On the charging of mesospheric dust

Polar mesosphere summer echoes (PMSE) are strong radar returns from the cold summer mesopause region, at altitudes ~ 80-95 km at high latitudes. The probing radar frequency can range from about 50 MHz to over 1 GHz. The prevailing theory to explain PMSE involves both neutral air turbulence and the presence of charged dust (probably ice) which slows down electron diffusion. This turbulence scattering theory requires a large value of the Schmidt number (the ratio of neutral viscosity to plasma diffusion) to explain observations. There have been questions recently about how large the Schmidt number has to be, and how realistic these values are, with regard to explaining PMSE at higher UHF frequencies. Recently, observations of PMSE at 33 cm using the Poker Flat Incoherent Scatter Radar (PFISR) have been reported1. The observations were made at night, and occurred with auroral particle precipitation which enhanced the D region ionization. We consider the charging of dust grains under these conditions, taking into account charging by thermal plasma and possible high energy particles, for a range of grain sizes. We also consider the effect of an enhanced dust surface area if the grains are irregularly structured. Schmidt numbers are estimated to explore whether turbulence scattering theory works in this case.