NONLINEAR VARIABILITY IN THE GEOMAGNETIC SECULAR VARIATION OF THE LAST 150 YEARS

A nonlinear forecasting analysis has been applied to the secular variation of the three-component annual means of 14 observatories, unevenly distributed over the Earth's surface (12 in the northern and 2 in the southern hemisphere) and spanning the last 150 years. All results were in agreement, either in terms of possible evidence of chaos (as opposed to the hypothesis of white or colored noise), or in terms of the Kolmogorov entropy, confirming previous results obtained with only three European observatories, i.e. it is practically impossible to predict the secular variation of the geomagnetic field more than six years into the future.