The magnetosphere of Saturn is the cavity created in the flow of the solar wind by the planet's internally generated magnetic field. Discovered in 1979 by the Pioneer 11 spacecraft, Saturn's magnetosphere is the second largest of any planet in the Solar System after Jupiter's. The magnetopause, the boundary between Saturn's magnetosphere and the solar wind, is located at a distance of about 20 Saturn radii from the planet's center, while its magnetotail stretches hundreds of Saturn radii behind it.
Saturn's magnetosphere is filled with plasmas originating from both the planet and its moons. The main source is the small moon Enceladus, which ejects as much as 1,000 kg/s of water vapor from the geysers on its south pole, a portion of which is ionized and forced to co-rotate with the Saturn's magnetic field. This loads the field with as much as 100 kg of water group ions per second. This plasma gradually moves out from the inner magnetosphere via the interchange instability mechanism and then escapes through the magnetotail.
The interaction between Saturn's magnetosphere and the solar wind generates bright oval aurorae around the planet's poles observed in visible, infrared and ultraviolet light. The aurorae are related to the powerful saturnian kilometric radiation (SKR), which spans the frequency interval between 100 kHz to 1300 kHz and was once thought to modulate with a period equal to the planet's rotation. However, later measurements showed that the periodicity of the SKR's modulation varies by as much as 1%, and so probably does not exactly coincide with Saturn's true rotational period, which as of 2010 remains unknown. Inside the magnetosphere there are radiation belts, which house particles with energy as high as tens of megaelectronvolts. The energetic particles have significant influence on the surfaces of inner icy moons of Saturn.
In 1980–1981 the magnetosphere of Saturn was studied by the Voyager spacecraft. Up until September 2017 it was a subject of ongoing investigation by Cassini mission, which arrived in 2004 and spent over 13 years observing the planet.
Discovery Immediately after the discovery of Jupiter's decametric radio emissions in 1955, attempts were made to detect a similar emission from Saturn, but with inconclusive results.Smith, 1959 The first evidence that Saturn might have an internally generated magnetic field came in 1974, with the detection of weak radio emissions from the planet at the frequency of about 1 MHz.
These medium wave emissions were modulated with a period of about , which was interpreted as Saturn's rotation period.Brown, 1975 Nevertheless, the evidence available in the 1970s was too inconclusive and some scientists thought that Saturn might lack a magnetic field altogether, while others even speculated that the planet could lie beyond the heliopause.Kivelson, 2005, p. 2077 The first definite detection of the saturnian magnetic field was made only on September 1, 1979, when it was passed through by the Pioneer 11 spacecraft, which measured its magnetic field strength directly.
Structure
Internal field Like Jupiter's magnetic field, Saturn's is created by a fluid dynamo within a layer of circulating liquid metallic hydrogen in its outer core. Like Earth, Saturn's magnetic field is mostly a dipole, with north and south poles at the ends of a single magnetic axis. On Saturn, like on Jupiter, the north magnetic pole is located in the northern hemisphere, and the south magnetic pole lies in the southern hemisphere, so that magnetic field lines point away from the north pole and towards the south pole. This is reversed compared to the Earth, where the north magnetic pole lies in the southern hemisphere. Saturn's magnetic field also has quadrupole, octupole and higher components, though they are much weaker than the dipole.
The magnetic field strength at Saturn's equator is about 21 μT (0.21 G), which corresponds to a dipole magnetic moment of about 4.6 T•m3.Belenkaya, 2006, pp. 1145–46 This makes Saturn's magnetic field slightly weaker than Earth's; however, its magnetic moment is about 580 times larger.Russel, 1993, p. 694 Saturn's magnetic dipole is strictly aligned with its rotational axis, meaning that the field, uniquely, is highly axisymmetric.Russel, 1993, pp. 717–718 The dipole is slightly shifted (by 0.037 Rs) along Saturn's rotational axis towards the north pole.
Size and shape Saturn's internal magnetic field deflects the solar wind, a stream of ionized particles emitted by the Sun, away from its surface, preventing it from interacting directly with its atmosphere and instead creating its own region, called a magnetosphere, composed of a plasma very different from that of the solar wind. The magnetosphere of Saturn is the second–largest magnetosphere in the Solar System after that of Jupiter.Blanc, 2005, p. 238
As with Earth's magnetosphere, the boundary separating the solar wind's plasma from that within Saturn's magnetosphere is called the magnetopause. The magnetopause distance from the planet's center at the subsolar point varies widely from 16 to 27 Rs (Rs=60,330 km is the equatorial radius of Saturn).Russel, 1993, p. 709, Table 4Gombosi, 2009, p. 247 The magnetopause's position depends on the pressure exerted by the solar wind, which in turn depends on solar activity. The average magnetopause standoff distance is about 22 Rs. In front of the magnetopause (at the distance of about 27 Rs from the planet)Gombosi, 2009, p. 206, Table 9.1 lies the bow shock, a wake-like disturbance in the solar wind caused by its collision with the magnetosphere. The region between the bow shock and magnetopause is called the magnetosheath.Russel, 1993, pp. 690–692
At the opposite side of the planet, the solar wind stretches Saturn's magnetic field lines into a long, trailing magnetotail, which consists of two lobes, with the magnetic field in the northern lobe pointing away from Saturn and the southern pointing towards it. The lobes are separated by a thin layer of plasma called the tail current sheet. Like Earth's, Saturn's tail is a channel through which solar plasma enters the inner regions of the magnetosphere.Gombosi, 2009, pp. 206–209 Similar to Jupiter, the tail is the conduit through which the plasma of the internal magnetospheric origin leaves the magnetosphere. The plasma moving from the tail to the inner magnetosphere is heated and forms a number of radiation belts.
Magnetospheric regions
Saturn's magnetosphere is often divided into four regions.Andre, 2008, pp. 10–15 The innermost region co-located with Saturn's planetary rings, inside approximately 3 Rs, has a strictly dipolar magnetic field. It is largely devoid of plasma, which is absorbed by ring particles, although the radiation belts of Saturn are located in this innermost region just inside and outside the rings. The second region between 3 and 6 Rs contains the cold plasma torus and is called the inner magnetosphere. It contains the densest plasma in the saturnian system. The plasma in the torus originates from the inner icy moons and particularly from Enceladus. The magnetic field in this region is also mostly dipolar. The third region lies between 6 and 12–14 Rs and is called the dynamic and extended plasma sheet. The magnetic field in this region is stretched and non-dipolar, whereas the plasma is confined to a thin equatorial plasma sheet.Andre, 2008, pp. 6–9 The fourth outermost region is located beyond 15 Rs at high latitudes and continues up to magnetopause boundary. It is characterized by a low plasma density and a variable, non-dipolar magnetic field strongly influenced by the Solar wind.
In the outer parts of Saturn's magnetosphere beyond approximately 15–20 RsMauk, 2009, pp. 317–318 the magnetic field near the equatorial plane is highly stretched and forms a disk-like structure called magnetodisk. The disk continues up to the magnetopause on the dayside and transitions into the magnetotail on the nightside.Gombosi, 2009, pp. 231–234 Near the dayside it can be absent when the magnetosphere is compressed by the Solar wind, which usually happens when the magnetopause distance is smaller than 23 Rs. On the nightside and flanks of the magnetosphere the magnetodisk is always present. The Saturnian magnetodisk is a much smaller analog of the Jovian magnetodisk.
The plasma sheet in Saturn's magnetosphere has a bowl-like shape not found in any other known magnetosphere. When Cassini arrived in 2004, there was a winter in the northern hemisphere. The measurements of the magnetic field and plasma density revealed that the plasma sheet was warped and lay to the north of the equatorial plane, looking like a giant bowl. Such a shape was unexpected.Gombosi, 2009, pp. 211–212
Dynamics
The processes driving Saturn's magnetosphere are similar to those driving Earth's and Jupiter's.Blanc, 2005, pp. 264–273 Just as Jupiter's magnetosphere is dominated by plasma co–rotation and mass–loading from Io, so Saturn's magnetosphere is dominated by plasma co–rotation and mass–loading from Enceladus. However, Saturn's magnetosphere is much smaller in size, while its inner region contains too little plasma to seriously distend it and create a large magnetodisk.Kivelson, 2005, pp. 303–313 This means that it is much more strongly influenced by the solar wind, and that, like Earth's magnetic field, its dynamics are affected by reconnection with the wind similar to the Dungey cycle.
Another distinguishing feature of Saturn's magnetosphere is high abundance of neutral gas around the planet. As revealed by ultraviolet observation of Cassini, the planet is enshrouded in a large cloud of hydrogen, water vapor and their dissociative products like hydroxyl, extending as far as 45 Rs from Saturn. In the inner magnetosphere the ratio of neutrals to ions is around 60 and it increases in the outer magnetosphere, which means that the entire magnetospheric volume is filled with relatively dense weakly ionized gas. This is different, for instance, from Jupiter or Earth, where ions dominate over neutral gas, and has consequences for the magnetospheric dynamics.Mauk, 2009, pp. 282–283
Sources and transport of plasma
The plasma composition in Saturn's inner magnetosphere is dominated by the water group ions: O+, H2O+, OH+ and others, hydronium ion (H3O+), HO2+ and O2+,Sittler, 2008, pp. 4, 16–17 although protons and nitrogen ions (N+) are also present.Young, 2005Smith, 2008 The main source of water is Enceladus, which releases 300–600 kg/s of water vapor from the geysers near its south pole. The released water and hydroxyl (OH) radicals (a product of water's dissociation) form a rather thick torus around the moon's orbit at 4 Rs with densities up to 10,000 molecules per cubic centimeter.Tokar, 2006 At least 100 kg/s of this water is eventually ionized and added to the co–rotating magnetospheric plasma. Additional sources of water group ions are Saturn's rings and other icy moons.Gombosi, 2009, pp. 216–219 The Cassini spacecraft also observed small amounts of N+ ions in the inner magnetosphere, which probably originate from Enceladus as well.Smith, 2008, pp. 1–2
In the outer parts of the magnetosphere the dominant ions are protons, which originate either from the Solar wind or Saturn's ionosphere.Gombosi, 2009, pp. 219–220 Titan, which orbits close to the magnetopause boundary at 20 Rs, is not a significant source of plasma.
The relatively cold plasma in the innermost region of Saturn's magnetosphere, inside 3 Rs (near the rings) consists mainly of O+ and O2+ ions. There ions together with electrons form an ionosphere surrounding the saturnian rings.Gombosi, 2009, pp. 206, 215–216