OBSERVATIONS OF FAINT, OUTLYING LOOP-SYSTEMS IN LARGE FLARES

Faintly visible, darkened regions in Her lying outside but adjacent to bright flare emission were found to occur in 10 of 31 major flares in vestigated. Without exception, the darkenings occur over 'magnetically neutral' areas, and these are usually bordered by ridges of oppositel y-poled field, where one border is shared in common with a hare ribbon . The darkenings probably result from the formation of faint, outlying loop systems, similar to post-flare loops seen in absorption, but whi ch are connected to magnetic features outside the flare and are unreso lved or only marginally resolved in patrol images. Simple models for p ost-flare loops incorporating the results of statistical equilibrium c alculations readily demonstrate that darkenings of several percent (co nsistent with our photometric measurements) can be produced by loop st ructures of cross-sectional diameter approximate to 10(2) km (unresolv ed by patrol instruments) and containing gas at densities 5 x 10(10)-5 x 10(11) cm(-3) and temperatures 8000-15000 K. Outlying loop systems might be formed by magnetic field reconnection, analogous to the mecha nism ascribed to eruptive two-ribbon flares, but associated with field structures adjacent to the flare. Alternatively, these outlying loop systems may not erupt but become visible as a result of heating and ch romospheric evaporation at the footpoints shared with the Rare ribbon. In either case, the observations presented here have interesting impl ications for both the spatial scale and the topology of the coronal ma gnetic fields in which eruptions occur.