Charged Aerosols in the Polar Summer Mesosphere

Trude Eidhammer

University of Bergen, Norway

Aerosols in the mesopause (at times seen as Noctilucent clouds) can carry charges. The aerosols can be charged due to collisions with plasma particles and will most likely obtain a few negative charges. The commonly accepted view about the composition of the mesospheric aerosols is that they probably consist of ice. Pure ice particles are not easily photo-ionized (the source of producing positively charged aerosols) and are unlikely to obtain positive charges. However, rocket measurements in 1996 (called DUSTY 2) (Havnes et al., 1996) showed the existence of strongly positively charged aerosols.

The aerosol temperature (Td), which is dependent on the aerosol size, may explain the existence of positively charged aerosols in the upper mesosphere. In general, Td increases with aerosol size (Grams and Fiocco, 1977). This increase of Td with size may lead to situations where there is a maximum size of the aerosol above which Td is too high for condensation of water vapor to occur. Aerosols of this size can still accrete other elements. If the aerosols accrete a sufficient amount of elements with low ionization potential as a surface "contamination", the photo-electric properties of the aerosols may change. It is suggested that the change of photoelectric effect can lead to the charges of the aerosols changing from the "normal" low negative value of pure ice particles to the positive charges which rocket in situ observations have shown to exist. In this situation we will have a mixture of small, newly created and negatively charged aerosol particles coexisting with larger and positively charged aerosols. Agglomeration of aerosols will then be an effective process.