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Hannes Alfvén died in 1995 at the age of 86. He was a Swedish physicist and Nobel laureate who was known for his work on magnetohydrodynamics, the study of the interaction of magnetic fields and plasmas. Alfvén's work helped to explain the structure and dynamics of the solar system, and he also developed Theory of magnetohydrodynamic waves, which are now known as Alfvén waves.