Positive ion depletion in a noctilucent cloud

With a Mattauch‐Herzog type of mass spectrometer the positive ion composition was measured in the mesosphere between 80 and 98.4 km over Kiruna, Sweden, in summer 1993. During the rocket flight an intense noctilucent cloud (NLC) was present and detected by a particle impact sensor at 83.0–83.6 km and 82.5–85.6 km on ascent and descent, respectively. For the first time a significant depletion of the main positive proton hydrates was observed on ascent and a complete disappearance of these ions on descent in the entire NLC‐layer. The observation is associated with an atypical low ion production of 0.01–0.1 cm−3s−1, which favors the ion loss by particle attachment and/or vertical transport. In order to explain the loss of mobile charge a second population of at least 105 cm−3 1–2 nm ice particles is required in the NLC.