Planetary science

Magnetic moments at Jupiter

The coming together of two spacecraft near Jupiter provided a unique opportunity to investigate the giant planet's magnetic field — and the results, collected in this issue, are stunning.

In January 2001, the Cassini–Huygens spacecraft rendezvoused with its sister craft Galileo in the vicinity of Jupiter. Planetary scientists seized the opportunity offered by this first-ever conjunction of two spacecraft at an outer planet, and were richly rewarded for their efforts. Observations by the two spacecraft were complemented by nearly simultaneous images from the Earth-orbiting Hubble Space Telescope and Chandra X-ray Observatory. As reported in a series of seven papers in this issue1,2,3,4,5,6,7 (beginning on page 985), this coordinated campaign has revealed several new clues to the complex dynamics of Jupiter's magnetosphere — the comet-shaped region of space that is filled by the planet's magnetic field.

Jupiter has by far the strongest magnetic field of any planet in the Solar System, and the most powerful magnetosphere. The Cassini–Huygens spacecraft flew within 135 Jupiter radii of the planet's centre (its equatorial radius is about 71,400 km). This distance of closest approach was fixed by the need to use Jupiter's gravity to boost the energy of the spacecraft just enough to guide it to its final destination, Saturn. Many scientists would have preferred a closer approach. Indeed, some (including myself) worried aloud that, at that distance, Cassini–Huygens would miss the magnetosphere altogether.

Jupiter's magnetosphere is easily the largest object in the Solar System, at around 20 solar diameters wide and several astronomical units in length (an astronomical unit is the mean radius of the Earth's orbit about the Sun, 1.5 × 108 km). But the size of a magnetosphere varies, and although Jupiter's is immense, it is usually not quite that immense. Nature chose to be kind on this occasion, however: on 9–10 January 2001, the magnetosphere was unusually large — just large enough to reach out and touch the passing Cassini–Huygens spacecraft on two brief occasions, as Kurth et al.1 report on page 991. In any case, the influence of Jupiter's magnetosphere is detectable far beyond its boundary surface (called the magnetopause) because it emits copious quantities of neutral gas, charged particles and electromagnetic radiation across the spectrum from radio to X-ray wavelengths.

The size of a magnetosphere is determined by the pressure balance at the magnetopause. The internal pressure is sustained by the planet's magnetic field and the plasma (hot ionized gas) trapped inside the field. The external pressure is provided by the solar wind — a fully ionized stream of plasma that flows continually, but variably, outwards from the Sun. The wind is highly supersonic, and its pressure variations tend to steepen into shock waves as they move away from the Sun. These interplanetary shocks produce sudden compressions and expansions of magnetospheres; when this happens in the Earth's magnetosphere, it causes disruptions of human technology, such as wireless communication, electric-power distribution and satellite navigation.

The two-spacecraft conjunction at Jupiter, coupled with a bit of good luck in the form of a passing interplanetary shock, allowed Kurth et al.1 to catch Jupiter's magnetosphere in the act of compression. Although the pressure of the solar wind determines the size of the magnetosphere, no single model magnetopause can describe what the two spacecraft saw during their almost simultaneous magnetopause encounters (Fig. 1). Kurth et al. argue plausibly that the magnetosphere must have been in the process of compression by an interplanetary shock that had passed Galileo's position but had not yet arrived at Cassini–Huygens. Plasma measurements taken by Cassini–Huygens outside the magnetopause, just before and just after its immersion in the magnetosphere, are consistent with the passage of a shock in the interim.

Figure 1: The conjunction of the spacecraft Cassini–Huygens and Galileo at Jupiter.

Jupiter's magnetic field is confined by the pressure of the solar wind to a region known as the magnetosphere (blue), bounded by the magnetopause. On 10 January 2001, both spacecraft encountered the magnetopause, within half an hour of each other. The dashed blue lines correspond to magnetopause equilibrium models for high and low solar-wind pressure. The near-simultaneous encounters do not fit a single model, but suggest a combination of the high- and low-pressure curves (solid blue line). Kurth et al.1 conclude that the magnetopause was caught in the act of adjusting from a low-pressure to a high-pressure state, as an interplanetary shock wave passed between the two spacecraft positions. (Even though the solar wind typically travels at around 400 km s−1, it would take 20 hours to cross this figure from right to left.)

The Cassini–Huygens spacecraft observed three such interplanetary shocks in the preceding two months, as it approached Jupiter from Earth. Gurnett et al.2 (page 985) provide evidence that one of these shocks was probably responsible for a subsequent magnetopause crossing by Galileo. They also show that all three shocks were followed by distinctive brightenings of Jupiter's aurorae — analogous to the northern and southern lights on Earth — and associated radio emissions after an appropriate time delay. This result settles a long-running debate over the sign of the effect — whether the aurorae would brighten or diminish. Additional clues about Jupiter's aurorae were provided by observations from Earth-based telescopes that were timed to coincide with the spacecraft conjunction (see Box 1).

Jupiter's interaction with the solar wind is by no means passive; its magnetosphere leaves a clear imprint on the solar wind, even far upstream. Io is the innermost of Jupiter's four largest moons — called the galilean satellites after Galileo who discovered them in 1610. Volcanoes on Io's surface supply heavy ions, mostly oxygen and sulphur, to Jupiter's magnetosphere (Fig. 2). There, the ions are energized and can escape their magnetic confinement if they are neutralized through charge-exchange interactions. These energetic neutral atoms (ENAs) form a planetary wind that can propagate upstream against the solar wind (the atoms are not coupled to the wind because they are uncharged). Eventually the ENAs re-ionize and become part of the solar wind — but the wind, like the Sun, is mainly hydrogen, so the presence of the heavy ions is a distinctive signature of their origin in the volcanoes of Io. Cassini–Huygens has now confirmed the existence of the long-hypothesized ENA wind in both its neutral and re-ionized forms (Krimigis et al.3 on page 994).

Figure 2: In orbit around Jupiter, the Galileo spacecraft caught this image of a volcanic eruption on Io, the innermost of Jupiter's largest moons.


The plume extends to a height of 140 km above Io's surface.

In addition to ENAs and charged particles, Jupiter's magnetosphere is also a prodigious source of non-thermal radio emissions. At frequencies below about 40 MHz, these emissions result from coupling processes between the magnetosphere and the ionosphere (the ionized level of Jupiter's atmosphere) that are associated with aurorae (see Box 1). Higher-frequency 'synchrotron' emission comes from relativistic (highly energetic) electrons trapped in the heart of Jupiter's radiation belt, only 0.5–3 Jupiter radii above the visible cloud level. The highest frequencies, corresponding to the greatest electron energies, are difficult to observe from Earth because they are lost in the thermal glow of the planetary disk. Taking advantage of the proximity of Cassini–Huygens, Bolton et al.4 (page 987) used its radar instrument to separate the high-frequency synchrotron emission (at 14 GHz) from the thermal glow, and discovered an unexpected population of ultrarelativistic electrons with energies up to 50 million electron volts (Fig. 3).

Figure 3: Synchrotron emission around Jupiter, picked up by instruments aboard the Cassini–Huygens spacecraft, reveals electrons with unexpectedly high energies4 — up to 50 million electron volts (yellow regions).


An image of the planet from the Hubble Space Telescope is superimposed.

The unique observations from the conjunction of two spacecraft are a fitting tribute to Galileo. In September 2003, after nearly eight years of successful orbital operations, the Galileo craft will be committed to the depths of Jupiter's atmosphere. But the work of Cassini–Huygens is just beginning. When it reaches Saturn in July 2004 the European Space Agency's Huygens Probe will separate from NASA's Cassini Orbiter and descend to the surface of Saturn's largest satellite, Titan, to investigate its intriguing, seemingly Earth-like atmosphere. Cassini will then begin an orbital tour, like that of Galileo at Jupiter, exploring Saturn's atmosphere, rings, satellites and magnetosphere. If its performance during the Jupiter flyby is any indication, Cassini–Huygens is ready for the task.


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Correspondence to Thomas W. Hill.

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Hill, T. Magnetic moments at Jupiter. Nature 415, 965–966 (2002).

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