Positive ion composition of the high‐latitude summer <i>D</i> region with noctilucent clouds

Abstract

Positive ion composition and electron and total positive ion densities were measured on July 30 (S26/1) and August 13 (S26/2), above Kiruna as part of a multinational rocket campaign to study noctilucent clouds (NLC). Two magnetic ion mass spectrometers took measurements at altitudes of 65–126 km in twilight at times of very low magnetic activity and of NLC sightings at 82.5–83.4 km. The hydration order of the most abundant proton hydrates increased with height from n = 3 below 76 km to n = 6 and n = 5 at 88 and 87 km, respectively. Temperatures inferred from ion composition revealed a minimum of 104° + 20°K at 90.5 km. At this altitude a special ion spectrum with heavy proton hydrates up to H + (H 2 O) 12 was measured, which seems to indicate the possibility for ice particle formation through ion nucleation in a narrow layer. The composition measurements were also used to infer mesospheric concentrations of nitric oxide, water vapor, and hydrogen peroxide. The water vapor mixing ratio in the NLC region was found to be around 3 parts per million by volume.