First 3-D observations of sun storms made with Ulysses
Posted: November 16, 2003

The sun's surface is a violent and turbulent place, where a fiery tempest always blows. Scientists are reporting in the journal Science that they have finally succeeded in getting a good three-dimensional view of it.

"The sun is huffing and puffing and blowing off steam," said Louis Lanzerotti, a member of an international team that used the Ulysses spacecraft to make the first-ever 3-D study of our parent star during solar maximum, the peak of the sun's 11-year activity cycle. "Ulysses gave us a chance to observe the sun from unique vantage points to better understand solar storms and their consequences."

Scientists have been trying to understand solar weather for years, in an effort to better predict terrestrial consequences of solar storms. Solar storms sometimes severely disrupt wireless telephone calls, satellite communications and electric power grids on Earth.

Ulysses, launched in 1990 by the shuttle Discovery as a joint mission of NASA and the European Space Agency, has an orbit that takes it over the solar poles, giving scientists a chance to look at the sun from all angles.

"No other spacecraft can do that," said Lanzerotti, a solar physicist who divides his time between Lucent Technologies' Bell Labs, which he joined in 1965 and where he is now a consultant, and the New Jersey Institute of Technology, where he is a distinguished research professor at the Center for Solar Terrestrial Research. "Many space missions have observed the sun near its equator, but only Ulysses has traveled from the solar equator to above the sun's polar caps."

Ulysses began its first solar orbit in 1992 and completed it in 1998, a period when solar activity was at a minimum. But during the second orbit, begun in 1998, the sun was at its most turbulent.

The scientists report that, during this period, huge explosions on the sun hurled vast amounts of solar material into space. Called coronal mass ejections, since the sun's outermost layer -- the corona -- throws them off, these swirling, boiling plumes travel out from the sun and are thought to be caused by the severest of solar gales.

"We just had a coronal mass ejection last week," Lanzerotti noted. "These are some of the most violent phenomena associated with the sun. We were able to look at a few that happened around the recent solar maximum."

The team also got to observe the solar wind - the stream of charged particles that are emitted by the sun. The solar wind blows out a giant bubble called the heliosphere within the interstellar medium, the dilute gas and dust that fills the space between stars. The sun's influence extends far beyond the orbits of the outer planets and the vast reservoir of periodic comets known as the Kuiper Belt because the solar wind fills the heliosphere and exerts an outward pressure on the interstellar medium. (The boundary between the heliosphere and the interstellar medium is the true edge of the solar system, a place where a lot of interesting physical phenomena take place. Last week, a separate team of scientists, of which Lanzerotti is also a member, reported in the journal Nature that Voyager 1 has reached the edge of the solar system.)

Data from Ulysses show that the solar wind originates in holes in the sun's corona, and the speed of the solar wind varies inversely with coronal temperature.

"This was completely unexpected," said Lanzerotti. "Theorists had predicted the opposite. Now all models of the sun and the solar wind will have to explain this observation."

Another surprising finding based on Ulysses' data is that the sun's magnetic field originates from a magnet that seems to be perpendicular to the sun's axis of rotation (instead of being parallel to it, as is the case with Earth).

"At solar maximum, the sun's polar cap magnetic fields reverse direction or sign," said Edward Smith of NASA's Jet Propulsion Lab at the California Institute of Technology, who is the US project scientist for the Ulysses mission. "Inward fields become outward and vice versa. Ulysses observations show that during this reversal, the Sun's magnetic poles are located near the solar equator instead of in the polar caps."

The sun has a powerful magnetic field -- the needle of a compass placed on the sun's surface would be deflected so strongly that it would require Herculean strength to push it back. It is thought that solar activity is strongly related to changes in the sun's magnetic field.

"We knew that the sun's magnetic field was dynamic and variable," said Lanzerotti. "But this shows that we still have a lot of understanding to do. No one really knows how it is formed and why it changes as it does."

Other members of the scientific team were: R.G. Marsden (European project scientist) and M. Landgraf of the European Space Agency in the Netherlands; A. Balogh of Imperial College, London; G. Gloeckler of the University of Maryland; J. Geiss of the International Space Science Institute in Switzerland; D. J. McComas of Southwest Research Institute; R.B. McKibben of the University of New Hampshire; R. J. MacDowall of NASA Goddard Space Flight Center; and N. Krupp and H. Krueger of the Max Planck Institutes in Germany.

The team's paper, "The Sun and Heliosphere at Solar Maximum," appears in the November 14, 2003 issue of Science on page 1165.

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