“I couldn't reduce the explanation to a freshman level. That means we really don't understand it."
Richard Feynman on the behavior of electrons
The first spectrometer was devised by Martin Deutsch (1917-2002) and Robley D. Evans (1907-1995) at the Massachusetts Institute of Technology (MIT). Electrons are a by-product of nuclear reactions and the first electron spectrometers were used to monitor the radiation from the nuclear tests that took place toward the end of World War II. They have since become a "must have" instrument on scientific space missions.
The fourth state of matter is plasma, where some of the outer electrons of the atoms have been knocked away, and move off freely in space. Electrons are relatively simple fundamental particles. Their mass, charge, and collision cross-section are well known.
That leaves their speed and direction of motion as unknowns. An electron spectrometer measures their kinetic energy by registering the way in which the trajectories of the electrons are bent as they pass through either an electrostatic or a magnetic field.
The Earth's upper ionosphere is plasma, as is the wind of particles ejected by the sun. Interesting auroral physical processes occur when this solar wind hits a planetary ionosphere; electron spectrometers are used to measure the distribution of electron energies. Placed on spacecraft, they can also measure how the electron energy spectrum varies with the direction in which the electrons are moving.
