Electrical structure of noctilucent cloud and polar mesosphere summer echo regions

Abstract

A number of recent coordinated rocket and ground-based investigations have concentrated on the cold summer mesopause region where the occurrence of noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSEs) provides excellent examples of layering processes observable over both relatively small and large altitude scales. This paper provides an overview of our recently flown instrumentation and the resulting measurements of mesospheric electrical structure during observed NLC/PMSE conditions. We begin with a brief description of our different probe techniques used to characterize this region's electrical structure, specifically, its polar components of conductivity, the mobilities and concentrations of different positively charged species, and the relative concentration of electron density, including its large- and small-scale vertical structure. Evidence for an electron “bite-out” at NLC/PMSE altitudes exists in our measurements of negative electrical conductivity and the Langmuir probe current data for collected electrons. A very challenging problem has been the detection of charged particles or aerosols, which we have addressed by using different probe geometries and charge collection schemes. Blunt probes operating under ram-flow conditions have measured current fluctuations suggestive of impacting aerosols/dust in both negatively and positively charged forms. Also, swept-voltage Gerdien condensers with stabilized flow geometries have identified very small mobility values that are believed to indicate the presence of sub-visible positive charged particles. The number densities of these species are particularly pronounced in the NLC/PMSE regions.