The heliosphere is the region of space dominated by the solar-wind plasma and its carried magnetic field as they interact with the interstellar medium. The solar wind slows at the termination shock and encounters the interstellar plasma at the heliopause.
This is a plasma-interaction boundary, not the outer reach of the Sun's gravity or light. Voyager observations combine particle, magnetic-field and plasma-wave measurements to diagnose the transition. An increased local electron density outside the heliopause supplied an important independent diagnostic, illustrating why a boundary cannot always be identified from one field direction alone.[ref]
NASA Spacecraft Embarks on Historic Journey into Interstellar Space by NASA. Solar/interstellar plasma distinction and Voyager plasma-wave diagnostic
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The heliosphere is a vast region of space that surrounds the Sun and is influenced by its solar wind—a stream of charged particles (mostly electrons and protons) emitted by the Sun. This bubble-like structure extends well beyond the orbit of the planets and marks the boundary between the solar system and interstellar space.