Saturation of Nonthermal Hard X-ray Emission in Solar Flares

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Number: 72
1st Author: Antoun Daou
2nd Author: David Alexander
Published: 24 January 2008
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Can the Sun be arbitrarily bright as a hard X-ray source? This question would have baffled astronomers even in the 20th century, but it seems important to us now because of the central role hard X-ray emission plays in solar flares. This Nugget discusses observations that suggest the saturation of the emergent hard X-ray intensity in large solar flares. Using the unprecedented spectral and spatial resolution of RHESSI, the integrated photon flux above 20 keV is observed to approach a limiting value with increasing flare intensity. We interpret this result by considering each analyzed large flare to be an aggregation of multiple fiducial energy release events identified in high-resolution RHESSI hard X-ray images.

Flare Selection and Analysis

We analyzed a range of flares spanning three orders of magnitude, from GOES class M1.8 to X17, to investigate how the nonthermal photon intensity varies with event magnitude, and how this relates to the energy-loss processes acting in the solar atmosphere. The selected flares (see the table below) show well-defined image substructures in the hard X-ray images. The analysis time intervals are concentrated around the peak of hard X-ray emission, ie at the time one would expect the most intense energy release. The excellent photometry of the Pixon image-reconstruction algorithm allows us to generate spectrally selected images for each flare covering 6-100 keV, with energy binning as good as 2 keV.

List of Analyzed Flares with Spectral Properties

After identifying the different independent image substructures for the flares, photon counts as well as effective areas are read and processed for every identified feature. The image-resolved spectra thus obtained are fitted and integrated to compute the total thermal photon flux above 3 keV and the total nonthermal photon flux above 20 keV. The obtained total thermal photon flux rises linearly (see Figure 1) with the total thermal energy, as expected. However, the presence of a plateau in the total nonthermal photon flux is evidence for saturation in nonthermal photon production in large flares.

Total thermal (diamonds) and nonthermal (asterisks) photon fluxes for the analyzed substructures plotted against total thermal energy. Horizontal dashed line represents a commonly derived numerical saturation limit.

Figure 1: Total thermal (diamonds) and nonthermal (asterisks) photon fluxes for the analyzed image substructures plotted against total thermal energy. The horizontal dashed line represents the suggested saturation limit.

Spectral Results

The image analysis suggests that above a certain limit, Ohmic dissipation dominates the energy loss mechanism in solar flares, and Coulomb collisional losses can be neglected. This would be due to the fact that the beam propagating through the background plasma causes a charge displacement, hence an electric field which will in turn drive a return current (see an earlier Nugget for more details about this). The return current is likely to be narrowly cospatial with the injected electron beam. The effect of this return current is the observed nonthermal photon flux saturation at a level determined by the accelerated electron flux, and the ambient conditions of the flaring solar atmosphere.

For more information, see: Saturation of Nonthermal Hard X-ray Emission in Solar Flares - Alexander D., & Daou A.G., 2007, ApJ 666:1268.

Biographical note: David Alexander (professor) and Antoun Daou (graduate student) are at Rice University (Houston, Texas). This Nugget is based on a published paper.

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