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- What defines the boundary of the solar system? Sun’s light? The influence of the Sun’s gravity? Or the influence of the Sun’s magnetic field & the solar wind? There is no definite boundary where the light or gravity stops or where they suddenly get weaker. The solar wind is however different from light or gravity. As it streams away from the Sun, it races out against the interstellar medium ― space between the stars permeated by hydrogen and helium gas).
- Even though the interstellar medium has a low density, it still has a pressure (similar to air pressure). The solar wind also has pressure. It blows against the interstellar medium and creates a bubble-like region. This bubble that surrounds the solar system is called the heliosphere (not a sphere in the true sense).
- Since the Sun is moving relative to the interstellar medium around it, the heliosphere forms a wave or shock in the interstellar medium like a boat in the ocean. This is called the bow shock or wave.
- The region where the solar wind begins to interact with the interstellar medium and begins to slow down is called the heliosheath.
- The heliosheath has a few parts: the termination shock (the innermost part of the boundary), the heliopause (the outermost part of the boundary) and the part in between the inner and outer boundary.
- The termination shock is the boundary where the solar wind particles slow down so that the particles are travelling slower than the speed of sound.
- At the heliopause ― the boundary of the heliosphere, the pressure from the interstellar medium is strong enough to slow down and eventually stop the flow of solar wind.
- Voyager 1 crossed the termination shock at 94 astronomical units (AU) and Voyager 2 crossed at 84 AU.