Big gamma-ray flare from star disturbs Earth's ionosphere
On Dec. 27, 2004, scientists detected the largest gamma-ray burst ever recorded. It came from a magnetar—a neutron star with an enormous magnetic field—50,000 light years away. Its powerful rays penetrated deep into the ionosphere, the electrically conductive layer encircling Earth. On Feb. 19 in St. Louis at the annual meeting of the American Association for the Advancement of Science (AAAS), Stanford electrical engineering Professor Umran Inan described what scientists learned from this rare and dramatic atmospheric disturbance.
"Enormous gamma-ray flares—such as this giant flare from magnetar SGR 1806-20—affect our lower ionosphere to such a massive degree that by simply watching and measuring its response to and recovery from the flare, we are bound to learn more about the dynamics of these upper atmospheric regions, which are ultimately so important for our quantitative understanding of space weather, as well as communication and navigation systems," said Inan in a recent interview. With more than 250 technical publications, he is a pioneer in the discovery of atmospheric electrical phenomena known as "elves" (horizontally expanding discharges at high altitudes), "red sprites" (diffuse blobs that begin at the base of the ionosphere) and "blue jets" (branches that shoot up from cloud tops).
His talk was part of a symposium, "A Giant Flare from a Magnetar: Blitzing the Earth from Across the Galaxy," that included astrophysicists Kevin Hurley of the University of California-Berkeley, David Palmer of Los Alamos National Laboratory, Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics and Lynn Cominsky of Sonoma State University.
For the astrophysicists, the colossal flare is a window into the workings of a neutron star. They observed the gamma-ray flare using two orbiting spacecrafts and will use new knowledge about the event to hone their theories about these distant objects.
Inan, in contrast, employs Earth-based equipment to measure very-low-frequency (VLF) radio waves that remotely detect ionospheric effects produced by lightning discharges, including precipitation of high-energy electrons from the Van Allen belts, and luminous high-altitude discharges such as elves, sprites or jets. He and his VLF research group in Stanford's Space, Telecommunications and Radioscience Laboratory (a.k.a. STAR Lab) continuously monitor the ionosphere for localized effects. They didn't expect to see the massive effect the flare had on the ionosphere, illuminating an entire half of the global ionosphere, but their vigilance enabled them to capture it nonetheless. "In the course of our studies of ionospheric effects produced by lightning of this type—sprites, elves and electron precipitation—this effect basically fell on our lap," Inan said.
Probing the 'ignore-o-sphere'
Solar wind is an important component of space weather. When the sun acts up through flares and coronal mass ejections, it transmits streaming energetic particles toward the Earth's magnetosphere, greatly increasing the fluxes of energetic electrons trapped in the Van Allen radiation belts and also causing large changes in the Earth's ionosphere. The ionosphere is the highest region of the upper atmosphere, which is maintained by ionization of neutral air by solar photons and cosmic rays. When cosmic rays hit the two nitrogen atoms bound together in a nitrogen gas molecule, the molecular components separate into positively charged nitrogen atoms and negatively charged electrons.
"At higher altitudes, there isn't enough air for ionized molecules to combine and become neutral again, so the region stays ionized," Inan said. "That's what the ionosphere is."
It's 60 to 90 kilometers up, where the atmospheric drag is too great for satellites to orbit. At the same time, the air is too thin for aircraft or research balloons. Ionization is too weak to provide detectable echoes for even the largest radars.
"This region has been called the 'ignore-o-sphere' because it isn't an easy region to measure," Inan said. "The VLF technique that we have developed is particularly suitable for looking at this altitude range, which is not otherwise measurable."
The Van Allen belts consist of energetic electrons trapped in the Earth's magnetic field, which extends into space and shields the Earth from cosmic radiation.
"One of the things that lightning does is to remove electrons from the radiation belts," Inan explained. Without electron removal, the belts would become more and more intense, he said.
When lightning flashes on the Earth's surface, it launches electromagnetic waves up to the Van Allen radiation belts. En route, the electromagnetic waves interact with energetic electrons trapped along the magnetic field lines of the Earth in the Van Allen belts and scatter these electrons into the ionosphere. Interactions between the electromagnetic waves change the energy and direction of momentum of the electrons, causing them to precipitate from the belts as a result of energy input by lightning discharges. The precipitating electrons in turn produce patches of enhanced electricity in the ionosphere.
Scientists detect these localized disturbances with VLF radio waves propagating along the Earth's surface. The ionosphere, like a metal, is a good electrical conductor. It acts as a guide for radio waves. That's why the Earth's curvature is no barrier, as VLF radio waves bounce off the ionosphere and can propagate to long distances around the globe, in the so-called Earth-ionosphere wave guide.
"[Guglielmo] Marconi discovered global communication because of reflections from the ionosphere," Inan explained. "Back at the turn of the century, he sent signals from England to the United States by reflecting them from the ionosphere."
Inan and his colleagues record VLF radio waves that propagate from one or more transmitters on the Earth's surface to 25 receivers whose locations include Japan, France, Israel, Greece, Turkey, Hawaii, Midway and Kwajalein islands, the continental United States, Alaska and Antarctica. The transmitters launch waves that propagate in the wave guide formed between the surface of the Earth and the ionosphere.
When lightning strikes, it launches an electromagnetic signal that propagates through the ionosphere to the Van Allen belts, and can propagate from one hemisphere to another along the Earth's magnetic field lines. When played through a loudspeaker, these signals have a distinct sound and are called whistlers.
At night, lightning has a notable effect in causing ionization at altitudes of 60 to 90 kilometers, as a result of precipitation of energetic electrons by whistler waves. During the day, however, solar ionization drowns out the effects of lightning-induced electron precipitation and renders them negligible.
For 2007, "The International Heliospheric Year," the United Nations has launched an initiative aimed at deployment of Inan's inexpensive receivers in every member country so even scientists and students in developing nations can host receivers, post data and access this rich data set. Efforts are currently under way for fundraising from private foundations to facilitate this global educational and outreach effort.
Flares hit Earth in 1998 and 2004
In a 1999 issue of the journal Geophysical Review Letters, Inan and his STAR Lab colleagues reported the ionospheric effects of a giant gamma-ray flare from another star. It occurred on Aug. 27, 1998, in the middle of the night (as recorded at Stanford in the Pacific Daylight Time zone), but it ionized the atmosphere to levels usually found only during daytime.
Like a lighthouse whose spinning beam hits a specific point on shore at regular intervals, this neutron star had a periodicity. It spewed gamma rays every 5.16 seconds. "We observed the ionosphere respond to that," Inan said. "The ionosphere was in fact pulsating at night."
The star responsible for the 2004 burst was about the same distance as the star responsible for the 1998 burst but was within 5 degrees of the sun as viewed from Earth. Therefore its gamma rays arrived on the day side of our planet. Neither star's gamma rays reached the Earth's surface, according to Inan. Neither flare posed a danger to people, he said.
"The amazing part for the new [daytime] event is even during daytime, even in this solar-illuminated ionosphere, the effect of the flare was huge," said Inan. "It was much, much more intense than the sun in terms of producing ionization."
Scientists didn't observe the ionosphere pulsating with the 2004 burst, although they did see that the gamma rays arrived in pulses. "Because the gamma rays were on the solar, day side, of the ionosphere, we didn't see the periodicity," Inan said. "We saw a massive effect that created new ionization." The pulsing was at lower levels than the initial peak and was drowned out by solar ionization, he said.
More powerful and brighter than the nighttime flare, the daytime flare pumped 1,000 times as much energy into the atmosphere, Inan said. "There's nothing like this [the magnetar that delivered flares in 2004], I understand from my astrophysics colleagues, in our part of the woods—in other words, near our galaxy," Inan said. If there was, he says, we would be inundated with gamma rays, which are high-energy X-rays from which the atmosphere shields us by creating ionization. "If the flare was intense enough, then it would penetrate—the atmosphere couldn't hold it."
The 2004 flare was brighter and more energetic than the sun but lasted for just a brief period. It ionized the atmosphere down to an altitude of 20 kilometers (about 50,000 feet), just above where airplanes fly. (Solar photoionization is not effective at such low altitudes because the atmosphere is too thick, Inan said.) Its most intense effects in ionizing the atmosphere (called the "peak") lasted a few seconds. The second-most-intense effects (the "oscillating tail") lasted five minutes. And the least intense effects (the "afterglow") lasted an hour.
The flare changed the ionic density at an altitude of 60 kilometers from 0.1 to 10,000 free electrons per cubic foot—an increase of six orders of magnitude. Normally, it takes hundreds of seconds for the ionosphere to recover from the electromagnetic waves launched by lightning.
"The remarkable thing is that it took an hour for it to come back from this disturbance," Inan said. "It's a very unusual event and was three orders of magnitude more intense than the  one, which we thought was very intense."
Of detectors, both satellite and planetary
In 1988, the journal Nature published a report of the first observed effect of a gamma-ray burst on the ionosphere by Inan and Gerald Fishman of NASA's Marshall Space Flight Center. On Dec. 27, 2004, it was Fishman who contacted Inan to tell him that a satellite had detected a colossal burst. The satellite's detector was designed to identify high-energy X-rays and gamma rays from the sun. That day, the detector counted a huge amount of gamma rays, became saturated and stopped counting. When impinging gamma rays from the flare began to wane, the detector began to count again.
During the period the detector wasn't counting, the astrophysicists had no data. But Inan's group, continuously monitoring VLF waves propagating across the planet to measure the ionosphere, did, and therefore had data to share. "Our response continued because the Earth of course didn't saturate. The Earth is much too large a detector to saturate," Inan said.
The 1998 gamma-ray flare had a low-energy component, Inan said. While satellite detectors look for high-energy rays (20keV and above), the earthly VLF system could detect low-energy rays as well.
"Our modeling told us that without presuming a low-energy component that was in place that was missed by our colleagues in the spacecraft measurements, we couldn't explain the ionospheric effect," Inan said. "That's not the case for this new event. For this new event, we are able to explain the ionospheric disturbance using the fluxes and energies that people have measured on spacecraft. So this particular flare might be different from the previous flare in terms of its energy content. Not in terms of its intensity—which has to do with the number and energy of photons— because we know that this new one is much more intense; but in terms of the energy of photons, the previous one may have had low-energy photons as well as higher-energy photons."
The scientists also saw for the first time a phenomenon—an intense, short-burst (less than a second), low-frequency signal—that they don't yet understand. To better understand the phenomenon, Inan will model the ionosphere and make a wish on a star—but it's a star rarer than a blue moon: "We are going to see whether we can get an effect like this theoretically. But we don't have it yet. And of course another event would be very useful."