Saturn's magnetic field, magnetosphere and auroras
Saturn's magnetic field is generated in its deep layer of liquid metallic hydrogen and is about 580 times stronger than Earth's in total magnetic moment, although at the cloud tops it is slightly weaker than Earth's field at our surface. Its most remarkable feature is its alignment: the magnetic axis is tilted by less than 0.01° from the rotation axis, a symmetry that dynamo theory struggles to explain. The field carves out a vast magnetosphere filled with water-derived plasma from Enceladus, and channels particles into the polar atmosphere to create ultraviolet and infrared auroras.
Contents
Discovery and strength
Saturn's magnetic field was first measured directly by Pioneer 11 when it flew past on 1 September 1979; until then its existence had been inferred only indirectly. The Voyager flybys of 1980 and 1981 refined the picture, and Cassini, in orbit from 2004 to 2017, mapped the field in detail, culminating in its 22 Grand Finale orbits between the planet and the rings.
At the equator, at the 1-bar level, the field is about 21 microtesla, a little weaker than the roughly 30 microtesla at Earth's equator. But Saturn is so much larger that its total magnetic moment, a measure of the strength of the whole magnet, is about 580 times Earth's. Only Jupiter's is stronger. The field is generated by a dynamo: convection and rotation in the electrically conducting metallic hydrogen layer, deep below the clouds, sustain electric currents that produce the field. See interior.
A field that is too symmetric
Earth's magnetic axis is tilted by about 10° from its rotation axis, Jupiter's by about 10°, and those of Uranus and Neptune by roughly 59° and 47°. Saturn's, as measured during the Grand Finale and published by Michele Dougherty and colleagues in 2018, is tilted by less than 0.01°. For practical purposes the field is perfectly axisymmetric. Its center is, however, shifted slightly north of the planet's center, by a few percent of Saturn's radius.
This symmetry is a puzzle. A theorem of magnetohydrodynamics, known as Cowling's theorem, states that a perfectly axisymmetric field cannot be sustained by a dynamo. The leading explanation, proposed by David Stevenson in the early 1980s, is that a stably stratified layer above the dynamo region, possibly related to helium rain, rotates differentially and filters out the non-symmetric parts of the field, letting only the symmetric component reach the outside. The dynamo itself may be tilted, but we cannot see it.
The symmetry had a practical consequence: with no wobbling field to act as a lighthouse, measuring Saturn's rotation period became extremely difficult, a problem solved only by ring seismology in 2019.
The magnetosphere and its plasma
The magnetic field creates a bubble in the solar wind, the magnetosphere, that shields the space around Saturn. On the side facing the Sun, its boundary, the magnetopause, typically lies about 20 Saturn radii from the planet, around 1.2 million km, but it moves in and out between roughly 17 and 30 radii as solar wind pressure changes. On the night side, the magnetosphere stretches into a long magnetotail. Titan orbits at about 20 Saturn radii, so it is sometimes inside the magnetosphere and sometimes exposed to the solar wind. See Titan.
Saturn's magnetosphere is dominated by an internal source. The geysers of Enceladus eject a few hundred kilograms of water vapor per second, forming a cloud of neutral gas around Saturn along the moon's orbit. Sunlight and collisions ionize these molecules into water-group ions (oxygen, hydroxyl, water and hydronium ions), which are picked up by the rotating magnetic field and dragged around the planet in a disk of plasma. Enceladus plays the same role at Saturn that the volcanic moon Io plays at Jupiter, although Saturn's magnetosphere contains far less plasma overall. The same ice grains feed the faint E ring. See Enceladus geysers.
Radiation belts and the rings
Saturn has belts of energetic trapped particles, but they are weaker than Jupiter's, partly because the moons and rings absorb particles crossing their paths. The main rings act as a barrier, wiping out radiation over their span. During the Grand Finale, Cassini discovered a separate, narrow radiation belt in the gap between the innermost D ring and the atmosphere, fed by particles produced when cosmic rays strike the rings.
The field also connects the rings to the planet. Charged ice grains and ions from the rings spiral along magnetic field lines into Saturn's upper atmosphere at mid-latitudes, part of the phenomenon called ring rain, which leaves a measurable imprint on the planet's ionosphere.
Auroras at both poles
Like Earth, Saturn has auroras: energetic charged particles guided by the magnetic field crash into the upper atmosphere around the poles and make the gas glow. They were first detected in ultraviolet light by Voyager and have been imaged repeatedly by Hubble since the 1990s; they also shine in infrared light from the H₃⁺ ion, observed from ground-based telescopes. Cassini even captured faint visible auroras, glowing in red and violet tones.
Saturn's main auroral ovals sit at high latitudes, roughly 70° to 80°. They are driven by a mix of influences: compressions of the magnetosphere by gusts of solar wind make them brighten and expand, while currents linked to the planet's rotation and its plasma disk modulate them with a period of about 10.6 to 10.8 hours. Cassini also found an auroral spot linked to Enceladus, where the moon's interaction with the magnetic field sends a current down to Saturn's atmosphere.
Auroral energy matters beyond the light show. Grand Finale data indicated that heat deposited by auroras spreads from the poles toward the equator, helping explain why Saturn's upper atmosphere is hundreds of degrees hotter than sunlight alone could make it. See temperature.
Radio emissions
The auroral regions also emit intense natural radio waves, the Saturn kilometric radiation. Voyager used its periodicity to define a 10 h 39 min rotation period, but Cassini showed that it varies over time and differs between the northern and southern hemispheres, so it reflects the dynamics of the magnetosphere rather than the spin of the interior. Lightning in Saturn's storms produces a separate class of radio bursts.
FAQ
Does Saturn have a magnetic field?
Yes. Saturn's field is generated in its metallic hydrogen interior. At the cloud tops it is slightly weaker than Earth's surface field, but because the planet is so large its overall magnetic moment is about 580 times Earth's.
Why is Saturn's magnetic field special?
It is almost perfectly aligned with the rotation axis, tilted by less than 0.01°. Standard dynamo theory says a perfectly symmetric field cannot be self-sustained, so scientists think a layer above the dynamo filters out the asymmetric parts.
Does Saturn have auroras?
Yes, around both poles. They are strongest in ultraviolet and infrared light and are driven by the solar wind and by currents tied to the planet's rotation and plasma from Enceladus. Faint visible auroras were recorded by Cassini.
How big is Saturn's magnetosphere?
On the Sun-facing side it typically extends about 1.2 million km, around 20 Saturn radii, varying with the solar wind. On the night side it stretches much farther into a long tail.
Sources
- Dougherty, M. K. et al. (2018), "Saturn's magnetic field revealed by the Cassini Grand Finale", Science 362
- Stevenson, D. J. (1982), "Reducing the non-axisymmetry of a planetary dynamo and an application to Saturn", Geophysical & Astrophysical Fluid Dynamics 21
- Pryor, W. R. et al. (2011), "The auroral footprint of Enceladus on Saturn", Nature 472
- NASA Science: Cassini mission