There are several approaches to how to define the boundaries of the solar system, which is why its size remains a matter of controversy. But astronomers have a clear criterion called the heliopause – this is the point in space where the strength of the solar wind is equalized with external pressure. Everything that is located before it is called the heliosphere, and outside – the endless interstellar space, and it suddenly turned out to be not empty at all.
Previous calculations determined the density of the void inside the Milky Way at 0.037 particles per cubic meter. see, and the plasma density behind the heliopause is 0.002 electrons per cubic meter. see These data were refuted when the Voyager 1 probe passed the heliopause, at a distance of about 18.1 billion km from Earth. His instruments made measurements on October 23, 2013 and revealed a plasma concentration of 0.055 electrons per cubic meter. cm.
The fact that there is no emptiness behind the heliopause was also confirmed by Voyager 2 measurements taken on January 30, 2019 at a distance of about 17.9 billion km from Earth. The density of the local space plasma was 0.039 electrons per cubic meter. see And the further the Voyagers flew, the more the concentration of particles grew. Voyager 1 received a figure of 0.13 electrons per cubic meter. cm at a distance of 21.2 billion km from the Earth, and Voyager 2 transmitted a level of 0.12 particles per cubic meter. see already at a distance of 18.5 billion km.
Today, there are two versions to explain this discrepancy. The first one says that the magnetic field of the galaxy in the heliopause creates electromagnetic instability. And Voyager 2 actually recorded an increase in the magnetic field at this point. According to another version, all the particles that the solar wind has driven to the heliopause are decelerated in it, and create a kind of “plug”, which explains their high concentration in this location.