Voyager Probes Find New Electron-Accelerating Physics in Deep Space

NASA launched the Voyager probes more than 40 years ago, and the fact we’re still talking about the impact of these spacecraft is a testament to how well-planned these missions were. Both Voyager 1 and 2 are outside the solar system now, but there’s plenty to see out there in the interstellar medium (ISM). A newly published study from the University of Iowa says that the Voyager probes have discovered an entirely new kind of “electron burst” related to coronal mass ejections on the sun.

The Voyager probes were launched within weeks of each other in 1977, taking advantage of a fortuitous alignment of the planets that occurs only once every 175 years. The spacecraft took different routes through the outer solar system, with Voyager 1 swinging past Jupiter and Saturn while Voyager 2 visited Jupiter, Saturn, Uranus, and Neptune. The probes got a gravity boost from these massive planets, flinging them out of the solar system. In 2012, Voyager 1 crossed the “heliopause,” the region of space where the solar wind dissipates and gives way to the ISM. Voyager 2, which took a more circuitous route, did the same in 2018.

Once in the ISM, the Voyager probes were able to look at the bubble of space dominated by the sun from the outside. That’s something no other spacecraft can do. Luckily, NASA planned ahead and equipped the robotic explorers with instruments that could probe the ISM. Cosmic ray detectors on the probes have been used to track the effects of coronal mass ejections (CME), which travel outward through space until they hit the heliopause. It takes about a year for these puffs of hot gas and energy to get there, and a few are powerful enough to punch through the heliosphere and into the ISM. That’s where Voyager 1 and 2 have noted some unexpected behavior.

Each time one of these big CMEs reaches the ISM, the researchers have noted an electron burst in advance — the shockwave itself didn’t arrive until 13 to 30 days after the high-energy cosmic ray electrons. It’s counterintuitive to see this signal showing up ahead of the shockwave, but the team says this is all thanks to the properties of magnetic field lines in the ionized gas of the ISM, which are apparently almost perfectly straight. Large CMEs punch through the heliopause and interact with these field lines, causing some of the electrons inside to accelerate along the magnetic straightaways. They can reach relativistic speeds, about 670 times faster than the shockwave that originally delivered them to the edge of the solar system. That’s why Voyager 1 and 2 see the electron burst before the CME shockwave.

Scientists have never seen electrons accelerated ahead of a shockwave like this. It’s an entirely new mechanism and one that could help us better understand the ISM. We’d never even know this was possible if not for two 40-something space probes.