Much later it struck me that the taxi is a nice metaphor for some of the science that triggered my visit. The gas of the Sun's atmosphere is sitting quietly, atoms moving around randomly in all directions, with a range of speeds but none of them going too much above the average speed. Then from somewhere above them comes an electron or an ion at enormous speed, like one of those São Paulo taxis heading from the airport into the city centre. Unlike the taxis it has no skilful driver, only speed, and it just crashes into all the rest of the traffic, sending them flying in all directions with much greater speeds than they had before; heating them up and making them glow in various ways. Exactly why they glow holds our detailed attention. But of course we'd really like to know, "who ordered the taxi?" And why was it moving so quickly?
If a taxi crashes into a car all that happens is car and taxi get mangled and some fragments go flying off in all directions. When we look more closely at the collisions in the Sun's atmosphere things get a bit stranger. Most of the time no actual damage is done. A very fast-moving proton crashes into a slow-moving proton. Afterwards the slow-moving one is going faster than before, because it's had a big dunt, but both protons are still protons. No lumps are knocked off them, both are still intact. But if the incoming proton is going fast enough, things get weirder, a bit like a surreal, Looney Tunes version of such a car-crash. The two protons crash into each other, there's a bit of a blur, but afterwards, different stuff comes out and moves off. It's as though the taxi crashed into the car and then, afterwards, the car shot off in a new direction and not just the original but two new taxis also get spat out the far end of the collision, only different sizes and maybe colours from the original (protons don't have a "colour". But in the "Big book of science" type books I read as a kid they were always drawn as little red billiard balls, red for positive electric charge. Electrons were blue (negative) and neutrons were green.)
Let's stick with the taxi metaphor. A fast red taxi crashes into an identical, slow-moving red car. Afterwards we'd expect a red taxi and a red car to come shooting out of this collision. Instead we get a red taxi, a green car and a funny wee yellow, sort of half-size taxi. Before our eyes, the funny new car travels a short distance then changes into something else different again.
The taxi metaphor's running out of steam. Here's what happens: accelerated to very high energy by an uncertain process, a proton collides with another proton. After the collision some of the energy of the proton has been transformed into a new particle, called a pion. After about one hundred millionth of a second the pion changes into a different, less massive particle called a muon, and finally the muon decays and spawns an electron or a positron (the antimatter counterpart of the electron). The electron or positron emits gamma-rays (like X-rays only even more penetrating) which we detect with instruments above the Earth's atmosphere on satellites. Once we detect those gamma-rays we can follow this sequence of events backwards to learn about the protons that were accelerated in the first place.
My colleagues here in Brazil lead the world at observing very short wavelength radio waves from the Sun. Recently they've begun to explore the wavelengths between radio and infra-red, what is called Terahertz radiation. This is a frontier and they have made some discoveries that we don't completely understand yet. The stories I've described above might be part of understanding Terahertz radiation in solar flares and that's a big part of why I'm in Brazil.
Since arriving I've had other taxi rides that were not nearly as exciting as that first one. Was the driver thinking of proton-proton collisions and pions? That question will probably go unanswered but we can be more optimistic about the reasons that solar flares glow in Terahertz radiation.
