The morphology of the solar upper atmosphere during the sunspot minimum

The solar upper atmosphere (SUA) is defined as the volume above the photosp here occupied by plasmas with electron temperatures, T-e, above approximate to 2x10(4) K. Until the Skylab era, only little was known about the morpho logy of the SUA, while the quality of the spectroscopic observations was co ntinually improving. A spherically symmetric atmosphere was assumed at that time, in which the temperature increased with height. With advances in the observational techniques, it became apparent that the morphology of the SU A was very complex even during the minimum of the magnetic activity cycle. In particular, spectroscopic measurements with high spectral and spatial re solution, which were made in the light of ultraviolet emission lines repres enting a variety of temperatures, led to the conclusion that most of the ra diation from the solar transition region could not be explained by assuming a continuous chromosphere-corona interface, but rather by a region of unre solved fine structures. Recent observational results obtained by modern ins truments, such as the Extreme-ultraviolet Imaging Telescope (EIT), the Larg e Angle Spectroscopic Coronagraph (LASCO), and the Solar Ultraviolet Measur ements of (SUMER) spectrograph on the Solar and Heliospheric Observatory (S OHO), as well as the Transition Region and Coronal Explorer (TRACE), and th eir interpretations will be presented in this review of our understanding o f the morphology of the SUA.