Coronal loops and active region structure

We intercompared synoptic Hα, Ca K, magnetograph and Skylab soft X-ray and EUV data for the purpose of identifying the basic coronal magnetic structure of loops in a 'typical' active region and studying its evolution. We focussed on a complex of activity in July 1973, especially McMath 12417. Our principal results are: (1) Most of the brightest loops connected the bright f plage to either the sunspot penumbra or to p satellite spots; no non-flaring X-ray loops end in umbrae; (2) short, bright loops had one or both ends in regions of emergent flux, strong fields or high field gradients; (3) stable, strongly sheared loop arcades formed over filaments; (4) EFRs were always associated with compact X-ray arcades; and (5) loops connecting to other active regions had their bases in outlying plage of weak field strength in McM 417 where Hα fibrils marked the direction of the loops. We conclude that a typical loop brightens in response to magnetic field activity at its feet, which heats the plasma. This suggests that the loop acts as a trap for gas convected from its base.