FAST flew through the aurora rather than photographing it. Its orbit carried it repeatedly through the narrow region a few thousand kilometres up where electrons are accelerated downward along magnetic field lines to strike the atmosphere and make it glow.
The instrument challenge was speed. The spacecraft crosses those structures in seconds, so the measurements had to be made at extremely high time resolution — thousands of samples per second — to resolve features that are thin and sharply bounded.
It found that the acceleration happens in discrete, structured regions with strong electric fields parallel to the magnetic field, a configuration that plasma physics does not readily produce and which had been debated for decades. It also identified the wave modes that accelerate particles in other regions.
The results apply well beyond Earth. The same processes operate in Jupiter's and Saturn's magnetospheres, and in astrophysical settings from pulsars to accretion disks, so measuring them in the one place a spacecraft can fly through them directly has broad value.
It was another Small Explorer, built quickly and cheaply, and operated for thirteen years until 2009. Its data continues to be used alongside later missions like MMS that apply the same in-situ approach to reconnection regions further out.
