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The TRACE Observatory is the first solar-observing satellite in the National Aeronautics and Space Administration's ͑NASA͒ Small Explorer series. Launched April 2, 1998, it is providing views of the solar transition region and low corona with unprecedented spatial and temporal resolution. The corona is now seen to be highly filamented, and filled with flows and other dynamic processes. Structure is seen down to the resolution limit of the instrument, while variability and motions are observed at all spatial locations in the solar atmosphere, and on very short time scales. Flares and shock waves are observed, and the formation of long-lived coronal structures, with consequent implications for coronal heating models, has been seen. This overview describes the instrument and presents some preliminary results from the first six months of operation.

We compare the morphology of active region structures observed in the 171 (T D 9 ] 105 K) and A Lya (T D 2 ] 104 K) lines. The coronal data were obtained by the T ransition Region and Coronal Explorer (T RACE) in support of the Very High Angular Resolution Ultraviolet Telescope (VAULT) sounding rocket launch, which acquired subarcsecond resolution images of an active region in the Lya line, on 1999 May 7. Using a pair of calibrated, nearly simultaneous images, we Ðnd that : (i) a very good correlation exists between the Lya and 171 intensities in the T RACE moss regions, (ii) we can identify A several identical structures in some (but not all) moss areas, and (iii) the correlations are greatly reduced at the footpoints of the 171 large-scale loops. We derive a lower limit for the Lya emission measure, A under the assumption of e †ectively optically thin emission, and compare it to the 171 emission A measure. As in previous studies, we Ðnd an excess of Lya material compared to the amount expected for a thermal conduction-dominated corona-chromosphere transition region, even for structures that appear to be identical in the two wavelengths. This result implies that some other mechanism besides classical heat conduction from the corona must contribute to the observed Lya intensities. The observations do not support the idea of a physically distinct cool loop component within active regions.