Evidence of heavy positive ions at the summer Arctic mesopause from the EISCAT UHF incoherent scatter radar

A confined layer characterised by narrow incoherent scatter spectra has been observed between 86 and 88 km altitude in the high latitude summertime D‐region with the EISCAT radar. Properties of the background plasma inferred from spectral measurements outside the layer are in close agreement with model predictions, but the localised minima in spectral width imply much heavier positive ions within the layer. This feature is interpreted as being due to the presence of large clustered ions with mass of the order of 500 amu, which are possibly hydrated protons with a mean hydration index of almost 30.