Evolution of direct XUV heating (upper panel), chemical heating (middle panel), and photoelectron heating (lower panel) within three altitude ranges in the Earth’s atmosphere. The quantities plotted are the total heating rates integrated over the relevant volumes and normalised to the values for the modern Earth. For photoelectron heating, only the values for altitudesbetween 200 km and the exobase are shown since at lower altitudes this process is negligible.