Heliopause

The heliopause is the outer boundary of the heliosphere, where the outward pressure of the solar wind balances the pressure of the surrounding interstellar medium.

Termination shock: A shock wave where the supersonic solar wind slows to subsonic speed. It marks an inner transition in the solar wind before the flow reaches the heliopause.
Voyager 1: A NASA spacecraft launched in 1977 to study the outer planets and interstellar space. It crossed the heliopause in 2012, providing the first in situ measurements beyond it.
Eugene Parker: An American astrophysicist who developed the theory of the solar wind and heliospheric magnetic field. His solar-wind theory supplied the outward flow that heliopause models must explain.
Cosmic ray modulation: Changes in cosmic-ray intensity and energy caused by transport through heliospheric magnetic fields and solar-wind plasma. The heliopause forms an outer boundary for much of the Sun's modulation of incoming cosmic rays.
Heliosheath: The region of slowed, heated solar wind between the termination shock and the heliopause. This outer solar-wind layer lies immediately inside the heliopause.
Voyager 2: A NASA spacecraft launched in 1977 that explored the outer planets and later entered interstellar space. It crossed the heliopause in 2018 and measured a second location on the boundary.
Solar wind: A continuous flow of charged particles released by the Sun into interplanetary space. Recognition of this flow made a boundary with interstellar space physically necessary.
Galactic cosmic rays: Energetic charged particles originating outside the Solar System. Their flux changes across the heliopause as particles enter the heliosphere.
Interstellar magnetic field: The magnetic field carried by interstellar plasma through the space between stars. Its pressure and orientation influence the heliopause's shape and structure.
Voyager Interstellar Mission: The extended phase of the Voyager spacecraft missions studying the outer heliosphere and interstellar space. Its instruments have sampled conditions on both sides of the heliopause.

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