Even more complicated than expected
The Sun's magnetic field appears to have a different shape than has been assumed for many years. With the help of a new mathematical model, some hitherto inexplicable observations could be described. Evidence is mounting that the Sun's magnetic field resembles a wild cyclone rather than a fancy lawn sprinkler, as has been the accepted scientific notion for nearly 40 years. The cyclone-like shape derives from a mathematical model first proposed last year by University of Michigan space scientist Len Fisk. The latest evidence to support Fisk's model can be found in a study published in the January 1, 2007 issue of the Journal of Geophysical Research - Space Physics. Released November 1997. The results of this study will also be presented at the American Geophysical Union meeting in San Francisco in December.
According to Fisk, understanding the sun and its surroundings is of great importance. "One of the links between the Sun and the Earth is the solar wind and the magnetic fields it carries. The energy it carries can affect the Earth's atmosphere, especially at higher latitudes." In addition, the sun's magnetic field influences cosmic rays, i.e. high-energy particles from space. Fisk's team is now trying to figure out what their model predicts about the interactions between cosmic rays and the Sun's magnetic field.
Previously, scientists believed that the sun's magnetic field lines spiraled, a bit like water ejected from a rotating lawn sprinkler. This view dates back to a 1958 paper by Eugene Parker of the University of Chicago. When Russian and American satellites began measuring the Sun's magnetic field in the 1960s, the first measurements agreed with Parker's model. However, data from the joint NASA-ESA mission Ulysses (the first mission to fly past the sun's poles) revealed inconsistencies that scientists could not explain with this model.
In particular, this model could not explain why Ulysses found particles with low energy charges near the sun's poles. Scientists knew these particles came from near the equator, but couldn't explain how they got to the poles. Just as railroad cars can only travel on rails, these particles mostly only travel on magnetic field lines. Consequently, they can only travel from the equator to the poles if the magnetic field lines follow the same path. However, according to Parker's model, field lines originating from the poles always remain at the poles. Likewise, field lines originating from the equator remain at the equator. There are no "tracks" for these particles to travel from one latitude to the other.
In Fisk's model, however, field lines pass through different latitudes, thus providing a route for the low-energy particles. Fisk developed the new concept by combining several well-known phenomena in new ways. First he recalled something scientists have known for almost 100 years, namely that the sun's poles rotate more slowly than the equator. Then he considered the fact that the Sun's magnetic field is constantly, but not uniformly, expanding. Finally he noticed that the axis of the magnetic field is slightly offset from the axis of rotation. These effects combine to cause magnetic lines to cascade from lower to higher latitudes.
In the new paper, Fisk and his colleagues Thomas Zurbuchen and Nathan Schwadron present data from Ulysses showing that the Sun's magnetic field depends on latitude exactly as predicted by the Fisk model.
In other unpublished work, the team shows how the model can also explain another observation that has been puzzling. This is about the solar wind, a continuous stream of particles emanating from the sun. The Ulysses observations revealed two types of solar wind, differing in speed, composition and source. One type, the slow solar wind, originates from around the Sun's equator. The other type, the fast solar wind, originates at higher latitudes. The researchers propose that the differences between the two types of solar wind can be explained by the reorganization of the Sun's magnetic field predicted by Fisk's model.
Team members acknowledge that their model - and the interpretation of the data that they believe supports it - is controversial. And that, they say, is a good thing.
"This is science as it should be," says Fisk."Someone observes a phenomenon, and that leads to thinking about things differently than before. Then you make a theory out of it that says what it's about, and people start poking around at it. You look at the evidence you observe and examines them. Then the model will no doubt change-models always do, after all."
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