Modelling of particle charging in the polar summer mesosphere: Part 1—General results

Abstract

Rocketborne measurements of electron and positive ion number densities in the vicinity of noctilucent clouds and/or polar mesosphere summer echoes frequently give evidence of pronounced disturbances relative to a smooth background profile. A model is applied to study the disturbances in terms of diffusional charging of aerosol particles with special regard to the background plasma conditions, i.e. the electron/ion production rate and the coefficient of dissociative recombination which depends on the positive ion composition. It is demonstrated that the disturbances in electron and positive ion profiles are a complex function of the aerosol radius, the aerosol number density, the production rate, and the recombination coefficient. However, if the latter two are known, the model allows the determination of aerosol radii and number densities from the measurement of positive ion and electron number densities. Furthermore, we show that nearly all combinations of electron biteouts, positive ion biteouts, and positive ion enhancements can exist depending on the properties of the aerosol particles and the background plasma conditions. Concerning electrons the presence of aerosol particles will always lead to a depletion. The magnitude of this depletion increases with increasing aerosol size and number density. Concerning positive ions both depletions and enhancements can occur. While small electron/ion production rates favour deep depletions in both electrons and positive ions, enhancements in positive ions are most likely created if the recombination coefficient is large implying large positive cluster ions. However, enhancements of electrons cannot be created by the variation of the above mentioned parameters. Thus observations of electron enhancements are a strong indication of a charging mechanism different from the diffusional charging discussed in this paper, e.g. photo emission. We have applied the charging model to in situ observations of electrons and positive ions in the vicinity of noctilucent clouds and polar mesosphere summer echoes. These results are presented in a companion paper by Lübken and Rapp (J. Atmos. Sol. Terr. Phys. (2001), 63, 771–780).