Properties of Intense Field‐Aligned Lower‐Band Chorus Waves: Implications for Nonlinear Wave‐Particle Interactions

Resonant interactions between electrons and chorus waves are responsible for a wide range of phenomena in near-Earth space (e.g., diffuse aurora and acceleration of > 1 MeV electrons). Although quasi-linear diffusion is believed to be the primary paradigm for describing such interactions, an increasing number of investigations suggest that nonlinear effects are also important in controlling the rapid dynamics of electrons. However, present models of nonlinear wave-particle interactions, which have been successfully used to describe individual short-term events, are not directly applicable for a statistical evaluation of nonlinear effects and the long-term dynamics of the outer radiation belt, because they lack information on the properties of intense (nonlinearly resonating with electrons) chorus waves. In this paper, we use the Time History of Events and Macroscale Interactions during Substorms and Van Allen Probes data sets of field-aligned chorus waveforms to study two key characteristics of these waves: effective amplitude B-w (nonlinear interaction can occur when B-w > 2) and wave packet length beta (the number of wave periods within it). While as many as 10-15% of chorus wave packets are sufficiently intense (B-w > 2-3) to interact nonlinearly with relativistic electrons, most of them are short (beta < 10) reducing the efficacy of such interactions. Revised models of nonlinear interactions are thus needed to account for the long-term effects of these common, intense but short chorus wave packets. We also discuss the dependence of B-w, beta on location (MLT and L-shell) and on the properties of the suprathermal electron population.