  Satellite observations of the geomagnetic field contain signals generated in
Earth's interior by electrical currents in the core and by magnetized rocks in
the lithosphere. At short wavelengths the lithospheric signal dominates,
obscuring the signal from the core. Here we present details of a method to
co-estimate separate models for the core and lithospheric fields, which are
allowed to overlap in spherical harmonic degree, that makes use of prior
information to aid the separation. Using a maximum entropy method we estimate
probabilistic models for the time-dependent core field and the static
lithospheric field that satisfy constraints provided by satellite observations
while being consistent with prior knowledge of the spatial covariance and
expected magnitude of each field at its source surface.
