When interplanetary shock waves hit the Cassini spacecraft and then Saturn in January 2004, it presented a unique opportunity to study the planet's magnetosphere and to compare it with that of Earth.
Saturn can be considered as the geometric mean of Earth and Jupiter in terms of the strength and extent of its magnetic field. Three papers in this issue — by Clarke et al.1, Kurth et al.2 and Crary et al.3 — describe the response of Saturn's magnetosphere to changes in the solar wind as observed by NASA's Cassini spacecraft and the Hubble Space Telescope (HST). The authors conclude that some aspects of the behaviour of Saturn's magnetosphere are similar to the behaviour of Earth's magnetosphere, some to that of Jupiter's and some are unique. Studies of Saturn's magnetic field and how it is driven by the solar wind are interesting in their own right, but they also allow researchers to compare different planetary magnetospheres and to test our understanding of Earth's system by applying the same principles to different conditions.
Earth's magnetic field forms a cavity in the solar wind — the stream of electromagnetic radiation and charged particles that flows outwards from the Sun. Earth's magnetosphere extends roughly 10 times the planet's radius towards the Sun and many hundreds of Earth radii away from the Sun, in a ‘magnetotail’ stretching downstream of Earth in the solar wind. Jupiter is much larger than Earth (by a factor of 11), and its magnetosphere is also vast, extending 50–100 jovian radii on the dayside, with a magnetotail that stretches out to its orbit distance. Saturn's magnetosphere (Fig. 1) is an intermediate case, extending about 20 Saturn radii towards the Sun (Saturn's radius is 9.4 times that of Earth). Most of the material in Earth's magnetosphere is a plasma of protons and electrons that has leaked in from the solar wind. By contrast, the magnetospheres of Jupiter and Saturn are mainly fed by plasma sources of heavy ions from their satellites.
The three papers1,2,3, beginning on page 717, describe observations of magnetosphere dynamics at Saturn. In Earth's magnetosphere, plasma circulates in a flow pattern that is primarily driven by the coupling of the planetary magnetic field to the solar wind. Within about 15° of the poles, Earth's magnetic field is directly connected to the solar wind. At lower latitudes the magnetic field topology is closed, with magnetic field lines connected at both ends to the planetary dynamo. At the outer boundary of the magnetosphere — the dayside ‘magnetopause’ — small regions of closed magnetic field couple to the solar magnetic field (which is swept towards the planet by the solar wind) in a process called magnetic reconnection. Once coupled to the solar wind, these tubes of magnetic flux are swept back over Earth's poles and down the magnetotail where they reconnect to closed field lines — as they must, to conserve the total magnetic flux from the planet.
The stresses associated with this process of coupling solar wind and magnetosphere drive electrical currents between the magnetopause and the ionosphere (the ionized upper part of the planet's atmosphere), leading to radio and auroral emissions. The terrestrial aurorae form in rings around Earth's magnetic poles, at the boundaries between regions of open and closed magnetic field. The orientation of the solar magnetic field is important for magnetic reconnection and therefore strongly modulates dynamical activity and auroral emissions.
Unlike the situation near Earth, Jupiter's intense aurorae are caused by stresses associated with rotation rather than coupling to the solar wind. Jupiter's strong magnetic field couples profuse plasma from its moon Io to the planet's rapid spin (a 9.9-hour period). As the plasma spreads out and inflates the giant magnetosphere, strong electrical currents between the magnetosphere and Jupiter's ionosphere keep the plasma rotating (against the tendency to slow down owing to conservation of angular momentum). Observations of Saturn in 1981 by the Voyager spacecraft indicated that its magnetosphere dynamics may be similar to those of Jupiter and be driven by Saturn's fast rotation (a 10.7-hour period). However, Saturn's icy satellites are much weaker plasma sources than Jupiter's volcanic Io, making Saturn's magnetosphere less inflated, the stresses less, and the aurorae weaker. Moreover, earlier HST studies hinted that the solar wind influences the aurorae. Thus, when Cassini approached Saturn, scientists grabbed the chance to test the hypothesis that its magnetosphere dynamics and associated aurorae are controlled by the solar wind, as occurs around Earth.
For 22 days, Cassini's instruments measured the magnetic field, plasma density and plasma velocity in the solar wind while the HST and Cassini radio antennas monitored Saturn's auroral activity. Nature cooperated and provided a couple of interplanetary shock waves that passed the Cassini spacecraft on 15 and 25 January and hit Saturn's magnetosphere some 17 hours later. Clarke et al.1 report HST observations of the subsequent brightening of auroral emission, and Kurth et al.2 report accompanying increases in radio emission. Crary et al.3 show a correlation of auroral intensity with solar-wind dynamical pressure, supporting the view that the solar wind has an Earth-like role at Saturn.
But further study showed that the solar-wind conditions that influence Saturn's magnetosphere dynamics are different from those that influence Earth's. Compression of the magnetopause by the solar wind is more important than reconnection of the solar and saturnian magnetic fields. Crary et al.3 point out that at Saturn's orbit (9.5 times Earth's distance from the Sun), the solar magnetic field is fairly closely confined to the plane of Saturn's orbit around the Sun; the solar and saturnian fields are therefore largely at right angles to each other, so that the optimum conditions for magnetic reconnection are not met.
But there are further unknown complications to Saturn's magnetospheric dynamics and auroral activity: what are the effects of Titan, Saturn's largest moon, and of the 26° tilt of Saturn's southern pole towards the Sun? Studies of the terrestrial magnetosphere have shown the need for long-term monitoring to properly distinguish between the causes of various auroral effects. Cassini will make many more observations in its 75 orbits of Saturn. But the imaging spectrograph onboard the HST is no longer operational and the space telescope's future looks bleak. Furthermore, these vital but unglamorous synoptic studies of planetary magnetospheres currently have little appeal for NASA, whose science budget is being squeezed to pay for human space exploration. So it may be some time before we can follow up on these latest findings from Saturn.
Clarke, J. T. et al. Nature 433, 717–719 (2005).
Kurth, W. S. et al. Nature 433, 722–725 (2005).
Crary, F. J. et al. Nature 433, 720–722 (2005).
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The Astronomy and Astrophysics Review (2014)