Termination shock

The termination shock is the boundary in the outer heliosphere where the supersonic solar wind abruptly slows to subsonic speeds as its kinetic pressure is balanced by the pressure of the interstellar medium. It is not a shock in the everyday sense but a collisionless shock wave mediated by electromagnetic interactions rather than particle collisions, because the solar wind plasma is too tenuous for direct collisional dissipation. The shock marks the inner boundary of the heliosheath, the turbulent region where the slowed wind is compressed, heated, and deflected before finally surrendering to the interstellar flow at the heliopause.

The location of the termination shock is not fixed. Voyager 1 crossed it at approximately 94 astronomical units in 2004, and Voyager 2 crossed at 84 AU in 2007 — the asymmetry reflecting the influence of the interstellar magnetic field and the local interstellar medium's pressure anisotropy. The shock's structure is modified by the 11-year solar cycle, retreating toward the Sun during solar maximum and expanding outward during solar minimum.

The termination shock is the heliosphere's heartbeat made audible: the place where the solar wind's supersonic scream is finally hushed by the interstellar medium's patient pressure. It is not an ending but a transformation — the sonic baptism of the solar wind into the galactic congregation.