The Sun and the Solar Wind

Abstract

Some 4.56 billion years ago, out of the gravitational collapse of a cloud in the interstellar medium with angular momentum L, a rotating star — the Sun — was formed, and began to burn hydrogen into helium and heavier nuclei. A gaseous disc in the plane orthogonal to L developed; its chemical composition, initially the same as in the solar photosphere, quickly differentiated, giving rise to a variety of planets according to the distance from the Sun. The Sun is an electromagnetically active body, with great variability, including a cycle of ≃ 11 y; it deeply affects the entire solar system, not only through its steady radiation, but also through its variable electromagnetic emission and the solar wind, a flow of charged particles heated and emitted at supersonic speed by the corona. This plasma drags the solar magnetic field outward and, coupling with matter in interplanetary space, carries away angular momentum. It interacts with the planets and their dipole magnetic fields in a variety of complex processes and produces local manifestations of solar activity. At still larger distances, beyond the planets, the solar wind merges through a termination shock with the interstellar medium, confining the heliosphere.