Emission measure in the limited range of temperature and corresponding spectral energy flux at 1 au. For better visualisation, the spectral energy fluxes in the bottom panel are averaged over 10 Å. Orange, green and blue lines correspond to the emission measures and spectral energy fluxes of T ≈ 10⁴ K (chromosphere), T ≈ 10⁵ K (transition region) and T ≈ 10⁶ K (corona), respectively. Black dotted lines show the full emission measure distribution and corresponding full spectral energy flux.