Recent developments in the study of mesospheric clouds

Abstract

Abstract Understanding the physics of ice particles at the cold summertime mesopause (85 km altitude) and their atmospheric environment represent a challenge to both experimentalists and theorists. This review emphasizes recent advances, both experimental and theoretical, in understanding the unique properties of this region of the atmosphere. Evidence is presented for mean cloud particle sizes to be less than 100 nm in effective radius. Charging of cloud particles by collisional attachment of electrons may cause local electron depletions. However this process appears to explain only partly the electron ‘bite-out’ phenomenon. Nor does it appear to explain all aspects of polar mesospheric summertime radar echoes (PMSE). It is possible that the particles acquire a large positive charge as a result of the photoelectric effect. However special ad hoc coatings of the ice surfaces by low-work function metals are needed. No satisfactory theory yet exists for PMSE. Observations of mesospheric phenomena over decadal time spans reveal significant cyclical and secular changes. In particular, changes in noctilucent cloud occurrence frequency, brightness and altitude offer important clues with regard to long-term compositional changes of the middle atmosphere.