Evidence of elusive ion source in space.

Evidence of elusive ion source in space

In a 1954 book about the formation of planetary systems, Hannes Alfven hypothesized that gas atoms having no electric charge can ionize -- acquire an electric charge -- by simply speeding through space. Ionized clouds formed by material ejected from a satellite last month now provide the strongest evidence yet for Alfven's much-sought phenomenon.

The Swedish physicist suggested that as an atom in space passes through a magnetic field, it can ionize -- provided its speed across a magnetic field "line" produces more than enough kinetic energy to strip an electron off the atom. (Stripping off one or more electrons produces a positively charged ion.) Alfven termed the required speed an atom's "critical ionization velocity" (CIV).

When ionized, gas atoms glow, rendering them visible not only to instruments on the ground and in aircraft, but also, in large numbers, to the naked eye. Unlike electrically neutral atoms, ions are susceptible to capture by Earth's magnetic field and by electric fields in space. Scientists studied the ionization of neutral atoms in space (usually by sunlight) several times during the 1980s. They used gas clouds ejected from sounding rockets -- most often as a way to probe the structure of these fields.

But a few tests were conducted expressly to test the phenomenon of critical ionization velocity. Researchers released powdered metals from rockets. Heat from the detonation of explosive charges provided the energy needed both to vaporize the ejected metals into a gas and to accelerate those gases beyond the critical ionization velocity. However, because the explosives' heat might also have induced some ionization, no one ever established how much, if any, of the ionization observed resulted from the proposed process.

Though many sounding rockets propel their clouds of neutral gas into sunlight, because sunlight quickly ionizes the neutral atoms, the rocket-launched CIV experiments attempted to limit ionization from such alternative sources by dispersing their gas clouds into darkness.

Two scientists from the University of Alaska in Fairbanks have for the first time conducted CIV-ionization experiments from an Earth-orbiting satellite. The results now indicate fairly conclusively that Alfven's CIV effect indeed exists, according to Eugene M. Wescott and Hans C. Stenbaek-Nielsen, who directed the tests.

Four canisters of powdered metal for their tests were carried into space aboard the Combined Release and Radiation Effects Satellite (CRRES), launched July 25. As in the sounding-rocket tests, barium, calcium and strontium powders were chosen for their ability to glow at visible wavelengths when ionized.

On Sept. 10, the researchers released canisters of strontium and barium; on Sept. 14, two more with calcium and barium. Each canister was propelled into space by a spring. About 20 minutes later, its powder was vaporized with explosives.

The gases were ejected from the canister when the satellite was in the fastest segment of its orbit, speeding at about 9.5 kilometers per second -- well in excess of the CIV for all three metals studied, Wescott observes. He and Stenbaek-Nielsen timed each canister's release so that its gas molecules were ejected in darkness. Free from any solar-induced effect, any glow recorded was probably due to Alfven's CIV effect, the researchers reasoned.

CRRES program scientist David Reasoner of NASA's Marshall Space Flight Center in Huntsville, Ala., is enthusiastic about the results but cautions that scientists may yet discover other explanations -- such as collisions with oxygen atoms in Earth's upper atmosphere. Wescott, Stenbaek-Nielsen and others are beginning detailed analyses of the CRRES data for evidence of non-CIV-initiated ionizations.