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The phenomenon of aurora borealis occurs when bursts of charged particles from the sun collide with the earth’s magnetic field. These jets of particles are an example of a special state of matter—plasma. Plasma is a gas comprised of electrons and ions (electrically charged atoms) that forms at high temperatures. From the late 1930s onward, Hannes Alfvén developed a theory about aurora borealis, which led to magneto-hydrodynamics; the theory of the relationships between a plasma’s movements, electric currents and fields, and magnetic fields.