Subauroral proton spots visualize the Pc1 source

Abstract

Recent observations from the IMAGE spacecraft revealed a new type of proton aurora – subauroral proton spots, which map onto the vicinity of the plasmapause. It has been suggested that this proton aurora is produced by energetic proton precipitation after the interaction of ring current particles with electromagnetic ion cyclotron (EMIC) waves in the equatorial plane of the magnetosphere. We prove this suggestion by comparing observations from IMAGE with geomagnetic pulsations Pc1, which are a ground signature of EMIC waves. We found that when the proton spot is nearly conjugated with the ground station equipped with a pulsation magnetometer, the station always observes Pc1. Moreover, there is a good agreement between the appearance/disappearance of the spot and the beginning/end of the Pc1 train. We conclude that the subauroral proton spots are images on the ionospheric “screen” of magnetospheric regions where the ion cyclotron instability develops leading to an intense scattering of energetic protons into the loss cone.