Saturn's Largest Sphere
The outer planets are much more than large gaseous planets. Rather they are highly complex systems of interacting components. The Saturnian system is an excellent example of this complexity! Almost everyone would recognize that an important part of Saturn's system is its illustrious rings. And, it is not difficult to understand that the large number of icy satellites, including Enceladus with its icy geysers are also an important part of the system. And, Titan, with its thick, murky atmosphere is certainly not to be a forgotten member of Saturn's domain. However, most would not recognize the largest, yet mostly invisible part of Saturn's system, it's magnetosphere.
What is a magnetosphere? This is a good question. The best way to think about Saturn's magnetosphere is that it is a magnetic bubble created by Saturn's internal magnetic field. This bubble exists within a supersonic wind originating at the Sun that blows at speeds of a million miles per hour past all of the planets. This solar wind is composed of a hot gas known as a plasma -- the electrons have been stripped off the atoms leaving a mix of negatively charged electrons and positively charged ions, all of which are threaded by magnetic fields originating in the Sun. So, Saturn's magnetosphere exists within the supersonic solar wind and is strongly influenced both by things going on inside it as well as variations in the solar wind blowing on it.
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The most obvious effect of the solar wind on Saturn's magnetosphere is to severely distort what would otherwise be a more-or-less spherically-shaped bubble into something resembling a wind sock. On the sun-facing side of the magnetosphere, the wind blows against the bubble, compressing it. Typically the edge of the bubble on the sunward side extends to just beyond Titan's orbit, somewhat more than 20 times the radius of Saturn. However, on the other side of Saturn, the anti-sunward side, the "tail" of the wind sock is formed, probably extending several thousand Saturn radii in the direction away from the sun. We don't have direct measurements of how long this tail is, but when Voyager 2 flew past Saturn in 1981, Saturn and its magnetosphere were sometimes embedded in Jupiter's magnetic tail, meaning Jupiter's tail stretched at least 5 astronomical units (one astronomical unit is the distance from the Sun to Earth).
Other effects of the solar wind on Saturn's magnetosphere can be easily imagined by thinking about blowing soap bubbles outside on a breezy day. The variations in the breeze often distort the bubbles in very dynamic ways. These same types of distortions have been observed in Saturn's magnetosphere when the solar wind blows particularly hard.
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As far as internal effects on Saturn's magnetosphere, two are of greatest importance. First, the magnetosphere is filled with plasma of its own, most of it thought to come from the icy moons, Titan, and the rings, with Enceladus being an obvious primary source. At Saturn, it appears that there is an enormous neutral torus surrounding the planet and that the mostly water-associated atoms are relatively slowly ionized (stripped of an electron). Second, the primary energy source is the rotational energy of the planet. As the neutral material is ionized, it is caught up by the magnetic field which rotates with the planet. As more and more ions are caught up by this process called mass loading, more and more torque is applied to the magnetic field. Hence, the magnetic field generated in the interior of the planet is continually stressed by the mass loading process as the planet's rotational energy is used to accelerate the plasma to rotate about the planet.
So, we see that even though the magnetosphere is a generally invisible component of the Saturnian system, it is indelibly involved in the workings of the system, often providing essential connections between the moons, rings, and the planet.
One of the aspects of the magnetosphere which is not invisible is the mark it leaves on the upper atmosphere of Saturn. Just as there are auroras at Earth, Saturn shows evidence of dramatic displays of "northern and southern lights". These eerily glowing lights are the result of energetic charged particles colliding with and exciting atoms in Saturn's upper atmosphere, particularly at high latitudes, near the poles. The excited atoms eventually drop to a lower energy state, but in the process, emit photons which can be viewed at various wavelengths.
Returning, briefly, to the influence of the solar wind on the magnetosphere, an especially important campaign which used both the Hubble Space Telescope and Cassini measurements as it approached Saturn in January, 2004 allowed us to study the relationship between the strength of the solar wind measured by Cassini and the brightness of Saturn's auroras, as measured both in ultraviolet light by Hubble and by the intensity of radio emissions generated above the auroras by Cassini. This experiment showed that Saturn's auroras become more intense when the solar wind is blowing harder.
Lest I leave you with the idea that the magnetosphere is a well-understood facet of the Saturnian system, we will address, briefly, the conundrum of the unknown length of a day on Saturn. As one might expect, it is not possible to determine the rotation rate of the deep interior of a gas giant planet because there is no solid surface visible which one could track to determine the rate of its rotation, or the length of a day. Worse, since there is an enormous system of winds on the planet, the period at which cloud features rotate around the planet strongly depends on the latitude of the cloud feature. In fact, without knowing the rate of rotation of the deep interior, atmospheric scientists cannot know the absolute wind speeds, just the relative speed between one latitude and another. Until recently, the rotation period of Saturn was thought to be given by periodicities observed in the radio emissions alluded to above that are associated with Saturn's auroras. Those radio emissions are known to be strongly related to the planet's magnetic field. And, the magnetic field is generated in the deep interior of the planet. Therefore, the logic went, the period measured in the radio emissions must be the period of the deep interior. In fact, the International Astronomical Union has adopted the Voyager-determined radio period of 10 hours, 39 minutes and 24 seconds as the length of a day on Saturn. This same technique has been used to infer the rotation periods of Jupiter, Uranus, and Neptune.
However, based on ongoing measurements of the radio period of Saturn by a spacecraft called Ulysses that have been confirmed by Cassini measurements, Saturn's radio period is currently longer than 10 hours, 47 minutes and is continuously varying. This dramatic change of period of several minutes in just a couple of decades would violate conservation of both angular momentum and energy, if the period really represented that of the deep interior of the planet. Hence, scientists do not think that Saturn has actually slowed its rate of rotation. Instead, it appears that the magnetic field observed exterior to the planet is slipping in some way with respect to its source deep in the interior of the planet. And, given that, we currently do not know what the rotation period of Saturn's deep interior is. This is one of the deepest mysteries remaining for Cassini to solve in its remaining tour of study of the Saturnian system.
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