The Height Structure of the Solar Atmosphere from the Extreme-Ultraviolet Perspective

We investigate the structure of the solar chromosphere and transition region using full Sun images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. The limb seen in the EIT coronal images (taken in lines of Fe IX/X at 171 AA, Fe XII at 195 AA and Fe XV at 284 AA) is an absorption limb predicted by models to occur at the top of the chromosphere where the density of neutral hydrogen becomes significant ($sim10^{10}$ cm$^{-3}$). The transition-region limb seen in He II 304 AA images is an emission limb. We find: (1) the limb is higher at the poles than at the equator both in the coronal images (by 1300 $pm$ 650 km) and the 304 AA images (by 3500 $pm$ 120 0 km); and (2) the 304 AA limb is significantly higher than the limb in the coronal images. The height difference is 3100 $pm$ 1200 km at the equator, and 6600 $pm$ 1200 km at the poles. We suggest that the elevation of the 304 AA limb above the limb in the coronal images may be due to the upper surface of the chromosphere being bumpy, possibly because of the presence of spicules. The polar extension is consistent with a reduced heat input to the chromosphere in the polar coronal holes compared with the quiet--Sun atmosphere at the equator.