Correlations of atmospheric dynamics with solar wind‐induced changes of air‐Earth current density into cloud tops

Abstract

We analyze reported correlations between solar activity and weather and climate and show that in six independent data sets there is a correlation of measured changes in atmospheric dynamics with measured or inferred changes in vertical atmospheric air‐earth current density. The current density changes are due to external modulation of the global electric circuit by the solar wind. We describe the several ways in which the solar wind modulates the global circuit, and the observations that support a simple model of the circuit, with two return paths in parallel. One return path is at low latitudes with relatively constant impedance and the other is at high latitudes and is responsive to solar wind modulation. The six independent data sets exhibiting the correlations include meteorological and air‐earth current density changes on the 10 to 12‐year solar cycle as well as on the day‐to‐day timescales of Forbush decreases of galactic cosmic ray flux and of heliospheric current sheet crossings. The geographic locations include northern and southern high latitudes as well as the tropics. In regions where these correlations are found, there exists free energy in the form of supercooled water droplets near the tops of clouds that are unstable with respect to precipitation. Laboratory data and models suggest that electrostatic charge accumulating on supercooled droplets and aerosols near cloud tops affects the probability of ice nucleation and droplet freezing, enhancing the rate of growth and sedimentation of ice crystals. This proposed mechanism is also an explanation for another longstanding meteorological problem, the discrepancy between measurements at cloud tops of initial concentrations of ice and of concentrations of ice‐forming nuclei. For light cloud cover the effect of increases in ice nucleation and sedimentation can be to reduce cloud opacity and albedo. For storm cloud systems the effect can be to enhance precipitation rates and latent heat release intensifying the storm. In several cases, measured or inferred storm intensification (or weakening) is directly related to measured or inferred increases (or decreases) of air‐earth current density. Thus electrical effects on cloud microphysics may serve as connecting links between the observed or inferred increases in air‐earth current density and the observed changes in atmospheric dynamics. In cases where thunderstorm electric fields are generated there are additional cloud microphysical effects that might contribute to the correlations. We discuss the present uncertainties regarding solar wind effects on the distribution of air‐earth current density in the global electric circuit and regarding the relevant cloud microphysics. Much work is required to quantify these effects and evaluate their importance relative to competing processes.