When it’s setting up a powerful shockwave in space made up of high-speed particles accelerated to nearly the speed of light, that’s when!
Back in February 2007 the Cassini spacecraft passed through Saturn’s magnetosphere and, just as it had done many times before, crossed through the “bow shock” created by the planet’s magnetic field interacting with the oncoming solar wind. But during that particular passage, the spacecraft detected surprisingly high speeds of charged electrons inside Saturn’s shock.
The particles were clocked at Alfvén Mach numbers of around 100 — this is considerably speedier than the typical 12 MA that are usually seen. (The Alfvén Mach scale is a way of measuring the strength of a shock in a space plasma environment.)
At the time that the measurements were made, Saturn’s magnetic field lines were aligned such that they were “quasi-parallel” to the flow of the incoming solar wind, as opposed to a more broadside “quasi-perpendicular.” In this orientation, the electrons interacting with Saturn’s magnetic field are boosted to near-relativistic velocities (that is, close to light speed) — a process that’s also found in the shockwaves surrounding recently exploded supernovae.
As the force from an expanding supernova spreads rapidly outward into space, it excites atomic particles within the interstellar medium. These charged particles then radiate energy in various electromagnetic wavelengths, creating the rings of glowing plasma that we see from Earth as supernova remnants.
Some of those accelerated particles — protons, mostly — are also expelled in the form of cosmic rays, high-energy particles that stream throughout the galaxy. While the majority of cosmic rays are blocked by Earth’s atmosphere, in space (and even at high altitudes) they can be damaging to sensitive electronics — as well as human DNA.
Since studying supernova remnants at close distance isn’t possible (at least for the time being) finding a similar behavior occurring around a planet right here in our own solar system is very exciting for scientists.
“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 Adam Masters of the Institute of Space and Astronautical Science in Japan.
Masters is the lead author of a paper titled “Electron acceleration to relativistic energies at a strong quasi-parallel shock wave,” published in the Feb. 17 issue of the journal Nature Physics.