Ice particles and electron “bite‐outs” at the summer polar mesopause

Rocket measurements of electron density profiles in the vicinity of the high‐latitude summer mesopause have frequently shown the existence of a sharply bounded layer of thickness ∼1 km in which the electron density is depleted by as much as an order of magnitude below the value that would be inferred from the densities above and below. The most plausible explanation of these “bite‐outs” is that they are caused by scavenging of the electrons by small ice particles that exist in the cold environment of the polar summer mesopause and that are related to those responsible for noctilucent and polar mesospheric clouds. This possibility is explored quantitatively using a simplified version of relations developed earlier (Natanson, 1960) to describe the charging of aerosols. It is shown that the radius of the ice particles must be of the order of 10 nm in order to satisfy the number density requirement and at the same time keep the water content within reasonable bounds. These particles would be too small to produce a visible cloud but may form a semipermanent substrate from which the larger cloud particles can develop under suitable conditions.