The main resource available for astronomical study is the spectrum of electromagnetic waves, such as visible light, radio, and X-rays, radiated by celestial bodies. In contrast, space physics relies on sending spacecraft to measure conditions on location, such as particle densities and temperatures and the local electric and magnetic fields. Unfortunately, these two scientific disciplines don't get much chance to overlap. Most objects of astronomical interest are beyond the reach of spacecraft, and electromagnetic fields are very difficult to study remotely. However, one object in our solar system is ideal for remote viewing from Earth, and we have had the good fortune to visit it repeatedly with unmanned satellites: The Io plasma torus, an immense cloud of ionized gas trapped in Jupiter's magnetic field.
The Galilean satellites found a natural place in the heavens once Copernicus and Kepler supplanted Ptolemy's geocentric cosmology with a heliocentric one and Newton explained that gravity governed all their motions. For though most wanderers were now understood to orbit the Sun, the conceptual breakthrough was profound enough that letting some of them orbit each other required no great leap. The Moon still orbits our demoted globe, while Galileo's quartet wheels about Jupiter in their own quiet circles, testimony to the Earth-shaking (or perhaps Earth-spinning) power of empirical evidence.
Centuries passed before physics progressed far enough to appreciate Galileo's moons further. In the 19th century physicists studied how electric and magnetic influences permeate space as "fields", in modern terminology. Maxwell discovered that disturbances in those fields could propagate as waves that continually swap energy back and forth between electrical and magnetic perturbations. This insight unified electricity and magnetism as separate facets of a single physical phenomenon, an astounding achievement comparable to our modern search for grand unified theories. It turned out that astronomers had been observing Maxwell's waves all along, but now they had good reason to look outside the visible portion of the electromagnetic spectrum.
In the mid 1950's modern astronomers followed Galileo's lead. However, this time the latest technological gadget was based on Maxwell's principles: a radio telescope. They were delighted to discover that Jupiter emits a prodigious quantity of radio waves. However, unlike the visible light from Jupiter, this signal is not simply reflected glory from the Sun. Something in Jupiter's vicinity produces a trillion watts of radio waves, a thousand times more powerful than a large electrical generating station on Earth. Furthermore, the measured spectra indicates a surprising source. These radio waves could only be produced by high-energy electrons trapped in Jupiter's magnetic field well above the cloud tops.
These observations were the first indication that Jupiter even had a magnetic field. It was only in the mid-twentieth century that magnetic origin of the Earth's aurora, or Northern and Southern lights, was discovered. Particles from the sun become trapped in the Earth's near-space magnetic field, now called the magnetosphere, along with particles that flow off the top of the Earth's ionosphere. Through a variety of systematic and turbulent processes, some particles are accelerated to high energies and eventually crash into the atmosphere. Collisions excite atmospheric atoms and make them emit the bright, colorful lights that dance across the polar skies. The process is quite similar to that in a neon sign, but aurora are totally natural and immensely more impressive. The spectacular visual displays seen overhead are merely the remote signature of electromagnetic tempests raging throughout a volume of space tens of thousand times larger than the Earth itself. However, even this pales in comparison with what's going on in Jupiter's magnetosphere.
Jupiter has the largest and most energetic magnetosphere of any planet in our solar system. Surprisingly, Io, the innermost moon discovered by Galileo, turns out to be of paramount importance, the insignificant-looking tail that wags the biggest dog in the neighborhood. What makes Io such a crucial dynamic element in the Jovian magnetosphere is its volcanism. Internal stresses driven by gravitational forcing make Io the most active volcanic body in the solar system. Evidence of past eruptions are immediately evident in pictures of Io's surface. Much of the ejected material, largely sulfur and oxygen, falls back to Io's surface, leaving huge reddish-orange blotches that make it look much like a sloppily constructed pizza.
More dramatically, great plumes of ejected material rising above the rim of Io's disk have been viewed directly by the Galileo spacecraft. The thin atmosphere and diminished gravitational acceleration, roughly a fifth of Earth's, permits volcanic ejecta to rise some 300 km above the surface. Some gets far enough away to escape Io's gravitational influence and populate immense clouds much larger than Io itself. Once ionized, the particles become part of the Io plasma torus.
The Io plasma torus can be observed quite readily from Earth. The figure below was taken by Nick Schneider and John Trauger using Catalina Observatory's 1.5-meter telescope. The central vertical strip masks the disk of Jupiter to prevent saturation, and the very bright spots are the Galilean moons themselves. A narrow-band transmission filter isolates light from singularly ionized sulfur, so this figure shows but one constituent of the torus. The torus itself is viewed here almost on edge, a huge fuzzy halo centered on Jupiter but canted slightly away from being level. This tilt shows that the magnetic field is not perfectly aligned around Jupiter's rotation axis. Instead, just as the case for Earth, Jupiter's magnetic poles are slightly offset from the geographic poles. As Jupiter rotates, so does its magnetic field, which makes the torus tilt back and forth like a wobbling hoop once every ten hours.
Remote observations give a good qualitative indication of Jupiter's field, but the first definitive measurements were obtained using magnetometers aboard the Pioneer spacecraft. Additional measurements taken by the Voyager, Ulysses, and Galileo spacecraft have given us excellent measurements of the magnetic field, much more detailed than remote observations could have provided. Using this precise, mathematical description of the magnetic field, remote observations can be carefully analyzed to study the interplay of plasma and magnetic field.
Mike Brown has studied the torus extensively this way using the 0.8-meter telescope at Lick Observatory with the Hamilton echelle grating spectrograph. The top frame of this figure indicates how the spectrograph's observing slit (long tilted rectangle) is aligned with the magnetic equator rather than Io's orbit (dashed curve). As Jupiter and its magnetic field rotate, this orientation must be adjusted continually to follow the rocking torus using the precise magnetic field parameters from spacecraft measurements.
The lower frame plots the intensity of light as a function of position along the slit (horizontal) and wavelength (vertical). Again, the center portion is obscured by a heavy filter to reduce Jupiter's overwhelming glare. The tilted dark lines on either side of the planet arise when excited sulfur ions relax and emit light at very particular wavelengths. Those lines are not flat because of varying Doppler shifts, which show that the velocity of the emitting ions varies substantially.
Because the Doppler shift increases steadily, velocity grows with distance. This feature is an immediate sign that this material does not simply orbit Jupiter because of gravity. Kepler's law requires that speed decreases with increasing orbital radius. For example, similar spectrographic observations of Saturn's rings indicate a gradual retreat back toward a zero Doppler shift, clearly indicating that the ring dynamics are governed by gravity, not electromagnetism. The linear relation apparent in the figure is dramatic evidence that the entire torus rotates around Jupiter as a coherent body and that plasma is indeed tightly bound to Jupiter's magnetic field.
Converting Doppler shifts into plasma velocities confirms that the torus rotates at approximately the same rate as the planet. However, in the vicinity of Io's orbit there is a persistent lag of about 4%. This figure plots the disparity between the observed rotational speed and perfect corotation with Jupiter. This phenomenon is remarkably stable and doesn't depend on whether Io itself is nearby or which way the magnetic field is tilted at that moment.
The explanation confirms the torus is not static but continually accepts new material. Atoms and molecules ejected from Io aren't charged, so each neutral particle acts as a tiny satellite held in orbit around Jupiter by gravity, just as Newton described. When ordinary mass near Io is ionized, it must immediately be accelerated from Keplerian speed (about 17 km/s) up almost to corotation speed (about 74 km/s). Accelerating a ton of plasma every second requires an enormous torque to supply the necessary angular momentum. Otherwise, the torus would gradually slow down and stop. The ultimate source of angular momentum is Jupiter itself, and the coupling mechanism is an immense electric circuit.
In contrast, another mechanism that also removes torus mass can be studied remotely. As described above, near Io's orbit neutral particles move around Jupiter at about 17 km/s, which implies an escape velocity of 24 km/s. This is much lower than the corotation speed of 74 km/s, which is how fast charged particles are moving. The force that keeps ions and electrons from flying away is electromagnetic, not gravitation. However, if an ion gains an electron, the resulting uncharged particle suddenly finds itself back in Newton's world, governed only by gravity. Because its speed greatly exceeds the escape velocity, the newly formed neutral escapes Jupiter on a ballistic trajectory like a stone hurled from sling.
This process occurs throughout the torus. Neutralized particles move away on tangents to their original paths, which lay fairly close to Io's orbital plane. The result is a spray of neutral particles fanning outward around Jupiter like an immense garden sprinkler. This extended cloud of outflowing sodium can actually be seen from Earth. The observations shown here were taken by Mike Mendillo and colleagues using Boston University's 0.1-meter telescope at McDonald Observatory. The color coding represents intensities, which are quite faint far from Jupiter where the particles are thinly spread out. Tracing ejected sodium trajectories back to the torus indicates the conditions under which plasma ions are neutralized.
To fully appreciate this figure one must project it onto the sky. The angular diameter of Jupiter's disk was about 45 seconds of arc during those observatures. Therefore, the cloud's total width of one thousand Jovian radii corresponds to 6 degrees on the sky, ten times the diameter of the full moon. From our perspective, the neutral sodium nebula would fit snugly into the bowl of the Little Dipper, which makes it one of the largest objects within the solar system visible to astronomers.
Using knowledge gleaned from this collaboration,
we can observe the torus indefinitely from our remote vantage and continue
learning more about this fascinating system, even after Galileo has transmitted
its final byte. The examples described here only hint at the wide variety
of ongoing investigations. Other spacecraft will visit Jupiter in the future
and allow us to continue our investigations on location. But the ready
accessibility of the Io torus for terrestrial observations makes it a uniquely
rich object for study.