An article published in the journal “Nature Astronomy” reports a study on the faint interstellar plasma waves detected by NASA’s Voyager 1 space probe. A team of researchers led by Stella Koch Ocker of Cornell University used a series of detections conducted by Voyager 1 while traveling a total distance that is about ten times the average distance of the Earth from the Sun which gives an idea of the characteristics of the interstellar plasma when it’s not altered by events related to solar activity.
The PWS (Plasma Wave Subsystem, in the image, a NASA diagram of its location and the antenna in common with the PRA instrument) is an instrument of the two Voyager space probes designed in particular with the hope that it would offer new information on the magnetospheres of the planets Jupiter and Saturn throughout their mission. This instrument records fluctuations in the plasma, therefore became useful again when the Voyagers entered interstellar space.
The new interstellar mission of the Voyager 1 space probe made it possible to collect data on the density of the plasma present in the interstellar medium. The influence of the Sun is no longer dominant but its emissions continue to have an influence even over 20 billion kilometers away. The solar activity produced eight events between 2012 and 2020 that had an influence that extended to the area where Voyager 1 was traveling. The interstellar plasma oscillations caused by solar activity lasted from two days to a year.
Starting from 2017, between one oscillation and the next, it was possible to detect what, according to the researchers, are signals attributable to what is almost the vacuum where there’s only a very tenuous interstellar plasma. The space probe conducted its measurements at intervals of about 30% of the Earth’s average distance from the Sun for a total of about ten times that distance.
Stella Koch Ocker compared those interstellar plasma emissions to a barely audible and monotonous but persistent hum. James Cordes, professor of astronomy at Cornell University, another of the authors of this study, compared it to a calm and gentle rain, adding that a solar outburst is like lightning during a storm and then it’s back to light rain.
The activity of interstellar plasma waves is greater than scientists thought. The possibility of making these direct detections brings information otherwise impossible to obtain. The radioisotope generators of the Voyager space probes will be able to power the instruments that remained active for a few more years, so they will still provide a lot of data on the interstellar environment as they continue their journey in what is truly the last frontier.