Donald L. Savage Headquarters, Washington, D.C. December 6, 1993 (Phone: 202/358-0883) Embargoed until 3 p.m. EST Jim Doyle Jet Propulsion Laboratory, Pasadena, Calif. (Phone: 818/354-5011) RELEASE: 93-217 ULYSSES REACHES UNEXPLORED REGIONS OF SUN The Ulysses spacecraft -- on its way to explore the polar regions of the sun --has become the first spacecraft to reach further south than the most southerly dip of the sun's magnetic equator. In this previously unexplored region, Ulysses observed that the fast-moving stream of charged particles, called the solar wind, is twice as fast, but less dense, than near the sun's equator. Measurements from the Ulysses' solar wind plasma experiment showed wind speeds of about 2 million miles per hour (800 kilometers per second) in this previously uncharted region of space, twice the speed at which the solar wind is known to flow in lower latitudes. The sun's magnetic equator is tilted and also characterized by a sheet of current extending into space. The current sheet rotates with the sun and has folds like the skirt of a whirling ballerina. Ulysses now is south of the folds of this current sheet. "This change in speed coincided with the spacecraft's passage south of a sheet of electrical current," said Dr. Edward J. Smith, NASA project scientist on the joint NASA-European Space Agency (ESA) mission. "This current sheet separates the solar wind that originates in the northern solar hemisphere from that originating in the southern hemisphere." In this sunspot cycle, magnetic fields are directed outward from the sun in the northern hemisphere and inward back towards the sun in the southern hemisphere, separated by the current sheet. Ulysses now has observed this change of direction, providing proof that its position is now below the sun's magnetic equator, according to Dr. Richard Marsden, ESA project scientist. "Far south of the sun's equator and well out of the equatorial region, Ulysses is observing magnetic field lines pointing inward only," Marsden said. - more - - 2 - A pair of magnetometers, each able to measure the magnetic fields above the sun's poles, will continue to measure the strength and direction of these field lines as Ulysses nears the region above the sun's southern pole. Ulysses also has observed the presence of shock waves emanating through this unexplored region of space. When a fast solar wind stream pushes against a slow flowing wind, a shock wave may be generated. A "forward" shock continues in the direction of the overtaking fast wind, while a so-called "reverse" shock propagates in the opposite direction. Shock waves in the solar wind are somewhat analogous to the acoustic waves, or sonic booms, generated in Earth's atmosphere when an airplane flies faster than the speed at which sound waves can travel. A lot of energy is concentrated in the compressed region close to the shock and charged particles passing through a shock can acquire part of this energy. "Acceleration at shocks is an important process, believed responsible for many of the high energy charged particles in the universe," Smith said. At low solar latitudes, within the domain occupied by the wavy current sheet, the interaction of fast and slow solar wind is a common occurrence because of the rotation of the sun, but it is not obvious that this shock- generating mechanism would be found at high solar latitudes. As Ulysses began to escape the wavy current sheet, quite a few reverse shocks, but very few forward shocks, were observed. "This interesting new observation is explained by noting that, because the current sheet is tilted with respect to the sun's equator, outward-propagating forward shocks travel toward the equator, while reverse shocks travel poleward," Smith said. Shock waves are believed to be responsible, too, for the acceleration of hydrogen, helium and certain other atoms which enter the solar system as low velocity neutrals from interstellar space which become ionized or charged in the solar wind. Eventually, they reach energies at which they appear as "anomalous" cosmic rays. Acceleration to cosmic ray energies was generally believed to take place in the outermost regions of the heliosphere. However, Ulysses has for the first time recorded acceleration of these particles by shock waves -- suggesting, Smith said, "that a significant increase in the energy of anomalous cosmic rays may take place much closer to the sun than was earlier believed." The spacecraft, launched from the Space Shuttle Discovery in October, 1990, begins its primary mission in June 1994, when it begins to pass over the sun's southern pole. - more - - 3 - Ulysses is measuring the magnetic fields and waves and ionized gas flow from the sun, particle radiation, radio waves, x- and gamma rays and dust in its highly inclined elliptical 5-year orbit around the sun. It is now about 45 degrees south solar latitude and will reach over 80 degrees south latitude for the first time next fall, and a maximum northerly latitude in 1995. Ulysses will be more than 100 million miles from the sun at its closest approach. The experiment teams responsible for these recent discoveries included the solar-wind plasma experiment, led by Dr. John Phillips of Los Alamos National Laboratory; the magnetometer experiment, led by Dr. Andre Balogh of Imperial College, London; and the solar wind-ion composition experiment, led by Professor Johannes Geiss of the University of Bern and Dr. George Gloeckler of the University of Maryland. New information about this region and other findings from Ulysses are featured today in 41 research papers presented at the 1993 fall meeting of the American Geophysical Union in San Francisco. The Jet Propulsion Laboratory manages the U.S. portion of the Ulysses mission for NASA's Office of Space Science, Washington, D.C. - end -