An analysis of Skylab X-ray pictures of a giant coronal arch

The limb event of 13/14 August, 1973, imaged by Skylab in soft X-rays, proved to be a giant arch, quite similar to those observed in 1980–1986 on SMM. High spatial resolution (by a factor of 4–5 better than in SMM data) made it possible to see the internal structure of the arch. Its brightest part consisted of loops very similar to, but higher than, post-flare loops, surrounded by a rich system of weak loop structures extending up to altitudes of 260 000 km. While the main brightest structure of the arch was newly formed, the weak very large loops had existed above the active region before and were only enhanced during the event.Skylab data support the model proposed by Kopp and Poletto that the giant arch is formed by reconnections high in the corona, different from the reconnection process in the underlying flare. However, contrary to Kopp and Poletto's suggestion, the data strongly indicate that the field lines that reconnect in the arch did not open before, as in the Kopp and Pneuman model: more likely, we encounter here an interaction of large-scale loops high in the corona. (The interaction of two of them is clearly seen.) Thus, while post-flare loops are formed by the Kopp and Pneuman mechanism, giant arches above eruptive flares may originate through interactive reconnections of large-scale magnetic field lines which form loops high in the corona. These loops are brought close to each other in consequence of changes in the coronal structure caused by the eruptive flare phenomenon. The arch-associated enhancement of the pre-existing large-scale active-region loops may be caused by electrons accelerated during the reconnection process and diffusing across field lines, as suggested by Achterberg and Kuipers (1984).