Fuminori Tsuchiya

The inner radiation belt ( L* < 3) has long been considered an impenetrable barrier to ultra‐relativistic electrons. Here we show that this barrier can be breached under extreme space weather conditions. Using continuous near‐equatorial observations from the Arase satellite, we show that the May 2024 superstorm rapidly generated multi‐MeV electrons deep inside the inner magnetosphere. Phase space density profiles reveal the development of a local maximum at L* ∼ 3, providing evidence that local in situ acceleration established multi‐MeV electron source before supplying inward and outward radial diffusion. Simultaneous observations of intense chorus waves under a strongly depleted plasma region, maintained by enhanced convection and a negative ionospheric storm, are consistent with chorus‐driven local acceleration. These findings demonstrate that local acceleration can establish a multi‐MeV electron source inside the impenetrable barrier under extreme space weather conditions.

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.

We investigated the spatial distributions and pitch‐angle distributions of 30–300 eV H ⁺ and O ⁺ ion fluxes using data obtained by the low‐energy particle experiments‐ion mass analyzer (LEPi) onboard the Arase satellite. A statistical analysis over 3.7 years reveals that the H ⁺ ion flux is enhanced at L > 4 from the premidnight sector through dawn to noon, with the peak location shifting inward during geomagnetic disturbances. The pitch‐angle distribution of H ⁺ ions varies with both L and geomagnetic activity, which can be interpreted as the influence of transport processes and the ring current effect. In contrast, the O ⁺ ion flux exhibits a pronounced enhancement in a confined region at L = 3–5 and 19–9 magnetic local time, with a strong dawn‐dusk asymmetry. The O ⁺ flux increases with geomagnetic activity at L = 3–5, while remaining nearly unchanged at L = 6. Its pitch‐angle distribution is consistently bidirectional and field‐aligned, with little dependence on geomagnetic conditions. These distinct spatial and pitch‐angle characteristics indicate that the observed H ⁺ ions correspond to the inner part of the warm plasma cloak, whereas the observed O ⁺ ions represent the high‐energy tail of the oxygen torus. The results further suggest that low‐energy O ⁺ ions are supplied directly from the nightside ionosphere along geomagnetic field lines, rather than being transported inward from the outer magnetosphere. Our findings demonstrate that the warm plasma cloak and the oxygen torus constitute likely independent plasma populations in the inner magnetosphere.

This paper provides the preparation activities for radio observations of Jupiter by Radio and Plasma Wave Investigation (RPWI) aboard ESA JUpiter ICy moons Explorer (JUICE), from the view for its high frequency radio observation capability in 20k –40MHz.This part enables the characterization of Jovian radio emissions (including gonio-polarimetry), passive radio sounding of the ionospheric densities of icy moons, and passive sub-surface radar measurements. It has an enough capability to detect Jovian radio emissions from magnetosphere (aurora etc.), atmosphere (lightning), and icy moons. Direction and polarization capabilities are first enabled in the Jovian system, to identify their source locations and characteristics.(1) Initial flux calibration was executed using solar type III radio bursts during the August 2024 Lunar-Earth Gravity Assist, using the measurements with Wind and PSP, and the sensitivity in 0.4-2 and 10-15 MHz were estimated (Vecchio et al., in printing). Caliblation of direction and polarization with antenna pattern is also now in preparation based on Cassini RPWS logics (Waters Collet et al.).(2) Distribution and occurrence characteristics of terrestrial Auroral Kilometric Radiation (AKR) and Jovian radiations are investigated for the detection capability in Earth flyby and Jovian observations (Yamanaka et al.; Sugawara et al.).(3) Evaluation of the sounding capability of the icy moons' ion density profiles were studied using Jovian auroral radiations, with ray tracing studies (Yasuda et al., 2024). This technique is also applied for Titan ionosphere (Yasuda et al, submitted; T. Katoh et al.).(4) Feasibility studies of the passive radar capability for the icy moons' surface and subsurface were studied with numerical and ray tracing studies, compared with Lunar AKR reflection detected by Kaguya (Kawakami et al.).We also summarize the radio observation plans for Jupiter and icy moons in 2030s.

Fluorescence line diagnostics of the Fe K α line at ∼6.4 keV observed in both solar and stellar flares can constrain the latitude and size of the flare loop, even in the absence of imaging observations. However, they are hampered by the unresolved origin of stellar Fe K α lines; i.e., it is unclear which of two mechanisms—photoionization by hard X-ray photons or collisional ionization by nonthermal electrons—is the dominant process. We present clear evidence for the photoionization origin based on simultaneous far-ultraviolet (FUV) and soft X-ray observations of a superflare on the RS Canum Venaticorum–type star UX Arietis with the Extreme Ultraviolet Spectroscope for Exospheric Dynamics instrument (900−1480 Å) on board the Hisaki and Neutron Star Interior Composition Explorer (0.2−12 keV). The flare started at 22:50 UT on 2018 November 15 and released 2 × 10 ³⁶ erg in the 900−1480 Å band and 3 × 10 ³⁶ erg in the 0.3−4 keV band. The FUV emission, a proxy for nonthermal activity, peaked approximately 1.4 hr before the soft X-rays. In contrast, the Fe K α line, detected at a statistical significance of 5.3 σ with an equivalent width of 6 7 − 20 + 28 eV, peaked simultaneously with the thermal X-ray maximum rather than the nonthermal FUV peak—strongly supporting the photoionization hypothesis. Radiative transfer calculations, combined with the observed Fe K α line intensity, further support the photoionization scenario and demonstrate the potential of this line to provide the flare geometry.

Cold plasma in the ionosphere-plasmasphere system controls various physical processes in the near-Earth space, including energization and loss of particles, coupling mechanisms and propagation of radio waves. The physical processes in the ionosphere and plasmasphere are different but tightly coupled. Ionospheric particles fill the plasmasphere on the dayside, while the plasmaspheric particles descend into the ionosphere at night and help sustain it in the absence of photoionization. However, how exactly these populations evolve and transfer into one another remains poorly understood due to two longstanding challenges. First, the measurements have been separated by a gap at altitudes of ~800-8000 km. Second, fundamentally different measurement techniques have been used in the two regions, and they have never been compared. We address both of these issues and show that two of the most prominent techniques to measure Ne in the two regions - using wave spectra and radio occultations - yield near-identical results. Using observations by the Arase mission at altitudes 440–32,000 km, we demonstrate that it is possible to bridge the gap between the ionosphere and plasmasphere. This result opens many opportunities for studying fundamental scientific questions that relate to the lifecycle of cold plasma in geospace, as well as coupling with the thermosphere and magnetosphere, influence of cold plasma on magnetic reconnection, and setting the inner boundary conditions for physics-based simulations, including global magnetospheric MHD codes. We additionally discuss the current state and the future improvements of empirical and physics-based models of the ionosphere-plasmasphere system.

Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly (LAPYUTA) is a future ultraviolet (UV) space telescope that is selected as a candidate for JAXA's 6th M-class mission. Launch is planned for the early 2030s. LAPYUTA will perform spectroscopic and imaging observations in the far-ultraviolet spectral range (110-190 nm) with a large effective area (>300 cm2) and a high spatial resolution (0.1 arcsec). LAPYUTA has the following four objectives: (1) atmospheres of solar system planets, (2) atmospheres of exoplanets around the habitable zone, (3) structures of present-day galaxies, and (4) synthesis process of heavy elements from observations of neutron star mergers. The key to addressing these scientific goals is the measurement of the physical state of hydrogen, oxygen, and carbon. These elements are common in the universe and are involved in understanding the structure and evolution of the universe at various spatial scales, from planets to stars to galaxies, and UV spectral measurement is adequate for measuring the physical state of the elements. LAPYUTA aims to achieve resolution and sensitivity in the far-UV wavelength range comparable to the Hubble Space Telescope (HST) while using JAXA’s small scientific satellite. The mission part consists of a Cassegrain telescope with a 60 cm aperture primary mirror, four focal plane instruments, a medium dispersion spectrograph (MRS), a high dispersion spectrograph (HRS), a UV slit imager (UVSI), and a wide-field fine guide sensor (FGS). To achieve a highly effective area and high angular resolution, we are developing three key technologies: UV mirror deposition, a large high-precision detector, and a pointing disturbance correction function, as well as studying the concept of the telescope structure. The key technologies for ultraviolet observations developed here will serve as a stepping stone for Japan's participation in the Habitable Worlds Observatory (HWO).

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

The Jupiter and Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA) will investigate the Jovian system with multiple instruments over several years, beginning in early 2031. This paper describes the historical context and state of knowledge, as well as JUICE’s scientific goals and measurement techniques of the satellites that will not be encountered in close flybys. These include the large volcanically active moon Io, the four small inner moons Metis, Adrastea, Amalthea, and Thebe, and the numerous small Irregular (outer) moons. JUICE will provide multiple opportunities to observe Io from relatively remote distances of hundreds of thousands of kilometers. These observations will enable monitoring of Io’s surface for changes, and for the study of its neutral clouds and plasma torus. Io observations will be performed with the four optical remote sensing instruments and with the Particle Environment Package. For the small inner moons it is planned to obtain complete geographic longitude (scales up to 8 km/px), solar-phase and multi-color coverage, oblique polar views, and UV to near-IR spectra. Astrometric measurements will also be performed. The Irregular moons will mostly appear unresolved to the JUICE instruments. Nonetheless, long-duration disk-integrated lightcurves will be acquired to derive rotation periods, object dimensions, pole-axis orientations, and colors for most objects for the first time. From these data, convex-shape models will be generated and phase curves determined. Furthermore, the precision of the orbital elements will be improved via accurate astrometry. UV and near-IR measurements will be attempted for the largest of these objects.

The Io plasma torus (IPT) is a dense plasma cloud that corotates around Jupiter near Io’s orbit. A dawn-to-dusk electric field shifts the IPT dawnward. The shift orientation has been previously measured, but the observing geometries have limited our understanding of the spatial extent of the dawn-to-dusk electric field in the day–night direction. This study probes the dawn-to-dusk electric field using Io’s auroral footprints. The position of the Io footprint corresponds to the travel time of the Alfvén waves from Io, which depends on the local plasma mass density along the magnetic flux tube connected to the moon. By modeling the Alfvén wave propagation, we retrieved the ion mass density and temperature at Io’s orbit from the footprint positions measured by the Juno Ultraviolet Spectrograph from 2016 to 2022, but the two parameters are degenerate. We found that the flux tube mass content (FTMC)—the total mass integrated along the magnetic field line—is a more robust proxy for variability in the plasma conditions at Io’s orbit. The deduced Io-FTMC is correlated with Io’s local time (LT) in the magnetosphere. The peak Io-FTMC is found at 03:42 ± 00:53 LT, suggesting the dawn-to-dusk electric field is oriented toward a predusk direction. The estimated dawn-to-dusk electric field is 3.0–20.0 mV m ⁻¹ , with an upper limit exceeding the previously observed values. This may reflect temporal variations in the solar wind dynamic pressure and in the local plasma conditions driven by Io’s volcanic activity over the Juno era.