Synchrotron radiation is the most common form of non-thermal radiation. Synchrotron radiation arises due to the acceleration of relativistic electrons within magnetic fields. As the relativistic electrons encounter a magnetic field, they spiral around it and emit radiation. The frequency of the emission is directly related to how fast the electron is traveling.
As the electron travels around the magnetic field, it gives up energy as it emits photons. The longer it is in the magnetic field, the more energy it loses. As a result, the electron makes a wider spiral around the magnetic field, and emits electromagnetic radiation at a longer wavelength.
(Ref : An Introduction to Radio Astronomy, Luke Woolfenden.
http://www.astro.cardiff.ac.uk/observatory/radioback.html)