NASA’s celebrated Voyager spacecraft have made the first detection of shock-wave-accelerated cosmic-ray electron bursts originating from large eruptions on the Sun.
The detection, reported by a team of physicists led by the University of Iowa, occurred as Voyager — which left Earth in 1977 — continues to drift through interstellar space, making it the first spacecraft to record this unique physics in the realm between the stars.
These newly detected bursts of electrons are like an advanced guard accelerated along magnetic field lines in the interstellar medium; the electrons travel almost at the speed of light, about 670 times faster than the shock waves that initially propelled them. The explosions were followed by oscillations of plasma waves caused by lower energy electrons that reached the Voyager instruments days later, and finally, in some cases, the shock wave itself until a month later.
The shock waves emanated from coronal mass ejections, ejections of hot gas and energy moving outward from the sun at approximately one million miles per hour. Even at these speeds, it takes more than a year for the shock waves to reach the Voyager spacecraft, which has traveled farther from the Sun (more than 22.5 billion kilometers and counting) than any human-made object.
“What we see here specifically is a certain mechanism by which when the shock wave first contacts the interstellar magnetic field lines that pass through the spacecraft, it reflects and accelerates some of the cosmic ray electrons,” says Don Gurnett, professor emeritus of physics and astronomy at Iowa and the corresponding author of the study. “We have identified through cosmic ray instruments that these are electrons that were reflected and accelerated by interstellar collisions that propagate outward from energetic solar events on the Sun. This is a new mechanism.
The discovery could help physicists better understand the dynamics behind the shock waves and cosmic radiation coming from exploding stars (whose brightness may vary briefly due to violent activity on their surface) and exploding stars. It would be important to consider the physics of such phenomena when sending astronauts on extended lunar or Martian excursions, for example, during which they would be exposed to cosmic ray concentrations far exceeding those we experience on Earth.
Physicists believe that these electrons in the interstellar medium are reflected in a strengthened magnetic field at the edge of the shock wave and are subsequently accelerated by the movement of the shock wave. The reflected electrons then spiral along the interstellar magnetic field lines, gaining speed as the distance between them and the impact increases.
In a 2014 article in the journal Astrophysical Letters, physicists J.R. Jokipii and Jozsef Kota described theoretically how ions reflected by shock waves could be accelerated along interstellar magnetic field lines. The current study analyzes the electron explosions detected by the Voyager spacecraft that are believed to be accelerated by a similar process.
“The idea that shock waves accelerate particles is not new,” Gurnett says. “It’s all about how it works, the mechanism. And the fact that we detect it in a new realm, the interstellar medium, which is very different from the solar wind where similar processes have been observed. No one has ever seen it with an interstellar shock wave, in a completely new pristine environment.
The findings were published online in the Astronomical Journal with the original “A Foreshock Model for Interstellar Shocks of Solar Origin: Voyager 1 and 2 Observations“.