Pc1 waves are known as continuous geomagnetic pulsations observed in the ultralow frequency range (0.2-5 Hz). These waves are mainly generated in the inner magnetosphere by anisotropic distributions of energetic ions and often subsequent nonlinear processes, and are generally accepted as representing electromagnetic ion cyclotron (EMIC) wave (

into the Earth's atmosphere via pitch angle scattering induced by resonant wave-particle interactions. Observations of precipitating protons (>30 keV) associated with Pc1 wave were first reported by Yahnina et al. (2000) using data from the NOAA-12 satellite and the Sodankylä ground magnetometer. Yahnina et al. (2003) investigated energetic proton precipitation with/without lower energy (<20 keV) counterparts during Pc1 wave activity and showed that the type of Intervals of Pulsations with Diminishing Periods (IPDP) Pc1 waves is mostly accompanied by lower-energy proton precipitations. Miyoshi et al. (2008) first reported simultaneous observations of relativistic electron and energetic proton precipitations caused by the EMIC wave.

Ducting Pc1 waves are observed over a wide latitudinal range by the low-Earth orbit satellites as compressional mode. In this paper, we present the first observation of the modulation of ducting Pc1 waves by equatorial plasma bubbles (EPBs) based on the Swarm satellites. We show two ducting Pc1 events propagating across EPBs that occurred on 7 April 2016 and on 27 September 2017. We found that the EPBs modulate the Pc1 wave propagation by setting up reflection boundaries and leakage holes in the ionospheric waveguide. We also found that changes of Pc1 wave intensities generally follow the electron density variation and that the intensity is stronger at higher density region. From the comparison between Swarm-A and Swarm-C observations, we conclude that ionospheric plasma plays an important role for Pc1 waveguide even though their density is significantly low in EPBs.Plain Language Summary Pc1 (or electromagnetic ion cyclotron) waves have been observed across the whole magnetospheric and ionospheric regions and on the ground with different characteristics. Typically, they propagate along the magnetic field line in the magnetosphere. But once the waves enter the ionosphere, plasma effects such as mode conversion and reflection complicate their propagation. One of the Pc1 wave propagation characteristics on the ionosphere is ducting (waveguide). Ionospheric ducting is a way radio waves can travel thousands of kilometers along an ionospheric layer. Ducting is controlled by various physical processes and ionospheric conditions, among which plasma density is one of the factors. In this study, we present the first observations of the ducting Pc1 wave controlled by ionospheric electron density, particularly by the equatorial plasma bubbles, which are localized density depletion near the geomagnetic equator. Our observation results show that the Pc1 wave intensity in the ionosphere strongly depends on the ambient electron density.