"Long before it's in the papers"
January 18, 2011

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Thunderstorms produce antimatter, scientists find

Jan. 10, 2011
Courtesy of NASA
and World Science staff

Sci­en­tists us­ing NASA’s Fer­mi Gamma-ray Space Tel­e­scope have de­tected beams of an­ti­mat­ter, a rare and bi­zarre mirror-image form of or­di­nary mat­ter, above thun­der­storms on Earth.

The phe­nom­e­non has nev­er been seen be­fore, re­search­ers say. An­ti­mat­ter is pract­i­cally non­ex­ist­ent on Earth, where it can be found only as a re­sult of cer­tain exotic pro­cesses which create, then quickly de­stroy it.

“These sig­nals are the first di­rect ev­i­dence that thun­der­storms make an­ti­mat­ter par­t­i­cle beams,” said Mi­chael Briggs, a mem­ber of Fer­mi’s Gamma-ray Burst Mon­i­tor team at the Uni­vers­ity of Al­a­bama in Hunts­ville. He pre­sented the find­ings Mon­day, dur­ing a news brief­ing at the Amer­i­can As­tro­nom­i­cal So­ci­e­ty meet­ing in Se­at­tle.

Most of the particles that are the bas­ic, sub­a­tom­ic build­ing blocks of mat­ter have an “an­ti­mat­ter” coun­ter­part: a par­t­i­cle that is ex­actly alike, ex­cept that it has an op­po­site elec­tri­cal charge. Charge comes in two forms, pos­i­tive and neg­a­tive, which at­tract each oth­er. The an­ti­mat­ter par­t­i­cles de­tected in the new ex­pe­ri­ments are pos­i­trons, ti­ny car­ri­ers of pos­i­tive charge. They are an­ti­mat­ter coun­ter­parts to elec­trons, the bas­ic charge car­ri­ers with­in atoms, which have neg­a­tive charge and are re­spon­si­ble for the cur­rents in ord­inary elec­tri­cal cir­cuits.

When an an­ti­mat­ter par­t­i­cle meets with its mat­ter coun­ter­part, they de­stroy each oth­er in a flash of en­er­gy. An­ti­mat­ter par­t­i­cles in the uni­verse ap­pear to be far out­num­bered by or­di­nary mat­ter par­t­i­cles, though sci­en­tists are un­sure why. 

None­the­less, cer­tain phys­i­cal pro­cesses can cre­ate an­ti­mat­ter. In the case of thun­der­storms, sci­en­tists think the an­ti­mat­ter par­t­i­cles were formed in a ter­res­tri­al gam­ma-ray flash, a brief burst pro­duced in­side thun­der­storms and shown to be as­so­ci­at­ed with light­ning. It is es­ti­mat­ed that about 500 of these flashes oc­cur daily world­wide, but most go un­de­tected.

The Fer­mi in­stru­ment is de­signed to mon­i­tor gam­ma rays, the highest-en­er­gy form of light. When an­ti­mat­ter strik­ing Fer­mi col­lides with a par­t­i­cle of nor­mal mat­ter, both par­t­i­cles im­me­di­ately are an­ni­hi­lat­ed and trans­formed in­to gam­ma rays. Fer­mi’s Gamma-ray Burst Mon­i­tor in­stru­ment has de­tected gam­ma rays with en­er­gies of 511,000 elec­tron volts, a sig­nal in­di­cat­ing an elec­tron has met its an­ti­mat­ter coun­ter­part, a pos­i­tron.

The re­search team has iden­ti­fied 130 ter­res­tri­al gam­ma-ray flashes since Fer­mi’s launch in 2008.

“In or­bit for less than three years, the Fer­mi mis­sion has prov­en to be an amaz­ing tool to probe the uni­verse. Now we learn that it can dis­cov­er mys­ter­ies much, much clos­er to home,” said Ilana Har­rus, Fer­mi pro­gram sci­ent­ist at NASA Head­quar­ters in Wash­ing­ton.

The space­craft was lo­cat­ed im­me­di­ately above a thun­der­storm for most of the ob­served flashes, but in four cases, storms were far from Fer­mi. In one case, the storm was even over the horizon, thus not directly detect­able by Fer­mi, scient­ists said. “Even though Fer­mi could­n’t see the storm, the space­craft nev­ertheless was mag­net­ically con­nect­ed to it,” said Jo­seph Dwyer at the Flor­i­da In­sti­tute of Tech­nol­o­gy in Mel­bourne, Fla. “The [flash] pro­duced high-speed elec­trons and pos­i­trons, which then rode up Earth’s mag­net­ic field to strike the space­craft.” 

Sci­en­tists long have sus­pected that the flashes arise from the strong elec­tric fields near the tops of thun­der­storms. Un­der the right con­di­tions, they say, the field be­comes strong enough that it drives an up­ward av­a­lanche of elec­trons. Reach­ing speeds nearly as fast as light, the high en­er­gy elec­trons give off gam­ma rays when they’re de­flect­ed by air molecules. Nor­mal­ly, these gam­ma rays are de­tected as a ter­res­tri­al gam­ma-ray flash.

But the cas­cad­ing elec­trons pro­duce so many gam­ma rays that they blast elec­trons and pos­i­trons clear out of the at­mos­phere, sci­en­tists said. This hap­pens when the gam­ma-ray en­er­gy trans­forms in­to a pa­ir of par­t­i­cles: an elec­tron and a pos­i­tron. It’s these par­t­i­cles that reach Fer­mi’s or­bit.

Sci­en­tists now think that all ter­res­tri­al gam­ma-ray flashes emit elec­tron-pos­i­tron beams. A pa­per on the find­ings has been ac­cept­ed for pub­lica­t­ion in the journal Geo­phys­i­cal Re­search Let­ters. “We still have to fig­ure out what is spe­cial about these storms and the pre­cise role light­ning plays,” said Ste­ven Cum­mer at Duke Uni­vers­ity.

* * *

Scientists using NASA’s Fermi Gamma-ray Space Telescope have detected beams of antimatter, a rare and bizarre mirror-image form of ordinary matter, above thunderstorms on Earth. The phenomenon has never been seen before, researchers say. Antimatter is almost nonexistent on Earth. “These signals are the first direct evidence that thunderstorms make antimatter particle beams,” said Michael Briggs, a member of Fermi’s Gamma-ray Burst Monitor team at the University of Alabama in Huntsville. He presented the findings Monday, during a news briefing at the American Astronomical Society meeting in Seattle. Most of the basic, subatomic building blocks of matter have an “antimatter” counterpart: a particle that is exact ly alike, except that it possesses an opposite electrical charge. Charge comes in two forms, positive and negative, which attract each other. The antimatter particles detected in the new experiments are positrons, tiny carriers of positive charge. They are antimatter counterparts to electrons, the basic charge carriers within atoms, which have negative charge and are responsible for the currents in our electrical circuits. When an antimatter particle meets with its matter counterpart, they destroy each other in a flash of energy. Antimatter particles in the universe appear to be far outnumbered by ordinary matter particles, though scientists are un sure why. Nonetheless, certain physical processes can create antimatter. In the case of thunderstorms, scientists think the antimatter particles were formed in a terrestrial gamma-ray flash, a brief burst produced inside thunderstorms and shown to be associated with lightning. It is estimated that about 500 of these flashes occur dai ly worldwide, but most go un detected. The Fermi instrument is designed to monitor gamma rays, the highest-energy form of light. When antimatter striking Fermi collides with a particle of normal matter, both particles immediate ly are annihilated and transformed into gamma rays. Fermi’s Gamma-ray Burst Monitor instrument has detected gamma rays with energies of 511,000 electron volts, a signal indicating an electron has met its antimatter counterpart, a positron. The research team has identified 130 terrestrial gamma-ray flashes since Fermi’s launch in 2008. “In orbit for less than three years, the Fermi mission has proven to be an amazing tool to probe the universe. Now we learn that it can discover mysteries much, much closer to home,” said Ilana Harrus, Fermi program scientist at NASA Headquarters in Washington. The spacecraft was located immediate ly above a thunderstorm for most of the observed flashes, but in four cases, storms were far from Fermi. In addition, lightning-generated radio signals detected by a global monitoring network indicated the on ly lightning at the time was hundreds or more miles away. During one flash, which occurred on Dec. 14, 2009, Fermi was located over Egypt. But the active storm was in Zambia, some 2,800 miles to the south. The distant storm was below Fermi’s horizon, so any gamma rays it produced could not have been detected. “Even though Fermi couldn’t see the storm, the spacecraft nevertheless was magnetic al ly connected to it,” said Joseph Dwyer at the Florida Institute of Technology in Melbourne, Fla. “The [flash] produced high-speed electrons and positrons, which then rode up Earth’s magnetic field to strike the spacecraft.” Scientists long have suspected that the flashes arise from the strong electric fields near the tops of thunderstorms. Un der the right conditions, they say, the field becomes strong enough that it drives an upward avalanche of electrons. Reaching speeds near ly as fast as light, the high energy electrons give off gamma rays when they’re deflected by air molecules. Normally, these gamma rays are detected as a terrestrial gamma-ray flash.. But the cascading electrons produce so many gamma rays that they blast electrons and positrons clear out of the atmosphere, scientists said. This happens when the gamma-ray energy transforms into a pair of particles: an electron and a positron. It’s these particles that reach Fermi’s orbit. Scientists now think that all terrestrial gamma-ray flashes emit electron/positron beams. A paper on the findings has been accepted for publication in Geophysical Research Letters. “We still have to figure out what is special about these storms and the precise role lightning plays in the process,” said Steven Cummer at Duke University.