Implications of solar flare hard X-ray "knee" spectra observed by RHESSI

Conway, A.J., Brown, J.C., Eves, B.A.C. and Kontar, E.P. (2003) Implications of solar flare hard X-ray "knee" spectra observed by RHESSI. Astronomy and Astrophysics, 407(2), pp. 725-734. (doi: 10.1051/0004-6361:20030897)

Abstract

We analyse the RHESSI photon spectra of four flares that exhibit significant deviations from power laws - i.e. changes in the "local" Hard X-ray spectral index. These spectra are characterised by two regions of constant power law index connected by a region of changing spectral index - the "knee". We develop theoretical and numerical methods of describing such knees in terms of variable photon spectral indices and we study the results of their inversions for source mean thin target and collisional thick target injection electron spectra. We show that a particularly sharp knee can produce unphysical negative values in the electron spectra, and we derive inequalities that can be used to test for this without the need for an inversion to be performed. Such unphysical features would indicate that source model assumptions were being violated, particularly strongly for the collisional thick target model which assumes a specific form for electron energy loss. For all four flares considered here we find that the knees do not correspond to unphysical electron spectra. In the three flares that have downward knees we conclude that the knee can be explained in terms of transport effects through a region of non-uniform ionisation. In the other flare, which has an upward knee, we conclude that it is most likely a feature of the accelerated spectrum.

• Item Type: Articles
• Keywords: Sun: X-rays, gamma-rays, Sun: flares.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Kontar, Professor Eduard and Brown, Professor John
• Authors: Conway, A.J., Brown, J.C., Eves, B.A.C., and Kontar, E.P.
• Subjects: Q Science > QB Astronomy; Q Science > QC Physics
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Astronomy and Astrophysics
• Publisher: EDP Sciences
• ISSN: 0004-6361
• Copyright Holders: Copyright © 2007 EDP Sciences
• First Published: First published in Astronomy and Astrophysics 407:725-734
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher