Foreword: Layered phenomena in the mesopause region

Layered phenomena in the mesopause region is the subject of an international working group whose main focus concerns the physics and chemistry of the summertime mesosphere and the processes involved in forming polar mesospheric summertime echoes (PMSEs) and polar mesospheric clouds (PMCs)—the global equivalent to ground observer's noctilucent clouds (NLCs). PMCs are ice clouds that occur in the summer mesosphere at altitudes typically between 81 and 86 km, and poleward of 50 degrees latitude. PMSEs are strong backscattered signals from the summer mesosphere region, recorded largely, although not exclusively, by VHF radars, that occur most often poleward of 50 degrees latitude and at altitudes typically between 82 and 88 km. The major areas of research within this working group involve understanding and numerically modeling the dynamical, thermal, and chemical processes related to the summer mesosphere; microphysical modeling of MCs; determining the scattering, composition, and shape of mesospheric particles; resolving sources of condensation nuclei; performing laboratory experiments of relevant reactions and related constituents involved in PMC and PMSE behavior, and determining the properties and conditions of PMC, PMSE, and the summer mesosphere by making rocket, satellite, and ground‐based radar and lidar measurements.