As a consequence of the smallness of the electronic fine structure constant, the characteristic time scale for the free diffusive decay of a magnetic field in a planetary core is much less than the age of the Solar System, but the characteristic time scale for thermal diffusion is greater than the age of the Solar System. Consequently, primordial fields and permanent magnetism are small and the only means of providing a substantial planetary magnetic field is the dynamo process. This requires a large region which is fluid, electrically conducting and maintained in a non-uniform motion that includes a substantial RMS vertical component. The attributes of fluidity and conductivity are readily provided in the deep interiors of all planets and most satellites, either in the form of an Fe alloy with a low eutectic temperature (e.g. Fe-S-O in terrestrial bodies and satellites) or by the occupation of conduction states in fluid hydrogen or 'ice' (H2O-NH3-CH4) in giant planets. It is argued that planetary dynamos are almost certainly maintained by convection (compositional and/or thermal).

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Planetary magnetic fields

The Pioneer Saturn vector helium magnetometer has detected a bow shock and magnetopause at Saturn and has provided an accurate characterization of the planetary field. The equatorial surface field is 0.20 gauss, a factor of 3 to 5 times smaller than anticipated on the basis of attempted scalings from Earth and Jupiter. The tilt angle between the magnetic dipole axis and Saturn's rotation axis is < 1°, a surprisingly small value. Spherical harmonic analysis of the measurements shows that the ratio of quadrupole to dipole moments is < 10 percent, indicating that the field is more uniform than those of the Earth or Jupiter and consistent with Saturn having a relatively small core. The field in the outer magnetosphere shows systematic departures from the dipole field, principally a compression of the field near noon and an equatorial orientation associated with a current sheet near dawn. A hydromagnetic wake resulting from the interaction of Titan with the rotating magnetosphere appears to have been observed.

/pdf/saturn-s-magnetic-field-and-magnetosphere-3kqfndom8g.pdf

Saturn's Magnetic Field and Magnetosphere

Initial observations by the Pioneer Venus mangnetometer in the sunlit ionosphere reveal a dynamic ionosphere, very responsive to external solar-wind conditions. The localtions of the bow shock and ionosphere are variable. The strength of the magnetic field just olutside the ionopause is in approximate pressure balance with the thermal plasma of the ionosphere and changes markedly from day, to day in response to changes in solar wind pressure. The field strength in the ionosphere is also variable from day to day. The field is often weak, at most a few gammas, but reaching many tens of gammas for periods of the order of seconds. These field enchantments are interpreted as due to the passage of spacecraft through flux ropes consisting of bundles of twisted field lines surrounded by the ionospheric plasma. The helicity of the flux varies through the flux tube, with lows pitch angles on the inside and very lage angles in the low-field outer edges of the ropes. These ropes may have external or internal sources. Consistent with previous results, the average position of the bow shock is much closer to the planet than would be expected if the solar wnd were completely deflected by the planet. In total, these observations indicate that the solar wind plays a significant role in the physics of the Venus ionosphere.

https://science.sciencemag.org/content/sci/205/4401/114.full.pdf

Initial Pioneer Venus Magnetic Field Results: Dayside Observations

With the purpose of estimating Neptune's magnetic field and its implications for nonthermal Neptune radio emissions, a new scaling law for planetary magnetic fields was developed in terms of externally observable parameters (the planet's mean density, radius, mass, rotation rate, and internal heat source luminosity). From a comparison of theory and observations by Voyager it was concluded that planetary dynamos are two-state systems with either zero intrinsic magnetic field (for planets with low internal heat source) or (for planets with the internal heat source sufficiently strong to drive convection) a magnetic field near the upper bound determined from magnetostrophic balance. It is noted that mass loading of the Neptune magnetosphere by Triton may play an important role in the generation of nonthermal radio emissions.

Magnetostrophic balance in planetary dynamos: Predictions for Neptune's magnetosphere

A new scaling law for the planetary magnetism was developed on the assumption that the Lorentz force balances the Coriolis force in the planetary core. In deriving the scaling law we decomposed the vector fields into the toroidal and the poloidal components, and dealt with them separately. From the scaling law, not only we can predict the dipole moments of the planets, but also can we estimate the toroidal magnetic field intensity in the planetary core; the result implies a typical toroidal magnetic field intensity of 100 [G], and the toroidal and the poloidal velocity fields respectively of the orders of 1 x 10-5 [ms-1] and 4 x 10-7 [ms-1] in the Earth's core. Since the present study is based on an αω-dynamo model, the resultant scaling law depends on the efficiency of the α-effect. If we adopt the dependence of the form α ∝ Ω (with Ω the angular velocity of the planet's self-rotation), the magnetic dipole moment M of a planet scales as M ∝ (characteristic length)7/2(mean density)1/2(angular velocity). The predictions agree well with the observations except Venus and Mars.

The Magnetic Fields of the Planets: A New Scaling Law of the Dipole Moments of the Planetary Magnetism

A dimensional analysis, based on a simple model, indicates that at present both Venus and Mercury have a fluid core and an intrinsic magnetic field, but that this is not the case for Mars nor for the Moon.

J. A. Jacobs

Papers

Planetary magnetic fields