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Electromagnetic ion cyclotron (EMIC) waves potentially cause precipitation loss of relativistic electrons from the outer radiation belt to the atmosphere through pitch angle scattering. However, the direct evidence of each EMIC wave element and burst of precipitation has not yet been reported. Here we show the temporal and spatial correspondence of the EMIC waves with relativistic electron precipitation (REP) during the geomagnetic storm of 27 March 2017. EMIC waves were observed at several stations in North America. REP was detected as a decrease of subionospheric radio amplitudes observed at Athabasca, Canada. When isolated proton aurora, observed at Athabasca, appeared on the radio propagation path, we found a good correspondence between the temporal variations of REP and EMIC waves, and REP preceded EMIC waves by 24s. This time lag is consistent with the travel time difference between relativistic electrons and EMIC waves from the magnetospheric equatorial plane to the ionosphere.

Plain Language Summary The flux of relativistic electrons in the outer radiation belt can vary in a time scale of hours to days during magnetically disturbed condition. One of the loss mechanisms of relativistic electrons is precipitation into the atmosphere due to interaction between electrons and plasma waves in the magnetosphere. Electromagnetic ion cyclotron waves, which are excited in the magnetosphere, are effective for causing precipitation loss of relativistic electrons. These waves can have rising spectral structure with an increase in frequency quasiperiodically. In this study, we made use of multiple ground-based observations and confirmed where and when relativistic electrons precipitated due to scattering by these waves. For the first time, we found direct evidence of correspondence between each wave element with rising tone and each burst of electron precipitation. We suggest that the precipitation of relativistic electrons occurred when the intensity of waves increased in the period of a few tens of seconds to minutes.