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
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.
