A ug 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”. The revisit is motivated not only by its supposed uniqueness despite accumulating observations of hard X-ray coronal emission, but also by the improvement of Yohkoh hard X-ray imaging, which was achieved after the intensive investigations on this celebrated event. We notice that the separation between the L-band (14–23 keV) loop top and the M1-band (23–33 keV) coronal source, in terms of centroid positions, has been significantly reduced in the calibrated Hard X-ray Telescope (HXT) images. This naturally accounts for the fact that the spectrum of the coronal emission was reported to be extremely hard below ∼20 keV in the pre-calibration investigations, whereas it has been considerably softer in the literature since Sato’s calibration circa 1999. In our re-analysis with both the MEM and Pixon algorithms, neither unique features of the original Masuda flare is clearly confirmed. Instead, (a) the coronal source is located only slightly above the soft X-ray loop top, enclosed by the L-band thermal loop defined by the 10% brightness contour, rather than distinctly above both loops as in the original HXT images; and (b) its spectral indices, softer than those of corresponding footpoints, accordingly fall into the normal range of impulsive-phase coronal emission. Despite the generally accepted belief that the Masuda coronal emission was nonthermal, we demonstrate that its spectrum can be reasonably 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 also discuss the long-standing difficulties in understanding the original Masuda flare, which have been more or less relieved with the calibrated data. Subject headings: Sun: flares—Sun: X-rays, gamma rays—Sun: Corona