Chasing the earth's magnetic tail.

The earth's magnetic field, chock full of charged particles trapped on its lines of force, would resemble a giant apple spinning on its axis--were it not for the sun. As the far larger particle stream, called the solar wind, "blows" by from the sun, it drags earth's magnetic field lines out into a vast "tail," stretching into space for millions of kilometers. It is a strange domain, where the incoming particles are somehow pumped up to electrical energies far higher than those with which they arrived, yet where some of those ions, instead of being swept away in the flow, are transported back "upstream" toward the earth.

The tail's rampant exotica, however, are almost entirely invisible -- detectable to spacecraft that fly through them, yet frustratingly inaccessible to scientists trying to stand back and look at the big picture.

Enter A,PTE, the international suite of satellites (the Active Magnetospheric Particle Tracer Explorers) that made headlines two days after Christmas when it generated a cloud of barium ions in space to produce the first artificial comet (SN: 1/5/85, p. 6). The geomagnetic tail is right down AMPTE's alley. Flying through the tail on March 21, a German satellite called the Ion Release Module (IRM) cast forth another barium cloud, while the U.S. Charge Composition explorer monitored the results from closer to earth Numerous ground-based observers in New Mexico, Arizona, Massachusetts, Alaska, Hawaii, Argentina and other locales, as well as aboard two aircraft flying over the Pacific, stood by with telescopes and other instruments to photograph the effects of the invisible tail on AMPTE's visible cloud, which was essentially serving as a tracer, like a dye marker in a stream.

One problem for the AMPTE researchers trying to decide whether to go ahead with the barium release was that they could not know in advance about the condition of the interplanetary magnetic field that transports the solar-wind particles that ultimately populate earth's magnetic tail. Until the summer of 1982, a satellite named ISEE-3 was stationed on the sunward side of the earth, where it could report on the oncoming solar wind to give as much as an hour's advance notice. But ISEE-3 (since renamed the International Comet Explorer) is now on its way to a September flight through the tail of Comet Giacobini-Zinner, leaving the sunwatching post unattended and AMPTE's planners little to go on but theoretical predictions. And indeed, the field conditions within the tail itself at the time of the cloud's release turned out to be less than optimum: A key index of magnetic activity dropped to almost zero, says Stamatios Krimigis of the Johns Hopkins Applied Physics Laboratory in Laurel, Md., and the cloud "kind of just sat there."

And yet, notes principal investigator Gerhard Haerendel of the Max Planck Institute for Extraterrestrial Physics in Munich, there was still plenty to see. Fortunately, Clear skies meant good viewing for the widespread observing teams (poor visibility had previously delayed the artificial comet experiment until the last possible day).

The cloud first appeared with the characteristic light-greenish hue of neutral barium, turning purplish as the barium became ionized. It was formed by detonating two barium-filled canisters released from the IRM satellite. The expanding cloud of barium ions, as expected, pushed out the earth's magnetic field lines to create a "cavity" that ultimately reached a diameter of about 560 kilometers, according to Haerendel. The IRM's instruments detected the cavity's presence for about 7 minutes, until the field lines returned to their undisturbed position and essentially flattened it out of existence. After the first couple of minutes or so, the photos show the expanding cloud developing a diffuse outer portion that lasted for about 35 minutes, well after the cavity was gone. This outer portion was made of barium ions too, so why did it keep growing even as the cavity shrank? "If I knew the answer to that," says Paul Bernhardt of Los Alamos (N.M.) National Laboratory, "I'd write a paper."

Also, Haerendel notes, the cavity's appearance developed a pattern of small, cellular features similar to the "granularity" seen in some images of the sun. "I think," he says of the earth's-tail version, "that nobody has ever seen such a thing," suggesting that AMPTE's data could aid studies of the solar corona and photosphere. The barium cloud also showed an irregular "edge," possibly indicating where it was pushed in and pulled out by "flux tubes" of the magnetic field.

Additional "clouds" in earht's magnetic tail may be attempted late this month. And in June, another artificial comet.