Geomagnetic and Solar Variability and Natural Climate Change

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 diurnal variation is due to currents flowing in the ionosphere where the atmosphere is ionised by the Sun's UV and X radiation. Controversy remains over what levels of solar variability are required to generate significant climate change and what the mechanisms are. We discuss here whether long-term changes in the two phenomena mentioned above can be useful proxies for changes in solar radiation, and thus be useful for studies attempting to answer these questions.