Heliosphere

The heliosphere is a bubble-like region of space dominated by the Sun's solar wind and magnetic field. It extends far beyond the orbit of the outer planets and forms a cavity in the interstellar medium created by the outward flow of charged particles from the Sun. The shape is influenced by solar activity and the pressure of the local interstellar medium, giving the heliosphere a roughly comet-like appearance with a nose pointing in the direction of the Sun's travel through the galaxy and a long tail downstream.

Inside the heliosphere, the solar wind expands outward at supersonic speeds until it encounters the termination shock, where it slows to subsonic speeds. Beyond this lies the heliosheath, a region of turbulent, slowed solar wind, and finally the boundary with the interstellar medium, the heliopause, where solar and interstellar pressures balance. The existence of a bow shock ahead of the heliopause has been debated, with some models suggesting only a bow wave rather than a strong shock.

Observations from spacecraft have confirmed key features. Voyager 1 crossed the termination shock in the mid-2000s and entered interstellar space, crossing the heliopause in 2012 at about 121 astronomical units. Voyager 2 followed with its own termination shock and heliopause crossings. The Interstellar Boundary Explorer (IBEX) mission maps energetic neutral atoms to study boundary regions, while ongoing research strives to understand the detailed shape, dynamics, and magnetic interactions of the heliosphere.

The heliosphere acts as a shield that modulates galactic cosmic rays and influences the space environment of the Solar System. Its study informs models of planetary protection, interstellar communication, and the interaction between stellar winds and the galactic medium.