Mechanism for higher harmonic radio emission from aurorae

It has long been known that the aurora emit radio waves at the second and third harmonics of the electron cyclotron frequency. A detailed theory explaining these emissions has been developed and tested in many experiments. However, recently (in 2012) the fourth and fifth harmonics were observed for the first time. While it is possible that the same theoretical explanation applies to them, there is some evidence that their polarization may be inconsistent with the mechanism that generates the lower harmonics. Colleagues have proposed and alternative, non-linear mechanism to explain the higher harmonics. We are working on observations as well as theoretical calculations to determine whether the polarization is indeed inconsistent with the mechanism established for the lower harmonics, and in either case to design experiments to establish the generation mechanism.