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.

Abstract. By comparing data from a globally distributed set of high-latitude magnetic observatories with optical data in the nightside auroral ionosphere, we studied the features of short-term activations (_<10-15 min) during periods of steady magnetospheric convection (SMC) observed between two substorms. We found that the occurrence of local auroral and current structures stretched along the meridian to several hundreds of kilometers is the characteristic feature of activations in the nightside ionosphere. At premidnight and night hours (2100-0100 LT) we observed auroral structures that expand rapidly equatorward (V>_ 1 km/s) from the discrete auroral oval and become north-south aligned (N-S aurora or auroral streamers). In the night-morning hours (0100-0500 LT) torch-like structures form at the poleward

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.

This paper reports two unique auroral features: postmidnight purple auroral rays and global Pc1 geomagnetic pulsations, observed before the onset of the corotating interaction region (CIR) storm of 21 March 2017, at the beginning of the first campaign of the new Particles and Waves in the Inner magnetosphere using Ground-based network observation (PWING) longitudinal ground network with the Arase satellite. The purple auroral rays were observed from~0315 to 0430 UT (~03-04 magnetic local time) in the northeastern sky at Husafell, Iceland (magnetic latitude: 64.9°N). We newly propose that the entry of high-density CIR plasma into the magnetotail created purple auroral rays in the sunlit ionosphere. Pc1 geomagnetic pulsations at frequencies of 0-0.5 Hz were observed after~00 UT over a wide local time range, of 13 hr, from midnight to afternoon sectors at subauroral latitudes associated with CIR arrival. These results indicate preconditioning of the magnetosphere due to crossing of a CIR. Plain Language SummaryWe report auroral ray structures, which show a unique purple color, and global geomagnetic pulsations observed on 17 March 2017. The purple auroral rays were observed in the northeastern sky at Husafell, Iceland. The geomagnetic pulsations at frequencies of below 0.5 Hz were observed over a wide longitudinal range extending from midnight through morning to afternoon sectors at subauroral latitudes. These phenomena took place associated with the arrival of the so-called corotating interaction region (CIR)" in the interplanetary space, which is characterized by high-density high-speed solar wind plasma. The CIR is one of the phenomena frequently occurring during the minimum phase of the 11-year solar cycle. We suggest that the CIR can cause these unique purple auroras and global geomagnetic pulsations. The present observations also suggest a possible mechanism for dropout of radiation belt electrons due to electromagnetic ion cyclotron waves (=geomagnetic pulsations) associated with the CIR arrival.

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