RELEASE NO: 98-69

ACE FIRST SCIENCE PUZZLES SOLAR RESEARCHERS

The first results from NASA's Advanced Composition Explorer (ACE) spacecraft are challenging the current understanding of the acceleration of particles by explosions on the Sun.

The researchers, led by Dr. Edward C. Stone of the California Institute of Technology (Caltech), Pasadena, Calif., used explosive solar events, called flares, to sample matter from the Sun with the ACE spacecraft. Solar flares that occurred between Nov. 5 and 7, 1997, shot particles from the Sun in quantities that differed from predictions. Some of the elemental isotope abundances which were observed differed from those which were predicted, implying that either the model for particle acceleration efficiency by solar flares is incorrect, that the solar element abundances are different than expected, or that some other phenomenon is affecting the measured abundances. Since the majority of the material in the solar system is contained in the Sun, studying the solar composition can tell researchers about the history and evolution of the Solar System.

"Trying to unravel the history and evolution of solar system material is comparable to trying to estimate how much gold is in a mountain by standing at the bottom and measuring the gold concentration in a stream flowing down from the top. You pick up other stuff along the way. Similarly, in the Solar System, geologic, chemical and nuclear processes all affect the original material composition," said ACE Project Scientist Dr. Jonathan F. Ormes of NASA's Goddard Space Flight Center, Greenbelt, Md.

The six high-resolution spectrometers on the ACE spacecraft determine the composition of particles of solar, local interstellar, and galactic origin, with collecting powers 10 to 1,000 times greater than previously available. Three other instruments provide real-time data to the NOAA space weather alert system.

The increased resolution of the instruments on ACE allows scientists to distinguish between the various isotopes of elements. The identity of an element is determined by the number of positively-charged protons in its atomic nucleus. The positive charge is generally balanced out by the atom's negatively-charged electrons. But these electrons are stripped off at the extremely high temperatures around the Sun, leaving a positively-charged ion which can be accelerated in the Sun's magnetic field.

In addition to the protons in an atomic nucleus, there are also neutrons. Each neutron has about the same mass as a proton, but no charge. The number of neutrons in an atomic nucleus determines the isotope. For a particular element, heavier isotopes result in ions which are less positively charged in relation to their mass. With this relationship between electric charge and mass, scientists are able to make predictions about the abundances of the various isotopes which are accelerated during a solar flare such as those which occurred Nov. 5 through 7.

ACE was launched on Aug. 25, 1997, and arrived at the L1 libration point in December. L1 is the point where the gravitational pulls of the Earth and Sun balance, keeping ACE at an ideal location for its observations, far outside the Earth's magnetic field and with a clear view of charged particles in the solar system. Its mission is expected to continue for at least five years, to the next maximum in solar activity.

ACE has begun its mission to provide high resolution observations of particles in space, providing new information about the origin of the elements, the formation of the Sun's corona and the acceleration of the solar wind, and allowing the detection of impending interplanetary geomagnetic storms which can affect satellite communications and electric power lines on Earth.

The ACE mission is managed by NASA Goddard, and the lead scientific institution is the California Institute of Technology (Caltech), Pasadena, Calif. The spacecraft was built by the Applied Physics Laboratory of Johns Hopkins University in Laurel, Md.

Information on the ACE spacecraft and science can be found at:   http://www.srl.caltech.edu/ACE/