CDS/SoHO multi-line observation of a solar active region: Detection of a hot stable loop and of a cool dynamic loop

We analyze a space-, time- and spectral-resolved SoHO/CDS observation of the evolution of an active region over a time lapse of approximately three hours in various spectral lines emitted in the interval of temperature 1.3 x 10(4) < T < 2.5 x 10(6) K. We identify and characterize two structures of interest: a longer coronal loop (approximate to5.5 x 10(9) cm), relatively steady and well visible in lines forming at coronal temperatures (e.g. Fe XIV 334.17 Angstrom, Fe XVI 360.76 Angstrom) and a smaller one (approximate to1.8 x 10(9) cm), transient and visible only in cooler lines (O IV 554.51 Angstrom, O V 629.73 Angstrom). In the hot lines, the longer loop has a bright apex and an emission distribution of constant shape, but of moderately variable absolute intensity; the region around the loop apex shows a distinct brightening practically in all lines. In the hot lines, the brightening appears as a minor perturbation over a steadily high emission level. In the same region the emission measure vs temperature of the hottest lines indicates a temperature of similar to2 MK, lower than the temperature obtained from Yohkoh data taken just before the CDS observation. Comparison with steady-state loop scaling laws and with plasma time scales, and connection to cooling or heating episodes are discussed. As for the cool loop, its whole evolution, from ignition to disappearance, is directly observed, confirming the highly transient nature of such structures. The O V line is blue-shifted at one footpoint, indicating an upflow associated with the loop ignition.