| Citation | Distinguished for his contributions to the theory of fluid mechanics particularly when magnetic fields are present and thermal convection occurs, as is the case in astrophysical and geophysical situations. His scientific work is marked by great versatility, critical insight into physical ideas and a well-developed mathematical technique, and is permeated by a remarkable scholarship and historical perspective. Deserving of special mention are his papers, alone or in collaboration, elucidating the nature of the geomagnetic field, the properties of rotating masses of fluid, the linear and non-linear theories of thermal instability. He has also made notable contributions to the theories of the solar wind and of the origin and behaviour of quantized vortex lines in liquid helium. |