The solar wind is the continuous stream of charged particles, mostly protons and electrons, flowing out of the Sun's corona at hundreds of kilometers per second. It carries the Sun's magnetic field along with it and inflates the heliosphere, the vast bubble enclosing the entire solar system. Where it meets Earth's magnetic field it lights the aurorae and, during severe storms, disturbs satellites and power grids. The Parker Solar Probe has flown through the corona itself to learn where the wind is born and why it accelerates.