From a conversation with Roland Crocker and Chris Lidman
The summer of 2014-2015 was an interesting one. I spent around two months in Canberra at the end of my honors degree, working on a eight-week summer project that somehow stretched into an entire PhD (the paper describing the work we started that summer, all five pages of it, was published this month in Monthly Notices of the Royal Astronomical Society Letters). It was also a summer where seemingly every night, intense thunderstorms would build up around Canberra, resulting in several young scientists nearly getting electrocuted while playing ultimate frisbee on more than one occasion. After two quieter summers, Canberra is again being treated to dramatic thunderstorms. The city is perfectly situated in a region where hot, dry air from the Australian interior slams into cooler, moister air driven up from the South, which, when combined with strong daytime heating, generates spectacular storm systems that tend to sweep across the city from South-West to North-East.
A typical thunderstorm contains a quantity of thermodynamic energy equivalent to around 10 15-kiloton nuclear bombs. A single bolt of lightning discharges around a million volts, with electrical currents peaking between 10,000 - 40,000 amps. A single lightning bolt heats the air to around 30,000 degrees Celsius, five times hotter than the surface of the sun. Particles in the air become instantly ionized, electrons ripped away by the immense electrical discharge, resulting in the bright flash of light. Heating of the air results in sudden expansion, which generates the sound we hear as thunder. The further away a thunderstorm is, the deeper the rumble can sound. This is because low pitched sounds tend to propagate further than high pitched sounds. If a thunderstorm is very far away, you may be aware of a very deep sound on the edge of your hearing - this is infrasound, vibrations so low pitched that we don’t perceive them as sound, almost more as a feeling. The feeling in your chest when you hear a nearby plane take off (especially something like a C-130, or even the Chinook helicopter) or at a loud concert is one way humans experience infrasound.
Thunderstorms don’t just produce optical light and sound. One way of detecting lightning is through the radio emission associated with the electrical discharge of lightning. However, as we know from astronomy, often where there is emission of radio waves from a high energy process, we also see the emission of gamma rays. In the 1990’s, NASA launched the Compton Gamma Ray Observatory to study the emission of gamma rays from the Milky Way galaxy and beyond. The BATSE instrument, which as designed to detect bright flashes of gamma rays from space, called “gamma ray bursts”, started to detect gamma ray flashes from Earth - a phenomenon called “Terrestrial Gamma Ray Flashes” or TGFs. In 1996, a Stanford university study connected the TGFs with intense thunderstorm activity and lightning flashes. More TGFs were subsequently discovered by the RHESSI satellite, named for pioneering gamma ray astronomer (and “father” of positron astrophysics) Reuven Ramaty.
The detection of TGFs by RHESSI is interesting in the context of some more recent work. One of RHESSI’s missions was to observe the production and annihilation of positrons (antimatter electrons) in the Solar atmosphere. However, it is RHESSI’s observation of TGFs that lead scientists back to studying the production and annihilation of positrons here on Earth. For a long time, it was hypothesised that the bright bursts of gamma rays that make up TGFs may result in pair production: two interacting gamma rays give rise to an electron-positron pair. These positrons of course don’t travel far - while a positron in space may live in excess 10 million years before bumping into an electron and annihilating, positrons produced in the atmosphere are surrounded by far more electron-bearing things (atoms, for example) and annihilate very quickly. However, these annihilations also produce gamma rays, which can be detected by gamma ray satellites. For a long time, it was assumed that positrons produced by thunderstorms mostly came from pair-production.
Even when we look at extreme environments in outer space, like jets launched by black holes, we aren’t really sure how many positrons we can produce via pair production. In an astrophysical context, we actually think most positrons come from the radioactive decay of a variety of nuclei that are produced by stars and the explosions that occur when stars end their lives. A recent study shows that a lot of positrons created by thunderstorms may also be coming from radioactive decay. TGFs, rather than producing positrons through the pair production mechanism, actually interact with nitrogen atoms in the atomosphere. This “photonuclear” reaction converts a standard, stable nitrogen atom (14N) that has seven protons and seven neutrons in it’s nucleus into 13N (nitrogen-13). This nucleus has one fewer neutron, and the nucleus is unstable. It will tend to decay through beta+ decay, which produces a positron (and a neutrino), and the nucleus transmutes into that of 13C (carbon-13). Other similar photonuclear reactions involving oxygen can also occur. In this work, recently published in the journal Nature, the authors detect positron annihilation gamma rays associated with the positrons produced in these photonuclear reactions. It makes a fascinating connection between nuclear reactions, lightning and antimatter. What’s more, when you have a hammer that models positron transport and annihilation, it’s another problem that looks like a nail. I’ll keep you posted.
(As an end note, I was revisiting a wonderful book by Paul Simons called “Weird Weather”, where the author mentions a 1994 report in Atmospheric Environment about the emission of Krypton-84 by nuclear power stations. This chemically inert but radioactive gas apparently makes the atmosphere conduct electricity more easily, but no study had been done to see if thunderstorms were more common around nuclear power stations. An interesting tidbit to follow up on, perhaps)