During NASA's Cassini spacecraft observations of an unusually strong solar system shock wave (Saturn’s bow shock), scientists
have detected particles being accelerated to ultra-high energies.
This is similar to the acceleration that takes place around distant supernovas.
Since we can't travel out to the far-off stellar explosions right now, the shockwave that forms from the flow of
solar wind around Saturn's magnetic field provides a rare laboratory for scientists with the Cassini mission --
a partnership involving NASA, the European Space Agency and the Italian Space Agency -- to observe this phenomenon
The findings, published this week in the journal
Nature Physics, confirm that certain kinds of shocks
can become considerably more effective electron accelerators than previously thought.
Click on image to enlarge
Magnetic Fields and Bow Shocks (Illustration)
This illustration shows "quasi-parallel" (top) and "quasi-perpendicular" (bottom) magnetic field conditions at a planetary bow shock. Bow shocks are shockwaves created when the solar wind blows on a planet's magnetic field.
Under quasi-parallel conditions, the planet's magnetic field is roughly pointing toward the shock surface, almost parallel to a vector at right angles to the shock front (red arrow). Under quasi-perpendicular conditions, the magnetic field is close to aligned with the shock surface, that is, almost perpendicular to the shock vector.
Shock waves are commonplace in the universe, for example in the aftermath of a stellar explosion as debris accelerate
outward in a supernova remnant, or when the flow of particles from the sun -- the solar wind -- impinges on the
magnetic field of a planet to form a bow shock.
Under certain magnetic field orientations and depending on the
strength of the shock, particles can be accelerated to close to the speed of light at these boundaries.
These may be the dominant source of cosmic rays, high-energy particles that pervade our galaxy.
Scientists are particularly interested in "quasi-parallel" shocks, where the magnetic field and the "forward"-facing
direction of the shock are almost aligned, as may be found in supernova remnants.
The new study, led by Adam Masters of the Institute of Space and Astronautical Science, Sagamihara, Japan, describes the first detection of significant
acceleration of electrons in a quasi-parallel shock at Saturn, coinciding with what may be the strongest shock ever
encountered at the ringed planet.
Click on image to enlarge
Cassini at Saturn's Bow Shock (Artist Concept)
This 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.
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. Credits: ESA
"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," said Masters.
"Electron acceleration to relativistic energies at a strong quasi-parallel shock wave. Nature Physics, 2013
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.