The international 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 3 February 2007, Cassini recorded a particularly strong shock (an Alfvén Mach number of approximately 100) under a ‘quasi-parallel’ magnetic field configuration, during which significant particle acceleration was detected for the first time. The findings provide insight into particle acceleration at the shocks surrounding the remnants of supernova explosions.Copyright ESA
During a chance encounter with an unusually strong blast of solar wind
arriving at Saturn, the international Cassini spacecraft detected
particles being accelerated to ultra-high energies, similar to the
acceleration that takes place around supernova explosions.
Shock waves are commonplace in the Universe, for example in the
aftermath of a stellar explosion as debris accelerates outwards 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, very 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 the orientation of the local magnetic field can
only be poorly measured 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 very
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.
Illustration of quasi-parallel (top) and quasi-perpendicular (bottom) magnetic field conditions at a planetary bow shock. Under quasi-parallel conditions, the 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. Copyright ESA
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 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. 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 the paper
reporting the results 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 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,” says Nicolas Altobelli, ESA’s
Cassini project scientist.
Notes for Editors
“Electron acceleration to relativistic energies at a strong quasi-parallel shock wave” by A. Masters et al. is published in Nature Physics, 17 February 2013.
The electron observations were carried out using the Electron Spectrometer of the Cassini Plasma Spectrometer, and the Low-Energy Magnetospheric Measurements System of the Cassini Magnetospheric Imaging Instrument. The high Alfvén Mach number of MA ~ 100 was measured on 3 February 2007.
The Cassini–Huygens mission is a cooperative project of NASA, ESA and ASI, the Italian space agency. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington.
The electron observations were carried out using the Electron Spectrometer of the Cassini Plasma Spectrometer, and the Low-Energy Magnetospheric Measurements System of the Cassini Magnetospheric Imaging Instrument. The high Alfvén Mach number of MA ~ 100 was measured on 3 February 2007.
The Cassini–Huygens mission is a cooperative project of NASA, ESA and ASI, the Italian space agency. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington.
Source: ESA
