The Heliosphere

The heliosphere is the domain of the Sun among the interstellar medium. It is, alike the Earth's magnetosphere, a comet-shaped region, with a bow shock in front of it, and a comet tail-shaped form in the trailing direction

The heliosphere may be divided into different zones and layers:

• the termination shock. The termination shock is the zone where the solar wind speed falls under sound's speed, from about 700,000-1.5 million mph to less than 100,000-250,000 mph (435,000-933,000 km/h to less than 62,000-156,000 km/h). At this distance from the Sun, the solar wind has become a mere thin plasma, but its speed remains very high. Due to fundamentals of supersonic fluid flow, the solar wind's speed abruptly falls when it reaches the interstellar medium (which itself is very thin and exerts a small pressure). The conversion of the kinetic energy yields heat and the temperature raises to more than a million degrees. Particles are accelerated. The termination shock is 90 AU from the Sun in direction of the leading edge, and about 170 AU in direction of the trailing edge
• the heliosheath (or heliotrail). After having crossed the termination shock, the solar wind further slows and turns into the ambient flow in the interstellar medium: it is this region which forms the comet-tail core of the heliosphere. This turbulent area is located approximately at 8.7 billion miles (14 billion km) from the Sun
• the heliopause. The heliopause is the outer border of the heliosheath. The heliosphere meets the interstellar medium. This frontier is about 110 AU from the Sun towards the leading edge, and about 260 AU crosswise
• the bow shock. The bow shock is the point, downstream, where the Sun's medium is plowing into the interstellar medium. The bow shock acts as a shield relative to the latter and deflects interstellar particles all along the bubble of the heliosphere. Bow shock is at 230 AU from the Sun, towards the leading edge. The bow shock should have the shape of a bow-shaped wall of hydrogen as the interaction between the neutral hydrogen being part of the interstellar medium and the plasma of the Sun medium, produces more neutral hydrogen, hence the wall. Neutral hydrogen is leaking inside the heliosphere, it be neutral at the origin, or ionized. When ionized it eventually turn neutral, via charge exchange. At the fore of the heliosphere, on the other hand, some atoms are heatep up and lose their electrical charge, thus no longer hindered by magnetic fields, flow back toward the bow shock

Some late observations by the SOHO solar satellite have seen that the flow of the neutral hydrogen atoms has been seen lately coming from a slightly different direction than other neutral matter like helium. This might point to a specific orientation of the local interstellar magnetic field which would not be aligned nor perpendicular to the direction of motion of the solar system. Further, this would mean that the shape of the heliosphere is not perfectly symmetric. This last point has been confirmed, on the other hand, by the two Voyager probes -Voyager 1 and Voyager 2- which are heading to the outside of the heliosphre's bubble in two different directions. They are reporting a large North-South asymmetry in the shape of the heliosphere, which could be due to an interstellar magnetic field pressing inwards on the southern hemisphere of it. Voyager 1 is reaching the first frontier of the heliosphere: at 8.4 billion miles (90 AU; 5.2 billion km) the craft is approaching the termination shock. It will take 10 to 20 years more to reach the next layer, the heliopause. Then beyond the bow shock, the craft will drift forever in the interstellar space. It should reach a star of the constellation of the Giraffe in about 40,000 years. Voyager 2, parent craft to Voyager 1 is too heading to the front side of the heliosphere, which it should begin to reach by the end of 2007, as is doing too Pioneer 11. Pioneer 10 is leaving the solar system in the opposite direction (Pioneer missions were launched in 1972 and 1973 towards Jupiter and Saturn). Voyager 2 should reach Sirius in 296,000 years. As the Pioneer are now inactive, both Voyager still are sending back data

Voyager 2 see the Heliosphere Assymetric
Voyager 2 has in turn reached the 'heliosheath', the beginning of the frontier of the 'heliosphere', this bubble equivalent for the Sun to the Earth's magnetosphere and protecting the whole solar system from the interstellar medium, by the end of the year 2007. With the second of the Voyager craft reaching there, the scientists, further, got the evidence that the heliosphere is not perfectly round, with its southern face closer to the Sun by one billion miles (1.6 billion km). The 'heliosheath' is the region where the solar wind ceases and leaves room to the interstellar medium