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Number 463, December 22, 1999 by Phillip F. Schewe and Ben Stein
THE SOLAR WIND DISAPPEARED for a day back on May 10/11, allowing Earth's magnetosphere to balloon out to the orbit of the Moon. Ironically, the greatly lowered solar wind flux of particles and solar magnetic field allowed high-energy electrons from the sun's corona to penetrate directly to our upper atmosphere unadulterate, where the electrons' characteristic x-ray emissions were observed by satellites over the North Pole for the first time. Such a "polar rain" had been predicted years before. Normally the coronal electrons (with energies of tens of keV, corresponding to temperatures of millions of degrees) lose much of their energy through scatterings with other particles on their ride from sun to Earth and in the topsy-turvy trajectories experienced at our magnetosphere. At last week's meeting of the American Geophysical Union in San Francisco, these results were reported by a number of speakers, including David Chenette of Lockheed, Jack Scudder of the University of Iowa, and Keith Ogilvie of NASA Goddard. (Images available at www-spof.gsfc.nasa.gov/istp/news/9912).
SPONGELIKE STRUCTURES NEAR THE SUN'S SURFACE, newly observed by the TRACE satellite (at extreme ultraviolet wavelengths) and the SOHO satellite (in x rays), lie between the 10,000-K chromosphere and the corona at a temperature of several million K. These filamentary structures (dubbed "solar moss" by Lockheed scientists reporting at the AGU meeting) are typically 6000-12,000 miles in size and about 1000-1500 miles above the photosphere, occur at various places around the sun's surface, usually near the footprint of huge coronal loops. The moss blobs seem to be stable for hours but can also change brightness over periods as short as 30 seconds. Thomas Berger of Lockheed said that the new structures may provide information on how the corona gets so hot, an issue that remains one of the great unsolved mysteries of solar physics.
THE RAREST NATURALLY OCCURRING ISOTOPE, tantalum-180, is rare because it is bypassed in the two processes that produced most of the heavy elements we dig out of the ground here on Earth: the so called s process (slow neutron capture in stars) and the r process (rapid neutron capture in supernova explosions). What little Ta-180 that is produced (in stars or in reactors) is quite robust; its halflife is more than 1015 years. Ta-180 is also unique in being the only naturally occurring isomer; it is essentially a nucleus in a perpetual excited state. A group of German physicists (Peter Mohr, 011-49-615-116-3221, mohr@ikp.tu-darmstadt.de), essentially working with the world's supply of this priceless substance, about 7 milligrams, try to jar the tantalum nuclei out of their customary states by shooting them with gamma photons, thus re-creating stellar conditions. They observed that depending on the temperature the Ta-180 halflife varied over a range of more than 1017! This rules out the nucleosynthesis of Ta-180 within the "canonical" s process; however, in a more realistic version of the theory, the tantalum can survive if it rapidly mixes with cooler layers of the star. (Belic et al., Physical Review Letters, 20 December 1999; Select Article.)
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