First detection of the atomic O18 isotope in the mesosphere and lower thermosphere of Earth

Abstract

The magnetic dipole transitions between the fine-structure ground-state levels of atomic oxygen residing in the mesosphere and the lower thermosphere of Earth are measured in absorption against the Moon. The small isotope shifts allow determining the ${}^{16}$O/${}^{18}$O abundance ratio in this remote environment, found to be typical of isotopic oxygen fractionations in the lower atmosphere and to significantly fall below the solar wind value.