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TRACE observations of active regions show a peculiar extreme ultraviolet (EUV) emission over certain plage areas. Termed `moss' for its spongy, low-lying, appearance, observations and modeling imply that the phenomenon is caused by thermal conduction from 3–5 MKcoronal loops overlying the plage: moss is the upper transition region emission of hot coronal loops. The spongy appearance is due to the presence of chromospheric jets or `spicules' interspersed with the EUV emission elements. High cadence TRACE observations show that the moss EUV elements interact with the chromospheric jets on 10 s time scales. The location of EUV emission in the moss does not correlate well to the locations of underlying magnetic elements in the chromosphere and photosphere, implying a complex magnetic topology for coronal loop footpoint regions. We summarize here the key observations leading to these conclusions and discuss new implications for understanding the structuring of the outer solar atmosphere. [ABSTRACT FROM AUTHOR]

Recent observations of solar active regions made with the Transition Region and Coronal Explorer (TRACE) have revealed finely textured, low-lying extreme ultraviolet (EUV) emission, called the moss. It appears as a bright, dynamic pattern with dark inclusions, structured on spatial scales of 1 to 3 Mm. The moss has been interpreted as the upper transition region above active region plage and below relatively hot loops. Here we study the temporal variability of the morphology of the moss using a 2-hr time sequence of high-cadence TRACE 171 Å images and G-band, Ca ii K-line and Hα filtergrams from the Swedish Vacuum Solar Telescope (SVST, La Palma) on 1 June 1999. The data provide a unique view of the connections between the photosphere, chromosphere, transition region and corona in an active region. We find that the moss is dynamic on time scales of 10–30 s due to intrinsic changes in brightness, obscuration by chromospheric jets and motion caused by physical interaction with these jets. The temporal variations of the bright moss elements occur on shorter time scales than those of the Ca ii K-line bright points. The bright moss elements generally do not occur directly above the G-band or Ca ii K-line bright points in the photosphere or lower chromosphere. This suggests that the upper transition region emission often occurs at the interface of neighboring flux tubes. The temporal variability of the moss brightness on 30 s time scales may suggest that the energy source of these intensity changes occurs relatively locally (height <10 000 km). [ABSTRACT FROM AUTHOR]

The Transition Region and Coronal Explorer (TRACE) – described in the companion paper by Handy et al. (1999) – provides an unprecedented view of the solar outer atmosphere. In this overview, we discuss the initial impressions gained from, and interpretations of, the first million images taken with TRACE. We address, among other topics, the fine structure of the corona, the larger-scale thermal trends, the evolution of the corona over quiet and active regions, the high incidence of chromospheric material dynamically embedded in the coronal environment, the dynamics and structure of the conductively dominated transition region between chromosphere and corona, loop oscillations and flows, and sunspot coronal loops. With TRACE we observe a corona that is extremely dynamic and full of flows and wave phenomena, in which loops evolve rapidly in temperature, with associated changes in density. This dynamic nature points to a high degree of spatio-temporal variability even under conditions that traditionally have been referred to as quiescent. This variability requires that coronal heating can turn on and off on a time 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 Å (∼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 weaving 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 discuss evidence that the heating occurs predominantly within the first 10 to 20 Mm from the loop footpoints. This causes the inner parts of active-region coronae to have a higher average temperature than the outer domains. [ABSTRACT FROM AUTHOR]

The Transition Region and Coronal Explorer is a space-borne solar telescope featuring high spatial and temporal resolution. TRACE images emission from solar plasmas in three extreme-ultraviolet (EUV) wavelengths and several ultraviolet (UV) wavelengths, covering selected ion temperatures from 6000 K to 1 MK. The TRACE UV channel employs special optics to collect high-resolution solar images of the H i Lα line at 1216 Å, the C iv resonance doublet at 1548 and 1550 Å, the UV continuum near 1550 Å, and also a white-light image covering the spectrum from 2000–8000 Å. We present an analytical technique for creating photometrically accurate images of the C iv resonance lines from the data products collected by the TRACE UV channel. We use solar spectra from several space-borne instruments to represent a variety of solar conditions ranging from quiet Sun to active regions to derive a method, using a linear combination of filtered UV images, to generate an image of solar C iv 1550 Å emission. Systematic and statistical error estimates are also presented. This work indicates that C iv measurements will be reliable for intensities greater than 1014 photons s−1 cm−2 sr−1. This suggests that C iv 1550 Å images will be feasible with statistical error below 20% in the magnetic network, bright points, active regions, flares and other features bright in C iv. Below this intensity the derived image is dominated by systematic error and read noise from the CCD. [ABSTRACT FROM AUTHOR]