What is the boundary where the solar wind meets the interstellar medium?

The heliopause is the boundary where the solar wind meets and is balanced by the interstellar medium. The solar wind is a continuous stream of charged particles flowing outward from the Sun's hot corona, creating a vast protective region called the heliosphere.

The heliopause is not the first boundary encountered by the solar wind. Farther out, the wind passes through the termination shock, where it slows from supersonic to subsonic speeds. It then travels through the turbulent heliosheath before reaching the heliopause, where pressure from the solar wind and interstellar space becomes comparable.

Voyager 1 crossed the heliopause in 2012, and Voyager 2 followed in 2018. Their measurements showed changes in particle populations and plasma conditions as they entered interstellar space. The boundary is dynamic rather than perfectly spherical: solar activity, the Sun's motion, and variations in the surrounding interstellar medium can change its shape and position.

A common mix-up is the termination shock, which is an inner transition in the solar wind, not the outer edge of the heliosphere.