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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'Standing on a comet': Rosetta mission will contribute to space weather research

A comet-bound spacecraft that's been in sleep mode for more than two years is scheduled to wake up on the morning of Jan. 20 -- beginning the home stretch of its decade-long journey to a mile-wide ball of rock, dust and ice.

If all goes as planned, Rosetta -- a European Space Agency-led mission that involves University of Michigan engineers and scientists -- will be the first craft to actually land on a comet as well as track it for an extended period of time.

The Philae lander will latch on to the core of comet 67P/Churyumov-Gerasimenko in November and the orbiter will operate until the end of 2015. No mission has ever attempted such an in-depth look at one of these relics of the earliest days of our solar system.

Engineers at U-M's Space Physics Research Lab built electronic components for an onboard instrument that's believed to be the most sensitive of its kind ever flown in space. And a team of researchers will be involved in the mission science as well.

While most of the big questions Rosetta aims to answer deal with the origin and evolution of the solar system, U-M scientists will make a unique contribution that could provide very practical insights into how the sun and planets interface today.

They'll analyze measurements taken at the comet to study solar wind interactions that can lead to solar storms. The solar wind is a stream of charged particles emanating from the sun. Solar storms are bursts of activity that can threaten astronauts and damage Earth's satellites and electric grid.

"How the solar wind operates is one of the biggest outstanding questions about the solar system today. By studying how it interacts with cometary gases, we can learn a lot about the composition of the solar wind," said Tamas Gombosi, the Rollin M. Gerstacker Professor of Engineering in the Department of Atmospheric, Oceanic and Space Sciences.

Gombosi and his research group are leaders in the field of space weather. A model they developed was recently adopted by the national Space Weather Prediction Center.

At the sun's equator, the wind travels rather slowly, Gombosi said. It moves faster at high latitudes. Interactions between the two varieties can lead to magnetospheric storms. Earth orbits near the equator, so it's hard to study the fast wind from our vantage point.

"But comets pass through all of it. With their help, we can study the fast solar wind," Gombosi said.

Gombosi and other U-M researchers will be involved in additional Rosetta goals. They'll study and simulate how quickly the comet outgases material from its nucleus to its tail as it rings around the sun. They'll be involved in examining what elements are in the comet's tail, atmosphere and ionosphere, as well as how fast the electrified particles in the ionosphere are traveling.

Michael Combi, the Freeman Devold Miller Collegiate Research Professor in the Department of Atmospheric, Oceanic and Space Sciences, is a co-investigator on several instruments. He'll look into the rate at which the comet's core is sublimating, or turning from a solid into a gas, and he'll also work on a team that's analyzing those gases. They'll explore the levels of carbon monoxide and carbon dioxide, for example. They can't detect carbon dioxide from Earth.

"It's very difficult to observe some of the chemical species when they're far away and faint. Carbon dioxide is probably the second most abundant species at most comets, but it's not been observed in the thousands we've looked at from Earth," said Combi, who has studied comets for more than 30 years.

Comets -- small rock and ice bodies -- were present in the nebula that spawned the solar system and have been orbiting ever since in far away, cold belts either just past the orbit of Neptune or a quarter of the distance to the nearest star. For scientists, they're archeological artifacts that help them understand how the solar system formed and evolved. They're believed to have delivered Earth's oceans and perhaps the seeds of life in organic materials.

"People use the analogy that it's been in the freezer for the past 4.5 million years and brought in for convenient study. So we're looking as much as we can at the way the way the solar system was 4.5 billion years ago," Combi said.

Comet 67P/Churyumov-Gerasimenko is one of the smallest bodies humans have ever tried to land on. Its gravity is about 1,000 times less than that of Earth.

"On the lander, there's a camera that can look straight down like you're standing up and looking at the ground. Then there's a panoramic camera that can look out and see a picture of the horizon. It'll be fun to see what this landscape looks like," Combi said. "It'll be like standing on a comet."

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