The V0 layer of Venus: A persistent NO+ layer controlled by solar X-rays

Anomalous electron density enhancements below 120 km altitude, known as the V0 layer, have long been observed in radio occultation measurements of the Venus ionosphere. Their physical origin, however, has remained unresolved. Using observations from Venus Express, supported by a photochemical model, we show that the V0 layer corresponds to a persistent NO+ ion layer whose density is controlled by solar X-ray flux and the ambient NO abundance. Enhanced X-ray irradiation increases CO2 photoionization, which drives O₂⁺ production. Subsequent charge exchange between O₂⁺ and NO produces localized NO+ enhancement at lower altitudes. The proposed mechanism successfully reproduces the observed V0 layer and demonstrates that it is not a sporadic feature, but a permanent component of the Venus ionosphere whose intensity varies with solar activity.