Solar Wind

Due to the supersonic expansion of the Sun’s corona, a type of conductive plasma called the solar wind is ejected from the Sun at a speed of about 500 km/s. This wind, which spreads through the interplanetary medium of the Solar System, is composed mainly of electrons and protons, but also contains 5% helium ions. The conductivity of this plasma is so high that the Sun’s magnetic field freezes in it, much like a superconductor.

The number density of electrons in the solar wind around the Earth is about 5 per cc, and the temperature is about 100,000 Kelvin. And the average strength of the interplanetary magnetic field is approximately 5 nanotesla.

When the solar wind is near the Earth’s dipolar magnetic field it cannot directly penetrate it, but flows slowly around the edge of the Earth’s magnetosphere as shown in the figure above. The supersonic speed of the solar wind then creates a bow shock, where a large portion of the particles’ kinetic energy is converted into thermal energy. The area behind the bow shock where the thermalized subsonic plasma resides is called the magnetosheath. The temperature and density of particles here are higher than in the solar wind, and the magnetic field is stronger.