Planetary magnetic fields averaged in their dynamo regions

S. V. Starchenko

Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences (IZMIRAN), 4 Kaluzhskoye highway, Troitsk, Moscow 108840 Russia

Abstract
The volume-averaged or mean arithmetical (MA) magnetic field vector in the whole dynamo region is already known once the observable magnetic dipole and region size of a planet/moon are known. This MA field is 1.4 of the root-mean-square (RMS) dipole field at the outer dynamo boundary. The Earth, Jupiter and Saturn have MA fields of 1 mT and small inclinations of their dipoles to the axis of rotation. This common feature for similar dynamos could be in these rather different planets. Uranus and Neptune have MA fields of  ∼  0.1 mT and their dipoles are strongly inclined requiring not so clear modifications in dynamo modelling. A similar MA field value is for Ganymede (a moon of Jupiter), whereas an almost symmetric to the rotation axis field and rather asymmetric to the equator is observed. Almost the same field spatial configuration is observed in Mercury, where the MA field is  ∼  0.01 mT. There is no active dynamo in Venus, Mars, Moon and in some other planets/moons, whereas an existence of ancient/future self magnetic fields is possible there. The geodynamo RMS field typical value is about a few mT, as it is estimated from the well-known Christensen-Aubert (2006) scaling law. That gives a geodynamo RMS/MA field ratio of the order 10. The same ratio could be expected in Jupiter and Saturn, whereas one of Starchenko (2018) scaling law gives a ratio  ∼  100 for such geodynamo-type system that is consistent with some observational estimations and dynamo models. Uranus, Neptune and Ganymede, perhaps, have the RMS field just slightly (up to a few times) exceeding the MA field. The intermittency of the internal magnetic field also could be roughly measured by this RMS/MA field ratio for the objects with a sufficiently different from zero MA field. Figs 1, Refs 20.