Monday, March 25, 2013

JUICE and Ganymede

Last year ESA decided that  JUICE (JUpiter ICy moons Explorer) was going to be the next L-class mission to be build and launched. This was a project long in the making, as I think that in 2002 or 2003 I went to a meeting in France at which a new Jupiter mission was going to be discussed; the name that wast proposed at that time was Laplace. A mission was developed, with the appropriate instrumentation planned.

Then it seemed that the NASA also wanted to send a large mission to Jupiter, so it was set out to make a joined project, two spacecraft that would arrive almost simultaneously at the largest planet of our solar system. First we would have two point measurements and at the end of the ~3 year mission the American satellite would go into orbit around Europa and the European one around Ganymede. The new (strange) name of the mission was EJSM (Europa Jupiter System Mission). However, a financial setback happened at NASA and they withdrew their part of the mission, leaving ESA with the JGO (Jupiter Ganymede Orbiter) which is now called JUICE.

The cover page of the JUICE proposal.
Last month the instrument teams for JUICE were chosen, and fortunately the magnetometer team in which my institute (Space Research Institute, Austrian Academy of Sciences) is involved was selected to fly. Now we just have to build these things and then wait until 2022 when the mission is launched and then wait another 8 years before JUICE arrives at Jupiter. Space physics is nothing for people in a hurry!

The ultimate goal of the mission is the investigation of the Jovian system; atmosphere, magnetosphere, moons, and in the end phase concentrate on the largest moon of the solar system: Ganymede.

Doing science with magnetometers, my main occupation, is more involved than you might think. It is not just measuring the magnetic field and say "okay it's got one", but through careful analysis of the data one can deduce a lot of information, which can come in handy when another instrument is not working, e.g. using waves in the magnetic field you can find out which ion species are present.

In preparation for the final phase of the JUICE mission (you can never be early enough) I decided to use the old Galileo magnetometer data to study this moon's magnetosphere. Early flybys in 1996 had shown that this moon is fully differentiated (i.e. a metal core, rocky mantle and icy shell) and has its own internal magnetic field. This field is strong enough to keep Jupiter's magnetic field at a distance, and thus creates a magnetosphere around Ganymede, similar to the Earth's magnetosphere in the solar wind. In 1997 and 2000 Galileo flew through the upstream magnetosphere (like the dayside of the Earth) and the magnetometer data showed some very interesting details.

A model for field line resonances for the
first (left) and second (right) harmonic,
with the the variations of the electric and
magnetic field shown.
Field line resonances: Magnetic field lines can act like strings on a violin, when you disturb them they start to oscillate with a certain spectrum of distinct frequencies, as shown in the figure. Naturally, one needs to be "on the same field line" to observe this spectrum when the spacecraft passes by. This is a bit easier than it sounds: when the spacecraft remains at basically the same distance of the moon this requirement is fulfilled. This happens for about 3 minutes around closest approach of Galileo to Ganymede. During both flybys such a spectrum was observed at different distances from the moon.

The frequencies of these field line resonances are dependent on the strength of the magnetic field and on the density of the plasma. The first we can measure with our magnetometer, the second we cannot find, because the plasma instrument on Galileo has some problems with the interpretation of the data. Therefore, we compared the observed peaks in the spectrum with a model for a dipole magnetic field and found that the peaks fitted within 5% with those of the model. Thus we could determine the density on the field line. This gave us 2 densities of the plasma in Ganymede's magnetosphere, and as one would expect the flyby that went in deeper measured a higher density. This is similar to the atmosphere of the Earth, where high on a mountain the air is thinner than at sea level.

Ion Cyclotron Waves: When particles get ionized in the presence of a magnetic field then they get "picked up", i.e. they start to gyrate around the magnetic field at a specific gyration period that is determined by the charge and mass of the particle and the strength of the magnetic field. When enough particles are picked up then we can observe waves in the magnetic field data at these specific frequencies. These waves are polarized because the (positive) ions only gyrate clockwise (or left-handed) around the magnetic field whereas electrons gyrate counter-clockwise (or right-handed). Knowing the magnetic field strength, one can search for specific cyclotron frequencies in the spectra of the magnetic field and thereby find out which particles around Ganymede are picked up. With Ganymede's icy surface, it is no wonder that in Galileo's measurements the presence of water-group ions was found. From the amplitude of the waves one can then infer a pickup density, and it was found that, compared to the density of Jupiter's magnetosphere, it was quite high, several times the background density.

If you want to find out all about details (shameless self promotion) you can read it in: Volwerk et al.: ULF waves in Ganymede's upstream magnetosphere published in open access journal Annales Geophysicae.

This was the first of (hopefully) bi-weekly space physics research stories, I hope you enjoyed it. Comments are welcome.


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