OFJ97 Field Journal from Duane Bindschadler - 2/20/97

But the magnetometer and the other fields and particles instruments are not quite finished for the day after Galileo has flown by Europa. These instruments will make another recorded observation, also for 45 minutes, starting at about 1 pm this afternoon.

Just a few minutes after our recorded flyby of Europa, MAG will need to be "reconfigured" (I talked about this process in my journal for yesterday, 3/19). This is because Galileo is venturing into the inner region of Jupiter's magnetosphere, where the magnetic field is very strong. The sensitivity of the magnetometer must be adjusted so that it can make accurate measurements.

Then, early this afternoon, Galileo will arrive at the point in its orbit where it is closest to Jupiter. In this orbit, we'll be about 580,000 km (360,000 miles) from the cloud tops of Jupiter. That's about one and a half times the distance from the Earth to the Moon. This may not seem "close" to Jupiter, but it is for Galileo.

"Why is that?" you might ask. Well, the complete explanation is a little bit complicated, but let me try anyway (if you get too confused, you can skip to the next to last paragraph in this journal to read the short answer). As you might know already, Jupiter has a very strong magnetic field. Jupiter's "magnetosphere" is the region of space. Inside this space, Jupiter's magnetic field is strong enough to deflect charged (ionized) particles that move outward from the Sun (these particles make up the "solar wind"). Earth also has a magnetosphere, which protects us from many of the effects of the solar wind. If the Earth didn't have a magnetosphere, we would probably not be here at all!

All of the Galilean satellites (Io, Europa, Ganymede, and Callisto) are found deep inside Jupiter's magnetosphere. This is important because of the constant volcanic eruptions on Io. These eruptions eject tons of material from Io into space. Once in space, Jupiter's radiation "ionizes" these small particles. When a particle is "ionized," it means that it now has an electric charge, either a positive charge or a negative charge. Magnetic fields can only affect charged particles, so now these particles from Io can be influenced by Jupiter's magnetic field.

Jupiter's magnetic field now tries to "grab" these particles. But the magnetic field and the particles are moving at different speeds. The particles are moving along with Io in its orbit, going once around Jupiter every 42 hours. Jupiter's magnetic field is rotating with Jupiter, so it spins completely around once every 10 hours. That means that Jupiter's magnetic field is moving about 20,000 miles per hour, or 5 miles per second faster than the particles ejected from Io! The result is a lot of interesting scientific phenomena which we would like to study.

Of course, it's not just the magnetic field and the slow particles that are involved. There are also particles that were picked up earlier, and that are moving along with Jupiter's magnetic field. The particles and their interactions form a doughnut-shaped region surrounding Io's orbit called the Io Torus. We would also like to study the Torus.

So what does all of this have to do with Galileo (which is still well over 100,000 miles away from Io) or with MAG? The interactions between the magnetic fields and the Io particles create radiation belts and a host of other phenomena that are unique to Jupiter. That radiation, which gets stronger as you get nearer to Jupiter, is a source of danger to Galileo. Too much of it can destroy electronic components, and render Galileo deaf and blind. Flying a mere 360,000 miles from Jupiter's cloud tops is therefore close enough!

At the same time, MAG and the other fields and particles instruments want to make measurements so that we can understand how Jupiter's magnetic field and Io's particles interact. So at 1 pm today, just as Galileo nears perijove (the point in its orbit that is closest to Jupiter), these instruments will record at their highest resolution for about 45 minutes. Although still far from Io, we will measure some of Io's effect on Jupiter's magnetosphere, and add to our understanding of the Io Torus.