The magnetic fields of Uranus and Neptune: Methods and models

We presennew models ofhe magnetic fields of Uranus and Neptune, based on data provided by l;he Voyager II magnel;ic field experiment. We find the simplesl; models l;hal; sal;isfy l;he dal;a, and use l;he observed surface heal; flow as a consrainonhe magnetic field srucure. Our models are similarohe previously described Q3 and 08 models far from l;he planel;s, bul; resolve smaller-scale strucl;ure close tohe planes' surfaces. The field of Neptune is much beter constrainedhan thaof Uranus, andhe field inhe northern hemisphere of Neptune is much better consl;rained than thai; in the soul;hern hemisphere, of importance for studies of Nepl;unian norl;hern hemisphere radio sources. Using exl;remal models, we show hat the large dipole tils and nondipole dominance of the fields are robust features required by the daa. Scaling analysis suggests thathe toroidal field that would be required for a magnetostrophic balance in the dynamo region would result in ohmic dissipal;ion greal;er l;han l;he observed surface heal; flow. Thus we suggest l;hat l;he dynamos of Uranus and Nepl;une are energy limited, and thai; l;he subsequent lack of magnetostrophic balance may account for the radically different field morphologies of Uranus and Nepl;une compared wil;h l;he Earl;h, Jupiter, and Saturn.