If conditions are right, the skies above America will explode with an unearthly glow tonight. At 8:43 p.m., a miniature version of the northern lights will suddenly erupt in the southern sky as a NASA research satellite begins bombing the heavens to create a luminous "artificial aurora."

Tonight's planned flare is the first of seven such events that will be visible across the United States during the next two weeks as the Combined Release and Radiation Effects Satellite (CRRES), a $170 million project of NASA and the Air Force, probes the workings of the Earth's magnetic field.

Approximately every other night until Jan. 25, the satellite will eject an explosive canister over South America at altitudes ranging from 3,000 to 21,000 miles, well above all but the faintest wisps of the Earth's atmosphere. Each blast will create a cloud of metallic vapor hundreds of miles wide that will "paint" the otherwise invisible lines of the Earth's magnetic field so that they can be studied with a combination of ground- and air-based sensors.

Because the moon will be down, tonight's flash "will certainly be the brightest feature in the sky," said Rick Howard, NASA's CRRES project manager. In Washington, the flash -- which may appear nearly as large as a full moon -- will be visible to the southeast at 53 degrees elevation above the horizon, if predicted partial cloud cover does not obscure it. It will first appear as a point of light, then expand into a greenish sphere that will turn blue or purple before dissipating in about 15 minutes. In the process, it may take the shape of "streaks" that follow the planet's magnetic field lines.

Interactions between that field and the "solar wind," a torrent of electrically charged particles that constantly streams off the sun, bring on magnetic "storms" thousands of miles above the Earth's surface. The field draws the charged particles, or ions, toward the Earth's magnetic poles, where they collide with atoms, causing them to glow and producing the familiar shimmering curtains of the aurora borealis (and its southern counterpart, the aurora australis). But they also cause severe disruptions in radio signals, including defense and satellite communications, and can even induce dangerous current spikes in power lines.

"To improve the effectiveness of future communications and space systems," said CRRES project scientist David Reasoner, scientists must learn to anticipate such disruptions more accurately. "The better we understand the links in that chain, the better we can predict solar weather."

The CRRES is designed to do that by creating, in effect, an artificial version of the aurora. The high-altitude satellite contains 24 canisters, each about the size of a fire extinguisher, which hold a thermite explosive and one of two metallic chemicals that become ionized when exposed to solar radiation. Of the seven January releases, three will contain lithium, an extremely light metal that produces a red glow visible for only 5 minutes because it expands rapidly, around 3 miles per second; and four will be barium, a heavier metal that produces a much denser ion cloud that turns green and purple, remaining visible for about 15 minutes.

The dates and times for each will depend on the condition of the magnetic field and the readiness of the monitors. They will be announced up to 30 minutes in advance. Callers can hear a NASA recorded message with updated information by telephoning 205-544-5356.