On the Size of Structures in the Solar Corona

Fine-scale structure in the corona appears not to be well resolved by current imaging instruments. Assuming this to be true offers a simple geometric explanation for several current puzzles in coronal physics, including the apparent uniform cross section of bright threadlike structures in the corona, the low EUV contrast ( long apparent scale height) between the top and bottom of active region loops, and the inconsistency between loop densities derived by spectral and photometric means. Treating coronal loops as a mixture of diffuse background and very dense, unresolved filamentary structures addresses these problems with a combination of high plasma density within the structures, which greatly increases the emissivity of the structures, and geometric effects that attenuate the apparent brightness of the feature at low altitudes. It also suggests a possible explanation for both the surprisingly high contrast of EUV coronal loops against the coronal background, and the uniform ‘‘typical’’ height of the bright portion of the corona (about 0.3R ) in full-disk EUVimages. Some ramifications of this picture are discussed, including an estimate (10Y100 km) of the fundamental scale of strong heating events in the corona. Subject headinggs: Sun: corona