Swedish astrophysicist who made fundamental contributions to plasma physics, particularly in the field of magnetohydrodynamics (MHD)—the study of plasmas in magnetic fields. He shared the 1970 Nobel Prize for Physics.
Alfvén was born in Norrköping, Sweden, and educated at the University of Uppsala. In 1940 he joined the Royal Institute of Technology, Stockholm, later dividing his academic career between that and the University of California, San Diego, from which he obtained a professorship in 1967. In 1972 he was among a group of Oxford scientists who appealed to governments to abandon fast-breeder nuclear reactors and concentrate efforts on nuclear fusion.
Alfvén formulated the frozen-in-flux theorem, according to which a plasma is—under certain conditions—bound to the magnetic lines of flux passing through it; later he used this theorem to explain the origin of cosmic rays. In 1939 he proposed a theory to explain aurorae and magnetic storms, which greatly influenced later ideas about the Earth's magnetosphere.
In 1942 Alfvén postulated that a form of electromagnetic wave would propagate through plasma; other scientists later observed this phenomenon in plasmas and liquid metals. Also in 1942 he developed a theory that the planets in the Solar System were formed from material captured by the Sun from an interstellar cloud of gas and dust.