April 30 (UPI) -- NASA's Galileo mission ended in 2003, but data collected during the spacecraft's first flyby of Ganymede -- conducted two decades ago -- has yielded fresh insights into the Jovian moon's unique electromagnetic field.
"We are now coming back over 20 years later to take a new look at some of the data that was never published and finish the story," Glyn Collinson, researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a news release. "We found there's a whole piece no one knew about."
Collinson is the lead author of a new paper on Ganymede's magnetosphere, which is shaped by a magnetic field unlike any in the solar system.
While reexamining the 20-year-old data, scientists discovered evidence of an explosive magnetic event triggered by the interactions between Jupiter's magnetosphere and the magnetic field of its satellite, Ganymede. The event saw plasma rain knock a barrage of high-energy particles onto Ganymede's icy surface.
New analysis of the event and its signatures, picked up by Ganymede's instruments, has helped scientists better understand the unusual shape and structure of the moon's magnetosphere.
Ganymede's magnetic field is unusual in that it exists almost entirely within the magnetic field of Jupiter, the gas giant around which Ganymede orbits. Like Earth, collisions between streams of high-energy particles and its magnetosphere yield dramatic auroras, but unlike Earth, these plasma streams come from Jupiter's electromagnetic field, not the sun.
As the latest study -- published Monday in the journal Geophysical Research Letters -- helped planetary scientists understand, Ganymede's poles are constantly bombarded by plasma particles accelerated by Jupiter's magnetic field. The moon's magnetosphere directs this plasma rain toward its surface.
"There are these particles flying out from the polar regions, and they can tell us something about Ganymede's atmosphere, which is very thin," said NASA scientist Bill Paterson. "It can also tell us about how Ganymede's auroras form."
By comparing the signatures of the plasma rain picked up by Galileo's instruments with observations collected by Hubble, scientists were able to pinpoint the origin of Ganymede's auroras and more accurately model the shape of the moon's magnetosphere. Scientists believe Ganymede's magnetic field looks like a giant bull horn, with the point stretching out in front of the moon's orbit.
Though Ganymede's magnetosphere is unique, researchers believe interactions between the moon and Jupiter's magnetospheres produce an electromagnetic phenomena that is quite common: the tangling and snapping of electromagnetic field lines that is known as magnetic reconnection.
Astronomers believe the reconnection occurring as the two magnetic fields interact helps explain why the moon's auroras are so bright.