Composition measurements of proton hydrate ions at the cold summer polar mesopause conducted by use of an improved experimental method reducing cluster ion dissociation revealed the presence of ions more massive than observed previously. Time constants for ion hydration derived from the data decrease steeply with decreasing temperature becoming as low as about 2 seconds at the mesopause for temperatures around 130 K. This is interpreted as being due to the occurence of indirect ion hydration involving N2-association association followed by CO2- and H2O-switching. Aeronomic implications are discussed.