Cluster ions and ice particle nucleation: Positive feedback at the summer mesospause

Large proton hydrate clusters H+(H2O)n can act as condensation nuclei for ice particles near the polar summer mesopause. However, laboratory data pose strong requirements on temperature and humidity for this nucleation mechanism to be feasible. Growth to sufficient cluster sizes is governed by competition between clustering reactions and recombination with free electrons. Since the abundance of free electrons can be strongly reduced by capture to ice particles, the presence of ice particles enhances the growth of cluster ions. This leads to a positive feedback mechanism with particle nucleation being favored in regions where particles already exist. Since this mechanism can amplify irregularities in the ionospheric particle and charge distribution, it is of potential interest for the understanding of polar mesosphere summer echoes (PMSE) and other layered phenomena. We demonstrate these ideas using a dusty ion‐chemical model of the D‐region.