Model for the production of CO Cameron band emission in Comet 1P/Halley

► First detailed chemistry-emission model for the production of Cameron band on comet Halley is developed. ► Photoelectron impact excitation of CO and dissociative excitation of CO2 are the major production processes of CO(a3Π). ► Contrary to earlier suggestions, the photodissociative excitation of CO2 is not the main source mechanism of CO(a3Π). ► Cameron emission can be used to derive the photoelectron density in the inner coma, rather than the CO2 abundance. ► Sensitivity of the Cameron band intensity to the solar EUV flux and electron impact excitation cross-sections is evaluated.