14.1 billion miles from Earth, Voyager 1 detects a new cosmic "hum"

A weak but persistent signal from the Voyager 1 spacecraft at first seemed like a fluke to Cornell University Ph.D. candidate Stella Koch Ocker.

“I was literally just kind of staring at a plot, and I thought I saw something really faint,” Koch Ocker tells Inverse. “And I was like, ‘am I starting to see things?’”

She wasn’t. In a new study, Koch Ocker and colleagues report a new signal coming in from the Plasma Wave System instrument aboard the craft. They used these observations to obtain a new measurement of the density of the interstellar medium, the thinly distributed mix of molecules, atoms, and ionized gas or plasma that fills the space between stars.

The research, published Monday in the journal Nature Astrophysics, provides the first continuous measurement of the density of the plasma in the interstellar medium, which previously had only been known in isolated spots.

By looking back at the last three years of observations from Voyager 1, the researchers were able to assemble consistent measurements of this elusive interstellar plasma. They continue to monitor it as Voyager 1 speeds out of the Solar System at more than 38,000 miles per hour. It is approximately 14.1 billion miles from Earth.

What’s new — Prior methods of measuring the plasma density relied on picking up strong, intermittent bursts of signal from Voyager 1, which seemed to occur about once every year. The intermittency of the signal made it akin to a once-a-year massive “storm” of large plasma oscillation events.

Emerging in data taken from 2017 to 2020, the new signal is only a few times stronger than the sensitivity of the instrument, making it barely noticeable. The Plasma Wave System allows Voyager 1 to measure vibrations of plasma at specific frequencies — the newly detected signal is a low-frequency “hum” at roughly 3 kHz.

And rather than being associated with intermittent events, the signal came across steadily over the entire duration of the three years, with little change in its character.

“When we looked between and below these plasma oscillation events, we found this very weak persistent signal of plasma waves that appears to not be related to solar activity,” says Koch Ocker.

The continuity of the new signal means the researchers can produce measurements of plasma density all along the trajectory of the spacecraft, rather than at the isolated locations of the “storms.” Discovering this trickle in the furthest reaches of space was almost entirely unexpected.

Here’s the background — Voyager 1 is one of two groundbreaking NASA missions launched in 1977 with a mission to explore the outer planets of the Solar System. Voyager 1 and 2 have obtained stunning pictures and information about the cosmos, and have both been in continuous contact with Earth since their launch. Voyager 1 is the most distant human-made object in space.

To accomplish these feats, mission planners took advantage of a particular alignment between the planets that happens only once every 175 years. This alignment allowed the spacecraft to consecutively slingshot between them purely through the use of natural, gravitational forces.

After their main missions ended in 1989, with Voyager 2’s encounter of Neptune, the craft were both transitioned to an interstellar phase on their path out of the Solar System. In 2012, Voyager 1 reached interstellar space — an area no longer dominated by the influence of the Sun.

Physicists always expected there to be tenuous plasma out there. Plasma is a phase of matter formed when a gas is heated so hot that its atoms and molecules are torn apart into their constituent ions and electrons. In the region of space closest to us, our solar neighborhood, plasma is formed with distinctive characteristics from the heat and radiation of the Sun.

Space isn’t a true void — it’s filled with stray molecules of gas and solid matter, which get turned by the Sun's heat and radiation into plasma.

Outside this bubble, known as the heliosphere, the plasma of the interstellar medium was expected to take on alternate characteristics. Voyager 1 showed exactly how.

How they did it — Prior to Voyager 1, researchers could only get clues on the interstellar plasma from observations with distant telescopes. But once Voyager 1 passed beyond the heliopause, separating the Sun’s neighborhood from interstellar space, it began registering direct signals from the plasma. These signals came in the form of occasional bursts of strong electrical discharge.

The bursts were detected by Voyager 1’s Plasma Wave System, one of the few instruments still operating on the craft. The Plasma Wave System works a bit like a voltage meter an electrician might use on a household electric outlet. (The spacecraft had a more robust plasma spectrometer that was defective on Voyager 1, though the one aboard Voyager 2 is still functioning.)

The occasional torrents of plasma picked up by the instrument were strong, like measuring a hundred pounds on a weight scale whose smallest measurement is one pound. Scientists gradually put together a picture of these signals as being caused by shock waves that occasionally barge out from the sun, making the plasma in the vicinity of the spacecraft vibrate, which in turn causes oscillations in its electric field.

However, when analyzing the overall electrical measurements collected from 2017 to 2020, researchers discovered a weak yet smooth signal hiding in the background of the data. Though this signal has the same fundamental electrical nature, it is more like a constant pitter-patter, rather than the occasional rush of a storm.

Why it matters — Both the strong, intermittent signals and the weak, steady signal provide important information on the nature of the interstellar plasma. In particular, the way that the plasma’s electric field fluctuates can be traced back directly to the plasma’s density.

Previously, researchers could only estimate the plasma’s density during the solar shock wave events. With the continuous data from the new signal, researchers can estimate the density of the plasma all along the spacecraft’s trajectory from 2017 to 2020. Moreover, the researchers find that the new density measurements are consistent with the old ones, having the same values when they were measured at the same time.

“It's pretty incredible how a mission that's so old can still produce such exciting new discoveries.”

The new results allow the researchers to turn a spotty record of plasma density with year-long gaps into a smooth, consistent record over space and time. The researchers find variations in the density with characteristic scales of 1 astronomical unit (AU), the distance between the earth and the sun. These variations have long been expected to arise due to turbulence in the interstellar medium.

The exact nature of the variations measured by the researchers will help inform whether the turbulence is primarily driven by huge, faraway sources like supernovae, or more local sources like the solar wind, Koch Ocker says.

What’s next — In addition to determining the nature of the interstellar turbulence, the researchers plan to find out more about the somewhat mysterious nature of the weak signal, which seemed to clearly emerge only in 2017, without any clear reason why.

For now, their best tool to attain further progress continues to be the 1970s era spacecraft of Voyager 1, the most loyal of probes, along with its sister probe of Voyager 2.

“It's pretty incredible how a mission that's so old can still produce such exciting new discoveries,” Koch Ocker says.

Abstract: In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium¹. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 au of interstellar space, revealing a large-scale density gradient^2,3 and turbulence⁴ outside of the heliopause. Previous studies of the plasma density relied on the detection of discrete plasma oscillation events triggered ahead of shocks propagating outwards from the Sun, which were used to infer the plasma frequency and, hence, density^5,6. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 data that persists from 2017 onwards and enables a steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling distance of 0.03 au. We find au-scale density fluctuations that trace interstellar turbulence between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and they could be clarified by new findings from Voyager or a future interstellar mission. The emission’s persistence suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.