Circularly Expanding Ring‐Shaped Pulsating Aurora Visualizing the Source of Plasma Waves in Space

Abstract Pulsating aurorae are prominent auroral emissions in the polar regions, typically occurring in the morning hours during the recovery phase of auroral substorms. These aurorae usually consist of round‐shaped patches of emission, with luminosity that pulsates at intervals ranging from less than a second to several tens of seconds. Here, we present, for the first time, a unique case of a pulsating aurora that expanded radially outward in all the directions and repeatedly formed a ring‐shaped structure. The speed of expansion, which was at least several tens of kilometers per second at ionospheric altitudes, cannot be attributed to the horizontal convective motion of plasma in the ionosphere. In the magnetosphere, corresponding to the expanding ring‐shaped aurora, the Arase satellite detected successive enhancements of natural electromagnetic waves known as a “chorus.” These chorus waves scatter energetic magnetospheric electrons into the ionosphere, resulting in pulsating diffuse aurorae. Notably, the satellite observed systematic delay in the timing of chorus detections, which suggests that a similar circularly expanding feature existed in space. These simultaneous observations of expanding features in both the ionosphere and the magnetosphere demonstrate that the temporal evolution of the shape of a pulsating aurora manifests the spatiotemporal evolution of the source of plasma waves in space.