Nonlinear Landau resonance with localized wave pulses

Wave-particle resonant interaction is responsible for many important space plasma processes and can be formally described theoretically for two main regimes: particle scattering (quasilinear diffusion) by low-intensity waves and particle trapping/phase bunching by sufficiently intense waves. There are important modifications of these regimes in inhomogeneous plasma systems. However, almost all theoretical results have been derived for harmonic waves and linear plasma modes. High-resolution spacecraft measurements of spatially localized plasma waves in Earth's magnetosphere demonstrate electron acceleration attributed to the Landau resonance mechanism. In the classical mechanism, the magnitude of the electric potential determines the maximum energy gain of electrons. However, a less reported mechanism of electron acceleration becomes possible in inhomogeneous plasma and magnetic field. In this Commentary, we discuss how combination of wave localization and plasma inhomogeneity modifies the nonlinear Landau resonance. We underline the important property of this resonance: an electron energy gain depends on the gradient in wave potential and not just its magnitude. We also consider this modified resonance mechanism in context of generation of hot field-aligned electron populations, particle transport, and energy transfer in the magnetosphere.