Heliopause
The heliosphere is divided into how many regions?
Solar System
The heliosphere is divided into two separate regions. The solar wind travels at roughly 400 km/s until it collides with the interstellar wind; the flow of plasma in the interstellar medium. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. Here the wind slows dramatically, condenses, and becomes more turbulent, forming a great oval structure known as the heliosheath. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field. The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space. Both "Voyager 1" and "Voyager 2" are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively. "Voyager 1" is also reported to have reached the heliopause. The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way. Due to a lack of data, the conditions in local interstellar space are not known for certain. It is expected that NASA's Voyager spacecraft, as they pass the heliopause, will transmit valuable data on radiation levels and solar wind back to Earth. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.