Capture rates of electrons and positive ions by mesospheric aerosol particles

Aerosol particles exist in the vicinity of the extremely cold polar summer mesopause and can be visually observed in the form of noctilucent clouds (Jesse (1885) Met. Zeit. 2, 311-312). Being embedded into the plasma of the D-region, these particles become charged by means of plasma diffusion and lead to huge VHF radar echoes known as Polar Mesospheric Summer Echoes (Ecklund and Balsley (1981) J. Geophys. Res. 86, 7775-7780). The diffusional charging of submicron particles has been treated in detail by Natanson (1960, Sov. Phys. Tech. Phys. 5, 538-551) and with some modifications by Fuchs (1963, Geophys. Pura. Appl. 56, 185-193). Previous model investigations of the charging of mesospheric aerosol particles (e.g., Rapp and Lübken (1999) Earth Planets Space 51, 799-807) applied analytical approximations to the rigorous Natanson theory which Natanson (1960) had derived for the case that the mean free path of the ions, (lambda), is much larger than the radius of the particles, rA. This conditions is satisfied for particles around the mesopause where (lambda)~1cm and rA< =100nm (Thomas and Mc Kay (1985) Planet. Space Sci. 33 1209-1244). The aim of this note is to investigate the applicability of Natansonʹs analytical approximations for models of mesospheric aerosol particles.