Discrimination between cosmic ray and solar irradiance effects on clouds, and evidence for geophysical modulation of cloud thickness

Abstract
Solar activity and galactic cosmic rays (CRs) are closely inversely correlated. In studies of cloud changes associated with solar changes, the close solar activity–CR relationship can cause ambiguity in attributing the cloud changes to one factor or the other. A method for discriminating between cloud effects from these causes is described, using the 1.68 yr periodicity present in surface-based measurements of CRs but absent in radio flux measurements of solar activity. The periodicity is present in CRs and other heliospheric parameters during, at least, solar cycle 21. Daily surface radiation measurements sensitive to cloud during a similar period (1978–1990) at Jersey and Lerwick exhibit 1.68 yr periodicities, indicating a CR, rather than solar irradiance, cloud effect. In the Jersey data, it is not possible to discriminate between ‘near-cloud’ (droplet charging) and ‘clear-air’ (ion nucleation) effects, but further study of the Lerwick cloud data shows that the periodicity is present only during thick overcast cloud conditions. Bandpass filtering the cloud and CR data (passband 1.55–1.81 years) shows that the cloud thickness responds in phase with the CR flux. This modulation suggests the near-cloud effect through geophysical modulation of the air–Earth current density. Cloud thickness modulation by the air–Earth current density can occur by the charge-enhanced activation (electroactivation) of cloud droplets on the upper and lower boundaries of stratiform cloud.