POSITIVE-ION DEPLETION IN A NOCTILUCENT CLOUD

With a Mattauch-Herzog type of mass spectrometer the positive ion comp osition was measured in the mesosphere between 80 and 98.4 km over Kir una, Sweden, in summer 1993. During the rocket flight an intense nocti lucent cloud (NLC) was present and detected by a particle impact senso r at 83.0 - 83.6 km and 82.5 - 85.6 lan on ascent and descent, respect ively. For the first time a significant depletion of the main positive proton hydrates was observed on ascent and a complete disappearance o f these ions on descent in the entire NLC-layer. The observation is as sociated with an atypical low ion production of 0.01-0.1 cm(-3)s(-1), which favors the ion loss by particle attachment and/or vertical trans port. In order to explain the loss of mobile charge a second populatio n of at least 10(5) cm(-3) 1-2 nm ice particles is required in the NLC .