Abstract
The INTERMAGNET program publishes each year a CD-ROM containing homogeneous series from a number of magnetic observatories (76 in 1999). These series are definitive one-minute values of the three components of the geomagnetic field. We transform these series using a simple nonlinear analysis tool able to characterize the activity of a signal, and we obtain a remarkably simple activity field, whose space and time variables separate over a large part of the Earth. The time function is almost identical for all observatories, and might be interpreted as an activity index. The-almost stationary-field geometry exhibits a dipole-like structure everywhere except in high latitudes. Key words: Geomagnetic field, irregular variations, INTERMAGNET, absolute derivative.
