Amplitude variations of electron cyclotron harmonic waves

Electron cyclotron harmonic (ECH) instabilities just outside the plasmapause and at frequencies near the cold upper hybrid frequency are a common feature of the Earth\u27s magnetosphere. These waves which have virtually no magnetic component, are believed to have an important role in the generation of weak diffuse aurora1,2. They are able to interact strongly with electrons in the hundred eV to several keV energy range which can result in pitch angle scattering and precipitation on magnetic field lines which map down into the auroral zone. On the dayside magnetosphere these waves can exhibit large amplitude variations of 30–40 dB and can also exist at steady amplitudes on time scales of the order of tens of seconds. Here we seek an explanation for the sporadic nature of such instabilities by performing linear stability calculations and extending the technique used in the accompanying paper3