aboard the SMM satellite imaged extensive coronal structures after occurrences of two-ribbon flares. The archlike structures extended along the neutral line of the longitudinal magnetic field to altitudes of 1 x 105 to 2 • 105 km.
The first arch observed by HXIS on 6-7 November appeared after the flare in AR 2779 at 3 9 29 UT of 6 November (Svestka et al., 1982b). It was observed during its late phase of development, when its X-ray flux was continuously decreasing and the maximum of brightness was rising with a velocity of ~ 8 km s-1. Subsequent tworibbon flares (at 14 : 44 UT and again at 4 : 40 UT and 20 : 50 UT of 7 November) were followed by revivals of this first arch: the new arches formed at the same site, apparently as a consequence of the renewed heating of the gas in the still existing coronal magnetic structure of the first arch (Svestka, 1984a, b; Fm-nik and Van Beek, 1984). Probably the first arch observed by HXIS also was a revival of an existing structure, as suggested by the homologous development of this arch and the subsequent revival (Svestka, 1984a). The two-ribbon flare which must have created the magnetic structure of the arch occurred at 13 : 41 UT of 5 November.
The arch on 21-22 May formed after the two-ribbon flare at 20 : 50 UT on 21 May (Svestka etal., 1982a). HXIS first imaged the arch two hours after the flare at a projected altitude of 100000 km above the neutral line. Contrary to the revivals on 6-7 November the maximum of brightness in the arch was stationary during its whole observed decay phase of ~ 10 hr (an upper limit of 1.1 km s -i could be established for the rise velocity; Hick and Svestka, 1985). This arch was not a revival of a pre-existing arch: no major activity occurred prior to the two-ribbon flare at 20 : 50 UT (Schadee et aL, 1983).