Solar wind modulation of lightning

Recent research by the Space & Atmospheric Electricity (SPATE) group at the University of Reading has demonstrated a clear modulation of lighting rates across Europe by the arrival of high-speed solar wind streams at Earth. Furthermore, the magnetic polarity of the solar wind has been shown to play a role. While the exact mechanism remains as yet unknown, evidence to date suggests that it could be through the modulation of galactic cosmic rays (GCRs) by the heliospheric magnetic field and/or by energetic particles within the solar wind. On arrival at Earth, these energetic particles are further affected by the Earth's magnetic field, which prevents all but the most energetic from being detected at the ground. Energetic particles in the lower atmosphere modify the electrical conductivity of the air, which in turn is thought to enable lightning to occur at much lower electric field strengths within thunderclouds.

Lightning forms part of the Global Electric Circuit, with thunderstorms acting like a battery, transferring charge from the Earth's surface to the lower ionosphere (the electrified part of the Earth's upper atmosphere starting at altitudes around 60 km). Since the ionosphere is electrically conducting, this charge is spread globally from the local thunderstorm regions, where it then leaks back to earth as a small vertical current in fair weather regions around the globe. Thus the ionosphere and the Earth's surface act as two plates of a spherical capacitor with thunderstorms charging the ionosphere to a potential of around 250 kV with respect to the surface and the weak conductivity of the atmosphere providing a pathway for this charge to leak back to the surface.

Prior research by the project supervisor has demonstrated that it is not just the lower ionosphere that is enhanced by lightning, with the concentration of electrified layers at around 100km being enhanced following lightning activity on the ground. Anomalies in the ionosphere density between hemispheres also hints at lightning as a cause.

There is clearly much to be learned about the influence of the solar wind on the Earth's upper and lower atmospheres. Since the upper atmosphere and ionosphere are dominated by variations in solar activity and solar wind, any mechanisms which link the upper and lower atmospheres provides a conduit by which solar activity can influence the lower atmosphere. In their work to date, researchers within the SPATE group have demonstrated that the passage of a high-speed solar wind stream can increase the observed lightning rate by as much as 30%. Since such solar wind streams rotate with the 27 day rotation of the Sun, their arrival at Earth is very predictable. Such information could potentially help in forecasting the severity of lightning storms.

Having identified this new area for research, in this project, the student will study the wealth of existing data from the heliosphere, ionosphere and lower atmosphere in innovative cross-disciplinary studies in order to identify the mechanisms involved, determine the global extent of the effects and provide insight into the potential for improving lightning forecasts.