  Current "Earth-like" numerical dynamo simulations are able to reproduce many
characteristics of the observed geomagnetic field. One notable exception is the
geomagnetic octupolar component. Here we investigate whether a stably
stratified layer at the top of the core, a missing ingredient in standard
dynamo simulations, can explain the observed geomagnetic octupole. Through
numerical simulations, we find that the existence of a stable layer has
significant influence on the octupolar-to-dipolar ratio of the magnetic field.
Particularly, we find that a 60 km stable layer with relatively strong
stability or a 130 km layer with relatively weak stability are compatible with
the observations, but a 350 km stable layer, as suggested by recent
seismological evidence, is not compatible with Earth's octupole field over the
past 10,000 years.
