Various Mechanisms of ULF Wave–Electron Interaction: Case Studies

Two case studies of ultra-low frequency (ULF) wave–electron interaction are presented. In both cases, the eastward propagating Alfvén wave with strong poloidal component was generated via drift resonance with energetic electrons, but the generation mechanisms were different: kinetic instability in one case and alternating current caused by movement of the electron cloud in the other. In the first case, the Van Allen Probe data were examined. The poloidal Alfvén wave was found simultaneously with 38 keV electrons injected into magnetosphere during a substorm onset. It was found that the wave was generated through an instability caused by the strong radial inhomogeneity of electron density (the gradient instability). In the second case, the wave was found in the MMS spacecraft data. The wave also was in the drift resonance with the particles, but the conditions for plasma instability were not satisfied. In this case, we arrive at a conclusion that the wave was apparently generated through the moving source mechanism, that is, by an alternating current created by a drifting substorm-injected electron cloud.