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The heliopause is the boundary where the solar wind’s outward pressure reaches equilibrium with the interstellar particles’ upward pressure. Its exact location and effects on the heliosphere and interstellar medium are still unknown. The heliosphere is not a perfect sphere and has two zones of compression and turbulence near the heliopause. Voyager 1 and 2 and Pioneer 10 may provide more information as they journey to interstellar space.
The heliopause is a boundary in space where the outward pressure of our sun’s solar wind, made up of gases and particles blown outward by forces generated by the solar furnace, reaches an equilibrium with the upward pressure. interior of interstellar particles pushing against the heliosphere. The heliosphere is the region around the Sun that is affected by the Sun’s magnetic field and where the solar wind exerts visible pressure outward from the Sun. An analogy would be a balloon, with the Sun inside. The solar wind is the air that inflates the balloon, the outside air is the pressure of the interstellar medium (ISM), and the balloon itself is the heliopause.
The existence of this boundary is accepted as fact by modern science. What is still undetermined is its exact location and how it affects the interaction between the heliosphere and the interstellar medium, which is the interstellar space between solar systems. Much has been deduced by astronomers and physicists about the exact location and profile of the heliosphere and, by extension, the heliopause, but no precise data is available so far. It is hoped that as man-made probes such as Voyager 1 and 2 and Pioneer 10 continue their journey from the solar system to interstellar space, more precise information will become available.
It is also generally accepted in science that the heliosphere is not a perfect sphere. This is because the Sun moves through interstellar space and, as with a flowing river, the “upstream” side of the heliosphere, in the direction of the Sun’s travel, is compressed, while the “downstream” side is stretched. Astronomers estimate the heliopause to be about 100-150 astronomical units (AU) from the sun in the “upstream” direction. An astronomical unit is the average distance from the Earth to the Sun, or about 93 million miles (149 million kilometers). The distance to the “downstream” boundary is unknown, but is thought to be somewhat further, perhaps in the 170 AU range.
As a boundary, the heliopause is not absolute. Particles from outside the heliosphere penetrate inward, and some solar wind particles are thought to pass over the boundary. Near the heliopause, on the “upstream” side, there are believed to be two zones of compression and turbulence, where the magnetic fields form “bubbles”. The solar wind, as it approaches the heliopause, reaches a region known as the termination shock, where it slows down markedly. Outside the heliopause, as the inflow of the interstellar medium approaches the boundary, it slows down markedly, a region known as the arc shock.
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