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]
NASA Spacecraft Embarks on Historic Journey into Interstellar Space by NASA. Plasma-wave observations and retrospective crossing date
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