Resonant Generation of an Alfvén Wave by a Substorm Injected Electron Cloud: A Van Allen Probe Case Study

We report the first observation of the resonant generation of an ultra‐low frequency wave by energetic elections in the magnetosphere. On 27 October 2012 in the morning side of the magnetosphere Van Allen Probe A registered an ULF wave with period of 100 s and an amplitude of 0.7 nT. A cloud of energetic electrons was observed simultaneously with the wave. It is established that the electron cloud was injected into the magnetosphere as a result of a substorm. Electron fluxes in several energy channels were modulated with the frequency of the observed wave. It is shown that these flux modulations are caused by the drift resonance of 38 keV electrons and the wave being a fundamental harmonic of an Alfvén mode with an azimuthal wavenumber m ∼ 110–115 propagating to the east, and generated through the gradient instability due to steep earthward density gradient of the resonant electrons. Plain Language Summary In recent decades, with the increase in the number of magnetospheric missions, information about wave phenomena has been significantly replenished. Most of ultra‐low frequency (ULF) waves observed by satellites interact with charged particles. A large part of the ULF waves are generated by high‐energy particles, usually protons, while there was no evidence of observations of generation with electrons. We report the first observation of resonant generation of an ULF wave by energetic electrons injected into magnetosphere during the substorm onset. We established that the found Alfvén wave was generated through instability caused by the strong radial inhomogeneity of density of 38 keV electrons being in the drift resonance with the wave (the drift velocity of the electrons coincides with the phase velocity of the wave). Key Points The resonant generation of an Alfvén wave by energetic electrons is observed The observed wave is the fundamental harmonic, propagating eastward with azimuthal wave number of about 110–115 The wave was generated through the gradient instability and interacted with electrons via the drift resonance