NASA's Fermi Mission brings deeper focus to thunderstorm gamma rays

Each day, thunderstorms around the world produce about a thousand quick bursts of gamma rays, some of the highest-energy light naturally found on Earth. By merging records of events seen by NASA's Fermi Gamma-ray Space Telescope with data from ground-based radar and lightning detectors, scientists have completed the most detailed analysis to date of the types of thunderstorms involved.

"Remarkably, we have found that any thunderstorm can produce gamma rays, even those that appear to be so weak a meteorologist wouldn't look twice at them," said Themis Chronis, who led the research at the University of Alabama in Huntsville (UAH).

The outbursts, called terrestrial gamma-ray flashes (TGFs), were discovered in 1992 by NASA's Compton Gamma-Ray Observatory, which operated until 2000. TGFs occur unpredictably and fleetingly, with durations less than a thousandth of a second, and remain poorly understood.

In late 2012, Fermi scientists employed new techniques that effectively upgraded the satellite's Gamma-ray Burst Monitor (GBM), making it 10 times more sensitive to TGFs and allowing it to record weak events that were overlooked before.

"As a result of our enhanced discovery rate, we were able to show that most TGFs also generate strong bursts of radio waves like those produced by lightning," said Michael Briggs, assistant director of the Center for Space Plasma and Aeronomic Research at UAH and a member of the GBM team.

Previously, TGF positions could be roughly estimated based on Fermi's location at the time of the event. The GBM can detect flashes within about 500 miles (800 kilometers), but this is too imprecise to definitively associate a TGF with a specific storm.

Ground-based lightning networks use radio data to pin down strike locations. The discovery of similar signals from TGFs meant that scientists could use the networks to determine which storms produce gamma-ray flashes, opening the door to a deeper understanding of the meteorology powering these extreme events.

Chronis, Briggs and their colleagues sifted through 2,279 TGFs detected by Fermi's GBM to derive a sample of nearly 900 events accurately located by the Total Lightning Network operated by Earth Networks in Germantown, Maryland, and the World Wide Lightning Location Network, a research collaboration run by the University of Washington in Seattle. These systems can pinpoint the location of lightning discharges -- and the corresponding signals from TGFs -- to within 6 miles (10 km) anywhere on the globe.

From this group, the team identified 24 TGFs that occurred within areas covered by Next Generation Weather Radar (NEXRAD) sites in Florida, Louisiana, Texas, Puerto Rico and Guam. For eight of these storms, the researchers obtained additional information about atmospheric conditions through sensor data collected by the Department of Atmospheric Science at the University of Wyoming in Laramie.

"All told, this study is our best look yet at TGF-producing storms, and it shows convincingly that storm intensity is not the key," said Chronis, who will present the findings Wed., Dec. 17, in an invited talk at the American Geophysical Union meeting in San Francisco. A paper describing the research has been submitted to the Bulletin of the American Meteorological Society.

Scientists suspect that TGFs arise from strong electric fields near the tops of thunderstorms. Updrafts and downdrafts within the storms force rain, snow and ice to collide and acquire electrical charge. Usually, positive charge accumulates in the upper part of the storm and negative charge accumulates below. When the storm's electrical field becomes so strong it breaks down the insulating properties of air, a lightning discharge occurs.

Under the right conditions, the upper part of an intracloud lightning bolt disrupts the storm's electric field in such a way that an avalanche of electrons surges upward at high speed. When these fast-moving electrons are deflected by air molecules, they emit gamma rays and create a TGF.

About 75 percent of lightning stays within the storm, and about 2,000 of these intracloud discharges occur for each TGF Fermi detects.

The new study confirms previous findings indicating that TGFs tend to occur near the highest parts of a thunderstorm, between about 7 and 9 miles (11 to 14 kilometers) high. "We suspect this isn't the full story," explained Briggs. "Lightning often occurs at lower altitudes and TGFs probably do too, but traveling the greater depth of air weakens the gamma rays so much the GBM can't detect them."

Based on current Fermi statistics, scientists estimate that some 1,100 TGFs occur each day, but the number may be much higher if low-altitude flashes are being missed.

While it is too early to draw conclusions, Chronis notes, there are a few hints that gamma-ray flashes may prefer storm areas where updrafts have weakened and the aging storm has become less organized. "Part of our ongoing research is to track these storms with NEXRAD radar to determine if we can relate TGFs to the thunderstorm life cycle," he said.

Video: https://www.youtube.com/watch?v=JgK4Ds_Sj6Q#t=66

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Flood gate in Thai capital focus of fear, rivalry (Reuters)

BANGKOK (Reuters) – Authorities in the Thai capital repaired a damaged flood gate on Wednesday that has become the focus of anger, fear and rivalry between arms of government battling the country's worst floods in decades.

The central government led by Prime Minister Yingluck Shinawatra, sister of the ousted populist premier Thaksin Shinawatra, is at odds with the city government dominated by the main opposition and former ruling Democrat party.

The floods that have killed 427 people since July are the first big test for Yingluck, who came to power in a July poll many Thais hoped would heal divisions that last year brought violent clashes in Southeast Asia's second biggest economy.

Inner Bangkok, protected by a network of dikes and sandbag walls, survived peak tides at the weekend and is mostly dry.

But huge amounts of water are bottled up to the north, west and east of the city, and new areas are being flooded daily as the water tries to find its way out to sea to the south.

Anger is seething in flooded communities on the wrong side of inner Bangkok's flood barricades.

Residents of the northeastern Bangkok suburb of Sam Wa took matters into their own hands this week and hacked away at the side of a canal flood gate, aiming to let the water flow out of their area toward the city center.

Yingluck ordered the gate opened in the face of the residents' demands. The Bangkok government objected on the grounds that the flow could endanger the city center.

But the city had to comply with Yingluck's order to open the gate by a meter (three feet), leading to fear among inner city residents that the disaster they thought they had dodged was looming again.

On Wednesday, city officials and workers went to the Sam Wa flood gate to repair the damage and limit the amount of water flowing through.

"We are here doing the repair work and the police are protecting us," said city administration spokesman Jate Sopitpongstorn.

"They have to accept it," he said of the neighborhood's residents. Several hundred policemen were on hand and there were no protests.

City governor Sukhumbhand Paribatra, watching workmen with heavy machinery fix the gate, played down the political clash and said everyone had to cooperate.

But, referring to the central government's change of heart and order to open the gate, he said everyone should stick to decisions.

"COMPLICATED SYSTEM"

The floods began in July, at the beginning of a particularly heavy rainy season.

Economic growth has been hit and investor confidence shaken as the water swamped industrial estates in the central Chao Phraya river basin, disrupting global supply lines for auto and computer parts.

Water is also approaching central Bangkok from the northern Don Muang district, where the city's domestic airport has been flooded and where one resident said the water had risen 5 cm (two inches) in his house on Wednesday.

City deputy governor Theerachon Manomaipaiboon said workers were building a wall of giant sandbags to try to stop the flow toward the city center from the north.

The wall is expected to be finished in three days but the flood is difficult to predict as it makes its way through the city's suburbs and a poorly maintained and often partly built-over network of canals and tunnels.

"We have a very complicated system. Water in one area can appear 20 km (12 miles) away," Theerachon told Reuters.

Bangkok's 12 million people account for 41 percent of Thailand's gross domestic product and neither the central government nor the city administration wants to be seen to be responsible for an inner city deluge.

Both sides will claim victory if the center can be saved.

But misery in outlying areas, especially north and west Bangkok, and provinces to the north will take the gloss off any success in the inner city, especially given a perception those areas have been sacrificed to save the well-to-do city-center.

Thonburi, on the west bank of the Chao Phraya, opposite central Bangkok's glittering Grand Palace and Chinatown, is mostly swamped with water chest-deep in places. It could be flooded for weeks, experts say.

To the north, Pathum Thani and Ayutthaya provinces have been largely inundated for weeks, along with seven industrial estates that have sprung up over recent decades on what used to be the central plain's rice fields.

Thailand is the second-largest exporter of computer hard drives and global prices are rising because of a flood-related shortage of major components used in personal computers.

Thailand is also Southeast Asia's main auto-parts maker and Japan's Honda Motor Co said car production could be difficult in the second half of its business year ending in March. Its Ayutthaya plant has suspended work indefinitely.

"The 'Motown' of Asia has become 'Waterworld' overnight," the Nation newspaper said in an editorial, referring to Thailand's position in the motor industry.

(Editing by Paul Tait)

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