 Spectrometer  Edit Entry  Download Preprint  Delete Entry 

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