A Brightening Coronal Loop Observed by TRACE. I. Morphology and Evolution

摘要

We analyze the transient brightening of a solar coronal loop observed, at high time cadence (30 s) and spatial resolution (05 pixel size), with the Transition Region and Coronal Explorer (TRACE) in the 171 ? band on 1998 June 26. The loop, located in AR 8253, is ≈1010 cm long and inclined with respect to the vertical to the solar surface. Its geometry and shape do not change significantly during the brightening, which lasts for ~2 hr and is preceded by highly dynamic events in nearby and perhaps interacting loops. The loop footpoints brighten first; after ~10 minutes, moving brightness fronts rise initially from the northern footpoint, and after another ~7 minutes from the southern one, at an apparent speed ~100 km s-1, the whole loop becoming clearly visible afterward. During the rising phase the loop evolves coherently as a single magnetic tube. The brightness profile is asymmetric with respect to the loop apex at all times; the brightness contrast between the footpoints and the apex decreases with time from a ratio of ~10 to ~3. After the loop has become all visible, the several parallel filaments which form it follow an independent evolution. Assuming a plasma temperature of ≈1 MK, we infer a plasma density of ~6 × 108 cm -3 and a pressure of ~0.2 dyn cm-2 close to the loop apex at the luminosity maximum. A companion paper is devoted to modeling the rising phase of this event.