In The Emptiness Of Space, Voyager I Detects Plasma 'Hum'


Voyager 1 composite plasma wave spectrum. Top: Frequency-time dynamic spectrum showing all of the Voyager 1 PWS wideband data available since Voyager 1 crossed the heliopause on August 25, 2012. The time resolution of the spectrum is 3 days and the frequency resolution is 0.011 kHz. Each column of pixels corresponds to a 1D spectrum that is the average of all ≈ 48 s long observations that fall within that 3-day time bin. The individual spectra that fall within a given time bin have been equilibrated to the same noise baseline. The 2.4 kHz supply interference line is masked out, and the spectrum is smoothed with a 1D Gaussian smoothing kernel with σ = 0.01 kHz. Bottom: Schematic showing relevant features in the PWS spectrum, including the locations of previously detected plasma oscillation events (POEs). Two events have direct associations with shocks detected by the magnetometer. A magnetic pressure wave was also detected in early 2017. The lower cutoff frequency of the plasma oscillations corresponds to the local plasma frequency. The approximate times of relativistic electron bursts detected by the cosmic ray instruments are also indicated. The model of the weak, narrowband plasma wave emission presented in this paper is shown in solid gray, and the plasma frequency inferred from POE activity between 2015 and 2017 is shown in dashed gray. A POE detected in June 2019 is also shown as a black circle, but was masked in our analysis because it coincided with a period of severely degraded telemetry performance.

Voyager 1 - one of two sibling NASA spacecraft launched 44 years ago and now the most distant human-made object in space - still works and zooms toward infinity.

The craft has long since zipped past the edge of the solar system through the heliopause - the solar system's border with interstellar space - into the interstellar medium. Now, its instruments have detected the constant drone of interstellar gas (plasma waves), according to Cornell University-led research published in Nature Astronomy.

Examining data slowly sent back from more than 14 billion miles away, Stella Koch Ocker, a Cornell doctoral student in astronomy, has uncovered the emission. "It's very faint and monotone, because it is in a narrow frequency bandwidth," Ocker said. "We're detecting the faint, persistent hum of interstellar gas."

This work allows scientists to understand how the interstellar medium interacts with the solar wind, Ocker said, and how the protective bubble of the solar system's heliosphere is shaped and modified by the interstellar environment.

Launched in September 1977, the Voyager 1 spacecraft flew by Jupiter in 1979 and then Saturn in late 1980. Travelling at about 38,000 mph, Voyager 1 crossed the heliopause in August 2012.

After entering interstellar space, the spacecraft's Plasma Wave System detected perturbations in the gas. But, in between those eruptions - caused by our own roiling sun - researchers have uncovered a steady, persistent signature produced by the tenuous near-vacuum of space.

"The interstellar medium is like a quiet or gentle rain," said senior author James Cordes, the George Feldstein Professor of Astronomy. "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."

Ocker believes there is more low-level activity in the interstellar gas than scientists had previously thought, which allows researchers to track the spatial distribution of plasma - that is, when it's not being perturbed by solar flares.

Cornell research scientist Shami Chatterjee explained how continuous tracking of the density of interstellar space is important. "We've never had a chance to evaluate it. Now we know we don't need a fortuitous event related to the sun to measure interstellar plasma," Chatterjee said. "Regardless of what the sun is doing, Voyager is sending back detail. 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 left Earth carrying a Golden Record created by a committee chaired by the late Cornell professor Carl Sagan, as well as mid-1970s technology. To send a signal to Earth, it took 22 watts, according to NASA's Jet Propulsion Laboratory. The craft has almost 70 kilobytes of computer memory and - at the beginning of the mission - a data rate of 21 kilobits per second.

Due to the 14-billion-mile distance, the communication rate has since slowed to 160-bits-per-second, or about half a 300-baud rate.

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NASA, the Jet Propulsion Laboratory and the National Science Foundation supported the work. Cordes, Chatterjee and Ockler are members of Cornell's Carl Sagan Institute.

Persistent Plasma Waves in Interstellar Space Detected by Voyager 1

S.K. Ocker, J.M. Cordes, S. Chatterjee, D.A. Gurnett, W.S. Kurth, S.R. Spangler
In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 astronomical units (au) of interstellar space, revealing a large-scale density gradient and compressive turbulence outside the heliopause. Previous studies of the plasma density relied exclusively on the detection of discrete plasma oscillation events that are triggered ahead of shocks propagating outwards from the Sun, and that can be used to infer the plasma frequency and hence density. We present the detection of a new class of very weak, narrowband plasma wave emission in the Voyager 1 Plasma Wave System data that persists from 2017 onwards and enables the first steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling time of 3 days, or 0.03 au. We find au-scale density fluctuations that trace turbulence in the interstellar medium between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The persistence of the emission suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

Comments: This is a preprint of an article published in Nature Astronomy. The final authenticated version is available online at: this https URL
Subjects: Astrophysics of Galaxies (astro-ph.GA); Earth and Planetary Astrophysics (astro-ph.EP); Plasma Physics (physics.plasm-ph); Space Physics (physics.space-ph)
DOI: 10.1038/s41550-021-01363-7
Cite as: arXiv:2105.04000 [astro-ph.GA] (or arXiv:2105.04000v1 [astro-ph.GA] for this version)
Submission history
From: Stella Ocker
[v1] Sun, 9 May 2021 19:18:36 UTC (1,422 KB)
https://arxiv.org/abs/2105.04000

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