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Evidence for the Full Hard X-ray Spectral Signature of Nonuniform Ionization in a Solar Flare View all abstracts by submitter

Yang Su   Submitted: 2011-03-21 13:22

The hard X-ray (HXR) emission from solar flares is observed primarily from the footpoints of flare magnetic loops, where nonthermal electrons are understood to emit thick-target bremsstrahlung as they stream from the fully-ionized hot corona to the denser, cooler, and partially ionized chromosphere. The change in the plasma ionization along the path of the electrons should result in a characteristic upward break and corresponding flattening of the X-ray spectrum with increasing energy at lower energies, and a downward break at higher energies. Due to the presence of thermal emission, the upward break usually cannot be observed. We report the first evidence for both breaks in spectra measured with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the GOES X1.2 class flare that happened on 31 October 2002. The RHESSI X-ray spectral analysis shows both the break-up at ~49 keV and the break-down at ~134 keV at the HXR peak time. The time evolution of both breaks also agrees with the nonuniform ionization model. Other possible explanations for the breaks are considered, but the nonuniform ionization model (NUI) provides the simplest explanation for the spectral shape and its time evolution. We find that the average column density of the fully ionized plasma changed from 2x10<sup>19</sup> cm<sup-2</sup> in the rise phase to 7x10<sup>21</sup> cm<sup>-2</sup> after the peak. This indicates that plasma in the target was heated and became ionized during the flare, in agreement with heating by the nonthermal electrons and chromospheric evaporation expected in the collisional thick-target model.

Authors: Su, Y., Holman, G. D., and Dennis, B. R.
Projects: RHESSI

Publication Status: Published in ApJ, 731:106, 2011
Last Modified: 2011-03-31 09:10
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