heliosheath

Heliosheath is the outer region of the heliosphere, the bubble of solar wind carved in interstellar space. It lies beyond the termination shock, where the supersonic solar wind slows to subsonic speeds, and extends outward to the heliopause, the boundary with the local interstellar medium. In the heliosheath, solar wind plasma is heated and compressed by the shock and flows subsonically toward the heliopause, interacting with interstellar plasma and magnetic fields. The magnetic field in this region is shaped by both solar and interstellar fields, creating a draped configuration and contributing to turbulence and mixing.

Plasma in the heliosheath has higher temperatures and lower speeds than the inner solar wind, and displays enhanced density relative to the supersonic wind. The region is highly variable, structured by solar activity, with fluctuations in speed, temperature, and magnetic field. The flow is ultimately diverted around the heliopause and deflected as it encounters the interstellar medium.

Observations: Voyager 1 and Voyager 2 traversed the termination shock in the mid-2000s and entered the heliosheath, providing in situ measurements of particle populations, magnetic fields, and plasma. Voyager 1 crossed into interstellar space near the heliopause in 2012; Voyager 2 did so in 2018. The Interstellar Boundary Explorer (IBEX) has mapped energetic neutral atoms emitted from the heliosheath, revealing large-scale structures such as the ENA ribbon and suggesting complexity beyond simple models. Numerical models using magnetohydrodynamics and kinetic effects continue to refine understanding.

Role: The heliosheath modulates galactic cosmic rays and solar energetic particles entering the inner solar system. It also serves as a laboratory for plasma processes at the boundary between decelerated solar wind and interstellar medium.