British Geological Survey, Edinburgh EH9 3LA, UK

The behaviour of the geomagnetic field as observed almost continuously at three European locations over the last 130 years is investigated by means of a non-linear forecasting approach. The analysis of the data in terms of first-differences (secular variation) of the horizontal magnetic components made in phase space with the simplex technique seems to exclude the pre-eminence of any stochastic or periodic behaviour. The dimensionality of the underlying non-linear process and the corresponding largest positive Lyapunov exponent are estimated. The results give some evidence that the geomagnetic field evolves as a non-linear chaotic system with unpredictable behaviour after times greater than a few years, confirming the common practice of updating global models of the geomagnetic field every 5 years.