Voyager 1 hears plasma ‘hum’

The most far-travelled thing humans have ever made, Voyager 1 is still transmitting trinkets of information to Earth. A team of US researchers has used this data to spot a plasma hum from the thin gas that occupies the space between stars.

“Voyager is sending back detail,” says Shami Chatterjee, a researcher at Cornell University, US, and author on a paper describing the research, published in Nature Astronomy.

“The craft is saying, ‘Here’s the density I’m swimming through right now. And here it is now. And here it is now. And here it is now.’ Voyager is quite distant and will be doing this continuously.”

Voyager 1 crossed the heliopause – the outer edge of the heliosphere – in 2012, entering the interstellar medium. This plasma that exists in the space between stars is fully ionised gas, which the Sun oscillates, creating clear ‘plasma waves’. Voyager 1 is fitted with a detector for these waves, so it can collect data on the waves and send it 22 billion kilometres back to Earth.

But between the solar noise that Voyager 1 sends back, this team of researchers has identified a persistent hum from plasma waves that don’t come from the Sun – it is generated by the interstellar gas around the craft, instead.

“It’s very faint and monotone, because it is in a narrow frequency bandwidth,” says Stella Koch Ocker, a doctoral student in astronomy at Cornell and lead author on the paper. “We’re detecting the faint, persistent hum of interstellar gas.”

Ocker adds that this means the interstellar gas might be more active than scientists previously thought, as the gentle plasma hum Voyager 1 hears is fairly consistant behind bigger signals.

“The interstellar medium is like a quiet or gentle rain,” says senior author James Cordes, professor of astronomy at Cornell. “In the case of a solar outburst, it’s like detecting a lightning burst in a thunderstorm and then it’s back to a gentle rain.”

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