A two-ion ice particle model of the polar summer mesopause region

A one-dimensional model is used to compute height dependent concentrations of molecular ions, cluster ions, positively and negatively charged ice particles, and electrons in the high-latitude summer mesopause region. In agreement with experimental results, the computed height profiles show a rapid transition from cluster to molecular ions between 85 and 90 km without ion depletion, whereas deep electron "bite-outs" occur within a particle layer. The model can be forced by turbulent neutral density fluctuations. It is assumed that gradient mixing generates perturbations in the concentration of all particles which are considered as passive conservative constituents so that their energy-wavenumber spectrum contains a viscous-convective subrange due to a high Schmidt number. In particular at the boundaries of the particle layer, small fluctuations in the neutral gas give rise to strong fluctuations in the ionized constituents. Computed radar reflectivity profiles are characterized by a double-peak structure and agree well with reflectivity profiles measured at frequencies of 53.5 and 224 MHz. Without assuming excessive water vapor mixing ratios, the model cannot explain measured reflectivities at frequencies near 1 GHz, supporting observational evidence that radar echoes in the VHF and UHF ranges are generated by distinct scatter mechanisms.