Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images

摘要

Non-linear force-free fields are the most general case of force-free fields,but the hardest to model as well. There are numerous methods of computing suchfields by extrapolating vector magnetograms from the photosphere, but very fewattempts have so far made quantitative use of coronal morphology. We present amethod to make such quantitative use of X-Ray and EUV images of coronal loops.Each individual loop is fit to a field line of a linear force-free field,allowing the estimation of the field line's twist, three-dimensional geometryand the field strength along it. We assess the validity of such a reconstruction since the actual corona isprobably not a linear force-free field and that the superposition of linearforce-free fields is generally not itself a force-free field. To do so, weperform a series of tests on non-linear force-free fields, described in Low &Lou (1990). For model loops we project field lines onto the photosphere. Wecompare several results of the method with the original field, in particularthe three-dimensional loop shapes, local twist (coronal alpha), distribution oftwist in the model photosphere and strength of the magnetic field. We findthat, (i) for these trial fields, the method reconstructs twist with meanabsolute deviation of at most 15% of the range of photospheric twist, (ii) thatheights of the loops are reconstructed with mean absolute deviation of at most5% of the range of trial heights and (iii) that the magnitude of non-potentialcontribution to photospheric field is reconstructed with mean absolutedeviation of at most 10% of the maximal value.