Photoelectron production rate at the solar zenith angle 0° as a functionof column density (left y-axis) or optical depth (right y-axis). Two cases are represented: (1) expanding comae: 1P during the Giotto flyby (red solid line) and 67P at two different heliocentric distances (green and blue solid lines, see legend and Table 2), (2) atmospheres in hydrostatic equilibrium like Earth at 1 au (black, solid line). The number density scale height H for the hydrostatic equilibrium case is set to 20 km, and the same photoabsorption cross section is assumed for all three cases. The dotted line corresponds to the production rate below the cometary surface. The circles identify the nucleus surface, and the dotted line corresponds to the hypothetical production rate below.