Coronal scattering as a source of flare-associated polarized hard X-rays H. S. Hudson, G. J. Hurford Space Sciences Laboratory, University of California, Berkeley, CA 94720 J. C. Brown Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, U.K. Abstract. We consider the scattering of flare-associated X-rays above 1 keV at coronal heights, particularly from regions of enhanced density. This includes a dis- cussion of the polarization of the scattered X-rays. Although the scattered radiation would not be bright by comparison with the total hard X-ray flux from a flare, its detectability would be enhanced for events located a few degrees behind the limb for which the dominant “footpoint” hard X-ray sources are occulted. Thus we predict that major flares occurring beyond the solar limb may be detectable via scattering in density enhancements that happen to be visible above the limb, and that such sources may be strongly polarized. Since thin-target bremsstrahlung will generally greatly exceed the scattered thick-target flux in flare loops themselves, these considerations apply only to coronal structures that do not contain significant populations of non-thermal electrons. Keywords: Corona, flares, X-rays, polarization 1. Introduction The possibility of observing polarized hard X-ray emission from solar flares has long tempted astrophysicists (Korchak, 1967; Elwert and Haug, 1970; H´ enoux, 1975; Bai and Ramaty, 1978). The motivation is that the polarization signature would provide information about the beaming of non-thermal electrons accelerated in the corona during a flare (Brown, 1972). These 10-100 keV electrons, which may carry a dominant fraction of flare energy (Neupert, 1968; Kane and Donnelly, 1971), interact in the chromosphere at the “footpoints” of the coronal magnetic flux tubes in which the acceleration takes place. This sig- nature would give direct knowledge of the pitch-angle distribution of the electrons. A complicating factor would come from the polarized “albedo patch” resulting from the Compton backscatter of a portion of the primary hard X-radiation (Brown et al., 1975; H´ enoux, 1975). In this paper we point out that polarization will also arise by scat- tering in the corona above the footpoint sources, especially if there are localized density enhancements. Structures such as filaments, flare loops themselves, flare ejecta, spicules, or any ambient coronal material c  2002 Kluwer Academic Publishers. Printed in the Netherlands. pol.tex; 29/12/2002; 12:21; p.1