Saturn's magnetic field

TThe magnetic field of Saturn, which was discovered and first analyzed from data collected by NASA’s Pioneer 11, Voyager 1, and Voyager 2 missions, reveals similarities to the magnetic fields of both Earth and Jupiter. There are, however, many significant differences. The Saturnian magnetosphere provides a cosmic laboratory for the study of important processes such as collision-free shocks, charge-accelerating processes, plasma-wave modes, particle trapping, diffusion, and a host of related phenomena. The Cassini spacecraft provided an orbital platform where those processes were studied on a regular basis for many years beginning in 2004.

Overview

In the early 1960s decimetric radio emissions of Jupiter were established to be emitted by high-energy electrons trapped in the giant planet’s intense magnetic field. A research effort was begun to determine if identical radiationin Saturn led to similar radio noise. Subsequently, the first model of Saturn’s magnetic field was developed. Data collected from NASA’s Cassini missions resulted in the greater fidelity of that model. Also revealed was that Saturn’s satellite Enceladus possesses its own magnetic field and emits radio waves of a complex nature.

Initial evidence for the existence of the Saturnian magnetic field was provided by Pioneer11’s magnetometer. The bulk of Saturn’s magnetic field is believed to be generated by the rapid internal motion of the metallic hydrogen that surrounds the planet’s rocky central core, forming a dynamo and resulting in a field that resembles that of the Earth and Jupiter. Field strength at cloud-top level over Saturn’s equator was found to be 0.2 gauss, roughly a third of the equatorial geomagnetic field. By comparison, Jupiter’s magnetic field at cloud-top level is ten times stronger than that of the Earth. Saturn’s magnetic axis and rotational axis are nearly coincident. The field is believed to originate at a greater depth than those of the Earth and Jupiter in relation to their respective radii. Moreover, the rotational and magnetic axes of both the Earth and Jupiter make a nearly 11 degree angle with their respective rotational axes. Pioneer 11 and Voyager data indicated that the center of Saturn’s magnetic dipole axis is offset by 4 percent of Saturn’s radius to the north of the center of the planet, and the polarity of the field is reversed with respect to the Earth’s polarity. These facts combine to give Saturn’s external magnetic field a perfect symmetry, with none of the wobbles that characterize the fields surrounding the Earth and Jupiter.

Another component of Saturn’s magnetic field is an extensive ring of current of 10 million amperes flowing from west to east. Near the axis, the ring-current field is not parallel to the main field but becomes parallel with distance. Moreover, field lines are outward-bound above the equatorial plane and inward-bound below that plane. Thus, Saturn’s magnetic field is made up of the main dipole field and the ring-current field, along with the boundary currents of the magnetopause (the border at which the solar wind meets the planetary magnetic field), thereby defining the magnetosphere. Because of the superposition of the ring-current field on the planet’s intrinsic dipole field, the shape of the overall magnetosphere is stretched outward along the equatorial plane, creating a bulge that distorts the poloidal aspect of the field.

As confirmed by Pioneer 11, the Voyagers, and the Cassini orbiter, interaction between Saturn’s magnetic field and the solar wind causes a magnetosphere accompanied by a well-defined bow shock, a plasma sheath, and a magnetotail, similar in many respects to those associated with the Earth and Jupiter. The shape of the magnetosphere can be schematically represented by a paraboloid of revolution around the Saturn-sun line. The size of the magnetosphere depends on the varying pressure of the solar wind. If this pressure is low, the magnetopause occurs at a greater distance, thus inflating the magnetosphere. The solar activity cycle, interplanetary conditions, and local variations influence solar wind pressures. Magnetospheric and myriad complex radiative processes that occur within it result primarily from the interaction between charged particles of the solar wind and Saturn’s intrinsic magnetic field. It is assumed that the transfer of energy from the solar wind to the magnetosphere occurs within the outer layer of the magnetosphere and the magnetotail. The magnetosphere effectively entraps, stores, and reradiates the energy of the solar wind.

With regard to Saturn’s magnetic field, Pioneer 11 and the Voyager probes found that there was a near alignment of the magnetic and rotational axes and a displacement of the magnetic dipole center compared to the planetary center. They also discovered the existence of an additional field produced by a ring of current flowing west to east in the equatorial plane. Because of the varied areas covered by these probes’ differing trajectories and the complementary data they provided, the Pioneer and Voyager missions provided a fairly accurate model of the Saturnian magnetic field configuration. Cassini’s magnetometer filled in a great many gaps and found some surprises, especially with regard to the satellite Enceladus.

Saturn’s ring systems are intricate, elaborate, and fascinating both in appearance and from a scientific point of view. The rings are thin and known to be composed of pieces of ice and chunks of rocky material, most of which are probably much smaller than ten meters in diameter. Stars can dimly be seen through the rings. The entire system rotates in the equatorial plane, within about 180,000 kilometers of the center of the planet. All of Saturn’s major satellites, starting with Mimas, are located beyond the principal ring systems. Smaller ones are embedded and inside the ring systems.

The regions surrounding the major rings of Saturn are devoid of trapped charged particles. Up to a radial distance of about 150,000 kilometers, the density of charged particles is found to be negligible. Thereafter it rises abruptly, with intermittent variations caused by the absorption of particles by the satellites and the tenuous E ring. The maximum density of ions and electrons is observed between the orbits of Tethys and Rhea, between 300,000 and 60,000 kilometers from Saturn’s center. The same region, which is rich in dense plasma, is lacking in low-energy electrons and protons. They are most likely absorbed by the neighboring satellites.

In this inner magnetosphere, rapidly spinning and charged particles are coupled to the magnetic field, resulting in a 60,330-kilometer-thick plasma sheet about 240,000 to 422,000 kilometers from the planet. From this distance to the orbit of Titan, there is a vast torus of neutral hydrogen, produced by the photochemical breakdown of methane escaping from the giant satellite’s atmosphere. Another source of charged particles is provided by the collision of neutral hydrogen atoms with the magnetosphere of Titan.

Saturn’s magnetospheric radio emission has three components, all of which are presumed to originate in the magnetic plasma surrounding the planet. Its kilometric radiation was found to have a fundamental periodicity of ten hours and forty minutes, presumably the rotational period of the magnetosphere. These magnetic storms seem to affect and are correlated with solar wind conditions, as well as the relative position of the satellite Dione. Saturn’s kilometric radio emissions, first discovered by Voyager 1 in January 1980, exhibit a wide range of period modulation (10.66 hours, 66 hours, and 22 days) along with intensity and power variations. The axial symmetry of Saturn’s magnetic field does not yield the kind of intensity modulation that is associated with the wobbly motion of Jupiter’s field. Saturn appears to radiate greater power toward its dayside than toward the night side; this phenomenon has its origin in midlatitude to polar cusps, regions where magnetic field lines enter or leave the planet. Magnetospheric kilometric radiations—short bursts of radio emissions caused by electrostatic discharges ranging in frequency from twenty kilohertz (kHz) to forty megahertz (MHz) with a periodicity of ten hours and ten minutes—were also detected by the planetary probes. The total power radiated by these discharges appears to be comparable to those of the kilometric emissions. The plasma trapped within the magnetosphere also has radio emissions, with a frequency exceeding two to three kHz and a recurrence period of ten hours and forty minutes. These low-frequency radio waves were detected by both Voyager probes. The total power radiated is estimated to be on the order of a million watts.

A dense atmosphere combined with a strong magnetic field indicates the presence of auroras. Voyager l’s detection of strong ultraviolet radiation above 76 degrees north latitude and below 78 degrees south latitude confirms the existence of Saturnian auroras similar to those in the polar regions of Earth and Jupiter, with possible periodic longitudinal variation in intensity. Based on these measurements, the power required to generate the Saturnian auroras is estimated at 200 billion watts, which is seven times the corresponding value for Earth’s auroras. The main source of this energy is the interaction between the solar wind and Saturn’s magnetosphere. Confinement of charged particles within the magnetic field and their being forced to travel along intense magnetic field lines near the polar regions cause the auroras. A more intense study of Saturn’s auroral activity was conducted using the Hubble Space Telescope and from the orbiting Cassini spacecraft.

Because Saturn’s magnetic field is highly symmetrical, there is a well-defined bow shock and a steady magnetopause. Saturn has the necessary ingredients for the presence of an active outer magnetic field: a planetary dipole field, a ring-current field, and contributions to the field from a magnetopause and tail currents. The outer magnetosphere of Saturn is supplied with a constant flow of hydrogen and nitrogen plasma by Titan and its magnetic field. This plasma torus is corotating with the magnetosphere, undergoing convective motion at the same time, and is estimated to have a temperature of 1 million kelvins—the hottest of planetary surroundings known to date. Neutral atoms escaping from Titan are photoionized, becoming part of the plasma torus. It is surmised that the corotations of plasma, combined with the continual radial movement of plasma torus caused by fluctuations in the pressure of the solar wind, are responsible for the plasma’s unusual heating, which leads to an increase in high-energy particles. In addition to the elevated plasma temperature that results in high-energy particles, there is a coincident rise in field strength. This jump in field strength appears to be sharper when the magnetosphere contracts. Voyager data indicate discontinuities between the region inside the magnetosphere and the outside, where the solar wind persists. An acceleration of electrons and ions occurs in the magnetotail. Voyager 1 detected low-energy ions streaming toward, and high-energy ions racing away from, Saturn at distances of 2 million to 2.7 million kilometers.

The bow shock, the magnetopause, the acceleration of ions in the magnetotail, a spectrum of energy, and the flux of charged particles appear to be common to the outer magnetospheres of Saturn and Earth. A major difference is the Titan-fed hot plasma torus of Saturn’s outer magnetosphere. There are also many unexplained phenomena in Saturn’s magnetosphere, one of them being the observed fluctuation in temperature at altitudes between 600,000 and 900,000 kilometers. It is not known whether the magnetotail drains the angular momentum of Saturn through the ejection of high-energy particles.

Knowledge Gained

The search for Saturn’s magnetic field began in 1955, with the accidental discovery of decimetric radio emissions from Jupiter. It was surmised that radio emissions from Saturn’s magnetic field would be weaker than those of Jupiter and hence not observable by ground-based radio telescopes. Long-wave radio bursts coming from the direction of Saturn were detected in 1975 by the Interplanetary Monitoring Platform (IMP) 6. Instruments on board Pioneer 11 in 1979, however, confirmed the existence of an extensive magnetic field surrounding Saturn. Detailed observations were carried out by more sophisticated instrumentations of Voyagers 1 and 2. Long-term studies of the magnetic fields and particle environment around Saturn continue to be performed by the Cassini spacecraft.

Data from Pioneer 11 and the two Voyagers suggest that the magnetic field of Saturn originates at a greater depth than does Jupiter. Unlike the fields of Earth and Jupiter, Saturn’s magnetic field is highly axisymmetric because of the overlapping of the dipole and the rotational axes. The main field is hemispherically asymmetrical because of a northward offset of the dipole axis. The field strength at an altitude of 60,330 kilometers is 0.2 gauss, roughly a third of the equatorial geomagnetic field. A second component of the magnetic field of Saturn is the extensive current ring that flows from west to east. The boundary currents of the magnetopause constitute a third component of the field.

The inner magnetosphere of Saturn has been found to be free of charged particles. The maximum density of ions and electrons occurs between the orbits of Tethys and Rhea. Between the orbits of Tethys and Titan, there is a vast torus of neutral hydrogen produced by the photochemical breakdown of methane escaping from Titan’s atmosphere. Saturn’s kilometric radio emission, along with a variety of periodic and nonperiodic bursts of radiation, is presumed to originate in the magnetic plasma surrounding the planet. The planet’s kilometric radiation has a periodicity of ten hours and forty minutes. Short bursts of radio emissions have been traced to electrostatic discharges.

Pioneer 11 and the two Voyagers also provided tantalizing information about Saturn’s auroras, phenomena that occur invariably in the presence of a dense atmosphere combined with a strong magnetic field. Saturn’s auroras, which occur above 76 degrees north latitude and below 78 degrees south latitude, are powered by the interaction between the solar wind and the magnetosphere.

Cassini’s magnetometer naturally was capable of determining the direction and strength of Saturn’s magnetic field with the best technology available at the time the spacecraft was designed. Called a dual-technique magnetometer, it also was able to assist in the determination of the size and nature of Saturn’s core. In order to have the sensitivity necessary for its intended research, the magnetometer assembly included a flux gate magnetometer and a vector/scalar helium magnetometer placed along an 11-meter-long boom to avoid interference from spacecraft electronics. The ability to determine three-dimensional magnetic field maps would be used not only on Saturn but also to search for magnetic fields of the ringed planet’s satellites. The magnetometer was intended to make new discoveries and to answer some of the outstanding questions raised by the Voyager results. Cassini confirmed the Pioneer 11 and Voyager magnetic field findings, also returning some surprising insights about Saturn’s satellite Enceladus.

One of these unexpected insights was evidence of emissions from the moon Enceladus. These discharges, which include plumes of water vapor, provide materials for Saturn’s E ring. These water emissions emanate from cracks in the moon’s icy surface. Enceladus is believed to have a salty sub-surface ocean, which may be able to support life.

Context

Study of Saturn’s magnetic field began with radio astronomers, whose findings and conjectures were to play a dominant part in the planning of planetary exploration probes of the 1970s. Pioneer and Voyager data vastly advanced scientists’ understanding of planetary magnetic fields in general and Saturn’s field in particular. For example, the axisymmetric nature of Saturn’s magnetic field and its effects on magnetospheric processes make it quite different from both Earth’s and Jupiter’s fields. However, data show that Saturn’s magnetosphere (like those of Earth, Jupiter, and Venus) has an extended magnetotail, in which interaction between solar plasma and the magnetic field produces a host of phenomena. The study of magnetospheric processes in general provides a basis for understanding cosmic plasma.

Saturn’s magnetic field is basically dynamo-driven, like similar fields in the solar system, including even that of the sun. Close to the planet, plasma depletion is caused by the ring system closer to the planet, but there is a vast torus of Titan-fed hot plasma at the outer magnetosphere. Saturn appears to possess three distinct regions of plasma: an inner plasma torus, an extended plasma sheet, and the hot outer plasma torus. Both the temperature and the thickness of the plasma disk increase with distance from the planet. The nature of the sources and sinks of these plasma regions is yet to be determined, although speculations abound. The plasma torus is corotating, with velocities decreasing by 10 to 20 percent beyond 480,000 kilometers of altitude.

Based on knowledge of Earth’s magnetic field and the data provided by Pioneer 11, Voyagers 1 and 2, and Cassini, a model of Saturn’s field has been constructed. There are undoubtedly numerous processes occurring in its distant magnetosphere that need to be further examined. The variation in the size and shape of the magnetosphere needs to be examined over an extended period. Quantitative studies of the plasma flow from the satellites of Saturn (Titan and Enceladus in particular) and research into the charge absorption properties of the ring system are projects of long-term interest as well.

The theoretical model of Saturn’s magnetic field is based on a solid foundation provided by spacecraft data. Yet there are numerous unanswered questions and doubts, and the model must be further refined. Continuing research into the atmospheres and magnetic fields of planets such as Jupiter and Saturn is vital.

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