篠原 育

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.

Abstract Electron conics are a distinct type of electron distribution observed in Earth’s magnetosphere, characterized by enhanced fluxes of upgoing electrons at several-keV energies, particularly in the auroral acceleration region. This study analyzes high-altitude (27,000–32,000 km) observations made by the Arase satellite to investigate the characteristics of electron conics after passing through the heating region, employing the high angular resolution of the low-energy particle experiments—electron analyzer (LEPe) onboard the satellite. We analyzed eight electron conic events between 2017 and 2022 to estimate their source altitudes using mirror ratios and potential differences and by comparing pre- and post-heating data to investigate heating properties. Our results show that the source region of conics has an upper boundary at 9,000–14,000 km, with the peak flux originating from a central altitude of 3,000–7,000 km. This region spatially coincides with the source of auroral kilometric radiation (AKR): the central altitude of the source of conics corresponds to the lower boundary of the AKR source, suggesting that a longer residence time of particles within the AKR source region leads to stronger heating. The comparison of pre- and post-heating populations demonstrated that upgoing conic electrons exhibit higher temperatures and lower densities. The number flux remains conserved, indicating the energization of a magnetospheric population, whereas the energy flux is enhanced by up to a factor of four, significantly higher than that reported in previous studies. A test particle simulation, using observed plasma parameters and incorporating stochastic perpendicular heating, reproduces the main features of observed conics in terms of both energy and pitch angle. Our simulation shows that electron conics evolve into narrow, field-aligned beams at higher altitudes, suggesting that some of the anti-Earthward-flowing beams observed in the magnetotail may actually be unresolved conics. These findings contribute to the understanding of energy transport between the auroral acceleration region and the magnetotail and show the importance of high-angular-resolution instrumentation. Graphical Abstract

Abstract Understanding how the properties of Pc1 waves change during their propagation from the magnetospheric source regions to the middle or low-latitude ionosphere have not yet been clearly revealed by observations. In this study, we present the first quantitative comparison of Pc1 wave power attenuation both along the geomagnetic field lines and in the ionospheric wave ducts, using simultaneous observations from the Arase satellite and dynamical variation of Particles and Waves in the INner magnetosphere using Ground-based network observations (PWING) ground magnetometers. One of our key findings is that the polarization sense of the waves changed from left-handed polarization (LHP) at the satellite to right-handed polarization (RHP) on the ground, providing observational evidence of polarization transformation from space to the ionosphere. By examining polarization angles, we confirm that the Pc1 waves observed at multiple ground stations originated from the same magnetospheric source as the EMIC waves detected by the Arase. Importantly, we quantify the wave power attenuation factor along the magnetic field line to be only 0.37 dB/1000 km, which is nearly an order of magnitude smaller than that in the ionospheric wave duct (4.7–8.2 dB/1000 km). This result establishes a previously unreported minimum Pc1 wave attenuation rate in the magnetosphere, highlighting that the wave energy loss occurs more rapidly in the ionospheric duct than in space. These findings provide new insights into Pc1 wave transmission mechanisms and emphasize the importance of combined space- and ground-based observations for characterizing wave propagation processes across geospace. Graphical Abstract