One of the most beautiful sights in the sky (at least, so I've heard, since I've never ^%$#&*# seen one) is an aurora. The Earth has a magnetic field that traps charged particles from the Sun, and due to complicated processes that are still being investigated these particles can slam into our air, causing it to glow (exactly) like a neon sign. But we're not the only planet with a magnetic field. And some moons have them, too. Check this image out:
That's the north pole of Jupiter as seen by Hubble Advanced Camera for Surveys back in 2007 (the vertical black line is part of the detector that blocks the view). It's an ultraviolet image, since the atmosphere glows brightest in the UV. The big oval is the main aurora, but see the dot with the curved tail to the right? That is due to Jupiter's mighty magnetic field interacting with the moon Io, a volcanic and violent satellite about the same size as our own Moon. The interaction is truly terrifying: over a million amps of current flows between Jupiter and Io as the magnetic field of the giant planet sweeps past the moon. To give you an idea of how much electricity that is, a typical car battery might crank at about 100-500 amps, so imagine 10,000 cars all lined up, powered only by the current flowing between the planet and its moon... Io doesn't have a magnetic field on its own, but the sulfur ions spewed out by Io's volcanoes get swept up by Jupiter's magnetic field, linking the two objects. So Io is connected to Jupiter magnetically, and a vast current flows from the moon down to Jupiter's poles. Where the current hits, the atmosphere glows, and you see that bright dot. As Jupiter spins, that footprint marches across the planet, fading with time, leaving the curved tail of emission. Ganymede is another moon of Jupiter, and as befits the massive planet, Ganymede is the largest moon in the solar system, bigger than Mercury! It too interacts with Jupiter's magnetic field, and it too leaves a footprint on the jovian atmosphere. That can be seen on the left of the picture above. Ganymede has a strong enough magnetic field on its own that it has what amounts to a protective bubble around it, much like the Earth's magnetic field protects it from the Sun's solar wind. As far as we know, Ganymede is the only moon to have such a bubble. And now, for the first time, astronomers have been able to measure the size of Ganymede's auroral footprint and see that it fits the expected size from what's known of Ganymede's magnetic field. They've also seen that it varies in strength over 3 different time intervals: 100 seconds, 10-40 minutes, and 5 hours. This means there are variables, changes in the magnetic fields of the two bodies, which is modulating the amplitude of the interaction. Most likely the longest is tied to Jupiter's rotation, which is over 9 hours. The other two may be due to local changes in the plasma environment of Ganymede, and to the way its magnetic field connects with Jupiter's. So what does all this mean? Well for one, it means Jupiter's raw might and power scare the crap out of me. I'm glad it stays 600 million kilometers away! Second, it means that the Jupiter system is incredibly complex and well worth studying, to understand how magnetic fields in space interact and produce the kind of environment we see. A manned mission to Jupiter is unlikely any time in the next century, but the more we understand about such things, the more we can apply them to places we can go, like Earth orbit, and possibly Mars too. Knowledge gleaned from scientific observations is never abstract; there are always applications for it. That's because everything really is connected to everything else... whether it's a physical connection blasting countless electrons into an alien atmosphere, or a connection we ourselves make by applying what we know to what we want to know. That's science.
