Cyclotron maser radiation is one of the two major emission mechanisms for radio waves generated in our solar system. Examples include Auroral Kilometric Radiation (AKR) from Earth's auroral regions, Jupiter's decametric radiation, solar microwave bursts associated with flares and solar active regions, planetary radio emissions from the other outer planets. This mechanism is also believed important for various stellar radio bursts.
Cyclotron maser emission involves generation near the electron cyclotron frequency 
and its harmonics in regions where 
. Considering the wave-particle
resonance condition
it is clear that generation at frequencies near 
requires the product
to be small (here the subscript
means parallel to the ambient magnetic field). This condition is
generally met by the radiation being produced with wavevectors almost perpendicular to the
magnetic field. Symmetry then implies that the radiation is produced in a cone with very large half-angle
degrees and symmetry axis along 
.
What particle distributions can drive cyclotron maser emission? It turns out that beam distributions
can, but that distributions with large gradients 
are more favoured.
So-called ``loss cone distributions'' have large gradients
since they are missing particles with small pitch-angles, as expected for particles bouncing back and
forth in the magnetic mirrors formed by dipole magnetic fields. Accordingly, cyclotron maser
radiation generated by loss cone distributions of electrons is expected to be ubiquitous from
planetary auroral regions and solar active regions, as is indeed observed.
Earth's AKR is one of the brightest radio sources in our solar system. Figure 13.18 illustrates the characteristic frequency range of the radiation. AKR brightens during magnetic and auroral substorms and can be used as a remote indicator of geomagnetic activity.