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Three Criteria to Discriminate between Elementary and Composite Coronal Loops

In this Letter, we establish three simple criteria to discriminate between elementary and composite loop structures, based on observations from high-resolution (TRACE) and low-resolution (EIT) imagers. Both data sets contain triple-filter data with a temperature diagnostic in the range of T ≈ 0.7-2.8 MK, which we use for forward-fitting of a differential emission measure (DEM) distribution to constrain the temperature range of each structure. From the TRACE data set, we find for the finest structures a mean width of w = 1.4 ± 0.3 Mm, a flux-to-background contrast of c ≈ 0.18 ± 0.13, and a temperature range of dT = 0.07 ± 0.10 MK, which we identify as elementary strands, defined in terms of their thermal homogeneity. The loop structures observed with EIT have loop widths w, flux contrasts c, and temperature ranges dT that are all about an order of magnitude larger and thus clearly constitute composite structures, consisting of many loop strands. These two contrasting observations resolve previous controversies about the basic thermal structure of coronal loops and yield a simple discrimination rule: elementary loop strands (1) are near-isothermal (dT ≲ 0.2 MK), (2) have a small width (w ≲ 2 Mm), and (3) have a faint contrast (c ≲ 0.3), while virtually all wider and higher contrast loop features are most likely multithermal composites and have a broad DEM.