Neutral Heating Efficiency in the Dayside Venusian Upper Atmosphere

The neutral heating efficiency is commonly defined as the ratio of the net local gas-heating rate to the rate of solar radiative energy absorption. It is a crucial parameter that determines the upper atmospheric temperature and the thermal escape rate on both solar system bodies and exoplanets. In this study, we construct a one-dimensional photochemical model to compute the neutral heating efficiency in the dayside Venusian upper atmosphere. This calculation involves a complex network of microscopic processes, including photon and photoelectron impact processes, as well as exothermic chemical reactions. Our calculations indicate that the major heat sources in the Venusian atmosphere are the photodissociation of CO₂ at lower altitudes and the dissociative recombination of O₂⁺ at higher altitudes. During solar maximum, the neutral heating efficiency remains relatively constant at approximately 35% between 110 and 160 km, declining to 20% near 220 km. Furthermore, we find that the heating efficiency at higher altitudes is enhanced by increased concentrations of background H₂, attributable to a higher abundance of O₂⁺.