The relation between hot and cool loops

In order to study the origin of the hot (greater than or similar to 3 MK) c orona above active regions, we compare Yohkoh/SXT X-ray images, which repre sent a broad temperature range above similar to 2.5 MK, with TRACE EUV coro nal images whose primary sensitivities are in the 1-2 MK range. Nearly simu ltaneous X-ray and EUV snapshots show that there are loops that appear simi lar in these images of different temperature sensitivities, but they are no t exactly cospatial with each other. A significant difference is noted in t he active region core, where bright loops are seen in X-ray images but not in EUV images, reflecting their high (similar to 5 MK) temperatures. In SXT time-sequence images, these loops are found to undergo repetitive minor br ightenings, suggestive of their flare-like origin. This is consistent with the absence of the EUV counterparts of the X-ray loops in TRACE time-sequen ce images at earlier and later times. We need to revisit the validity of th e assumption that coronal loops are steady.