Distinguished Visiting Lecture Programm

California State University Northridge

Department of Physics and Astronomy


The Intriguing Structure of a Sunspot


John H. Thomas


Department of Mechanical Engineering and Department of Physics and Astronomy

University of Rochester


18 April 2007 3:30 PM


Thousand Oaks Room, University Student Union


Sunspots, which have fascinated astronomers since the time of Galileo, mark the locations where strong magnetic fields emerge through the surface of the Sun. They provide the ideal proving ground for the theory of magnetohydrodynamics under astrophysical conditions, for nowhere else in astrophysics is the theory confronted with such a wealth of detailed observations.  Recent advances in high-resolution observations have provided us with a new picture of the magnetic structure of a sunspot, especially its outer part (the filamentary penumbra) which involves two components with different magnetic field inclinations. The darker component, with a more nearly horizontal magnetic field, contains `returning' magnetic flux tubes that dive back down below the solar surface near the outer edge of the penumbra. The submergence of these flux tubes, which is surprising in view of their inherent magnetic buoyancy, can be understood to be a consequence of downward pumping of the magnetic flux by turbulent granular convection in the `moat' surrounding the sunspot. The effectiveness of this flux-pumping process is demonstrated in three-dimensional numerical simulations of fully compressible thermal convection. The flux-pumping mechanism turns out to be an important key to understanding not only the curious magnetic structure of the penumbra but also its formation and maintenance.

Video1, Video2

 
Jack

John H. Thomas


Department of Mechanical Engineering and Department of Physics and Astronomy

University of Rochester


John H. Thomas received his bachelor's (1962), master's (1964), and Ph.D. (1966) degrees in Engineering Sciences at Purdue University. After a year as a NATO postdoctoral fellow at the University of Cambridge, England, he joined the faculty of the University of Rochester, where he has remained and is currently Professor of Mechanical and Aerospace Sciences and of Astronomy. He also served eight years as Rochester's dean of graduate studies. His long career at Rochester has been interspersed with visiting appointments at several institutions, including the universities of Oxford, Cambridge, and Sydney, the Max Planck Institute for Astrophysics in Munich, the National Solar Observatory in New Mexico, and the High Altitude Observatory in Colorado. He is a Fellow of the American Physical Society and the Royal Astronomical Society (UK). He has held a Guggenheim fellowship and has served as chair of the Solar Physics Division of the American Astronomical Society and scientific editor of the Astrophysical Journal. His research interests are in the general area of astrophysical fluid dynamics and magnetohydrodynamics, especially as applied to the Sun. In addition to his theoretical work, he has carried out several observational studies of sunspots at the National Solar Observatory.