By Michelle Starr
Published in ScienceAlert
In November 2018, after an epic 41-year journey, Voyager 2 finally crossed the border that marked the limit of the Sun’s influence and entered interstellar space. But the little probe’s mission is not yet complete – it is now sending home information about space beyond the Solar System.
And it is revealing something surprising. As Voyager 2 moves further and further away from the Sun, the density of space is increasing.
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It is not the first time that this increase in density has been detected. Voyager 1, which entered interstellar space in 2012, detected a similar density gradient at a different location.
The new data from Voyager 2 shows that not only was the detection of Voyager 1 legitimate, but that the increase in density may be a large-scale feature of the local interstellar medium (MIL).
The edge of the Solar System can be defined at a few different limits, but the one traversed by the Voyager probes is known as the heliopause and is defined by the solar wind – a constant supersonic wind of ionized plasma that exits the sun in all directions. Heliopause is the point at which the external pressure of the solar wind is no longer strong enough to repel the interstellar wind.
The space inside the heliopause is the heliosphere, and the space outside it is the MIL. But the heliosphere is not a round sphere. It is more like an oval shape, with the Solar System at one end and a tail at the back; the “nose” is pointed in the direction of the Solar System’s orbit in the Milky Way.
Both Voyagers crossed the heliopause through the nose, but with a difference of 67 degrees in heliographic latitude and 43 degrees of difference in longitude.
Space is generally considered to be a great vacuum, but it is not – at least, not completely. The density of matter is extremely low, but it still exists. In the Solar System, the solar wind has an average density of protons and electrons of 3 to 10 particles per cubic centimeter, but decreases as you move away from the Sun.
The average electron density of the interstellar medium in the Milky Way, between stars, was calculated to be about 0.037 particles per cubic centimeter. And the density of the plasma in the outer heliosphere is about 0.002 electrons per cubic centimeter.
As Voyager probes crossed beyond the heliopause, their instruments called Plasma Wave Science detected the electron density of the plasma by means of oscillations.
Voyager 1 crossed the heliopause on August 25, 2012, at a distance of 121.6 astronomical units from Earth (121.6 times the distance between Earth and the Sun, that is, about 18.1 billion km) .
When Voyager 1 first measured plasma oscillations after crossing the heliopause at a distance of 122.6 astronomical units (18.3 billion km), it detected a plasma density of 0.055 electrons per cubic centimeter .
Voyager 2, which traveled the long way, passing Jupiter, Saturn, Uranus and Neptune, crossed the heliopause on November 5, 2018 at a distance of 119 astronomical units (17.8 billion km). She measured plasma oscillations on January 30, 2019 at a distance of 119.7 astronomical units (17.9 billion), finding a plasma density of 0.039 electrons per cubic centimeter, very close to the Voyager 1 measurement.
And both instruments reported an increase in density. After traveling another 20 astronomical units (2.9 billion km) through space, Voyager 1 reported an increase to about 0.13 electrons per cubic centimeter.
But the detections made by Voyager 2 in June 2019 showed a much more marked increase in density of about 0.12 electrons per cubic centimeter, at a distance of 124.2 astronomical units (18.5 billion units).
Given that plasma at Earth’s atmospheric pressure has an electron density of 10 ^ 13 per cubic centimeter, these amounts may seem tiny, but they are significant enough to justify our interest – especially since it is unclear what causes this.
One theory is that the lines in the interstellar magnetic field become stronger as they descend from the heliopause. This could generate an instability of electromagnetic ion cyclotron that depletes the plasma in the coverage region. Voyager 2 detected a stronger than expected magnetic field when it crossed the heliopause.
Another theory is that the material blown by the interstellar wind must slow down when reaching the heliopause, causing a kind of congestion. This was possibly detected by the new New Horizons probe from the Solar System, which in 2018 captured a weak ultraviolet glow resulting from an accumulation of neutral hydrogen in the heliopause.
It is also possible that both explanations are correct. Future measurements made by both Voyager probes, while continuing their journey in interstellar space, may help to discover this. But it’s a long shot.
“It is not certain,” the researchers wrote in their article, “that Voyagers will be able to operate far enough to distinguish between these two classes of models.”
The research was published in The Astrophysical Journal Letters.