The Origin Of The Geomagnetic Field
Alternate Models For Planetary Processes, Part 6
This is the sixth of seven posts in support of the PROM article “An integrated physical model characterizing planetary magnetism and heat”, which proposes an alternative origin for the geomagnetic field versus the consensus geodynamo theory.
Each PSI post documents an alternate model to the existing scientific consensus on planetary processes. Post 6 deals with the main topic of the PROM paper: the magnetic fields of the planets are generated by a Solar Wind Induced Electromagnet (SWIEM) powered by Solar Wind Induced Currents (SWICs) and not a thermal or mechanical dynamo.
The prevailing theory on planetary magnetogenesis
All the magnetic fields of the planets and most of their satellites are also commonly thought to be generated by a dynamo, whereby convection cells convert thermal and mechanical energy to magnetic energy via induced electric currents [1,2]. A magnetogenic dynamo requires:
- A highly-conductive fluid medium, such as the Earth’s ferromagnetic Outer Core
- Kinetic energy from the planet’s rotation (Coriolis force)
- A thermal or mechanical energy source powering the fluid convection
- A planetary magnetic field that induces currents in the convecting fluid
The Problems with a Planetary Dynamo model
The following discussion deals only with the significant magnetic fields in our solar system: Earth, the Gas Giants, and Jupiter’s Galilean Satellites. Mercury is omitted as it has an unusually weak field, and Saturn’s satellites are omitted due to a lack of comprehensive datasets. All proposed planetary dynamos require highly speculative scenarios.
Part 1: a highly-conductive fluid medium
Jupiter, Saturn, Uranus, Neptune, and the Galilean Satellites all likely have solid, ferrous cores [4,5]. This is not a problem under the alternate SWIEM theory, which only requires a – solid or fluid-highly conductive ferromagnetic core, but is a problem under dynamo theory, which requires a highly-conductive, convecting fluid layer.
Additionally, the dynamo fluids must be relatively shallow, as consensus theory indicates much of the magnetic energy generated by the dynamo will be absorbed by the overlying (Mantle) layers, which again is not a problem under SWIEM theory as non-ferromagnetic materials – such as the Mantles of most planets and satellites – do not significantly absorb the low-frequency SWIEM-generated magnetic energy (PSI Post 4).
Most authors generally assume a dynamo is generating the planetary field and propose highly-speculative fluid layers in the outer regions of the planets as the dynamo source layers, for example, water-rich layers at 0.7-0.8 planetary radii for Neptune and Uranus [1]. While theoretically possible, such layers are highly unlikely dynamo sources, as their low conductivity – typically on the order of 1-10 S/m – ensures that any significant magnetic energy generated by such a dynamo will require immense power sources that generate an enormous amount of waste heat (see PSI Post 4), that is heat energy that is multiple orders of magnitude larger than what has been observed from the planet’s radiated heat, and existentially-uncertain power sources that are numerous orders of magnitude larger than the planet-incident solar energy.
Part 2: Coriolis Force
Uranus’ rotational axis roughly lies in its orbital plane, while its magnetic field axis is almost perpendicular to its orbital plane [6], suggesting that Coriolis forces only have minor to absent roles in its magnetogenic process. Uranus has a stronger surface magnetic field than Saturn, whose magnetic and rotational axes coincide, indicating – if anything – that a negative correlation exists between a planet’s Coriolis force and its magnetic field strength, and that it cannot play a major role in a magnetogenic dynamo.
Part 3: energy source
Dynamo theory requires a planet-internal energy source that powers the dynamo’s convection. The PROM article extensively reviews all proposed planet-internal candidates, such as original heat, tidal dissipation of kinetic energy, radioactivity, gravitational energy, and mechanical convection energy. All proposed planet-internal energy candidates can be shown to be able to power a dynamo under various often highly speculative assumptions, though their magnitude, and often even their existence, is highly uncertain, and most are directly contradicted by observations and commonly-accepted Earth and planet models.
All are diffuse, and therefore unlikely to be able to focus their energy on the energy gradients that power the fluid convections that drive a dynamo. A quick look through a selection of geodynamo articles demonstrates little consensus exists among present-day authors on the dominant Earth-internal power source, and Merrill et al. [2] caution that “all calculations and relevant observational data on this subject contain a sufficient number of problems”.
All planet-internal candidates are likely to be relatively constant over sub-millennial timescales: even the most variable, for example, radioactive decay, only varies significantly over extremely long periods, as the half-lives of the most common radioactive isotopes (U-235, U-238, Th-232, K-40) are on the order of billions of years. This steady nature is in fact a requirement of the thermal geodynamo: any Earth-internal thermal source needs to be continuous enough to fuel the relatively steady-state geomagnetic field over billions of years, yet variable enough to effect significant changes over sub-millennial time scales, as the Outer Core is likely unable to permanently store magnetic energy. This strongly argues for a solar power source.
Part 4: internal magnetic field
Dynamo theory predicts that a highly conductive fluid convecting through a magnetic field will generate the electrical currents that generate the field. Which leads to an infinite regression of cause versus effect: how does the dynamo self-start, that is how does it generate the field that is required to generate the field?
Part 5: magnetic field geometry
Only Earth and Jupiter’s magnetic field geometries are explainable under the dynamo theory. “Standard” dynamo models cannot generate Saturn’s highly axisymmetrical magnetic field, or Uranus and Neptune’s offset dipole field, or Uranus and Neptune’s non-axially dipolar fields.
Saturn is highlighted in PSI Posts 1, so will be explained further here: Cowling’s anti-dynamo theory [2] states that a dynamo’s fluid flow direction and magnetic field direction cannot both be axisymmetric, as such would generate an axially symmetric current, which implies that axisymmetric fields – such as Saturn’s – cannot be modeled by dynamo theory.
An evaluation of the dynamo theory
The functioning of a magnetogenic dynamo can only be evaluated via computer models, though such numerical simulations are highly complex and poorly constrained: state-of-the-art dynamo models need to solve 10 or more equations, which in turn requires making numerous – and often physically unrealistic – assumptions, applying speculative initial and boundary conditions, and estimating numerous parameters that can vary up to several orders of magnitude [2].
For example, numerically describing the pressure gradients that drive a convection cell’s viscous flow may require solving a 3D Navier-Stokes equation, which in turn may require determining its 64 coefficients, which in turn may require setting 64 boundary conditions.
The numerical simulation of a geodynamo, therefore, necessitates substantial simplifications, for example, the magnetohydrodynamic assumption, incompressible fluids, etc.
And even then, a fairly recent review of 155 geodynamo models determined that the basic morphological properties of the geomagnetic field can only be reproduced under input parameters that are “remote from Earth’s core values” [3]: the reviewed models can only produce Earth-like geometries using input parameters that are orders of magnitude different from realistic values. In fact, most models only focus on reproducing “accurate force balances even if the quantitative values for the parameters are not correct“ [1]. “Standard” geodynamo models cannot reproduce [3]:
- The geomagnetic field strength: most models simulate non-dimensional field strength, so their realizations must be scaled
- The geographical locations of geomagnetic features and anomalies, such as the Earth’s non-dipole anomalies
- Observed heat anomalies
Dynamo models use a set of equations that balance Coriolis force, pressure, viscosity, gravity, and the Lorentz force, and should also include but often omit forces due to buoyancy and heat flows, which is problematic as these forces are driving convection.
In particular, the amount and geographical distribution of heat lost at Earth’s Core-Mantle boundary should be one of the most important boundary conditions constraining realistic dynamo simulations and represents an important test of whether the model is accurately reproducing the observed geothermal data (see PSI Post 2) and whether the hypothesized dynamo’s power requirements are realistic. There are therefore numerous long-lived, intractable problems with dynamo simulations that strongly suggest an alternative, non-dynamo magnetogenic process is preferable.
Alternative model: the Solar Wind Induced Electromagnet (SWIEM)
Fig. 2: SWIEM model-predicted versus observed planetary magnetic field strength (Note: log scale).
The PROM article develops an alternative magnetogenic process – the Solar Wind Induced Electromagnet (SWIEM) – from an integrated analysis of all relevant geomagnetic, geothermal, geologic, and geo-seismic data. This newly described Outer Core process credibly simulates the strength and location of Earth’s magnetic and heat anomalies, is plausibly responsible for most of the decennial to centennial surface-observed secular geomagnetic variability, and can be used to predict the magnetic field strength (Fig. 2), geometries and heat signatures of other planets.
Furthermore, the described process is derivable from the Maxwell equations, is physically far simpler than dynamo theory, and is, therefore, more likely to reproduce the relatively simple and similar planetary magnetic field geometries. The model requires no speculative assumptions: Occam’s razor, therefore, suggests the SWIEM physical model constitutes a substantial improvement over the dynamo theory.
[1] Stanley, S., 2014, Magnetic Field Generation in Planets. In: Encyclopedia of the Solar System (Third Edition), Academic Press, 121-135, ISBN 978-0-12-415845-0
[2] Merrill, R.T., McElhinny, M. W., McFadden, P. L., 1998, The magnetic field of the earth: paleomagnetism, the core, and the deep mantle. Academic Press. ISBN 978-0-12-491246-5.
[3] Christensen, U., Aubert, J., Hulot, G., 2010, Conditions for Earth-like geodynamo models, Earth and Planetary Science Letters, 296, 487-496, doi://doi.org/10.1016/j.epsl.2010.06.009.
[4] Marley, M.S., Fortney, J.J., 2014, Interiors of the Giant Planets. In: Encyclopedia of the Solar System (Third Edition), Academic Press, p. 743-758, ISBN: 978-0-12-415845-0
[5] Collins, G., Johnson JT.V., 2014, Ganymede and Callisto. In: Encyclopedia of the Solar System (Third Edition), Academic Press, ISBN: 978-0-12-415845-0, p. 813-829
[6] Kivelson, M.G., Bagenal, F., 2014, Planetary Magnetospheres In: Encyclopedia of the Solar System (Third Edition), Academic Press; ISBN: 978-0-12-415845-0, p. 137-157
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