The nucleation and growth of ice particles in the upper mesosphere

Abstract

The highest clouds and the coldest temperatures in the Earth's atmosphere exist in the region of the summer mesopause at high latitudes. The presence of ice particles in the region leads to several unique phenomena, including sharply bounded layers in which electrons, and occasionally positive ions, are severely depleted (“biteouts”), and intense radar echoes. This paper reviews some of the recent advances in our understanding of the ways in which these ice particles form and grow. Nucleation can occur on heavy positive ions, but the smoke and dust particles resulting from meteor ablation are more likely condensation nuclei. Ice particles probably form mainly near the mesopause and grow as they sediment downward, but the presence of strong horizontal and vertical winds in the region complicate this simple picture. While biteouts are now generally recognized as being due to scavenging of electrons by particles, the reasons for their existence in narrow sharply bounded layers remain unclear. While the subvisible ice particles are likely to be negatively charged under normal conditions, it is pointed out that under conditions of low ionization in the region, negatively and positively charged particles probably exist in roughly equal numbers, leading to the possibility of enhanced growth by coagulation of oppositely charged particles.