Things that go bang from below

In the 1960s Cold War paranoia lead to the discovery of the most violent explosions in the Universe. Now instruments intended to study these massive cataclysms have detected signals coming from the Earth. While not the covert nuclear tests the original satellites were originally built to identify, these signals raise a whole set of questions about the physics of some of the most violent events on our planet.

At the height of the Cold War the US, concerned about the possibility of covert Soviet nuclear tests sent up a series of satellites to look for tell-tale flashes of radiation. These satellites began seeing something strange, they saw flashes. The Soviet military scientists hadn’t been working overtime, these were signals of astronomical origin. After decades of debate, it was shown that these were caused by the explosion of the most massive stars. Since then a succession of satellites have been flown to study light at the extreme end of the electromagnetic spectrum. One such instrument is the AGILE satellite, an Italian space observatory capable of surveying large chunks of the sky at once. As well as detecting distant Gamma Ray Bursts, it has also mapped events in our own Galaxy, such as the sudden brightening of the Crab Nebula in gamma rays last year. However in a strange completion of the historical circle it has also been detecting signals from the Earth.

Terrestrial Gamma-ray flashes are associated with spectacular lightning events in the upper atmosphere similar to this red Sprite. Credit:NASA

Terrestrial Gamma-ray Flashes (TGFs) are short bursts of gamma-ray radiation. These were first noticed by the Compton satellite and are associated with thunderstorms. Strong updrafts in clouds cause the formation of layers of positive and negative charge. There are typically eased by lightning strikes removing the net charge from one or more layers. However the strong electric fields in the clouds can accelerate electrons to velocities close to the speed of light. When a fast moving electron such as this (or from a source such as a cosmic ray) interacts with another electron it can accelerate it too leading to a run-away growth of fast-moving electrons. These are then diverted by interactions with atomic nuclei in the cloud releasing “braking radiation”. As the electrons are moving so fast, this radiation takes the form of extremely energetic gamma-rays.

As the AGILE satellite passes along its orbit it is capable of detecting TGFs from below. However an orbital inclination of 2.5 degrees limits the area where the satellite can detect these flashes to close to the equator. Happily this is where some of the Earth’s largest thunderstorms happen. From June 2008 to January 2010, AGILE scientists isolated over a hundred TGFs detections. The largest number of detected events came from Africa with others being found over Indonesia and a few over South America. However when it came to associating these events with lightning events they found something striking (excuse the pun). At first glance it appeared that the distributions of Terrestrial Gamma-Ray Flashes and lightning matched rather well. But when the distributions were studied in more detail, it was found that while in South America there was an 87% chance that that TGFs came from a random sub-sampling of lightning, this probability dropped to 3% in Africa. The authors don’t go on to explain this discrepancy, but it is clear there is still something unknown about the mechanisms driving some of the most violent events on the planet.

Source
AGILE Observations of Terrestrial Gamma-Ray Flashes , M. Marisaldi et al., 2011 Fermi Symposium proceedings