What are the long-lasting waves detected by Voyager 1? Our first working interstellar probe — admittedly never designed for that task — is operating beyond the heliosphere, which it exited back in 2012. A paper just published in Nature Astronomy explores what’s going in interstellar space just beyond, but still affected by, the heliosphere’s passage through the Local Interstellar Medium (LISM).

We have a lot to learn out here, for even as we exit the heliosphere, the picture is complex. The so-called Local Bubble is a low-density region of hot plasma in the interstellar medium, the environment of radiation and matter — gas and dust — that exists between the stars. Within this ‘bubble’ exists the Local Interstellar Cloud (LIC), about 30 light years across, with a slightly higher hydrogen density flowing from the direction of Scorpius and Centaurus. The Sun seems to be within the LIC near its boundary with the G-cloud complex, where the Alpha Centauri stars reside.

Image: Map of the local galactic neighborhood showing the Sun located near the edge of our local interstellar cloud (LIC). Alpha-Centauri is located just over 4 light-years away in the neighboring G-cloud complex. Outside these clouds, the density may be lower than 0.001 atoms/cc. Our Sun and the LIC have a relative velocity of 26 km/sec. Credit: JPL.

But if the interstellar medium is a sparse collection of widely spaced particles and radiation, it proves to be anything but quiet. We learn this from Voyager 1’s Plasma Wave Subsystem, which involves two antennae extending 30 meters from the spacecraft (see image below). What the PWS can pick up are clues to the density of the medium that show up in the form of waves. Some are produced by the rotation of the galaxy; others by supernova explosions, with smaller effects from the Sun’s own activity.

Vibrations of the ionized gas — plasma — in the interstellar medium have been detectable since late 2012 by Voyager 1 in the form of ‘whistles’ that show up only occasionally, but offer ways to study the density of the medium. The new work in Nature Astronomy, led by Stella Koch Ocker (Cornell University), sets about finding a more consistent measure of interstellar medium density in the Voyager data.

Image: An illustration of NASA’s Voyager spacecraft showing the antennas used by the Plasma Wave Subsystem and other instruments. Credit: NASA/JPL-Caltech.

A weak signal appearing at the same time as a ‘whistle’ in the 2017 Voyager data seems to have been the key finding. Ocker describes it as “very weak but persistent plasma waves in the very local interstellar medium.” When whistles appear in the data, the tone of this plasma wave emission rises and falls with them. Adds Ocker:

“It’s virtually a single tone. And over time, we do hear it change – but the way the frequency moves around tells us how the density is changing. This is really exciting, because we are able to regularly sample the density over a very long stretch of space, the longest stretch of space that we have so far. This provides us with the most complete map of the density and the interstellar medium as seen by Voyager.”

So we have an extremely useful instrument, Voyager 1’s Plasma Wave Subsystem, continuing to return data with increasing distance from the Sun. Analyzing the data over time, we learn that the electron density around the spacecraft began rising in 2013, just after its exit from the heliosphere, and reached current levels in 2015. These levels, which persist to the end of 2020 through the dataset, show a 40-fold increase in electron density. Up next for Ocker and team is the development of a physical model of the plasma wave emission that will offer insights into its proper interpretation.

As we begin to think seriously about interstellar probes in this century, it’s striking how much we have to learn about the medium through which they will pass. Voyager 1 is helping us learn about conditions immediately outside the heliosphere. A probe sent to Alpha Centauri will need to cross the boundary between the Local Interstellar Cloud and the G-cloud, a region we have yet to penetrate. The nature of and variation within the interstellar medium will require continuing work with our admittedly sparse data.

The paper is Ocker et al., “Persistent plasma waves in interstellar space detected by Voyager 1,” Nature Astronomy 10 May 2021. Abstract / Preprint.