THEMIS observations of long‐lived regions of large‐amplitude whistler waves in the inner magnetosphere

Recent reports of large‐amplitude whistler waves (>100 mV/m) in the radiation belts have intensified interest in the role of whistler waves in accelerating radiation belt electrons to MeV energies. Several critical parameters for addressing this issue have not previously been observed, including the occurrence frequency, spatial extent and longevity of regions of large‐amplitude whistlers. The THEMIS mission, with multiple satellites in a near‐equatorial orbit, offers an excellent opportunity to study these waves. We use data from the Electric Field Instrument (EFI) to show that in the dawn‐side radiation belts, especially near L‐shells from 3.5 to 5.5, the probability distribution of wave activity has a significant high‐amplitude tail and is hence not well‐described by long‐term time averages. Regions of enhanced wave activity exhibit four‐second averaged wave power above 1 mV/m and sub‐second bursts up to several hundred mV/m. These regions are spatially localized to at most several hours of local time azimuthally, but can persist in the same location for several days. With large regions of space persistently covered by bursty, large‐amplitude waves, the mechanisms and rates of radiation belt electron acceleration may need to be reconsidered.