A measure of the disturbance of the Earth’s magnetic field caused by fluctuations in the strength of the solar wind; the name comes from the German planetarische Kennziffer, meaning ‘planetary index’. It is compiled from magnetometer readings at ground-based observatories around the globe and is reported on a scale from 0 to 9; a level of 5 or more indicates a geomagnetic storm. At times of high Kp readings the auroral ovals expand towards the equator, resulting in aurorae being seen from lower latitudes than usual. The Kp index was introduced in 1949 by the German geophysicist Julius August Bartels (1899–1964) and was adopted internationally in 1951.