The Temporal Evolution of Coronal Loops Observed by GOES SXI

We study the temporal evolution of coronal loops using data from the Solar X-Ray Imager (SXI) on board the Geosynchronous Operational Environmental Satellite 12 (GOES-12). This instrument provides continuous soft X-ray observations of the solar corona at a high temporal cadence permitting detailed study of the full lifetime of each of several coronal loops. The observed light curves suggest three evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases [I/(dI/dt), where I is the loop intensity] are much longer than a cooling time and indicate that the loop-averaged heating rate increases slowly, reaches a maintenance level, and then decreases slowly. This suggests that a single heating mechanism operates for the entire lifetime of the loop. For monolithic (uniformly filled) loops, the loop-averaged heating rate is the intrinsic energy release rate of the heating mechanism. For loops that are bundles of impulsively heated strands, it relates to the frequency of occurrence of individual heating events, or nanoflares. We show that the timescale of the loop-averaged heating rate is proportional to the timescale of the observed intensity variation, with a constant of proportionality of approximately 1.5 for monolithic loops and 1.0 for multistranded loops. The ratios of the radiative to conductive cooling times in the loops are somewhat less than 1, putting them intermediate between the values measured previously for hotter and cooler loops. This provides further support for a trend suggesting that all loops are heated in a similar way.