Modulation of Whistler-mode Waves in the Solar Wind in the Vicinity of Earth’s Moon

Abstract Ultra-low-frequency (ULF) waves are known to modulate high-frequency whistler-mode waves within the Earth’s magnetosphere. Periodic bursts of whistler-mode waves are excited in association with ULF perturbations of plasma density and magnetic field. In this paper, we present a multievent study of whistler-mode waves modulated by ULF waves (0.05–1 Hz) observed by ARTEMIS in the vicinity of Earth’s Moon, when it was in the solar wind region upstream of the quasiparallel bow shock. The events exhibit periodic whistler-mode wave bursts between 0.2 and 0.4 f ce (electron gyrofrequency) coupled with ULF perturbations. Similar to modulation events observed within Earth’s magnetosphere, some of the strongest whistler-mode wave bursts were observed when electron density and magnetic field perturbations were anticorrelated. In one of the events, a strong positive correlation between electron density and magnetic field perturbations was observed during the excitation of whistler-mode waves. The ULF waves develop a periodic enhancement in transverse electron flux anisotropy, which is positively correlated with the whistler-mode wave bursts, thus providing conditions favorable for their generation. The observation of these events on the quasiparallel side of bow-shock suggests a possible connection between the observed ULF waves and the upstream waves in the Earth’s foreshock region. Finally, we examine the possibility that whistler-mode waves are trapped within localized regions of enhanced plasma density and magnetic field gradients produced by ULF waves in the solar wind.