Solar Linkages to Atmospheric Processes (SLAP) is an International Polar Year project investigating the links between changes in solar output and weather and climate.

Thunderstorms and lightning strikes drive electricity around the world and form part of a global ‘atmospheric electric circuit’ that flows between the ground and the lower reaches of the ionosphere — about 80km up.

Thunderstorms and electrified clouds are the ‘batteries’ of the atmospheric electric circuit, which drive the current from the ground to the ionosphere, while lightning is a visual representation of the current. The flow of current around the world is modulated by cosmic rays, which control atmospheric conductivity. (Cosmic rays are in turn modulated by the solar wind). The circuit is completed when the current trickles back to Earth, in regions remote from thunderstorm activity, such as Antarctica.

Through the International Polar Year (IPY) project, Solar Linkages to Atmospheric Processes, Australian Antarctic Division scientist, Dr Gary Burns, and his colleagues Drs Oleg Troshichev and Alexandr Frank-Kamenetsky of the Arctic and Antarctic Research Institute, are measuring the atmospheric circuit high on the Antarctic plateau at Vostok, near the centre of East Antarctica. Instruments have also been deployed at three sites in West Antarctica by Dr Martin Jarvis of the British Antarctic Survey and another will be deployed at the French-Italian station, Concordia, at Dome C, in January 2009.

Meteorological and solar variability influences on the atmospheric circuit are well established (see box below). The question now is whether the electric circuit actively links solar variability and weather, or if it responds passively to both meteorological and solar variations. Understanding this interaction is important because changes in the global electric circuit, caused by solar variability, could alter the conditions under which thunderstorms develop. Recent results from Dr Burns and his colleagues support an active link.

A theory under investigation is that current flowing through the atmospheric electric circuit influences cloud formation. Measurement of the circuit will enable investigation of the cloud microphysics processes and meteorological responses (such as thunderstorms) at sites around the world.

Accurate measurements of the atmospheric electric circuit could also enable scientists to monitor changes in global thunderstorm activity as the world warms.

'It’s thought that an increase in temperature of one degree Celsius could increase meteorological electrical activity by 10 percent,' Dr Burns says.

'So changes in the global electric circuit could provide an indication of the way the Earth’s weather is changing.'

A model of the global electric circuit has been developed by collaborators at the University of Texas, Dallas, incorporating variations in cosmic rays, energetic particles, natural radioactivity and aerosols. Outputs from the model will be compared with measured atmospheric circuit responses to these variations, to refine understanding of the processes involved.

Measuring the atmospheric electric circuit

The atmospheric electric circuit responds quickly to global thunderstorm activity. Thunderstorms preferentially occur in summer, over land in the equatorial and mid-latitude regions and in the local afternoon.

Despite this, the global variation in the atmospheric circuit, due to meteorological influences, is the same everywhere at the same time (because the ionosphere and the earth are so conductive that the influence of all meteorological electrical activity is rapidly integrated).

At good sites (those with low local daily variations in atmospheric conductivity, such as the Antarctic plateau) the average daily variation in the electric circuit reflects the distribution of land across the globe. For example, in the figure at right, 20UT (Universal Time or Greenwich Mean Time) corresponds to when thunderstorm activity peaks over the Americas, 15UT is Africa and Europe, 8UT is Australia and Asia and the minimum around 3UT corresponds to local afternoon in the Pacific, where there is little land area and thus relatively few thunderstorms.