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 Å , Fe XII at 195 Å and Fe XV at 284 Å ) is an absorption limb predicted by models to occur at the top of the chromosphere where the density of neutral hydrogen becomes significant ( \sim 10 ^ { 10 } cm ^ { -3 } ) .
The transition-region limb seen in He II 304 Å 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 Å images ( by 3500 \pm 1200 km ) ; and ( 2 ) the 304 Å 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 Å 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 .