Cassini-Huygens At Saturn's Bow Shock [Image]

While crossing the bow shock on Feb. 3, 2007, Cassini recorded a particularly strong shock in a "quasi-parallel" orientation, where the magnetic field and the direction of the front of the shock's movement are almost aligned. Under these conditions, significant particle acceleration was detected for the first time. The findings provide insight into particle acceleration at the shocks surrounding the remnants of stellar explosions.

The artist's impression by the European Space Agency shows NASA's Cassini spacecraft exploring the magnetic environment of Saturn. The image is not to scale. Saturn's magnetosphere is depicted in grey, while the complex bow shock region -- the shock wave in the solar wind that surrounds the magnetosphere -- is shown in blue.

"Cassini has crossed Saturn's bow shock hundreds of times, recording typical Alfvén Mach numbers of around 12. But during one particular crossing in early 2007, we measured a value of ~100, during which time the shock was quasi-parallel," describes Adam Masters of the Institute of Space and Astronautical Science, Japan, and lead author of a paper reporting the surprising results and published a few days ago in Nature Physics. 

The findings confirm that, at high Mach numbers like those of the shocks surrounding supernova remnants, quasi-parallel shocks can become considerably more effective electron accelerators than previously thought. This result sheds new light on the complex process of cosmic particle acceleration.

"Cassini has essentially given us the capability of studying the nature of a supernova shock in situ in our own Solar System, bridging the gap to distant high-energy astrophysical phenomena that are usually only studied remotely," adds Dr Masters.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for NASA's Science Mission Directorate, Washington. JPL is a division of the California Institute of Technology, Pasadena.