The Temperature Structure of Active Region Loops

Previous solar observations have shown that coronal loops near 1MK are difficult to reconcile with simple heating models. These loops have lifetimes that are long relative to a radiative cooling time and densities that are large relative to thermodynamic equilibrium. Models proposed to explain these properties generally rely on the existence of small scale filaments that are in various stages of heating and cooling, suggesting a distribution of temperatures within a loop. We present the observation of an evolving coronal loop observed with EIS/Hinode and TRACE. This loop has a high density, a narrow distribution of temperatures, and a lifetime that is long relative to a radiative cooling time. These properties will be difficult to reconcile with physical models of coronal loops.