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 1980-19 86 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 structure s extending up to altitudes of 260 000 km. While the main brightest st ructure of the arch was newly formed, the weak very large loops had ex isted above the active region before and were only enhanced during the event. Skylab data support the model proposed by Kopp and Poletto tha t the giant arch is formed by reconnections high in the corona, differ ent from the reconnection process in the underlying flare. However, co ntrary to Kopp and Poletto's suggestion, the data strongly indicate th at 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 inter action of large-scale loops high in the corona. (The interaction of tw o of them is clearly seen.) Thus, while post-flare loops are formed by the Kopp and Pneuman mechanism, giant arches above eruptive flares ma y originate through interactive reconnections of large-scale magnetic field lines which form loops high in the corona. These loops are broug ht close to each other in consequence of changes in the coronal struct ure caused by the eruptive flare phenomenon. The arch-associated enhan cement of the pre-existing large-scale active-region loops may be caus ed by electrons accelerated during the reconnection process and diffus ing across field lines, as suggested by Achterberg and Kuipers (1984).