Moon mav radio cosmic ravs’ bieeest hits Now and then, ultra-high-energy cos-

mic rays from space slam Earth’s atmo- sphere with energies too huge to be ex- plained. Scientists suspect they may have exotic origins, such as yet-to-be-dis- covered relics of the Big Bang. They rank among the major puzzles of astrophysics (SN: 8/15/98, p. 101).

Only a handful of such rays has been detected, but researchers hope to soon observe greater numbers of them with a supergiant detector-the moon. Building on theories from the 1960s, scientists sus- pect that ultra-highenergy particles strik- ing the moon may interact with lunar soil to produce detectable bursts of mi- crowaves. A new experiment simulating that burst-generating effect in a giant sandbox suggests that a moon-based de- tector might just work.

“We’ve shown that the effect is real,” says David Saltzberg of the University of California, Los Angeles. At the Stan- ford (Calif.) Linear Accelerator Center, Saltzberg and his colleagues fired gamma rays into 3.5 metric tons of sand and measured a strong microwave signal com- ing out. Their findings appear in the March 26 PHysicAL REVIEW L E ~ .

In the sand, gamma rays initiate cascades of particleejecting reactions, Saltzberg explains. Similar chain reactions should take place in lunar soil or other

materials such as Antarctic ice. Cosmic rays may be protons, neutrinos, or other particles. They strike Earth’s atmosphere or the lunar surface at nearly the speed of light and with up to 100 million times the energy that can be produced in the most powerful accelerators.

The microwaves emitted by the sand are a form of sc-called Cerenkov radiation. Although electromagnetic radiation slows

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Where the highestenergy cosmic rays graze the moon (light-gray area), microwaves may shoot Earthward.

down as it passes through many materials, including sand, highly accelerated charged particles may actually traverse the materi-

Touching legs turns shy locusts gregarious Researchers wielding artists’ paint-

brushes have tickled some insects and come up with a new insight into how a plague of locusts gets started.

Left to itself, the desert locust of biblical fame stays camouflage green and shuns company, explains Stephen Simpson of the University of Oxford in England. How- ever, when their population spikes be- cause of such factors as abundant food, shy Schistocerca gregaria locusts become yellow-and-black partygoers.

Researchers have known that touch plays a central role in converting reclu- sive locusts into crop-destroying mobs. Now, Simpson and his colleagues say that touching one part in particular-the fe- mur of the hind leg-triggers the shift. The other 10 parts the researchers stroked didn’t evoke gregarious behavior, they report in the March 27 PROCEEDINGS OF THE NATIONAL ACALXMY OF SCIENCES.

Elizabeth A. Bernays of the University of Arizona in Tucson welcomes the new find- ing as “a fascinating detail.” As she puts it, “Imagine if sitting on a crowded train with thighs touching made people gregarious or made their skin change color.”

The paper “opens up the whole field of neurophysiology,” Bernays adds. Narrow- ing the sensitive area should help find the specific nerves and chemical signals driv-

MARCH 31,2001

ing the dramatic shift. That shift separates a true locust from

the rest of the grasshoppers, Simpson ex- plains. About a dozen species around the world switch from recluses to swarmers. “You hear millions of beating wings,” he says. ‘‘It’s an assault on all the senses.”

His lab tested possible cues for the switch. Solitary locusts didn’t respond much to air blown from a swarm or to the sight of a swarm they couldn’t touch. However, a locust in a cage with a ball rolling back and forth-to simulate bump ing from other locusts-changed the in- sect’s behavior. After 4 hours, such a lo- cust given a choice either to move toward a crowd or to retreat to a hideaway opted to congregate. Changes in color and subtle shifts in body shape followed more slowly.

Touch-sensitive hairs cover the desert locust’s body, so Simpson’s lab tried to localize the effect, an effort he derides as “mind-bogglingly tedious.” Researchers poked fine-tipped paint- brushes through cage mesh to stroke a body part every 60 seconds for 4 hours on a total of 170 insects.

The hind legs “are a very good place to have [a crowd sensor]” remarks entomol- ogist David Hunter at the Australian Plague Locust Commission in Canberra. He points out that a single locust going

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a1 faster than the electromagnetic radia- tion does. When this happens, Cerenkov radiation is emitted. Scientists already rou- tinely use visible forms of this radiation to detect neutrinos in large underground tanks of water (SN 1/30/99, p. 76).

The Stanford experiment confirmed that invisible, microwave Cerenkov radiation does exist. What’s more, compared with ordinary cosmic rays, ultra-high-energy cosmic rays elicit particularly intense and detectable bursts of microwave Cerenkov radiation, according to the researchers. “That’s why the lunar observations ought to be possible,” Saltzberg says. So far, he notes, radio telescopes haven’t spotted the cosmic ray-induced microwaves.

The new data should also help scien- tists refine models of microwave Cerenkov radiation, says George M. Frichter of Flori- da State University in Tallahassee.

Neutrino astronomy is a closely related area that may benefit from the newly measured effect. Recently, a decadelong effort to build a Cerenkov-light-detecting neutrino telescope has finally begun to pay off. In the March 22 NATURE, Francis Halzen of the University of Wisconsin-Madison and his colleagues report the first neutrino detections by their array of photomulti- plier tubes embedded in Antarctic ice (SN: 3/27/99, p. 207). In the same spot, Frichter and his colleagues have placed microwave detectors that may glimpse higherenergy neutrinos that the photomultiplier tubes can’t pick up. -P Weiss

The shy green Form OF a desert locust nymph (right) can switch into a multicolored crowd lover ifjostled.

about its daily business of eating and hopping doesn’t bump the outside of its rear legs as much as when it jostles around in a crowd. The real benefit of the new research will be in refining scientists’ models for predicting locust behavior, Hunter says.

Grasshopper ecologist Gregory Sword of the U S . Department of Agriculture in Sidney, Mont., adds that the shift from solitary to gregarious behavior offers in- sights into warning colors.

He and Simpson have shown that desert locusts don’t seem affected by their bud- dies’ colors. The researchers propose that the bright hues warn predators away. Lc+ custs often have gutfuls of plants that other animals reject. To a predator, there- fore, locusts “are like Twinkies, except with a filling of noxious plants,” Sword says. The loud color makes a fine warning label.

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