heliopause

The heliopause is the boundary that marks the outer edge of the Sun’s influence in interstellar space. It is where the solar wind, a stream of charged particles emitted by the Sun, meets the local interstellar medium. Inside this boundary lies the heliosphere, a bubble shaped by the solar wind and solar magnetic field, while outside lies the relatively untouched plasma and magnetic environment of other stars.

Between the Sun and the heliopause, the solar wind undergoes two major regions. The termination shock is where the wind slows from supersonic to subsonic speeds. Beyond that lies the heliosheath, a turbulent region of slowed solar wind plasma, until the solar wind pressure is balanced by the interstellar medium at the heliopause.

Distance to the heliopause is not fixed; it varies with direction and solar activity. Estimates commonly place it around 120 astronomical units from the Sun, though measurements differ by region. Voyager 1 crossed the heliopause in 2012 at about 122 AU, providing the first direct evidence of the boundary. Voyager 2 followed in 2018 at about 119 AU.

The heliopause acts as a catalytic boundary between solar and interstellar plasmas and magnetic fields. It influences how galactic cosmic rays propagate into the inner solar system and serves as a focus of ongoing observational and theoretical studies, including data from spacecraft and remote sensing projects addressing the structure and dynamics of the outer heliosphere.