Toward a consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines

Abstract

Simultanous Freja observations of precipitating keV electrons and electromagnetic emissions around half the local proton gyrofrequency are analyzed. The observed wave fields are used for reconstructing the energy density in wave vector space, the wave distribution function (WDF), and detailed observations of the electron distribution are used to analyze the linear stability of the plasma. We find that a local Landau resonance with precipitating electrons can generate electromagnetic waves having their wave vector (κ) pointing obliquely relative to the background magnetic field. However, the observed parallel electron drift energy suggests that the unstable frequencies are located above the peak in the observed power spectrum and that the κ spectrum predicted from the linear stability analysis does not overlap with the reconstructed WDF. This apparent inconsistency between the proposed instability mechanism and the available Freja data is resolved by placing the source region within 2400 km above Freja. By using a ray‐tracing mapping of an instability at an assumed generation altitude down to Freja altitudes, we show that an electron Landau instability occurring above Freja can be made consistent with the observed wave frequency as well as the reconstructed WDF. The agreement that we obtain between data and theory leads to consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines.