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The heliopause is the boundary separating the predominantly solar-wind plasma of the heliosphere from the surrounding interstellar plasma. It is set by the interaction of flows, magnetic fields and pressure, rather than by a fixed spherical surface. It differs from the termination shock, where the solar wind slows abruptly inside the heliosphere.

Voyager 1 supplied a direct test. Plasma-wave oscillations observed in 2013 indicated a much greater local electron density, supporting a retrospective crossing in August 2012. Because the characteristic plasma frequency depends on the square root of electron density, the observed oscillation frequency served as a density diagnostic.^[ref]

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]