Probing Jupiter’s big magnetic bubble

Simultaneous measurements by two spacecraft have probed in greater detail than ever before the invisible bubble of charged particles that surrounds Jupiter.

The bubble, or magnetosphere, is the solar system’s largest structure with sharp boundaries and has a diameter greater than 100 times that of the giant planet.

The data were obtained early last year when the Galileo probe, which has orbited Jupiter and its moons since 1995, was briefly joined by the Cassini craft, which flew past the planet on its way to Saturn. The combined information shows that the magnetosphere contracts in response to shock waves from the sun.

The two craft were uniquely situated to probe the magnetosphere, notes Scott Bolton of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. On Jan. 10, 2001, Galileo resided inside the bubble and Cassini was 7.7 million kilometers away, outside the bubble. The Saturn-bound craft was monitoring the solar wind, the stream of particles that blows from the sun.

In response to shock waves carried by the solar wind, each craft encountered the boundary of the Jovian magnetosphere while it contracted. This is the first two-point measurement of the Jovian magnetosphere caught in the act of responding to the solar wind, notes William Kurth of the University of Iowa in Iowa City.

Kurth, Bolton, and their collaborators, along with other planetary-science teams, describe findings from the joint Galileo-Cassini observations in the Feb. 28 Nature.