The magnetic field of stars and planets is generated by the turbulent
motions of internal electrically conducting fluids.
This mechanism is named dynamo instability. Untill now, it was believed
that this instability required flows of complicated geometry. Physicists
at Laboratoire de Physique Statistique at ENS (LPS, CNRS / ENS / Univ. Paris Diderot/UPMC) have shown that such a complicated flow is not necessary if the
electrical conductivity of the fluid is not assumed to be uniform. In
that case, a new induction mechanism takes place so that planar flows
can create a dynamo, which is impossible if the conductivity is uniform.
This result provides an explanation for the spatial structure of the
magnetic field of the iced giants (Neptune and Uranus) which magnetic
field has a strong equatorial dipole component.
This picture shows the dipole field generated at the core-mantle boundary in one of our geodynamo simulations. In this model, fluctuations of the electrical conductivity of the fluid, due to variations of temperature in the core for instance, act as a new source for astrophysical magnetic fields.
[ credit : C. Gissinger]
CNRS : http://www.cnrs.fr/inp/spip.php?article5065
Physical Review Letters : http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.161102
Arxiv : https://arxiv.org/abs/1604.00469
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