Voyager Spacecraft Detect an Increase in The Density of Space Outside The Solar System

 

In November 2018, after an epic, 41-year voyage, Voyager 2 finally crossed the boundary that marked the limit of the Sun's influence and entered the region. But the small probe's mission isn't done yet - it's now sending home information about the space beyond the system.

And it's revealing something surprising. As Voyager 2 moves farther and farther from the Sun, the density of space is increasing.

It's not the primary time this density increase has been detected. Voyager 1, which entered the region in 2012, detected an identical density gradient at a separate location.

Voyager 2's new data show that not only was Voyager 1's detection legit but that the rise in density is also a large-scale feature of the very local interstellar space (VLIM).

The Solar System's edge will be defined by some different boundaries, but the one crossed by the Voyager probes is thought because of the heliopause, and it's defined by the solar radiation.

This is a continuing supersonic wind of ionized plasma that streams out from the Sun altogether directions, and therefore the heliopause is that the point at which the outward pressure of that wind is not any longer strong enough to push into the wind from the part.

(NASA/JPL-Caltech)

The space inside the heliopause is that the heliosphere, and therefore the space outside it's the VLIM. But the heliosphere isn't a round sphere. It's more like an oval, with the system at one end, and a streaming tail behind; the "nose" is pointed within the direction of the Solar System's orbit within the extragalactic nebula.

Both Voyagers crossed the heliopause at the nose, but with a difference of 67 degrees in heliographic latitude and 43 degrees difference longitude.

Space is usually thought of as a vacuum, but it's not, not completely. The density of matter is extremely low, but it still exists. within the scheme, the solar radiation has a mean proton and electron density of three to 10 particles per ml, but it grows slower the farther out you go from the Sun.

The mean electron density of the interstellar space within the Milky Way System, out among the celebs, has been calculated to be around 0.037 particles per metric capacity unit. and therefore the plasma density within the outer heliosphere is around 0.002 electrons per cc.

As the Voyager probes crossed beyond the heliopause, their Plasma Wave Science instruments detected the electron density of the plasma through plasma oscillations.

Voyager 1 crossed the heliopause on 25 August 2012, at a distance of 121.6 astronomical units from Earth (that's 121.6 times the space between the planet and Sun, so roughly 18.1 billion km).

When it first measured the plasma oscillations after crossing the heliopause on 23 October 2013 at a distance of 122.6 astronomical units (18.3 billion km), Voyager 1 detected a plasma density of 0.055 electrons per cubic centimeters.

Voyager 2, which took the great distance around, flying by Jupiter, Saturn, Uranus, and Neptune, crossed the heliopause on 5 November 2018 at a distance of 119 astronomical units (17.8 billion km). It measured the plasma oscillations on 30 January 2019 at a distance of 119.7 astronomical units (17.9 billion), finding a plasma density of 0.039 electrons per milliliter, very near the Voyager 1 measurement.

And both instruments reported a rise in density. After traveling another 20 astronomical units (2.9 billion km) through space, Voyager 1 reported a rise to about 0.13 electrons per metric capacity unit.

But detections made by Voyager 2 in June 2019 showed a way sharper increase in density to about 0.12 electrons per milliliter, at a distance of 124.2 astronomical units (18.5 billion units).

Given that plasma at Earth's air pressure has an electron density of 10^13 per metric capacity unit, those amounts could appear tiny, but they're significant enough to warrant our interest - especially since it isn't clear what causes them.

One theory is that the interstellar field of force lines become stronger as they drape over the heliopause. this might generate an electromagnetic ion cyclotron instability that depletes the plasma from the draping region. Voyager 2 did detect a stronger flux than expected when it crossed the heliopause.

Another theory is that material blown by the interstellar wind should slow because it reaches the heliopause, causing a form of hold up. This has possibly been detected by outer system probe New Horizons, which in 2018 picked up the faint ultraviolet glow resulting from a buildup of neutral hydrogen at the heliopause.

It's also possible that both explanations play a job. Future measurements taken by both Voyager probes as they continue their journey out into part could help figure it out. But that may be a protracted bet to require.

"It isn't certain," the researchers wrote in their paper, "whether the Voyagers are going to be able to operate far enough to tell apart between these two classes of models."

We believe you, space probes!