Solar wind — a stream of high-energy charged particles from the Sun — is constantly hitting the Earth, and much of it is deflected by the Earth's magnetic field. In some cases, a fraction of this flux reaches the ionosphere and causes auroras. But this constant interaction between the solar wind and the magnetosphere has another consequence:the leakage of part of the Earth's ionospheric plasma into space; a phenomenon called polar wind or plasma fountain.
Continuously, the Sun emits bursts of plasma in the direction of the Earth, composed of ions and electrons whose energy and intensity depend on solar activity. When these particles hit the magnetosphere, an area dominated by the Earth's magnetic field, they are deflected along field lines on either side of the planet. Thus, the geomagnetic field constitutes a real shield against the solar wind.
However, solar particles possess enough energy to penetrate the Earth's ionosphere and transfer some of this energy to the molecules that make up the upper atmosphere. These interactions tear electrons from the latter, which are then ionized, and acquire sufficient kinetic energy to be ejected into space.
Indeed, in order to be able to escape, these ions must reach the terrestrial escape velocity, i.e. 11.2 km/s. A large majority of the ions ejected into space remain bound to the magnetic field of the Earth, where they constitute part of the Van Allen radiation belt, the other part being composed of the particles of the solar wind captured and trapped by the field. terrestrial magnetic.
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This mechanism of ionospheric plasma leakage into space was originally proposed by physicists Ian Axford, Peter M. Banks, and Thomas E. Holzer in 1968, as an explanation for the Earth's helium budget paradox. The latter consists in the fact that atmospheric helium is produced faster than it escapes from the upper atmosphere. The fact that helium can be ionized by the solar wind and be ejected would be a possible solution to the paradox.
The term "polar wind" was chosen because the process is similar to how the solar wind escapes from the solar corona. The particles emitted from the latter follow a well-defined path along the lines of the solar magnetic field, before escaping into space. The phenomenon is similar on Earth, where the ejected ions also follow the lines of the geomagnetic field.
