= Heliopause
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.<NASA Spacecraft Embarks on Historic Journey into Interstellar Space by NASA>[Plasma-wave observations and retrospective crossing date]{ref}
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