Atmospheric Ionization and Clouds as Links Between Solar Activity and Climate

Observations of changes in cloud properties that correlate with the 11-year cycles in space particle fluxes are reviewed. The correlations can be understood in terms of one or both of two microphysical processes; ion mediated nucleation (IMN) and electroscavenging. IMN relies on the presence of ions to provide the condensation sites for sulfuric acid and water vapors to produce new aerosol particles, which, under certain conditions, might grow into sizes that can be activated as cloud condensation nuclei (CCN). Electroscavenging depends on the buildup of space charge at the tops and bottoms of clouds as the vertical current density (Jz) in the global electric circuit encounters the increased electrical resistivity of the clouds. Space charge is electrostatic charge density due to a difference between the concentrations of positive and negative ions. Calculations indicate that this electrostatic charge on aerosol particles can enhance the rate at which they are scavenged by cloud droplets. The aerosol particles for which scavenging is important are those that act as insitu ice forming nuclei (IFN) and CCN. Both IMN and electroscavenging depend on the presence of atmospheric ions that are generated, in regions of the atmosphere relevant for effects on clouds, by galactic cosmic rays (GCR). The space charge depends, in addition, on the magnitude of Jz. The magnitude of Jz depends not only on the GCR flux, but also on the fluxes of MeV electrons from the radiation belts, and the ionospheric potentials generated by the solar wind, that can vary independently of the GCR flux. The roles of GCR and Jz in cloud processes are the speculative links in a series connecting solar activity, the solar wind, GCR, clouds and climate. This article reviews the correlated cloud variations and the two mechanisms proposed as possible explanations for these links.