  Observational constraints on geomagnetic field changes from interannual to
millenial periods are reviewed, and the current resolution of field models
(covering archeological to satellite eras) is discussed. With the perspective
of data assimilation, emphasis is put on uncertainties entaching Gauss
coefficients, and on the statistical properties of ground-based records. These
latter potentially call for leaving behind the notion of geomagnetic jerks. The
accuracy at which we recover interannual changes also requires considering with
caution the apparent periodicity seen in the secular acceleration from
satellite data. I then address the interpretation of recorded magnetic
fluctuations in terms of core dynamics, highlighting the need for models that
allow (or pre-suppose) a magnetic energy orders of magnitudes larger than the
kinetic energy at large length-scales, a target for future numerical
simulations of the geodynamo. I finally recall the first attempts at
implementing geomagnetic data assimilation algorithms.
