Modulation of Whistler Waves by Ultra‐Low‐Frequency Perturbations: The Importance of Magnetopause Location

Ultra‐low‐frequency (ULF, 0.001–1 Hz) perturbations are important aspect of the dynamics in the inner magnetosphere: They are responsible for radial transport (diffusion) of high‐energy electrons and for energizing the ionosphere with field‐aligned currents. This study is devoted to properties of ULF perturbations generated at the magnetopause, their propagation into the inner magnetosphere, and their modulation of whistler‐mode very‐low‐frequency (VLF) waves. Taking advantage of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission configuration in 2019, we investigate ULF perturbations simultaneously captured by three spacecraft at different distances from the magnetopause. Combining ground‐based and THEMIS measurements of ULF perturbations to separate temporal and spatial variations of their properties, we show that their intensity decays exponentially with distance from the magnetopause as close as the geostationary orbit. Near the magnetopause ULF perturbations can modulate whistler‐mode (VLF) waves effectively: Close to the magnetopause, VLF wave bursts have the same periodicity as the ULF perturbations. Our results demonstrate that almost the entire outer magnetosphere (from the geostationary orbit to L ∼ 12), including the outer radiation belts, is significantly influenced by ULF perturbations excited by magnetopause dynamic responses to the solar wind. Key Points We conduct statistics on ULF perturbations generated at the magnetopause In the dawn sector, the intensity of ULF perturbations generated at the magnetopause decays with the distance δR as ∼1.8RE/δR Within 5RE from magnetopause, ULF perturbations can effectively modulate VLF whistler waves, producing quasi‐periodic electron precipitation