Electromagnetic radiation emitted by a charged particle moving in a magnetic field at an appreciable fraction of the speed of light. It is similar to cyclotron radiation, except that relativistic effects cause most of the energy to be emitted at higher multiples of the gyrofrequency. The slowly varying radio emission received from active regions on the Sun may consist of gyrosynchrotron radiation. Gyrosynchrotron radiation is believed to be responsible for the intense radio emission received from solar flares occurring in the magnetic fields above groups of sunspots.