  We present a theoretical analysis of the ability of atomic magnetometers to
estimate a fluctuating magnetic field. Our analysis makes use of a Gaussian
state description of the atoms and the probing field, and it presents the
estimator of the field and a measure of its uncertainty which coincides in the
appropriate limit with the achievements for a static field. We show by
simulations that the estimator for the current value of the field
systematically lags behind the actual value of the field, and we suggest a more
complete theory, where measurement results at any time are used to update and
improve both the estimate of the current value and the estimate of past values
of the B-field.
