Takeshi Takashima

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.

Abstract Analyzing the dynamics of trapped electron fluxes in the Earth's outer radiation belt is a complex task, due to the presence of insufficiently known parameters and the long runtimes of multi‐dimensional radiation belt codes, preventing a thorough examination of dependencies on all parameters. Here, we present an approximate eigenfunction modeling of whistler‐mode wave‐driven electron pitch‐angle diffusion, slightly generalized compared to previous work. This new model can approximately describe, in an easy, flexible, and fast way, both the asymptotic electron pitch‐angle distribution (PAD) at all pitch angles and its temporal evolution toward this final state, in both weak and strong diffusion regimes, in the presence of a finite, time‐varying electron source. In this model, wave‐driven pitch‐angle diffusion is assumed to prevail over energy diffusion and radial diffusion, limiting its applicability to the plasmasphere or intervals of smooth decay of the electron flux outside the plasmasphere, during moderately active periods. We propose a new method, based on this model, for estimating the energy spectrum and temporal variation of the electron source. We investigate the dynamics of the electron flux measured by the Van Allen Probes and Arase spacecraft during two events in 2018 and 2022 in the outer radiation belt. We demonstrate that the new model can reproduce the evolution of the measured electron flux and of its PAD, provided that the magnitude of diffusion rates is normalized to the observed decay timescale in the 300–600 keV range and that a finite electron source term is included below 300 keV.

Abstract The May 2024 geomagnetic superstorm provided the opportunity to explore how strong wave‐particle interactions affect energetic electron precipitation under intense driving. Using coordinated measurements from a balloon‐borne Timepix‐based X‐ray detector, ground‐based riometers and magnetometers, and Arase satellite observations, we identified quasi‐periodic bursts of energetic electron precipitation coincident with Pc5 ultra low frequency (ULF) wave oscillations. Arase satellite data revealed energy‐dispersed trapped energetic electron flux modulations in the “seed” energy range, indicating that trapped electron flux was likely modulated by ULF waves. This letter reveals that these flux enhancements surpassed the Kennel‐Petschek (K‐P) limit, creating intense chorus waves and driving periodic electron precipitation. Drift‐dispersion analysis traced these modulations back to a source in the post‐noon magnetospheric sector, matching balloon and ground‐based measurements. Here, we propose a novel indirect ULF wave‐driven mechanism for modulated energetic electron precipitation, whereby periodic modulations of “seed” electron fluxes enhance electron losses.