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This artist’s concept shows one of NASA’s Voyager spacecraft entering interstellar space, or the space between stars. This region is dominated by plasma ejected from the deaths of giant stars millions of years ago. The warmer and more scattered plasma fills the environment within our solar bubble. Credit: NASA
Physicists report accelerated electrons bound to cosmic rays.
More than 40 years after its launch, the Voyager probe is still making discoveries.
In a new study, a team of physicists led by the University of Iowa report the first detection of electron bursts of cosmic rays accelerated by shock waves originating from large eruptions of the sun. The detection, carried out by instruments aboard the Voyager 1 and Voyager 2 spacecraft, took place as the Voyagers continue their outward journey through interstellar space, thus making them the first aircraft to record this unique physics in the realm among the stars.
These newly detected flashes of electrons are like an accelerated advance guard along the magnetic field lines in the interstellar medium; electrons travel at nearly the speed of light, about 670 times faster than the shock waves that initially propelled them. The outbursts were followed by plasma wave oscillations caused by low-energy electrons arriving at Voyager instruments days later – and finally, in some cases, the shock wave itself up to a month later.
The Voyager spacecraft continues to make discoveries even as it travels through interstellar space. In a new study, physicists at the University of Iowa report on Voyagers’ detection of electrons from cosmic rays associated with eruptions from the sun, more than 14 billion miles away. Credit: NASA / JPL-Caltech
The shock waves emanated from coronal mass ejections, ejections of hot gas and energy moving outward from the sun at about one million miles per hour. Even at those speeds, it takes more than a year for the shockwaves to reach the Voyager spacecraft, which has traveled farther from the sun (more than 14 billion miles and beyond) than any man-made object.
“What we see here in particular is a certain mechanism whereby when the shock wave first contacts the lines of the interstellar magnetic field passing through the spacecraft, it reflects and accelerates some of the electrons of the cosmic rays, ”says Don Gurnett, professor emeritus of physics and astronomy in Iowa and the corresponding author of the study. “We have identified through cosmic ray instruments these are electrons that have been reflected and accelerated by interstellar shocks that propagate outward from solar energy events to the sun. This is a new mechanism. “
The discovery could help physicists better understand the dynamics behind shockwaves and cosmic radiation emanating from flare stars (which can vary in brightness 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 concentrations of cosmic rays far in excess of those we experience on Earth.
Physicists believe that these electrons in the interstellar medium are reflected by 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 the lines of the interstellar magnetic field, gaining speed as the distance between them and the shock increases.
In a 2014 article in the journal Astrophysical Letters, physicists JR Jokipii and Jozsef Kota theoretically described how ions reflected by shock waves could be accelerated along the lines of the interstellar magnetic field. The current study examines the electron bursts detected by the Voyager probe which are believed to be accelerated by a similar process.
“The idea that shock waves accelerate particles is not new,” says Gurnett. “Everything has to do with how it works, the mechanism. And the fact that we have detected it in a new realm, the interstellar medium, which is very different from the solar wind where similar processes have been observed. Nobody saw it with an interstellar shockwave, in a completely new and pristine medium. “
Reference: “A Foreshock Model for Interstellar Shock of Solar Origin: Voyager 1 and 2 Observations” by DA Gurnett, WS Kurth, EC Stone, AC Cummings, B. Heikkila, N. Lal, SM Krimigis, RB Decker, NF Ness and LF Burlaga, 3 December 2020, The Astronomical Journal.
DOI: 10.3847 / 1538-3881 / abc337
Co-authors include William Kurth of Iowa; Edward Stone and Alan Cummings of the California Institute of Technology; Bryant Heikkila, Nand Lal and Leonard Burlaga del NASA Goddard Space Flight Center; Stamatios Krimigis and Robert Decker of the Johns Hopkins University Applied Physics Laboratory; and Norman Ness of the University of Delaware.
NASA funded the research.
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