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.

This paper presents the highlights of joint observations of the inner magnetosphere by the Arase spacecraft, the Van Allen Probes spacecraft, and ground-based experiments integrated into spacecraft programs. The concurrent operation of the two missions in 2017–2019 facilitated the separation of the spatial and temporal structures of dynamic phenomena occurring in the inner magnetosphere. Because the orbital inclination angle of Arase is larger than that of Van Allen Probes, Arase collected observations at higher $L$ L -shells up to $L \sim 10$ L ∼ 10 . After March 2017, similar variations in plasma and waves were detected by Van Allen Probes and Arase. We describe plasma wave observations at longitudinally separated locations in space and geomagnetically-conjugate locations in space and on the ground. The results of instrument intercalibrations between the two missions are also presented. Arase continued its normal operation after the scientific operation of Van Allen Probes completed in October 2019. The combined Van Allen Probes (2012-2019) and Arase (2017-present) observations will cover a full solar cycle. This will be the first comprehensive long-term observation of the inner magnetosphere and radiation belts.

Stable auroral red (SAR) arcs are optical events with dominant 630.0-nm emission caused by low-energy electron heat flux into the topside ionosphere from the inner magnetosphere. SAR arcs are observed at subauroral latitudes and often occur during the recovery phase of magnetic storms and substorms. Past studies concluded that these low-energy electrons were generated in the spatial overlap region between the outer plasmasphere and ring-current ions and suggested that Coulomb collisions between plasmaspheric electrons and ring-current ions are more feasible for the SAR-arc generation mechanism rather than Landau damping by electromagnetic ion cyclotron waves or kinetic Alfvén waves. This work studies three separate SAR-arc events with conjunctions, using all-sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes [RBSP]) during non-storm-time substorms on December 19, 2012 (event 1), January 17, 2015 (event 2), and November 4, 2019 (event 3). We evaluated for the first time the heat flux via Coulomb collision using full-energy-range ion data obtained by the satellites. The electron heat fluxes due to Coulomb collisions reached ∼10 9 eV/cm 2 /s for events 1 and 2, indicating that Coulomb collisions could have caused the SAR arcs. RBSP-A also observed local enhancements of 7-20-mHz electromagnetic wave power above the SAR arc in event 2. The heat flux for the freshly detached SAR arc in event 3 reached ∼10 8 eV/cm 2 /s, which is insufficient to have caused the SAR arc. In event 3, local flux enhancement of electrons (<200 eV) and various electromagnetic waves were observed, these are likely to have caused the freshly detached SAR arc.Plain Language Summary Stable auroral red (SAR) arcs are aurora with an optical red emission from oxygen atoms at latitudes slightly lower than the auroral oval and often occur during storm-time substorms. The oxygen excitation is caused by low-energy electrons transferred from the inner magnetosphere to the ionosphere. Past studies concluded that these low-energy electrons were generated INABA ET AL.