The pressure at the borders of the solar system exceeded all forecasts

UNITED STATES, WASHINGTON (OBSERVATORY) — Voyagers, the only spacecraft to transmit a signal from interstellar space, presented scientists with another mystery. Astronomers measured the pressure and speed of sound at the borders of the solar system and concluded that we did not know anything important about these boundaries.

So, the pressure was significantly higher than experts expected.

The discovery is described in a scientific article published in the Astrophysical Journal by scientists from three US research centers.

Let us explain that space is not an absolute void. So, the solar system is filled with solar wind. This is a stream of charged particles (mainly protons and electrons mixed with helium nuclei and other elements), continuously emitted by our star. Of course, the concentration of this plasma is many orders of magnitude lower than that of air in the Earth’s atmosphere. Therefore, compared with the usual conditions, it’s almost empty. But even such a rarefied medium conducts sound and puts pressure on surrounding objects.

Recall that Voyager 1 and Voyager 2 were launched in 1977. Despite their venerable age, the devices still communicate with the Earth, in which engineers diligently help them . In 2012, Voyager 1 , and in 2018 Voyager 2 reached heliopause. This, recall, the front of collision of the solar wind flow with the interstellar medium. It is located about a hundred times farther from the Sun than the Earth, and more than twice as far as Pluto.

Heliopause is the boundary of the heliosphere, that is, the area from which the solar wind displaces interstellar gas. Many astronomers identify the heliosphere with the solar system. Thus, the Voyagers are the first and so far the only vehicles transmitting to Earth the scientific data collected in interstellar space.

For this reason, any bit received from Voyagers is unique. However, a new discovery will stand out even from this series. The fact is that it was obtained thanks to a fantastic combination of circumstances.

Just a few months after Voyager 1 left the solar system, it was caught up by the sun’s farewell greetings in the form of a global united interaction region (in English global merged interaction region, or GMIR). Let us explain what it is.

The solar wind moves at different speeds. There is a so-called slow solar wind in the region of the Earth with a speed of about 400 kilometers per second. There is also a fast solar wind that rushes through the Universe twice as fast. Finally, there are plasma clouds that are occasionally ejected by our star , and they can move even faster.

When a fast solar wind stream catches up to a slower one, they collide. At the boundary between them, a region of compressed plasma is formed with increased concentration and magnetic field strength. This is the area of ​​interaction.

When several such collisions occur next to each other, their interaction regions merge and a united interaction region forms. Well, the largest of the latter are called global.

It was such a zone of seething plasma that first covered Voyager 2, and four months later Voyager 1, which flew further from the Sun. At the same time, Voyager 1, as already mentioned, managed to leave the heliosphere. But Voyager 2 was located in its boundary layer, the so-called internal heliosheath (from English heliosheath). This is the area where the solar wind has a subsonic speed (unlike the rest of the solar system, where it is supersonic).

“This event was truly a unique moment, because we have seen it immediately after the” Voyager 1 “entered the local interstellar space, – explains the study’s first author Jamie Rankin ( by Jamie Rankin ) from Princeton University.

Researchers recorded a meeting of GMIR with probes due to the fact that their detectors began to detect much less cosmic rays coming from the vastness of the Galaxy. A cloud of dense plasma simply replaced extraneous charged particles.

Knowing the distance between the probes and the speed of their movement, astronomers calculated the speed of the wave that caught up with them. Together with the data of other instruments, this made it possible to calculate the pressure and speed of sound in the helioshis, that is, the outermost shell of the solar system.

It turned out that the sound propagates there at a speed of 314 ± 32 kilometers per second, that is, almost a thousand times faster than in air. This is due to the fact that charged particles act on each other at a great distance by their electric fields. Therefore, such a particle does not have to collide with a neighbor in order to transmit an oscillation to it.

The plasma pressure in the helioshis turned out to be equal to 267 ± 55 femtopascals. To call it huge, of course, does not work: atmospheric pressure on Earth is more than about 10 18 , that is, a million trillion times. However, astronomers did not expect to fix such pressure on the desert outskirts of the solar system.

Another mystery is associated with this data. When GMIR caught up with Voyager 2, located in the helioshis, the flux of cosmic rays decreased from all directions (as physicists expected). But at Voyager 1, located in the interstellar medium, the flux of cosmic rays decreased only from the direction perpendicular to the direction of the local magnetic field. Scientists do not yet understand what makes this difference.


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