CHARGING OF MESOSPHERIC PARTICLES - IMPLICATIONS FOR ELECTRON-DENSITY AND PARTICLE COAGULATION

Measurements of the electron density at high latitudes in the summertime show a pronounced "bite-out" at about 87 km. Using a numerical model of polar mesospheric cloud (PMC) formation. we have investigated the effects of ion and electron capture by ice particles on the ionization near the mesopause. The microphysical model includes all physical processes believed to be important, and the predicted PMC optical properties agree well with polarization and albedo measurements. We have calculated statistical distributions of charges on ice particles, including collection of ions and electrons by particles, and photoemission. Given these charge distributions, ionization loss rates due to capture by particles and the resulting steady-state electron and ion densities are calculated. Our calculations indicate that aerosol concentrations associated with visible PMC are unlikely to cause a severe electron density depletion. The bite-out could possibly be caused by a large concentration of small ice particles existing in a narrow cold layer near the mesopause. However, concentrations of ice nuclei (either dust particles or water cluster ions) on the order of 1000 cm-3 would be required. We have also found that the net negative charge on mesospheric aerosols may severely inhibit coagulation. The importance of this result is that mesospheric dust particles would not grow significantly. If this analysis is indeed correct, then mesospheric dust particles will be smaller than previously estimated, and a higher supersaturation with respect to water vapor would be required for heterogeneous nucleation of ice crystals. Hence, the existence of ice particles at the mesopause may require lower temperatures than previously assumed.