Jupiter's Dazzling Northern Lights

As Jupiter twirls its magnetic skirt in a cosmic fandango, astronomers have watched patiently, hoping to catch a glimpse of the electric clasps which they believe hold the skirt in place. Now, the waiting may be over. In today's Nature, a team of planetary scientists reports finding the first direct evidence of the electric fasteners, known as "auroral electrojets."

A giant plasma sheet of charged particles and magnetic fields permanently fans out from Jupiter's surface. The charged magnetic fabric drapes over the volcanic moon Io and its edges even tickle the rings of Saturn. It requires a huge amount of energy to keep the sheet spinning, but scientists have long realized that if the skirt and planet were linked, Jupiter's rotation could easily supply the necessary energy. And many argued that the magnetic tension from field lines in auroral electrojets, which, like our own northern lights, become visible when bursts of charged particles streak along the magnetic fields at the Jovian poles, could provide the necessary connection. But no one had ever detected auroral light on Jupiter.

Now, a team led by planetary scientist Steve Miller of the University College London has caught the first glimpse of an electrojet with NASA's Infrared Telescope Facility on Mauna Kea. Miller's team tracked charged hydrogen molecules in two parallel electrojets as they rocketed around the magnetic fields of Jupiter's north pole faster than the speed of sound. "The magnetic fields in the electrojet act like wires" connecting the planet to the plasma sheet, says Miller. These "wires" pull on the Jovian atmosphere, which spins more slowly near the poles.

"This is a nice first observation" of electrojets, says space physicist Margaret Kivelson of the University of California, Los Angeles. But to completely understand the dynamics of the clasps on Jupiter's magnetic skirt, "we need to measure the total electric current in the electrojets," says Kivelson. The current study, she says, only shows that such a current exists.