The photodissociation of water vapor in the mesosphere

An analysis of the photodissociation of water vapor has been made for mesospheric conditions. The simultaneous action of two spectral ranges, namely the Lyman‐alpha radiation and the O2 Schumann‐Runge bands, must be considered. The H2O photodissociation frequency owing to Lyman‐alpha is represented by J(H2O, Ly‐α) = J∞ exp [−4.17 × 10−19N0.917], where N is the total number of O2 absorbing molecules, and J∞ = (3.5 ± 1.0) × 10−6 s−1 for quiet sun conditions and reaches (1.25 ± 0.35) × 10−5 s−1 for very active sun conditions. The H2O photodissociation frequency resulting from the O2 Schumann‐Runge band spectral range is represented by J(H2O, SRB) = J∞(H2O, SRB) exp [−4.8 × 10−10N0.466] for N ≤ 1020 cm−2, and J(H2O, SRB) = J∞(H2O, SRB) exp [−4.6 × 10−8N0.366] for 1020 < N ≤ 1022 cm−2. The mean value which has been adopted for J∞(H2O, SRB) = (1.3 ± 0.3) × 10−6 s−1 while a median value could be (1 ± 0.5) × 10−6 s−1 based on all observational data.