Out of the ambient noise reigning inside the Solar System, the Voyager probes found a little more calm once they entered the interstellar medium. However, calm does not necessarily mean silent. Indeed, the data returned by Voyager 1 indicates the existence of a slight hum from interstellar plasma waves. Although astrophysicists do not yet know the cause, this phenomenon should help researchers to map interstellar plasma in detail as well as the dynamics of the interaction between solar wind and interstellar medium.
Voyager 1, having spent more than 43 years moving away from Earth since its launch in 1977, is now more than 150 times the distance between Earth and the Sun. It takes over 21 hours for transmissions traveling at the speed of light to reach Earth. It officially passed the heliopause – the limit where the solar wind is stopped by the interstellar medium – in 2012.
In the latest analysis of data from the probe, at a distance of nearly 23 billion kilometers, astrophysicists discovered, starting in 2017, a constant hum of plasma waves in the interstellar medium, the diffuse ionized gas present between stars. ” It is very weak and monotonous because it is in a narrow frequency bandwidth. We detect the persistent faint hum of interstellar gas Says Cornell University astronomer Stella Koch Ocker.
A hum from interstellar plasma waves
Voyager probes are the first human-made objects to enter interstellar space and therefore represent a unique opportunity to directly sample the interstellar medium. Even so far from the Sun, and even out of reach of the solar wind, it is not that easy. The Sun is always a bright, noisy object, releasing solar flares that can saturate the surrounding physical conditions.
” The interstellar medium is like a calm rain, which can be easily disturbed Says Cornell University astronomer James Cordes. This gentle rain, according to the team, suggests that there may be more low-level activity in the interstellar medium than scientists thought.
The cause of this activity is not entirely clear; it may be thermally excited plasma oscillations, or quasi-thermal noise generated by the movements of electrons in the plasma, producing a local electric field. Whatever the cause, the discovery has several implications.
Map and study the dynamics of the interstellar medium
The hum can be used to map the density of plasma as the two Voyager probes move deeper into interstellar space. It can also be used to better understand the interaction between the interstellar medium and the solar wind. We know there is an increase in electron density just on the other side of the heliopause – both Voyager probes detected it as they passed.
Knowing more precisely the density of the interstellar medium can help us understand why. The show’s discovery and persistence also suggests that Voyager will continue to be able to detect it, providing us with ongoing data that will help us understand the large-scale turbulence and structure of the interstellar medium.
” We never had the opportunity to assess it. Now we know that we don’t need a fortuitous Sun-related event to measure interstellar plasma. Regardless of what the Sun is doing, Voyager returns details. The machine says: “Here is the density in which I am evolving at the moment. And here it is now. And here it is now. And here it is now Says Cornell University astronomer Shami Chatterjee. But not forever. The radioisotopic thermoelectric generator that powers the probe’s instruments deteriorates a little more each year. Around 2025, he may no longer be able to operate them.