The Origin Of Saturn And Neptune’s Magnetic Fields

Written by Koen Vogel, Ph.D. on January 3, 2023. Posted in Current News

Alternate models for planetary processes, part 7

This post is the last of seven in support of the PROM article “An integrated physical model characterizing planetary magnetism and heat”, which proposes an alternate 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 7 deals with the magnetic fields of Saturn and Neptune.

The prevailing theory on Saturn and Neptune’s magnetogenesis

All the magnetic fields of the planets and most of their satellites are commonly thought to be generated by a planet-internal dynamo, wherein fluid convection cells convert thermal and mechanical energy to magnetic energy via induced electric currents.

However, dynamo theory cannot explain Saturn and Neptune’s magnetic field origins: [¹; PROM Article]

• Saturn’s axisymmetric and Neptune’s offset and severely tilted dipolar magnetic field geometries cannot be generated by a standard dynamo.
• Their proposed fluid magnetic source layers are highly speculative: an outer mantle metallic hydrogen shell is assumed for Saturn, and a water-ammonia-methane mantle shell for Neptune. Due to the low conductivity of these shells, any magnetogenesis would require immense power sources and generate enormous amounts of heat, both of which are not in evidence.
• No plausible planet-internal magnetogenic power sources exist: for Saturn, a speculative “Helium Rain” falling through its metallic hydrogen mantle is proposed, while for Neptune even such a highly imaginative energy source cannot be invented.
• Both planets have “energy crises”, that is they emit significantly more heat than they receive from solar radiation. Both planets show heat anomalies that vary with the seasons: their polar heat anomalies are maximal during their summer solstices. It is therefore extremely likely that solar energy – the solar wind – is powering these anomalies, as well as their heat crises and magnetic fields.

Alternative model: the Solar Wind Induced Electromagnet (SWIEM)

The PROM article determines a more evident magnetogenic process – the Solar Wind Induced Electromagnet (SWIEM) – from an integrated analysis of all relevant geomagnetic, geothermal, geologic, and geo-seismic data.

PSI Post 6 demonstrates the SWIEM physical model constitutes a substantial improvement over dynamo theory: it can predict surface magnetic field strengths from best estimates of solar wind strength and planetary geometries (Fig. 1) and accounts for the temporal and spatial variation of planetary heat anomalies.

The model explains the great similarity (in essence more similar than dissimilar) between the magnetic fields of the planets: a highly conductive, highly magnetically permeable fluid or a solid ferromagnetic core; a low-conductivity, low magnetic, permeable, non-ferromagnetic mantle; and significant diurnal solar wind-generated magnetic flux energy are the only requirements for a planetary magnetic field, which is the reason why most planets have one.

Under the SWIEM model:

• The solar wind generates synchronous Northern (NH) and Southern hemisphere (SH) Solar Wind Induced Currents (SWICs) that circuit in the outermost shell of the planets’ ferromagnetic cores, thereby generating both external poloidal and internal toroidal fields
• These synchronous NH and SH SWIC circuits are coupled inducers, whose strength therefore jointly increases or decreases.
• Ohmic heating along the circuit causes core heating, which explains why most planets with a magnetic field radiate more energy than they receive from high-frequency solar radiation alone: magnetogenic waste heat travels from the core to the planet’s surface where it is radiated to space. This Ohmic heating will be focused in the areas where the SWICs are strongest, for example in the polar region of the hemisphere experiencing its summer solstice.

Figure 1: SWIEM model-predicted versus observed planetary magnetic field strength (Note: log scales).

In contrast to Earth, however, the SWIC circuits on other planets are possibly not entrenched by long-lived heat anomalies and may therefore be more mobile, which in turn suggests that planetary magnetic field geometries may be significantly more variable than the geomagnetic field.

Saturn’s Field

Saturn’s proposed magnetogenic power source under dynamo theory – convection due to negative buoyancy of “Helium Rain” falling through the metallic hydrogen Mantle – is diffuse, highly speculative, and likely orders of magnitude lower than its magnetogenic power requirements.

In contrast, the PROM article demonstrates that solar wind-generated magnetic flux is a credible magnetogenic energy source that can power Saturn’s magnetic field (Fig. 1). Saturn’s magnetic field geometry cannot be explained under dynamo theory: its axisymmetric field geometry cannot be generated by a “standard” dynamo.

Cowling’s anti-dynamo theorem states that a dynamo’s fluid velocity vector and the magnetic field vector cannot both be axisymmetric, which implies that axisymmetric fields – such as Saturn’s – cannot be the result of a dynamo ^[2]. In contrast, an axisymmetric field is the most basic, easily-explained geometry under SWIEM theory: any axisymmetric SWIC circuit configuration will generate an axisymmetric magnetic field geometry.

The most basic axisymmetric SWIEM configuration is one whereby a single NH/SH circuit pair is not entrenched but remains on the sunward or leeward side of the planet.

Fig. 2 presents an alternative configuration, whereby 6 entrenched SWIC circuit pairs generate an axisymmetric field geometry and a hexagonal north polar heat anomaly during Saturn’s northern summer, similar to that observed by Cassini’s Composite Infrared Spectrometer (Downloaded September 22, 2022).

During Saturn’s northern summer, the solar wind-generated magnetic flux is maximal at its north polar area, and therefore its NH SWICs are at maximum intensity. This has little effect on Saturn’s overall field geometry or strength, due to the coupled inductance of its NH and SH SWIC circuits, but does have an effect on its heat anomalies that will be maximal where the SWICs are strongest, that is at its north pole.

Figure 2: Schematic of a possible axisymmetric SWIEM geometry for Saturn. Solid lines represent notional SWIC circuits; hatched polar area represents hexagonal polar heat anomaly; the dashed arrow is Saturn’s magnetic axis (roughly coincident with its rotational axis).

Neptune’s Field

Neptune’s magnetic field cannot be explained under dynamo theory ^[1], and credible dynamo source layers and Neptune-internal heat sources are absent. That some internal heat source is active is irrefutable: Neptune radiates around 2.6 times the heat energy that it receives from shortwave solar radiation ^[3], which strongly implies that a large Neptune-external energy source – the solar wind – is supplementing Neptune’s internal energy.

Neptune’s magnetic field can be modeled (Fig. 3) as a magnetic dipole that shows a relatively large tilt of 47° and is offset by a relatively large amount of 0.55 RN from its center ^[4].

Fig. 1 demonstrates that the solar wind can adequately power Neptune’s magnetic field, while Fig. 3 demonstrates that the SWIEM can easily explain both Neptune’s magnetic field geometry as well as its surface heat anomalies.

A Neptune year lasts 165 Earth years ^[5] indicating each Neptune season lasts just over 41 Earth years. Its southern summer solstice was in 2005 ^[5], so Neptune had just entered its SH summer at the time the Voyager 2 flew by in 1989. During Neptune’s southern summer, its solar wind-generated magnetic flux is maximal at the south polar area, and therefore its SH SWICs are at maximum intensity.

Neptune’s southern summer SWIEM geometry (Fig. 3) is dominated by strong SH SWICs that have little, coupled inductance with the far-weaker NH SWIC circuits, likely indicating Neptune’s SWIC circuits are not defined by long-lived core heat anomalies that can survive its long seasons.

The SWIEM model predicts that the strong SH SWICs generate a “reverse” field magnetic moment that is significantly offset from Neptune’s center and is at a 45° angle to its rotation axis (Fig. 3), which almost exactly matches the observed 47° tilt ^[4]. In addition, the model predicts significant heating occurs where the SWICs are strongest, that is at Neptune’s south polar area.

Observations indicate Neptune’s south pole temperature is currently hotter than the rest of the planet, and that its temperature peaked in 2005, during its summer solstice ^[5] indicating the SWIEM model also effectively explains Neptune’s heat anomalies. Note that the SWIEM model predicts that Neptune’s magnetic field geometry and heat anomalies will be completely transposed to its Northern Hemisphere during its northern summer in 2087.

Figure 3: Schematic of Neptune’s magnetic field (after Ness et al. 1989). Thin-dashed lines represent magnetic field lines; the thick-dashed line is the rotational axis; the dot-dash line is the magnetic equator; dark sphere=core. Dashed white arrows indicate SWIC circuits. Circled Xs reflect electric circuit flow into the page; dotted circles electric circuit flow out of the page. Not to scale.

[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] Pearl, J.C., Conrath, B.J., 1991, The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data. Journal of Geophysical Research: Space Physics. 96 (18) 921–30. doi:doi:10.1029/91ja01087.

[4] Ness, N.F., Acuña, M.H., Burlaga, L.F., Connerney, J.E.P., Lepping, R.P.; Neubauer, F.M., 1989, Magnetic Fields at Neptune. Science. 246 (4936): 1473–78. doi:10.1126/science.246.4936.

[5] Roman, M.T., Fletcher, L.N., Orton, G.S., Vatant d’Ollone, J., Sinclair, J.A., Rowe-Gurney, N., Moses, J.I., & Irwin, P.J., 2020, Sub-Seasonal Variations in Neptune’s Stratospheric Infrared Emission from VLT-VISIR, 2006-2018. The Planetary Science Journal, 3, 41 pp, https://doi.org/10.3847/PSJ/ac5aa4

[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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