Takeshi Takashima

Abstract Determination of electron phase space density (PSD) at a wide range of L‐shells with high temporal resolution is central to understanding acceleration, transport, and loss processes in Earth's radiation belts. However, cross‐mission comparisons of PSD are complicated by instrumental biases and differing orbital coverages. In this study, we develop and validate a cross‐calibrated PSD data set combining measurements from the Arase and Van Allen Probes missions across overlapping energy ranges. Using conjunctions between Arase's MEP‐e and HEP instruments with Van Allen Probe's MagEIS instrument, we quantify systematic differences in PSD observation for fixed adiabatic invariants and derive energy channel‐specific correction functions. The cross‐calibration procedure reduces systematic biases between instruments to below 5% and leads to statistically meaningful reductions in random error, with uncertainty estimates provided for each channel. This results in a continuous combined PSD product spanning energies from 35 keV to 1.6 MeV. The resulting data set provides internally consistent cross‐mission PSD observations, extending reliable coverage of the outer belt to higher L‐shells accessible to Arase.

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.