Solar Wind, Heliosphere & Space Weather: Observation, Measurement & Evidence

The heliosphere includes the supersonic solar wind close to the Sun, the termination shock where that wind slows, the heliosheath beyond it, and the heliopause marking the boundary where solar plasma pressure balances the surrounding interstellar medium. Near Earth, the solar wind interacts with the magnetosphere rather than striking the atmosphere uniformly.

Spacecraft measure solar-wind speed, density, temperature, and magnetic field directly. Auroras, geomagnetic storms, radio disruptions, satellite drag, and induced currents in power systems correlate with changes in solar-wind conditions and eruptive solar activity. Voyager spacecraft have crossed major heliospheric boundaries.

The corona is hot enough that some plasma escapes the Sun's gravity and accelerates outward along open magnetic field lines. As the Sun rotates, its magnetic field is carried outward by the flowing plasma into a spiral structure. Disturbances such as high-speed streams and CMEs compress or reconnect with planetary magnetic fields.

Spacecraft such as Parker Solar Probe, Solar Orbiter, ACE, DSCOVR, and the Voyagers measure plasma and magnetic fields directly. Auroral activity, geomagnetic indices, energetic-particle detectors, and solar observations provide complementary evidence. Voyager crossings of the heliopause demonstrate that the heliosphere has a measurable outer boundary.