Observations Support New Model Of Sun's Magnetic Field

November 10, 1997

ANN ARBOR--Evidence is mounting that the sun's magnetic field looks more like a wild cyclone than a tidy lawn sprinkler--the image scientists had accepted for almost 40 years. The cyclone-like shape comes from a mathematical model first proposed last year by University of Michigan space scientist Len Fisk.

The latest pieces of evidence in support of Fisk's model are in a study published in the Nov. 1 issue of the Journal of Geophysical Research--Space Physics. Results of the study will also be presented in December at the American Geophysical Union meeting in San Francisco.

Understanding the sun and its surroundings can have important implications, says Fisk. "One of the coupling mechanisms between the sun and the Earth is the solar wind and the magnetic fields that are contained in it. The energy transmitted by this mechanism can influence the Earth's atmosphere, particularly at high latitudes." The sun's magnetic field also affects galactic cosmic rays--very high energy particles from outer space. Fisk's team is now looking to see what their model predicts about interactions between cosmic rays and the sun's magnetic field.

Until Fisk's model, scientists thought the sun's magnetic field had a spiral shape, something like water twirling from a rotating lawn sprinkler. That notion came from a paper published in 1958 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 were consistent with Parker's model. But data from the joint NASA/European Space Agency mission Ulysses--the first mission to fly over the poles of the sun--revealed inconsistencies that scientists couldn't explain using that model.

In particular, the model couldn't explain why Ulysses found low-energy charged particles near the sun's poles. Scientists knew that the particles originated near the sun's equator, but couldn't understand how they could get to the poles. Like railroad cars that can run only on tracks, these particles tend to travel only on magnetic field lines. The particles can't move from the equator to the poles unless the magnetic field lines also follow that path. But in Parker's model, field lines that originate at the poles stay at the poles, and field lines that originate at the equator stay at the equator. There are no "tracks" for particles to travel from one latitude to another.

In Fisk's model, field lines pass through different latitudes, providing a route for the low-energy particles. He came up with the new configuration by combining several well-known phenomena in a new way. He first considered something scientists have known for almost 100 years--that the sun's poles rotate more slowly than its equator. Then he took into account the fact that the sun's magnetic field is constantly expanding, but not uniformly. Finally, he also noted that the axis of the sun's magnetic field is offset slightly from the sun's axis of rotation. These effects all combine to make magnetic field lines that swoop from low to high latitudes.

In the new paper, Fisk and research fellows Thomas Zurbuchen and Nathan Schwadron present data from Ulysses showing that the sun's magnetic field exhibits just the kinds of latitudinal variations that Fisk's model predicts.

In other work not yet published, the team shows how the model can account for another previously puzzling observation, this one about the solar wind--a continuous flow of particles streaming out from the sun. Observations from Ulysses revealed two kinds of solar wind which differ in speed, composition, and source. One type, the slow solar wind, originates near the sun's equator. The other, fast solar wind, is made at higher latitudes. The researchers argue that the observed differences in the two types of solar wind can be explained by the continuous reorganization of the sun's magnetic field that Fisk's model predicts.

The team admits that their model--and their interpretation of the data they believe support it--are controversial. And that, in their opinion, is a good thing. "This is science at its best," says Fisk, who is professor and chair of the U-M Department of Atmospheric, Oceanic and Space Sciences and was NASA's associate administrator for space science and applications before coming to the U-M in 1993. "Someone observes a phenomenon, and that causes you to think about things in different ways. Then you put out a theory that says what it is, and people poke at that. You look at the observational evidence and test it out. The model will undoubtedly change -- models always do."
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University of Michigan

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