Sept. 29, 1994
Vol. 14, No. 3

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    Simpson's 'eyes': Over the poles of the sun

    , graphic Ulysses experiment latest in lifetime of research When the spacecraft Ulysses reached the peak of its historic climb over the south pole of the sun on Sept. 13, it marked the summit of a lifetime of research into cosmic rays for John Simpson, the Arthur Holly Compton Distinguished Service Professor Emeritus in Physics.

    Simpson, who has supervised the design, construction and operation of more than 35 space experiments in the past 35 years -- including nine spaceprobe encounters with planets and two with comets -- is the principal investigator for the Cosmic and Solar Particle Investigation (COSPIN), one of 11 experiments on board Ulysses.

    Cosmic rays -- the nuclei of atoms emanating from distant sources in our galaxy and traveling at nearly the speed of light -- hold a key to understanding the lives and deaths of massive stars. But because cosmic rays are deflected by magnetic fields, scientists are unable to tell where they originate. Although the results of the experiment won't be known for several months, Simpson said Ulysses' voyage over the sun's pole may offer a unique opportunity to detect cosmic rays as they stream in from outer space, before they are deflected by the magnetic fields of the sun.

    "Over the pole is where the field lines may connect into the interstellar medium -- they are like pipes channeling the lower-energy cosmic rays into the polar regions," Simpson said.

    Scientists do know that cosmic rays comprise all the natural elements from hydrogen to uranium, and by determining the relative abundances of the different elements and their isotopes, Simpson and his colleagues hope to gain insight into how the elements were formed deep within the nuclear furnaces of stars.

    Ulysses was launched in October 1990 and passed close to Jupiter in February 1992, when scientists used the giant planet's powerful gravity to sling the spacecraft out of the plane of the ecliptic -- the plane in which the planets orbit the sun -- and over the sun's south pole.

    "All of the space research up until now has been in or near the ecliptic plane," Simpson said. "Intellectually, it's like trying to describe nature while believing that the earth is flat. Now for the first time we are carrying instruments, which are our eyes and our senses, into the third dimension -- the polar regions of the solar system. And already we are changing our basic concepts and theories of how high-energy radiation reaches us from the galaxy."

    Simpson said they already have seen some surprises in the mission. "The biggest one so far is that during the Jupiter fly-by we were able to correlate charged-particle emissions with radio waves emanating from Jupiter's upper atmosphere," he said. "We saw waves of particles, beginning with electrons, then the highest-energy protons, and finally lower-energy particles. We don't know what it means, but it was extremely satisfying to be able to link the two for the first time."

    Bruce McKibben, Senior Scientist in the Enrico Fermi Institute, who has worked closely with Simpson on the Ulysses experiment, said that the numbers of cosmic rays detected by COSPIN as the spacecraft neared the pole did not increase as much as the models predicted. "There is an effect," McKibben said, "but it's much less than we expected."

    After its journey over the sun's south pole is completed, Ulysses will orbit around the sun, passing over its north pole in July 1995.

    "Now the really interesting part starts," McKibben said. "It took two and a half years to go from the plane of the ecliptic to the maximum latitude -- now within six months we'll be back at the equator, and five months later we'll be over the north pole. On the way we'll get a snapshot of how cosmic rays vary with the sun's latitude, as opposed to the time exposure we had earlier."

    These two polar passages occur during the least active period of the 11-year cycle of solar activity. Scientists hope to have funding to continue the mission through the year 2001, when Ulysses will again pass by the sun's poles, during the most active period of the solar cycle.

    -- Diana Steele