On the influence of electrostatic charging on coagulation of dust and ice particles in the upper mesosphere

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Abstract
The upper mesosphere, at heights above about 80 km, is the region of the atmosphere in which most ablation of incoming meteoric material takes place, and is thought to contain tiny nanometer‐size smoke and dust particles with concentrations of several thousand per cubic centimeter. In the extremely cold summer conditions at high latitudes, these particles probably form the condensation nuclei on which ice particles grow, giving rise to a variety of unique phenomena, including noctilucent clouds and localized electron depletions (“biteouts”). Recent work has shown that the particles become electrically charged by scavenging free electrons and positive ions from the background ionosphere. Under normal daytime conditions, calculations of charge balance have shown that most of the particles will carry a single negative charge, and it has been pointed out that electrostatic repulsion will inhibit coagulation of the particles, which would otherwise be an important mechanism for their growth. It is shown here that the opposite is true when ionization rates are very low, for example at night in the absence of energetic‐particle precipitation. In this case, positively and negatively charged particles tend to exist in equal numbers, and coagulation is enhanced by charging, instead of being inhibited. Because of the major unknown factors other than charging that affect coagulation, only idealized semi‐quantitative estimates of coagulation rates can be made. Electrical charging, however, appears to be a much less serious barrier to coagulation than had been thought, and may actually be a powerful stimulant.