Since arriving at Jupiter in the summer of 2016, NASA’s Juno probe has been exploring the giant planet’s environment, capturing amazing vistas of its swirling bands and taking measurements of its atmosphere, interior, and magnetic field.
Now, Mura et al. have published an analysis of infrared images of Jupiter’s impressive auroras, captured by Juno as it made its first flyby of the planet on 27 August 2016. These images reveal that the auroral ovals ringing the poles are made up of multiple intense arcs within, giving clues about the structure of currents running through Jupiter’s magnetosphere. They also capture a long auroral “tail” caused by its moon Io and its unique magnetic influence within the Jovian system.
Like at Earth, Jupiter’s auroras are caused by energetic charged particles being channeled along the planet’s magnetic field to the poles. When they strike atmospheric molecules, they make them glow, forming a dynamic oval around the planet’s poles. But unlike at Earth, where the most spectacular auroral displays are driven by particles from the solar wind streaming into Earth’s magnetic field, Jupiter has its own local source of plasma: the volcanic moon Io, which spews sulfur dioxide gas into space.
As Io moves with respect to Jupiter, it acts as an obstacle to the plasma and magnetic field flow; this disturbance propagates back to Jupiter, traveling along the magnetic field lines as Alfvén waves. This propagation generates an additional auroral signature—a glowing spot called a “footprint” at the point where it enters the planet’s atmosphere. As the planet rotates, the tube’s end point drags across the surface of Jupiter, creating an auroral tail. The same thing happens at the moons Ganymede and Europa, but in a much weaker fashion as the source of the plasma is not volcanic activity but ice sublimating off the moons’ surfaces.
These observations were carried out by an instrument on Juno specifically designed to image the infrared auroral infrared glow given off by molecular hydrogen, called the Jovian Infrared Auroral Mapper (JIRAM). The mapper has captured dramatic images of Io’s tail on Jupiter: 500 kilometers thick and extending a quarter of the way around the planet in longitude. Europa’s and Ganymede’s tails were also visible. The instantaneous footprints of Io and Europa were beyond the horizon, but future flybys should image them. That should prove helpful: Because the footprints are magnetically connected via the flux tubes back to the moons, they can help map out the geometry of the planet’s magnetic field.
JIRAM’s images also captured a detailed view of the main auroral oval, revealing that its structure is not uniform but banded, made up of several thin, nested arcs. This structure suggests that instead of one loop of current connecting the planet to its magnetic field, there may be several ribbons of current in a filamentary structure.
Juno also carries an ultraviolet spectrometer that has returned images published separately. Future studies will compare the imagery to learn more about the structure of the aurora in Jupiter’s atmosphere. (Geophysical Research Letters, https://doi.org/10.1002/2017GL072954, 2017)
—Mark Zastrow, Freelance Writer