The solar forcing of climate is presently a high-profile area of scientific research (see e.g., Gray et al., 2010 for a recent review) as climate scientists attempt to disentangle the effects of solar variability on climate from the effects of anthropogenic global warming. The most obvious, best understood and most well-tested mechanisms proposed to explain the Earth’s climate response to solar variability are the effect of variations in total solar irradiance and ultraviolet irradiance on the Earth’s radiation budget. The former provides heat input to the lower atmosphere and oceans, whilst the latter affects stratospheric ozone production and temperature. In addition, solar variability is thought to affect the Earth’s weather and climate through other mechanisms that are unrelated to variations in solar irradiance. In particular, it has been proposed that variations in the interplanetary magnetic field (IMF) can affect weather and climate through its influence on the global electric circuit (GEC) in the Earth’s atmosphere (see reviews by Tinsley, 2008; Siingh et al., 2007; Harrison and Carslaw, 2003; Rycroft et al., 2000), although the associated mechanisms are presently poorly understood.
According to this model, the GEC is predominantly driven by the current output from thunderstorms around the globe which maintain a vertical potential difference in the atmosphere of ~250 kV between the ground and the ionosphere. This atmospheric potential difference drives a small vertical current Jz through non-thunderstorm regions of the atmosphere. Proposed mechanisms as to how variations in this vertical current density affect weather and climate remain underexplored and hence, controversial. The atmospheric current is thought to cause weak electrification of stratified clouds affecting the production of space charge at the boundaries of cloud and aerosol layers, with the charges being transferred to droplets and aerosol particles with consequences to cloud lifetime, precipitation, and the radiative balance and dynamics of the atmosphere (e.g., Tinsley et al., 2006). Hence, any processes (solar-driven or otherwise) that modulate either the Earth-ionosphere potential difference or the atmospheric conductivity in the lower and middle atmosphere will consequently vary the vertical atmospheric current density in the GEC with consequences for meteorological and climate processes (see Figure 1).
Indeed, experimental evidence for the modulation of this current by changes in solar activity exists (Markson and Muir, 1980; Harrison and Usoskin, 2010). The atmospheric conductivity is argued to be controlled by atmospheric ionization from a range of sources such as galactic cosmic rays, solar proton events, and relativistic electron flux, all of which have some relationship to solar activity. The Earth-ionosphere potential difference is mainly determined by the output of the internal generator of highly electrified clouds and thunderstorms, but also, and especially at higher latitudes, by the external generator of solar wind electric fields penetrating into the ionosphere in the geomagnetic polar cap region. This latter process is shown in red in Figure 1 and accurate quantification of the magnitude of this effect is still outstanding and forms the basis of this proposal.
The solar wind and IMF are continually interacting with the magnetosphere of the Earth through the process of magnetic reconnection, such that solar wind mass, momentum and energy are transferred into and around the magnetospheric system. This interaction drives a plasma velocity field V across the magnetospheric magnetic field B which generates an electric field -V x B and a dawn-to-dusk potential difference of ~30-150 kV. The exact magnitude depends on the strength of the solar wind driver, which is highly dependent on the IMF magnitude and direction. This electric field and potential pattern map down conducting magnetic field lines into the high-latitude polar cap ionosphere. The ionospheric potential pattern has maxima and minima in the dawn and dusk segments of the polar cap, respectively, and is responsible for varying the otherwise globally uniform vertical atmospheric potential difference (~250 kV) in these regions (see Figure 2).