[from Forefronts, Newsletter of the Cornell Theory Center, Volume 9, Number 3, Winter 1994: "Computational Science and Engineering Research Group: Profiles of Member Research, Part II", by Melissa Jacobs, Science Writing Intern, Cornell University]
Solar Convection---Steven R. Lantz
The newest member of the Strategic Applications consulting group at the Theory Center is Steven R. Lantz, an astrophysicist by training. Lantz is not a newcomer to the Theory Center, however, having been a user of the facility almost from its beginning. At Cornell, he has been a National Science Foundation postdoctoral research associate, holding a joint appointment with the Center for Radiophysics and Space Research (CRSR) and with the Theory Center's Computational Science and Engineering Research Group (CSERG), and before that, a graduate student in the field of applied physics, receiving his doctorate from Cornell University in January 1992. His Ph.D. dissertation described aspects of magnetic field dynamics within the solar convection zone, a line of research that he continued during his postdoctoral work. In his new position, he plans to explore the dynamics of solar convection still further, and also to use his experiences to help others, as he moves his applications to the IBM SP1 parallel environment.
In collaboration with Peter J. Gierasch, director of CRSR, Lantz studies fundamental fluid dynamics problems inside the sun's convection zone, such as magnetoconvection, the interaction of convection with magnetic fields, a phenomenon occurring in most stars. Magnetoconvection can be indirectly observed at the solar photosphere, but it really extends deep inside the sun. In general, lab experiments cannot be used to study this large-scale, high- temperature phenomenon. "Numerical simulation is practically the only way we have to find out about the interior of the sun," he said.
Since the field of numerically simulated magnetoconvection is still in its infancy, the numerical experiments are idealized, Lantz said. In nearly all cases, researchers substitute for the whole sun an idealized piece of the star, which shows the simplified dynamics of a fluid flowing with magnetic fields. Often, though, by neglecting magnetic fields in a simulation, researchers treat the ionized plasma as if it were a simple fluid like water, a move that risks ignoring the local structural effects of things like sunspots, and ionization zones, he said. But such simplifications may be necessary to make the problem tractable even on very fast computers; a 2D simulation can run for a prohibitively long time, while a 3D simulation "exponentiates the problem" by increasing the amount of data storage (i.e., grid points) by a large factor, he said. Thus, scientists working on 3D simulations have had to wait days to weeks for results. "Three-dimensional simulations generate enormous amounts of data that are too cumbersome even for most mass storage systems. Three-dimensional is not unfeasible, but if one wants to tackle it, one should be ready to adopt a pioneer mentality---and a parallel computing system," he explained.
Lantz is now wrapping up a collaboration with Cornell engineering professor Ravindra N. Sudan by writing a series of papers, with an accompanying video, about magnetoconvection dynamics in a 2D stratified layer. Most convection models deal with perturbations that are a small departure from constant background levels, Lantz said. In contrast, Lantz's and Sudan's model has a stratified background in which the pressure and temperature are not constant with depth. In the model, density can vary by more than a factor of 20, for instance. A similar model could be used for the Earth's atmosphere, he said. Although the stratified model is closer to true sun conditions, it is not as realistic as compressible simulations, which involve sound waves. These waves' high speed of propagation is precisely characteristic of compressional disturbances, he said. But taking into account sound waves complicates the simulations, since smaller time steps are needed to resolve the rapidly-moving waves. If sound waves are not energetically important, they can be self-consistently ignored, a tactic that allows simulations to be more efficiently produced, he said. This is the situation that arises if the bulk motions are occurring at a low Mach number; then the sound waves can be ignored because they should not influence the overall dynamics. The computational speedup the model provides is then tremendous.
In addition to his work on magnetoconvection, Lantz is currently developing a new 2D simulation code that takes ionization into account. So far, his models have involved ideal gases, which consist of electrons and ions moving so fast that they do not interact much. But in reality, the sun's gases ionize and recombine continuously, which has a significant impact on the thermodynamics of the system, he said. Lantz continually tries to address real-world questions in his research. While simulations must be idealized, Lantz would rather "try to talk about the real sun and answer questions about what we observe and don't understand."
Although Lantz currently focuses on solar convection, his interests include related areas of physics. In November 1992, for example, Lantz traveled to Seattle, Washington, for a meeting of the American Physical Society's Division of Plasma Physics. There he discussed surprising similarities between his models of solar convection and some recent research into the physics of controlled thermonuclear fusion in a device called a "tokamak." Although nuclear fusion research has yet to be successful in generating economically useful power, it is an intense and still-promising field of research, Lantz said.
Over the next few years, Lantz hopes to address a number of questions concerning the physics and dynamics of solar convection. He is also interested in global processes on the sun, and the solar cycle, in which sunspots reach a minimum and maximum in their number every 11 years.
(Photograph of Steven Lantz)