Shock waves are commonplace in the universe: in the aftermath of a stellar explosion as debris accelerates 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. Indeed, strong shocks at young supernova remnants are known to boost electrons to ultra-relativistic energies and may be the dominant source of cosmic rays — high-energy particles that pervade our galaxy.
Space telescopes reveal evidence for accelerated electrons at supernova remnant shocks as X-ray emission, but these observations are made at great distances, and, thus, scientists can only poorly measure the orientation of the local magnetic field at best. Without this crucial information, it is difficult to gain a full understanding of the shock acceleration process.
Scientists want to understand how the acceleration of electrons in strong shocks with large “Mach numbers” depends on the angle between the magnetic field and a vector at right angles to the shock front. In particular, they are interested in what happens in a “quasi-parallel” shock where the field and vector are almost aligned, as may be found in supernova remnants.
Shocks in the solar wind in the solar system are much more accessible and can be studied in greater detail. To date, however, particle acceleration has only been seen in “quasi-perpendicular” shocks where the magnetic field and shock vector are almost perpendicular.
But this new study by Cassini scientists 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.
“Cassini has crossed Saturn’s bow shock hundreds of times, recording typical Alfvén Mach numbers of around 12,” said Adam Masters of the Institute of Space and Astronautical Science in Japan. “But during one particular crossing in early 2007, we measured a value of about100, during which time the shock was quasi-parallel.”
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,” said Masters.
“The Cassini observations have given us a glimpse of a process never before seen directly, providing new information on how high-energy particles, like cosmic rays, are accelerated to such high velocities by magnetic fields throughout the universe,” said Nicolas Altobelli from the European Space Agency.