Gamma rays make up a considerable amount of the cosmos. They can be emitted from objects within our own Milky Way galaxy or come from as far back in time as the Big Bang. Gamma rays are the most energetic kind of radiation, and relate to supermassive black holes, merging neuron stars, streams of hot gas, pulsars, and blazars. NASA scientists want to know what gives birth to the diverse spectrum of gamma rays, and that is where Fermi steps in.
Some of the mystery behind this extreme universe of high-energy astrophysics is answered through NASA’s Fermi Gamma-Ray Space Telescope. Fermi is designed to monitor gamma rays—the highest energy form of light—and help scientists solve some of the most compelling mysteries of modern astrophysics. Fermi surveys the sky every day and explores the most extreme environments in the universe.
NASA’s gamma-ray observatory orbits the Earth every 95 minutes and investigates a variety of astrophysical phenomena. Fermi’s expected lifetime is between 5-10 years. Its height is 2.9 m (9.2 ft.) with a width of 1.8 m (4.6 ft.) across spacecraft bus. It weighs 4,303 kg (9,487 lbs.). It has a download link of 40 megabits/second and 1,500 watts of power. Fermi was launched June 11, 2008.
Fermi is managed by NASA’s Goddard Space Flight Center in Greenbelt, Md. It was developed in partnership with the U.S. Department of Energy, with contributors from academic institutions and partners in Sweden, Japan, Italy, France, Germany, and the United States. It is considered an astrophysics and particle physics partnership.
Fermi monitors gamma rays. According to NASA, “[W]hen antimatter striking Fermi collides with a particle of normal matter, both particles immediately are annihilated and transformed into gamma rays.”
The Large Area Telescope (LAT) is Fermi’s primary instrument. It is viewing “an enormous 20 percent of the sky at any given time and is detecting the arrival time and direction of gamma rays broadly ranging from 20 GeV (20 billion electron volts) to 300 GeV (300 billion electron volts). The LAT’s field-of-view is four times that of its predecessor instrument, the Energetic Gamma-ray Equipment Telescope (EGRET), which operated on board the Compton Gamma-Ray Observatory (CGRO) from 1991-2000. The sensitivity of the LAT is 30 or more times that of EGRET, depending on energy.”
Gamma-ray bursts are very hard to find, even with a telescope as sophisticated as the Large Area Telescope (LAT) on the observatory. The Gamma-ray Burst Monitor (GBM) provides all-sky coverage with a variety of 12 low- and 2 medium-energy gamma ray detectors strategically places around the spacecraft in different directions. The detectors are tracking the direction and time histories of GRBs and other gamma ray sources. The GBM is detecting around 200 GRBs each year, along with solar flares and other chaotic cosmic events.
Gamma rays with energies of 511,000 electron volts had been detected by the GBM on Fermi. This happens when an electron meets a positron, its antimatter counterpart.
In January 2011, NASA Scientists using the Fermi Gamma-Ray Space Telescope detected beams of antimatter produced above thunderstorms on Earth. This had never been seen before. According to NASA: “[S]cientists think the antimatter particles were formed in a terrestrial gamma-ray flash (TGF), a brief burst produced inside thunderstorms and shown to be associated with lightning. It is estimated that about 500 TGFs occur daily worldwide, but most go undetected.”
The spacecraft, designed to uncover mysteries of high-energy events in the universe, is also providing incredible insights into the mysteries that take place close to home, like thunderstorms. Thunderstorms around the world produce about a thousand quick gamma-ray bursts.
Recently, scientists merged records of events viewed by Fermi with data from ground-based radar and lightning detectors, and in doing so they completed the most detailed analysis to date of the kinds of thunderstorms involved in the phenomena.
The GBM Instrument Operations Center resides at the National Space Science Technology Center in Huntsville, Ala. The team is a collaboration of scientists from University of Alabama in Huntsville (UAH), the Max Planck Institute for Extraterrestrial Physics in Germany, NASA’s Marshall Spaceflight Center in Huntsville, and other institutions.
Since Fermi’s launch in 2008, the GBM team identified 130 TGFs.
“Remarkably, we have found that any thunderstorm can produce gamma ray, even those that appear to be so weak a meteorologist wouldn’t look twice at them,” said Themis Chronis. Chronis led the research at the UAH.
Terrestrial gamma-ray flashes were discovered in 1992 by NASA’s Compton Gamma-Ray Observatory, which operated until 2000. At the time, TGFs still remained poorly understood. They occurred unpredictably, with durations lasting less than a thousandth of a second. In late 2012 the GBM received an upgrade that made it 10 times more sensitive to TGFs. This allowed Fermi to record weak events that may have been overlooked in the past.
Fermi is located above a thunderstorm for most observed TGFs. There were four cases where Fermi recorded details from far away. One of which occurred on Dec. 14, 2009, when Fermi was located above Egypt and detected signals from a distant storm some 2,800 miles to the south in Zambia.
“Even though Fermi couldn’t see the storm, the spacecraft nevertheless was magnetically connected to it,” said Joseph Dwyer in the 2011 NASA press release. “The TGF produced high-speed electrons and positrons, which then rode up Earth’s magnetic field to strike the spacecraft.”
Continuing past Fermi, the beam reached a mirror point where its motion was reversed and then collided with the spacecraft again 23 milliseconds later. Positrons in the beam collided with electrons in the spacecraft, which then caused the particles to annihilate each other. The emitting gamma rays are detected by the observatory’s GBM.
“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.
Chronis, Briggs, and their colleagues derived a sample of approximately 900 TGFs accurately located by the Total Lightning Network operated by Earth Networks in Germantown, Md., and the World Wide Lightning Location Network run by the University of Washington in Seattle. These systems can show the exact location of lightning discharges, as well as the corresponding signals from TGFs, to within 6 miles (10 km) anywhere on the planet.
The team identified 24 TGFs from the group that occurred within areas covered by Next Generation Weather Radar (NEXRAD). These sites were in Louisiana, Texas, Florida, Guam, and Puerto Rico. Through sensor data collected by the Department of Atmospheric Science at the University of Wyoming in Laramie, researchers obtained more information about atmospheric conditions for eight of these storms.
Scientists believe that TGFs are birthed from strong electric fields near the tops of thunderstorms. This is because updrafts and downdrafts within the storms force snow, rain, and ice to collide and become electrically charged. Positive charge accumulates at the top of the storm, and negative charge accumulates at the bottom. A lightning charge occurs when the storm’s electrical field becomes strong enough to break down the insulating properties of air.
Previous findings indicating that TGFs occur near the tops of thunderstorms had been confirmed by the new study. TGFs tend to occur between 7 and 9 miles (11 to 14 km) high.
“We suspect this isn’t the fully story,” said Briggs in the release. “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.”
Scientists turn to Fermi statistics when estimating how many TGFs occurred each day. Currently, 1,100 TGFs occur each day, but this number may be higher if low-altitude flashes are not detected.
According to Chronis, there are a few subtle clues that gamma-ray flashes may prefer storm areas where updrafts and the storm have become less stable. He and the GBM team plan to track the storms with NEXRAD to determine if they can tie TGFs to the thunderstorm lifecycle.