The Science of Discworld IV

something was obviously going wrong with CGRO. Except – it wasn’t. The observatory was functioning perfectly. Somehow, the Earth’s atmosphere was generating gamma rays.
    At first, these rays were thought to be generated about 80 kilometres up, well above the clouds. It had just been discovered that strange glowing lights, known as sprites and resembling huge jellyfish, existed at that height. They are thought to be an unexpected effect of lightning in thunderclouds below. At any rate, it seemed clear that sprites must be producing the gamma rays, or at least, associated with them. Theoreticians produced several explanations; the most plausible was that avalanches of electrons produced by lightning were colliding with atoms in the atmosphere, generating both the sprites and the gamma rays. The electrons could move at almost the speed of light and create a chain reaction in which each electron could kick others out of atoms.
    From 1996 onward, physicists added bells and whistles to this theory, predicting the energy spectrum of the gamma rays. Data from CGRO fitted these predictions, and confirmed that the rays originated at very high altitudes. It all looked pretty good.
    Until 2003.
    That year, Joseph Dwyer was in Florida, on the ground, measuring x-rays from lightning, and he observed a huge burst of gamma rays from the storm clouds overhead. The burst had exactly the same energy spectrum as those that were thought to come from much higher. Even so, no one really imagined that the rays that CGRO was detecting came from thunderclouds: they were much too energetic. The energy needed to propel the rays through an atmosphere was too large to be credible.
    In 2002 NASA had launched a satellite called RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) to observe gamma rays from the Sun. David Smith hired a student, Liliana Lopez, to look through the data for evidence of gamma rays from the Earth. There was a burst every few days, far more often than CGRO was detecting. This new instrument provided far better information about the energy spectrum, and it showed that these gamma rays had traversed a lot of atmosphere. In fact, they originated at altitudes ofabout 15-25 kilometres – the tops of typical thunderclouds. As new evidence piled up, it became ever harder to deny that thunderstorms generate gamma rays in huge quantities. Sprites, on the other hand, do not.
    How do thunderclouds produce such energetic radiation? The answer is straight out of
Star Trek
: antimatter. When ordinary matter and antimatter come together, they annihilate each other in a burst of energy – almost total conversion of mass to energy. Antimatter powers Starfleet’s vessels. Its commonest form is the positron, the anti-electron, which is naturally produced by radioactive decay and is routinely used in medical PET scanners (Positron Emission Tomography). However, naturally produced antimatter is rare, and thunderclouds are not renowned for their radioactive atoms. Nevertheless, there is strong evidence that gamma rays from thunderclouds involve positrons.
    The idea is this. The electric field inside a cloud is negative at the bottom and positive at the top. This field can sometimes generate runaway electrons with high energies. Being negatively charged, these electrons are repelled by the field at the bottom of the cloud and attracted by that at the top, so they go upwards. They then hit atoms in air molecules and create gamma rays. If such a ray hits another atom, it can produce a positron-electron pair. The electron keeps going upwards, but the positron, having a positive charge, goes
downwards
, attracted by the field at the bottom of the cloud. On the way down it bumps into an air atom and knocks out new electrons … and the process repeats. Again there is a kind of chain reaction, which spreads sideways, across entire banks of storm clouds.
    It’s a bit like a naturally formed laser, in which cascades of photons shuttle to and fro between mirrors, triggering the production of ever more photons as they do so – until they get so energetic that they escape through one of the mirrors. The mirrors are the top and bottom of the cloud, but instead of bouncing photons to and fro, the cloud sends electrons up and positrons down. By 2005 thistheory was pretty much firm. The Fermi Gamma-ray space telescope has now detected beams of charged particles, produced by thunderclouds and travelling thousands of miles along the