The project investigated one of the potential indirect effects of solar variability on atmospheric processes, which is the effect of electrical charge on clouds. Charge is created in the atmosphere by ionisation from Galactic Cosmic Rays (GCRs) (highly energetic particles from outside our solar system). When GCRs approach Earth, they are deflected by both the Solar and Earth's magnetic field, which act as a selective energy barrier to GCRs. The sun's magnetic field varies mainly on an 11 year timescale, therefore GCR fluxes, and thus charge in the atmosphere is controlled by solar activity. Due to constant vertical flow of charge in the atmosphere, charge accumulates at the upper and lower boundaries of layer clouds (the very common sort of clouds that you see on an overcast day). The charge sticks to the cloud droplets, which is thought to influence the behaviour of the droplets, such as how they grow and stick together, which can be seen in large scale cloud properties like cloud height. Since such clouds control heating and cooling in the atmosphere, and cover around 40% of the Earth's surface at one time, charge effects on clouds may have implications for climate. The project investigated the factors that control charge in the atmosphere, determine whether charge plays a role in cloud processes, and ultimately determine whether this is important for climate.

In order to characterise the factors controlling charge in the atmosphere, and the typical charge present inside layer clouds, measurements were made using a suite of newly developed sensors which have been designed to fly alongside conventional weather balloons. These lightweight, disposable sensors provide a cost effective method of obtaining extra science data above the surface, from weather balloons which are already being launched around the world by global meteorological services. These airborne measurements were combined with surface measurements of charge and atmospheric electricity at various sites around the world to understand the global response of charge to changes in solar variability. Such measurements are rare and are vital to understand the physical mechanisms responsible for modulating vertical charge flow and therefore coupling between Space Weather and the lower atmosphere.

Objectives: This project aims to investigate one of the proposed mechanisms by which natural solar variability may influence climate, through the effect of energetic particles on clouds. A combination of new measurements, both at the surface and in the free atmosphere, was used to better characterise the flux of energetic particles entering Earth's lower atmosphere, and asses their influence on cloud properties. Energetic particles create current flow in the atmosphere, which is responsible for generating charge at the edges of non thunderstorm clouds. The first set of objectives was therefore concerned with characterising vertical current flow in the atmosphere and how it varies with solar activity. The second set of objectives deals with the charge response of clouds to such current flow.