heliosfärens

The heliosfären, or heliosphere in English, is the bubble-like region of space dominated by the solar wind and the Sun's magnetic field. It marks the outer boundary of the Sun's direct influence and extends far beyond the orbit of Pluto. The size and shape vary with solar activity and the interstellar environment, but estimates place the termination shock where the solar wind slows from supersonic to subsonic at about 80–100 astronomical units (AU) from the Sun, and the heliopause, the boundary with interstellar space, at roughly 120–150 AU. The heliosphere is not a perfect sphere; it is compressed on the sides toward the direction of the Sun's travel through the galaxy and stretched into a tail that points opposite that motion.

Within the heliosphere lies the solar wind plasma and embedded magnetic field created by the Sun, extending outward as the Parker spiral. The region between the termination shock and the heliopause is called the heliosheath. The existence and location of a bow shock in the interstellar medium are debated; some models predict a bow wave rather than a strong bow shock.

Observations from the Voyager probes provided direct measurements: Voyager 1 crossed the termination shock in 2004 and the heliopause in 2012, becoming the first man-made object to enter interstellar space. Voyager 2 followed with a termination shock crossing in 2007 and heliopause passage in 2018. Instruments like IBEX have mapped the boundary region, while New Horizons continues to operate within the heliosphere.

The heliosphere partly shields the Solar System from galactic cosmic rays, modulating their flux in step with the solar cycle, and thereby influencing space weather conditions for planets and spacecraft.