Answer
Hannes Alfvén was a pioneer in plasma physics and MHD. His research led to the development of the magnetohydrodynamic dynamo, which is now used to explain the magnetic fields of planets and stars. Alfvén also proposed the existence of magnetohydrodynamic waves, which are now known as Alfvén waves. These waves play an important role in the dynamics of plasmas and have been observed in space and in laboratory experiments.