FINE-STRUCTURE AND THE EMISSION FILLING FACTOR

There is observational evidence for an extreme fine structure in the s olar transition region, much smaller than 1'' in size (Dere et al., 19 87, 1988) Corresponding to this extreme fine structure there appear to be an equally complex dynamical structure. We review the evidence for such dynamical extreme fine structure as demonstrated by the frequent appearance of multiple velocities, i.e. distinctly different velociti es in the transition region occurring within the angular resolution el ement. Multiple velocities are prominent in active regions and particu larly near sunspots, where velocity components may be supersonic. Howe ver, multiple velocities are frequent also in quiet regions. The conse quences of such fine structure for modeling the transition region will be outlined. Finally the appropriate. CDS and SUMER observations need ed to extend our knowledge of a finely structured transition region an d corona, spatially or in time, are discussed.