Kunihiro Keika

Earth's main magnetic field forms the magnetosphere, shielding the planet from solar wind plasma. Southward interplanetary magnetic fields erode the magnetosphere, a process known as magnetopause erosion, allowing solar wind penetration close to the Earth. While magnetic reconnection is considered a key driver, the mechanism during super geomagnetic storms remains unclear due to limited in situ observations. Here, we report a magnetopause crossing by Arase at 4.96 R _E during the May 2024 superstorm, one of the innermost magnetopause crossings ever detected by magnetospheric spacecraft. We observe an anomalous and global reduction in magnetic field intensity extending from the inner magnetosphere to the magnetopause, suggesting global weakening by magnetospheric currents. This weakened magnetic field is responsible for the enhanced magnetopause erosion.

During geomagnetic storms, energetic ions are injected from the Earth's magnetotail along the plasma sheet boundary layer (PSBL). Owing to its off‐equatorial inclined orbit, the Arase satellite is in a prime position to observe these injections in the PSBL on the nightside at the apogee. The injections are observed as energy and pitch angle dispersed features in the Arase LEP‐i and MEP‐i data. While these injections have often been observed in H ⁺ , reports of O ⁺ dispersions are rare. During the main phase of a geomagnetic storm on 16 July 2017 we identify frequent strong earthward injections of O ⁺ . The distance to the injection location varied from about 10 to 40 Earth radii. Some successive injections showed an increasing trend in distance, indicating that the ion acceleration region moved down the magnetotail over time. The earthward flux of these ions was comparable to the flux of nightside auroral outflow during this time, indicating that these injections may be a significant contributor to the hot O ⁺ population in the storm‐time near‐earth plasma sheet. Additionally, we present statistics derived from 180 individual O ⁺ injections during storms in 2017–2020, confirming that the injection source is mainly located in 10–40 R _E with a median at 20 R _E .

Field line resonance (FLR) is an important source of Ultra‐low‐frequency (ULF) waves observed in the inner magnetosphere. In this study, we present multi‐spacecraft observations of a toroidal ULF wave, detected by the Arase, Van Allen Probe (Radiation Belt Storm Probe, RBSP) ‐A, and GOES‐14. During the event, Arase and RBSP‐A were positioned on two closely spaced magnetic field lines in the midnight sector (magnetic local time ≈23) and on opposite sides of the magnetic equator, providing a rare opportunity to examine the latitudinal structure of FLR. The temporal profiles of these toroidal components exhibited distinct, separated wave packets, with each packet persisting for ∼10 min and one‐to‐one correspondence in timing and amplitude across all three spacecraft. Based on the phase relationships and harmonic eigenfrequency analysis, the observed waves are identified as the fundamental toroidal mode. The observed waveforms resemble transient toroidal waves associated with impulsive disturbances in the near‐Earth magnetotail. This interpretation is further supported by four corresponding enhancements in the Wp index and Pi2‐type pulsations recorded by ground magnetometers. The observed proton phase space density oscillations may result from ULF‐wave‐induced E × B convection of a background phase‐space‐density gradient, as suggested by their phase relation and pitch‐angle distribution.

At subauroral latitudes, discrete auroras, such as strong thermal emission velocity enhancement and stable auroral red arcs, have been extensively studied. On the other hand, diffuse emissions at subauroral latitudes have not been studied well. In this paper, we report eight events of conjugate observations of faint diffuse 557.7‐nm emissions with intensities of 300–1,000 rayleighs without discrete structures equatorward of the auroral oval, using the Arase satellite and high‐sensitivity all‐sky airglow imagers in North America. Ground‐magnetosphere conjugate observations of such diffuse emissions have not been previously reported. Diffuse 557.7‐nm emissions were observed during both geomagnetically disturbed and quiet conditions. Preference for magnetic local time was not found in these eight events. Arase observed plasma‐sheet electrons with energies from several hundred eV to beyond 10 keV in the source magnetosphere, which is located inside the plasmapause. Electromagnetic waves, such as plasmaspheric hiss and lower‐band chorus waves, were also observed there by the Arase satellite. For five events, the resonance energies estimated from the observed wave frequencies ranged from 30 eV to 30 keV, suitable for generating the 557.7‐nm emission, though the pitch‐angle diffusion coefficient estimated for one event was not so large. We discuss the possibility that plasma‐sheet electrons in the plasmasphere precipitated into the ionosphere through wave‐particle interaction, forming faint diffuse 557.7‐nm emission equatorward of the auroral oval.

Electron and proton precipitation into the nightside ionosphere is a key process in magnetosphere–ionosphere coupling. Although auroral electron precipitation at –10 keV is traditionally considered the main driver of auroral emissions, recent studies show that keV electrons and protons can significantly contribute to ionospheric energy deposition, especially during storms. While energetic electron precipitation has been studied extensively, the role of energetic proton precipitation remains poorly quantified. Here, we use coordinated ELFIN and DMSP nightside observations during two substorm events, resolving electron and proton precipitation spectra from eV to ∼MeV. We show that keV proton precipitation, consistent with curvature scattering, can have an impact on E‐region ionization comparable to that of energetic electrons. These analyses are further supported by magnetospheric observations from the Arase and THEMIS spacecraft and ground‐based incoherent scatter radar measurements from PFISR.

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.

Super geomagnetic storms are characterized by extreme intensification of the ring current in near-Earth space. The origin of the ions that carry the ring current is key to understanding its development. In situ measurements of ring current ions by the Arase satellite demonstrate an unprecedented dominance of heavy ions originating from the Earth during the May 2024 super geomagnetic storm, despite the high solar wind density. The solar wind, another expected source of ions, contributes little to the energy density of the ring current. This observational evidence highlights the critical role of ion supply processes from the Earth and transport in the magnetosphere in developing the ring current for the super geomagnetic storm. Furthermore, the super-intense ring current penetrated close to the Earth, strongly deforming the local geomagnetic field and driving unusual outward transport of electrons, which led to the loss of radiation belt electrons from the near-Earth region.

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.