December 3, 1997
In a paper to appear in the December 4 issue of Nature, John H. Thomas and collaborator Benjamin Montesinos of Madrid's Laboratory for Space Astrophysics and Fundamental Physics present a more realistic version of the siphon-flow model, which predicts how gas flows from sunspots into the solar atmosphere. The results of their model match in considerable detail the observations reported by another research group in the September 4 issue of Nature. Thomas calls this recent observational evidence "the clincher" in support of the siphon-flow model.
The work could also offer insights into other astrophysical processes that involve strong magnetic fields and jets of gas, such as when stars form or die -- an area that's the focus of much research. "Unlike most gas or plasma flows in space, sunspots can be observed in great detail," says Thomas, former chairman of the Solar Physics Division of the American Astronomical Society and a scientific editor of the Astrophysical Journal. "They can serve as a test bed for our understanding of similar flows throughout the universe."
Indeed, some astrophysicists think that siphon flows analogous to those in sunspots may occur on a galactic scale.
Sunspots have mystified humans ever since Galileo's first telescopic observations in 1611. Sunspots mark areas on the Sun's surface where the star's magnetic field becomes so intense that a buoyant tube of magnetism literally pops through the Sun's surface. The magnetic field disrupts the outward convection of heat, resulting in dark Earth-sized splotches that are some 2,500 degrees Celsius cooler than the rest of the solar surface. As many as 30 sunspots may be visible on the Sun's surface at any given time; they last anywhere from one hour to three months.
Sunspot activity exhibits an 11-year cycle, with the number of sunspots next expected to peak in 2000 or 2001. Periods of high sunspot activity also usher in an increase in the number of solar flares -- intense bursts of magnetic energy hurling energetic particles out from the Sun. When these flares reach the Earth's magnetic field, they can wreak havoc with electrical lines, communications satellites, and even automatic garage door openers.
The solar puzzle that has absorbed Thomas is the Evershed flow, a flow of gas emanating from sunspots that has perplexed astrophysicists for nearly 90 years. The September observations, published by researchers from the Canary Islands Institute of Astrophysics and the High Altitude Observatory in Boulder, Colorado, demonstrate -- as Thomas first hypothesized in 1988 -- that the flow follows along arched magnetic field lines that emerge from within the sunspot and then dive back down below the solar surface near its outer edge.
"An ionized gas like that in the Sun's atmosphere flows strictly along lines of magnetism," says Thomas, a professor of mechanical engineering and of astronomy. "The magnetic field in a sunspot is shaped roughly like a sheaf of wheat -- with the base rooted beneath the Sun's surface and each stalk representing a discrete tube of magnetic flux. The original siphon- flow model held that gas would flow outward along field lines extending well beyond the sunspot, but little of this outward flow was ever detected outside the sunspot. Now we know that most of the flow returns into the Sun at the outer edge of the sunspot, along low-lying, arched magnetic field lines."
The original siphon-flow model was first proposed in 1968 by Friedrich Meyer and Hermann Schmidt of the Max Planck Institute for Astrophysics. But Thomas, who spent a year in the early 1970s working with Meyer and Schmidt in Munich, realized in the mid-1980s that the early siphon-flow model had some shortcomings.
"The original theory did not accurately describe the flow of gas in the photosphere, the region of the solar atmosphere closest to the sun's surface," Thomas says. "It assumed that the flowing gas had no effect on the configuration of the magnetic field, but this is not true in the photosphere, where most of the flow occurs."
Since Thomas' 1988 modifications to Meyer and Schmidt's theory, he has become the siphon-flow model's main proponent, penning a half-dozen scientific papers -- most of them with Montesinos -- that have steadily filled in the model's details.
Thomas' sunspot research is funded by NASA.