浅村 和史

During the first Earth gravity assist maneuver of JUICE on 20 August 2024 the spacecraft passed through the Earth plasmasphere for about 2 h. Before closest approach at a distance of 2.1  R _E the Jovian Electron and Ion spectrometer JEI of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm ³ (observed by the RPWI plasma wave instrument) led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion populations. Since the ions are only corotating with the Earth at a velocity of about 1 km s ⁻¹ at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 km s ⁻¹ . For this reason ions with different mass appear at different energies in the energy spectrum observed by the JEI sensor. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for these specific observing conditions it is possible to quantify densities of the different ion species. This response calculation depends critically on the response of the JEI channel electron multipliers to the ion velocity and mass. Since the sensor was operated with a low post-acceleration further laboratory calibrations may be needed for this specific setup. Nevertheless we can already conclude from the observations that ions with mass > 4 amu q ⁻¹ contribute at least 25 % to the observed total ion density. A flux peak observed in the energy spectrum at 15 eV can only be explained by the presence of heavy molecular ions with mass ∼ 30 amu q ⁻¹ . Molecular ions have only been rarely detected in the outer Earth plasmasphere. The observations indicate that the JEI sensor can also be used to achieve ion composition measurements in the exospheres of the Jovian moons.

Abstract 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 .

Abstract 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.

The Juice flyby of Earth in August 2024 gave us the first chance to evaluate the performance of the Jovian Plasma Dynamics and Composition analyzer (JDC) in environments similar to those expected at Jupiter. JDC is one of the sensors belonging to the Particle Environment Package (PEP) on the Juice spacecraft. It measures positive and negative ions as well as electrons in the energy range 1 eV q ⁻¹ to 35 keV q ⁻¹ . One of the most challenging observations at the final destination is the one of the low-energy ion populations in the tenuous ionospheres of Jupiter's icy moons. During the Juice flyby of Earth we discovered that the energies of the positive ions observed by JDC were not easy to interpret due to a problem with the energy sweep. Using measurements made on ground, we were able to reconstruct the observed energies and construct a new sweeping scheme that solves the problem and that will greatly improve future observations. We also used a simulation to explain the effects of the spacecraft velocity and spacecraft potential on the recorded positive ion fluxes when Juice passed through the Earth's plasmasphere. The study highlights the importance of in-flight calibrations for optimizing the scientific return. Planetary flybys give access to multiple low-energy particle populations besides the mono-energetic and highly directional solar wind.

Abstract 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.