Magnetic activity: a flare for research.

Like giant accelerators in the sky, the arching magnetic fields that pierce the upper atmosphere of stars can unleash vast amounts of energy Colliding, breaking apart, and reconnecting, the fields accelerate charged particles high in the atmosphere, triggering an explosive brightening, or flare, near the visible surface of the star below. New observations of the sun and of a nearby Milky Way star reveal the profound role that magnetic fields and protons play during a flare - and in its high-energy afterglow.

For years, researchers have speculated about how magnetic energy released in a star's upper atmosphere, or corona, heats the lower depths, where visible-light flares occur. Some scientists proposed that beams of electrons in the corona, excited by the magnetic fields, rain down on the star and carry the energy Others suggested that proton beams could transport the energy more efficiently

In 1976, two U.S. astronomers predicted that if downward-moving proton beams were indeed the carriers, these particles would collide with hydrogen atoms to produce a brief but telltale type of ultraviolet radiation. At a press conference last week, researchers announced that the Hubble Space Telescope had detected such radiation from a Milky Way flare star called AU Microscopium.

Located 30 light-years from Earth, this star undergoes flares many times more intense than those on the sun. Hubble's Goddard High-resolution Spectrograph monitored the star for some three hours last Sept. 3, but it detected enhanced emission of a particular wavelength of ultraviolet light for a mere 3.2 seconds - just at the onset of a flare, when astronomers had predicted a higher intensity would occur.

According to theory, as protons stream down from the corona, they knock into hydrogen atoms, robbing each of its solitary electron. Each electron then pairs with a proton to form a new, downwardmoving hydrogen atom. The newly created atoms briefly emit a type of ultraviolet light called Lyman-alpha emission.

And because the atoms are moving toward the surface of the flare star, away from Earth, the ultraviolet light they radiate appears to have shifted to a slightly redder, or longer, wavelength. Bruce E. Woodgate of NASA's Goddard Space Flight Center in Greenbelt, Md., reported that Hubble measured redshifted Lyman-alpha emissions as the flare began, indicating that hydrogen atoms were streaming toward the star at about 1,800 kilometers per second.

Woodgate notes that the finding doesn't preclude the possibility that electron beams also carry some of the energy unleashed by magnetic activity But the observations hint that protons serve as the dominant energy carrier in AU Microscopium - and perhaps in other stars, including the sun.

In June 1991, the Compton Gamma Ray Observatory (GRO) recorded high-energy emissions from the sun during a month of spectacular flares (SN:6/22/91, p.388). The findings, also reported last week, indicate that magnetic fields in the corona help create an afterglow of gamma rays and neutrons for many minutes to hours after flares begin. James M. Ryan of the University of New Hampshire in Durham announced that GRO's EGRET telescopes detected solar gamma rays for more than five hours after a flare began on June 11, 1991. Four days later, GRO's Compton Telescope detected gamma rays and neutrons for more than 90 minutes after the onset of another flare.

The findings, says Ryan, support the theory that some protons get trapped in the magnetic arches of the corona, rattling back and forth inside a kind of magnetic slinky Earth's Van Allen radiation belts trap protons in a similar way, he adds. As the protons slowly leak out of the slinky they strike atoms near the solar surface, accounting for the extended neutron and gamma-ray emissions.

In imaging the spray of neutron particles from the sun, the Compton Telescope made astronomical history The blurry picture marks the first time that researchers have used neutrons to image any celestial object. The spectra of neutrons, Ryan adds, may provide more information than gamma rays about the energy of the trapped protons and the nature of the magnetic fields that accelerated them.