A brightening coronal loop observed by TRACE. I. Morphology and evolution

We analyze the transient brightening of a solar coronal loop observed, at h igh time cadence (30 s) and spatial resolution (0."5 pixel size), with the Transition Region and Coronal Explorer (TRACE) in the 171 A band on 1998 Ju ne 26. The loop, located in AR 8253, is approximate to 10(10) cm long and i nclined with respect to the vertical to the solar surface. Its geometry and shape do not change significantly during the brightening, which lasts for similar to 2 hr and is preceded by highly dynamic events in nearby and perh aps interacting loops. The loop footpoints brighten first; after similar to 10 minutes, moving brightness fronts rise initially from the northern foot point, and after another similar to 7 minutes from the southern one, at an apparent speed similar to 100 km s(-1), the whole loop becoming clearly vis ible afterward. During the rising phase the loop evolves coherently as a si ngle magnetic tube. The brightness profile is asymmetric with respect to th e loop apex at all times; the brightness contrast between the footpoints an d the apex decreases with time from a ratio of similar to 10 to similar to 3. After the loop has become all visible, the several parallel filaments wh ich form it follow an independent evolution. Assuming a plasma temperature of approximate to 1 MK, we infer a plasma density of similar to 6 x 10(8) c m(-3) and a pressure of similar to 0.2 dyn cm(-2) close to the loop apex at the luminosity maximum. A companion paper is devoted to modeling the risin g phase of this event.