A new view of the solar outer atmosphere by the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer (TRACE) - described in the compa nion paper by Handy et al. (1999) - provides an unprecedented view of the s olar outer atmosphere. In this overview, we discuss the initial impressions gained from, and interpretations of, the first million images taken with T RACE. We address, among other topics, the fine structure of the corona, the larger-scale thermal trends, the evolution of the corona over quiet and ac tive regions, the high incidence of chromospheric material dynamically embe dded in the coronal environment, the dynamics and structure of the conducti vely dominated transition region between chromosphere and corona, loop osci llations and flows, and sunspot coronal loops. With TRACE we observe a coro na that is extremely dynamic and full of flows and wave phenomena, in which loops evolve rapidly in temperature, with associated changes in density. T his dynamic nature points to a high degree of spatio-temporal variability e ven under conditions that traditionally have been referred to as quiescent. This variability requires that coronal heating can turn on and off on a ti me scale of minutes or less along field-line bundles with cross sections at or below the instrumental resolution of 700 km. Loops seen at 171 Angstrom (similar to 1 MK) appear to meander through the coronal volume, but it is unclear whether this is caused by the evolution of the field or by the weav ing of the heating through the coronal volume, shifting around for periods of up to a few tens of minutes and lighting up subsequent field lines. We d iscuss evidence that the heating occurs predominantly within the first 10 t o 20 Mm from the loop footpoints. This causes the inner parts of active-reg ion coronae to have a higher average temperature than the outer domains.