2 More on the determination of the coronal heating function from Yohkoh data

Two recent works have analyzed a solar large and steady coronal loop observed with Yohkoh/SXT in two filter passbands to infer the distribution of the heating along it. Priest et al. (2000) modelled the distribution of the temperature obtained from filter ratio method with an analytical approach, and concluded that the heating was uniform along the loop. Aschwanden (2001) found that a uniform heating led to an unreasonably large plasma column depth along the line of sight, and, using a two component loop model, that a footpoint-heated model loop (with a minor cool component) yields more acceptable physical solutions. We revisit the analysis of the same loop system, considering conventional hydro-static single loop models with uniformly distributed heating, and with heating localized at the footpoints and at the apex, and an unstructured background contribution extrapolated from the region below the analyzed loop. The flux profiles synthesized from the loop models have been compared in detail with those observed in both filter passbands with and without background subtraction; we find that background-subtracted data are fitted with acceptable statistical significance by a model of relatively hot loop (∼ 3.7 MK) heated at the apex, with a column depth ∼ 1/10 of the loop length. In discussing our results, we put warnings on the importance of aspects of data analysis and modeling, such as considering diffuse background emission in complex loop regions.