3-DIMENSIONAL KINEMATIC DYNAMOS WITH EQUATORIAL SYMMETRY - APPLICATION TO THE MAGNETIC-FIELDS OF URANUS AND NEPTUNE

Previous studies of kinematic dynamo theory have sought solutions domi nated by the axial dipole component of magnetic field, so as to model the field of the Earth. Motivated by the observed magnetic fields of t he planets Uranus and Neptune, we obtain 3D kinematic dynamos with a d ifferent morphology: specifically, fields possessing equatorial symmet ry, and so lacking an axial dipole component, where the axis is that o f the differential rotation present in most of the flows studied. We b ase our investigations on the standard Kumar-Roberts flow, and find th at with a small alteration this flow is capable of supporting a wide r ange of dynamos with equatorial symmetry, albeit in a different region of parameter space from the Earth-like solutions. A particularly inte resting example is a dynamo generated by a flow with no toroidal compo nent. We include an impenetrable inner core within the dynamo region, but find its effect to be slight compared with small alterations in th e chosen flow, and observe no effect on the preferred symmetry of the solution. Solutions for a conducting or insulating inner core are very similar, although there is evidence that this is due to our particula r choice of flow. On the evidence of kinematic dynamos, the axial dipo le morphology seems to be strongly preferred only in the presence of s trong axial differential rotation. However, our results are sensitive to the ad hoc choice of steady flow, which may limit the applicability of our conclusions. (C) 1997 Elsevier Science B.V.