On the role of charged aerosols in polar mesosphere summer echoes

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Abstract
Submicron aerosols, as evidenced by the occurrence of polar mesospheric and noctilucent clouds, exist at heights from which polar mesosphere summer echoes (PMSE) are observed. We investigate the role of positively and negatively charged aerosols in the scattering processes proposed in the literature. These aerosols, if charged substantially, can account for the remarkably high radar reflectivity at both VHF and UHF by raising the electron Schmidt number through the ambipolar effect. A positively charged component may be responsible for enhanced UHF radar scatter by increasing the incoherent scatter power through a dressed dust effect, although such a process is not realistic as an explanation for VHF scatter during PMSE. Such an enhanced UHF scatter will be associated with extremely narrow backscatter spectra. We propose a model in which both negatively and positively charged aerosols are present to explain both the radar properties and the rocket probe observations of charged particle depletions. Finally, we point out that the Poker Flat 50‐MHz long‐term data, which contrary to accepted dynamical theory show average downward velocities in the summertime upper mesosphere, can be attributed to the fall speed of the aerosols responsible for PMSE.

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