Dynamics of Transition Region 'Moss' at high time resolution

Recent observations of solar active regions made with the Transition Region and Coronal Explorer (TRACE) have revealed finely textured, low-lying extr eme ultraviolet (EUV) emission, called the moss. It appears as a bright, dy namic pattern with dark inclusions, structured on spatial scales of 1 to 3 Mm. The moss has been interpreted as the upper transition region above acti ve 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 hig h-cadence TRACE 171 Angstrom images and G-band, Ca II K-line and H alpha fi ltergrams from the Swedish Vacuum Solar Telescope (SVST, La Palma) on 1 Jun e 1999. The data provide a unique view of the connections between the photo sphere, 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 ch anges in brightness, obscuration by chromospheric jets and motion caused by physical interaction with these jets. The temporal variations of the brigh t 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 ab ove the G-band or Ca II K-line bright points in the photosphere or lower ch romosphere. This suggests that the upper transition region emission often o ccurs at the interface of neighboring flux tubes. The temporal variability of the moss brightness on 30 s time scales may suggest that the energy sour ce of these intensity changes occurs relatively locally (height < 10 000 km ).