Voyager 1 and 2 detect new kind of solar electron burst

Data collected by the Voyager spacecraft, Voyager 1 and 2, has revealed a new type of solar electron burst -- the satellites' instruments detected speeding cosmic ray electrons accelerated by shock waves produced by solar eruptions.

The phenomenon was described Thursday in the Astrophysical Journal by a team of physicists led by the University of Iowa.

The Voyager spacecraft were launched in 1977. In 2012, Voyager 1 left the heliosphere and entered interstellar space. Its younger sibling, Voyager 2, escaped the solar system in 2018.

The two probes are now 14 billion miles from the sun, farther than any human-built objects.

While traveling through interstellar space, the two craft observed electrons accelerating along magnetic field lines, some moving 670 times faster than the shock waves that initially triggered their acceleration.

The cosmic burst events were followed by plasma wave oscillations, detected by the same instruments several days after the electrons zipped past the spacecraft.

The shockwaves that accelerated the electron bursts detected by Voyager 1 and 2 were produced by coronal mass ejections from the sun. These solar explosions propel hot gas and energy at speeds one million miles per hour.

It took more than a year for the shockwaves emanating from the sun to reach the two Voyager spacecraft.

"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," Don Gurnett, the study's corresponding author, said in a news release.

"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," said Gurnett, a professor of physics and astronomy at Iowa.

Previously, physicists have been forced to study only cosmic ray bursts moving the opposite direction -- those propelled toward Earth by explosions on distant variable stars.

Researchers suggest the detections made by Voyager 1 and 2 could help scientists better understand the physics underlying the propulsion of shock waves and cosmic radiation.

Scientists suspect electrons are first reflected off a localized magnetic field strengthened by the bow of the shockwave, and subsequently accelerated by the motion of the shockwave itself.

The reflected and accelerated electrons zip along interstellar magnetic field lines, getting faster as they separate from the shockwave.

This theoretical sequence of events has previously been described in the scientific literature, but -- for obvious reasons -- has never before detected in interstellar space.

"The idea that shock waves accelerate particles is not new," Gurnett said. "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," Gurnett said.

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