Modelling of positively charged aerosols in the polar summer mesopause region

Abstract

We present a model based analysis of rocket borne common volume measurements of electron number densities and aerosol charge densities during the ECHO campaign in 1994. During that campaign a sounding rocket was launched into a noctilucent cloud (NLC) as detected by a ground based lidar. At NLC altitudes a particle impact detector gave strong evidence for positively charged aerosols, and an electron probe measured a significant electron enhancement. We have applied a model of aerosol charging to these measurements and find that the existence of positively charged aerosols can be explained if they mainly consist of a substance with a sufficiently low work function. The electron enhancement as well as the aerosol size and number density deduced from our model are consistent with the electron probe and lidar measurements, respectively. Considering the photoelectrical properties of various metals we conclude that only sodium and potassium have a sufficiently low work function to allow for significant photoemission. Even under very favourable conditions the maximum positive charge accumulated on the aerosols is only approximately 4 elementary charges which is much less than discussed in some of the current theories for the creation of polar mesosphere summer echoes. We note that the amount of sodium or potassium required to form these particles is far above the natural abundances at NLC altitudes. The exact abundance and composition of the aerosols need to be known at the time of the in situ measurements in order to make more sophisticated comparisons between measurements and models.