Elementary loop structures in the solar corona analyzed from TRACE triple-filter images

This study represents the first quantitative analysis of the multithread structure of coronal loops. We analyzed a set of 234 fine loop threads observed with TRACE triple-filter images at wavelengths of 171, 195, and 284 angstrom. The cross-sectional flux profiles are simultaneously forward fitted in all three filters with the superposition of a cospatial Gaussian and linear background functions. We fit a general multitemperature differential emission measure (DEM) distribution to each cross section, aswell as the special case of a single-temperature or isothermal DEM. We perform these forward fits at approximate to 18,000 loop positions and find that this geometric model could be fitted in approximate to 3500 cases (within a chi red <= 1.5), while all other cases require more complex geometric models of the loop cross section, secondary loops, and background. The major result of this study is that the vast majority (84%) of the acceptable DEM fits are isothermal. Temperatures are measured over the whole sensitivity range of 0.7 - 2.8 MK, but with a higher probability near the peak sensitivities of the three filters. We conclude that we indeed resolve "elementary" or "monolithic" loop strands with TRACE, in terms of isothermal homogeneity. Virtually all earlier studies detected ensembles of multiple strands, while our detected loop strands exhibit much smaller widths (w approximate to 1: 4 +/- 0.3 Mm) and also smaller signal-to-background ratios (14% +/- 10%). We suggest that the temperature homogeneity of coronal loops up to widths of w less than or similar to 2000 km is related to their magnetic mapping to photospheric granulation (convection) cells.