2 7 M ay 2 00 9 A Revisit of the Masuda Flare

We revisit the flare on 1992 January 13, which is now universally termed the “Masuda flare”. Our work is motivated by the remarkable improvement of Yohkoh hard X-ray imaging, which was only achieved after the intensive investigations on this famous event. With the calibrated Hard X-ray Telescope (HXT) data, we find that the hard X-ray coronal source is indeed a loop-top source, located slightly above the soft X-ray loop but within the 10% brightness contour of the HXT L-band thermal loop, rather than distinctly above the loop top as in the original HXT images. This critical change relieves the three major difficulties of understanding the original Masuda flare, viz., (1) the lack of L-band emission at the coronal source location and consequently the extremely flat spectrum of the coronal source below ∼20 keV, (2) the lack of soft X-ray signature of hydrodynamic response at the presumed field lines that connected the hard X-ray coronal source to the chromosphere, and (3) the apparently low density at the location of the coronal source due to the lack of soft X-ray emission. Despite the generally accepted belief that the coronal emission in the Masuda flare was nonthermal, we find its spectrum can be explained either by thermal bremsstrahlung of a superhot plasma (80 MK), or by nonthermal bremsstrahlung at a thick-thin target, with the injecting electron index derived from the coronal emission approximately equal to that from the footpoints. We propose a simplified picture, featuring the field line shrinkage effect and the resultant increase of the loss-cone angle at the coronal trap, to qualitatively explain that as time progresses the hard X-ray coronal source diminishes in brightness relative to the footpoints, that its centroid position moves to higher altitudes, and that the footpoint spectrum is successively hardening, with the coronal spectrum softening simultaneously. Subject headings: Sun: flares—Sun: X-rays, gamma rays—Sun: Corona