Planetary Science Institute researcher Jeff Morgenthaler has been awarded a $356,000 NASA Outer Planets Research Program grant to use nearly 20 years’ worth of ground-based telescopic observations to study, among other effects, the flow of energy in and out of Jupiter’s Io plasma torus.
The Io plasma torus is composed of ionized gas that is swept up in Jupiter’s powerful magnetic field. The source of the gas is Jupiter’s innermost Galilean satellite, Io. The plasma forms a torus-shaped structure that encircles Jupiter with the radius of the torus approximately the same as the radius of Io’s orbit. Hence it is called the Io plasma torus.
One of the long-term puzzles of the Io plasma torus concerns the observed imbalance between the well-known sources of energy input and output from the Io plasma torus. It is possible to calculate how much energy the torus receives when newly formed ions join the torus. It is also possible to observe how much energy leaves the torus by looking at the total energy of all of the photons it emits. The “energy crisis” is that about twice as much energy leaves the torus than is currently known to be added to it.
Morgenthaler’s working hypothesis is that the source of the “missing” input energy is related to material leaving the torus. As torus material diffuses outward, it pulls the magnetic field with it. The magnetic field can withstand some of this, but eventually something has to give. The magnetic field reconfigures in a process called “flux tube interchange,” which allows magnetic field laden with plasma to continue to diffuse outward and brings magnetic field with no plasma inward. These interchange events seem to produce high-energy beams of electrons which have been detected by spacecraft like Voyager and Galileo. These electrons might provide the missing energy.
Morgenthaler has been part of a team of researchers, originally from the University of Wisconsin, which has been collecting observations of Io and the plasma torus for more than two decades. These observations include spectra and images of the plasma torus and spectra of Io. The observations have been recorded at the McMath-Pierce Solar Telescope on Kitt Peak, Arizona.
Using a solar telescope — chosen because there is not a high demand for a solar telescope at night — has allowed continuous multi-week studies of the Io plasma torus to be conducted in many of the past 20 years. This time coverage is essential for sorting out the variations in the magnetic field configuration that might lead to a solution to the “missing energy” problem.
Spacecraft visits to Jupiter, like the upcoming Juno mission, provide invaluable information that is not available from the ground. Interestingly, the spectroscopic observations of Io that Morgenthaler is working with essentially turn this moon into a spacecraft with a plasma sensor. Using these spectra, Morgenthaler and collaborator Ronald Oliversen of NASA Goddard Space Flight Center have been able to detect an emission line of oxygen in Io’s atmosphere, which changes brightness as Io moves in and out of the denser regions of the torus. The brightness changes are mostly well understood, however, occasionally there is a period of an hour or so where the oxygen emission from Io is unusually bright. Morgenthaler and his colleagues suspect that these “departure events” are triggered by flux tube interchange events and therefore may help solve the missing energy problem.
Morgenthaler’s co-investigators on this project are William Smyth and Max Marconi, of Prisma Basic Research. Smyth and Marconi bring to this project expertise in calculating the torus plasma density, effects relating to the tenuous atmosphere of Io, and the distribution of non-ionized material in the vicinity of the Io plasma torus.
