Resonant Whistler-Electron Interactions: MMS Observations Versus Test-Particle Simulation

We present a novel technique to analyze VDF, which allows to capture their fine structure including wave-particles resonances. By applying the technique to magnetospheric multiscale (MMS) data, the simultaneous observation of characteristic three-dimensional (3-D) signatures in the electron velocity distribution function (VDF) and intense quasi-monochromatic waves in the terrestrial magnetosheath is investigated. The intense wave packets are characterized and modeled analytically as quasi-parallel circularly polarized whistler waves and applied to a test-particle simulation in view of gaining insight into the signature of the wave-particle resonances in velocity space. Both the Landau and the cyclotron resonances were evidenced in the test-particle simulations. The location and general shape of the test-particle signatures do account for the observations, but the finer details, such as the symmetry of the observed signatures, are not matched, indicating either the limits of the test-particle approach or a more fundamental physical mechanism not yet grasped. Finally, it is shown that the energization of the electrons in this precise resonance case cannot be diagnosed using the moments of the distribution function, as done with the classical E · J “dissipation” estimate.