Geomagnetic Variability and Climate Change: Is there a link?

The Earth's magnetic field varies over many time scales. Whilst the slow secular variation of the strength and direction of the field over years to centuries is governed by processes in the fluid outer core of the Earth, the shorter variations, on time scales of seconds to years, are driven by the Sun. These external field variations are classified as irregular or regular. The larger irregular variations, commonly known as geomagnetic activity or storms, occur as a consequence of extreme events on the Sun such as coronal mass ejections or (usually with less intensity) as a result of regions of increased solar wind speed from coronal holes. The (relatively) regular is due to currents flowing in the ionosphere where the atmosphere is ionised by the Sun's UV radiation. In this paper we discuss whether long-term changes in can be useful proxies for changes in solar radiation. The Sun's significance for climate change appears to be clear, since its radiation is the Earth's source of external energy. However, there is controversy over what levels of solar variability are required to generate significant climate change and what the mechanisms are. Indices are often used to characterise geomagnetic activity and are well correlated with indices characterising solar activity. The index is derived from measurements made at near-antipodal magnetic observatories: one in the south of England, currently Hartland magnetic observatory, operated by BGS, and the other in Australia, currently Canberra magnetic observatory, operated by Geoscience Australia. The aa data set extends back to 1868, one of the longest geophysical time series, and is evidently related to solar activity, parameterised by the sunspot number, throughout the period (left). In particular during the minimum phase of the Sun's 11-year cycle there has been a steady increase in geomagnetic activity.