The Solar Wind & Heliosphere

The steady, high-speed stream of the solar wind — around seven to eight hundred kilometres per second — that flows out along open magnetic field lines from coronal holes, especially over the poles at solar minimum. It is smoother and less dense than the slow wind.

The denser, more variable component of the solar wind — around three to five hundred kilometres per second — associated with the streamer belt near the solar equator. Its exact sources and release mechanisms are still being pinned down by close-in missions.

The turbulent outer region of the heliosphere, between the termination shock and the heliopause, where the slowed solar wind piles up and is deflected by the interstellar medium. Both Voyager spacecraft spent years traversing it before reaching interstellar space.

The disturbance ahead of the heliosphere as it moves through the surrounding interstellar cloud. Earlier work expected a sharp bow shock, but measurements from IBEX and the Voyagers suggest the Sun moves too slowly through the local medium for a strong shock — a gentler bow wave instead. The exact nature of the boundary is still being studied.

The spiral shape the Sun's magnetic field takes as it is carried outward by the solar wind while the Sun rotates, like water from a spinning sprinkler. Predicted by Eugene Parker in 1958 and since confirmed by spacecraft, the Parker spiral sets the geometry of the interplanetary magnetic field and how solar storms reach the planets.

The boundary where the supersonic solar wind abruptly slows as it runs into the pressure of the interstellar medium, roughly ninety astronomical units from the Sun. Voyager 1 crossed it in 2004 and Voyager 2 in 2007 — the first direct measurements of this shock.