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Cyclotron radiation

Cyclotron radiation is electromagnetic radiation emitted by moving charged particles trapped in a magnetic field. The Lorentz force on the particles acts perpendicular to both the magnetic field lines and the particle's motion through them, creating an acceleration of charged particles that causes them to emit radiation (and to spiral around the magnetic field lines).

The name of this radiation derives from the cyclotron, a type of particle accelerator used since the 1930s to create highly energetic particles for study. The cyclotron makes use of the circular orbits that charged particles exhibit in a uniform magnetic field. Furthermore, the period of the orbit is independent of the energy of the particles, allowing the cyclotron to operate at a set frequency, and not worry about the energy of the particles at a given time. Cyclotron radiation is emitted by all charged particles travelling through magnetic fields, however, not just those in cyclotrons. Cyclotron radiation from plasma in interstellar space or around black holes and other astronomical phenomena are an important source of information about distant magnetic fields. The planet Jupiter in particular is a large source of cyclotron radiation.

Cyclotron radiation would likely be produced in a high altitude nuclear explosion. Gamma rays produced by the explosion would ionize atoms in the upper atmosphere and those free electrons would interact with the earth's magnetic field to produce cyclotron radiation in the form of an electromagnetic pulse (EMP). This phenonenon is of concern to the military as the EMP may damage solid state equipment.

Cyclotron radiation has a spectrum with its main spike at the same fundamental frequency as the particle's orbit, and harmonics at higher integral factors. Harmonics are the result of imperfections in the actual emission environment, which also create a broadening of the spectral lines. The most obvious source of line broadening is non-uniformities in the magnetic field; as an electron passes from one area of the field to another, its emission frequency will change with the strength of the field. Other sources of broadening include collisional broadening from the electron failing to follow a perfect orbit, distortions of the emission caused interactions with the surrounding plasma, and relativistic effects if the charged particles are sufficiently energetic. When the electrons are moving at relativistic speeds, cyclotron radiation is known as synchrotron radiation.