Physicists led by the University of Iowa, USA, report the first detection of bursts of cosmic ray electrons accelerated by shock waves originating from major eruptions on the Sun.
The detection, made by instruments onboard both the Voyager 1 and Voyager 2 spacecraft, occurred as the Voyagers continue their journey outward through interstellar space. These newly detected bursts of cosmic ray electrons travel at nearly the speed of light, 670 times faster than the shock waves that initially propelled them.
The shock waves emanated from expulsions of hot gas and energy, known as coronal mass ejections, that move outward from the Sun at about one million miles per hour. Even at those speeds, it takes more than a year for the shock waves to reach the Voyager spacecraft, which have travelled further from the Sun than any human-made object.
Don Gurnett, professor emeritus in physics and astronomy at Iowa and the study’s corresponding author, said: “What we see here specifically is a certain mechanism whereby when the shock wave first contacts the interstellar magnetic field lines passing through the spacecraft, it reflects and accelerates some of the cosmic ray electrons. We have identified through the cosmic ray instruments these are electrons that were reflected and accelerated by interstellar shocks propagating outward from energetic solar events at the sun. That is a new mechanism.”
The team at the University of Iowa believe these electrons in the interstellar medium are reflected off a strengthened magnetic field at the edge of the shock wave and subsequently accelerated by the motion of the shock wave. The reflected electrons then spiral along interstellar magnetic field lines, gaining speed as the distance between them and the shock increases.
Gurnett said: “The idea that shock waves accelerate particles is not new. It all has to do with how it works, the mechanism. And the fact we detected it in a new realm, the interstellar medium, which is much different than in the solar wind where similar processes have been observed. No one has seen it with an interstellar shock wave, in a whole new pristine medium.”