西野 真木

Abstract Though the Moon does not possess a global magnetic field like the Earth, there are localized crustal magnetic fields on the lunar surface. Because of the plasma interaction with the crustal magnetic fields, electrostatic and electromagnetic environments near magnetized regions can differ from those near non‐magnetized regions on the Moon. Previous studies observationally revealed the difference in the electrostatic potential on the lunar surface between magnetized and non‐magnetized regions of the Moon in the solar wind, which was attributed to upward electric fields formed by electron‐ion decoupling above the magnetic anomaly regions. However, these inhomogeneous distributions of surface potentials associated with lunar crustal magnetic fields remain uncharacterized in plasma regimes different from the solar wind. In this study, we use a large number of observations by Kaguya and a numerical model of photoelectrons emitted from the sunlit lunar surface to investigate the horizontal distributions of the lunar surface potential in the terrestrial magnetotail lobes. We estimate the relative surface potential variations from the measured energy shift of lunar surface photoelectrons. The results indicate that photoelectrons emitted from relatively strong crustal magnetic field regions tend to be more decelerated, suggesting more positive potentials on the magnetized surface. This implies that upward electric fields are formed by the interaction of terrestrial magnetotail plasma with the lunar crustal magnetic fields in a similar manner to the solar wind interaction with lunar crustal magnetic fields.

Abstract Although the Moon does not have a global intrinsic magnetic field, lunar crustal magnetic anomalies (LMAs) are nonuniformly distributed over the lunar surface. The interaction between the solar wind and LMAs leads to the formation of mini-magnetospheres. Since the spatial scales of LMAs are very small, below several tens of kilometers, solar wind ions are demagnetized while electrons are still magnetized, forming Hall electric fields typically at low altitudes ($$<\sim$$30 km). Since direct observations of these interaction regions are challenging from typical nominal altitudes of lunar orbiters ($$>\sim$$100 km), the solar wind-LMA interaction has not been fully understood. In this study, we analyze low-altitude data obtained by Kaguya over various LMAs to comprehensively characterize the plasma environment and electromagnetic fields in the solar wind-LMA interaction region. We observe strong solar wind ion reflection and whistler mode waves at 1–10 Hz under high solar wind dynamic pressure and strong interplanetary magnetic field conditions, respectively. These trends are particularly clear over spatially extended LMAs. Over both spatially isolated and extended LMAs, strong Broadband Electrostatic Noise at 1–10 kHz tends to be observed when the spacecraft is magnetically connected to the lunar surface. In addition, our results suggest that anti-moonward electrostatic fields at low altitudes contribute to the acceleration, deceleration, and reflection of incident solar wind particles, and the resulting modification of particle velocity distribution functions can strongly influence the nature of the solar wind-LMA interaction including plasma wave excitation. Based on Kaguya data, we also develop a predictable indicator of the central interaction region where solar wind ions and electrons are decoupled. We propose that this indicator can be utilized to define regions of interest for future low-altitude or lander missions to LMA. Graphical Abstract

Abstract We present observations on 24 April 2023 by the Magnetospheric Multiscale spacecraft at the dayside, mid‐latitude magnetopause, when an interplanetary magnetic cloud (MC) with sub‐Alfvénic flows and northward and dawnward interplanetary magnetic field components impacted Earth's magnetosphere. The aim is to reveal the processes of solar wind‐magnetosphere interaction under sub‐Alfvénic solar wind with northward magnetic field. Our analysis of electron and ion data suggests that magnetopause reconnection occurred near both polar cusps, forming boundary layers on closed magnetic field lines on both the solar wind (i.e., MC) and magnetospheric sides of the magnetopause. Grad‐Shafranov, electron‐magnetohydrodynamics, and polynomial reconstructions of magnetopause current layers show that local (equator‐of‐the‐cusp) reconnection occurred in a sub‐ion‐scale magnetopause current sheet with a low magnetic shear angle (30°). Interestingly, the local reconnection was observed between the two (MC‐side and magnetosphere‐side) layers of closed field lines. It indicates that reconnected field lines from double cusp reconnection were interacting to induce another reconnection at the mid‐latitude magnetopause. Our results suggest that magnetopause reconnection was more efficient or frequent under sub‐Alfvénic solar wind with much lower beta plasma conditions than typical conditions. We discuss the role of such efficient reconnection in the formation of low‐latitude boundary layers.

Abstract We analyze data acquired by the Kaguya satellite on 14 October 2008 when the Moon was in the terrestrial magnetotail lobe to gain new insight into the energization of ions originating from the Moon. The Moon‐originating ions were detected over a broad range of latitudes from −80° to 50° above the Moon's dayside at ∼100 km altitude. The fluxes of the Moon‐originating ions were observed at energies from ∼50 to ∼1,000 eV. Additionally, these ions exhibited a wide distribution pitch angle spanning from ∼45 to 90°. The energy levels of ions originating from the Moon show rapid changes, either increasing or decreasing by a factor of ∼10 within 8 min without the solar zenith angle dependence. Such rapid energy changes were observed over the highland regions. These observations are discussed in light of possible acceleration mechanisms of Moon‐originating ions, including temporal and spatial effects.

Abstract In the tenuous atmospheric bodies of our solar system, space weathering on the celestial surface is an important process for its chemical and physical evolution and ambient environment on timescales of celestial evolution. Space plasma is a dominant energy and material source for space weathering. Plasma irradiation experiment in the laboratory is an effective method for modeling space weathering driven by space plasma. However, comprehensive modeling of plasma space weathering has not yet been conducted because the capabilities of the earlier facilities were not optimized for realistic space weathering; for example, the incident electron and ion were not irradiated in the same condition. Here, we developed a plasma irradiation system, Plasma Irradiation Emulator for Celestial Environments (PIECE) of the solar system bodies, which reproduces plasma space weathering in tenuous atmospheric bodies by the electron and ion irradiations in the same condition. We successfully developed a system with high electron and ion number fluxes of $$\sim 10^{13} - 10^{16} {\text{ particles cm } }^{ { - {2 } } } {\text{s } }^{ { - {1 } } }$$ at any acceleration energy in the range of 1–30 keV, which leads to a fluence of e.g., $$\sim 10^{18} - 10^{21} {\text{ particles cm } }^{ { - {2 } } } {\text{s } }^{ { - {1 } } }$$, with a 1-day irradiation time. This fluence corresponds to a plasma irradiation time of ~ 10³–10⁶ years on Europa. Graphical Abstract

Abstract Due to the lack of a dense atmosphere, the Moon directly interacts with ambient plasmas and solar radiation, leading to lunar surface charging. Solar X‐rays drive the emission of photoelectrons and Auger electrons from the lunar surface to space. The Auger electrons have characteristic energies intrinsic to the photo‐emitting atoms and were recently identified at the Moon by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) observations. In this study, we developed a numerical model of the energy spectrum of lunar photoelectrons and Auger electrons, thereby comparing the predicted and observed energy spectra. By adjusting a scaling factor, the model well reproduces the ARTEMIS observations obtained in the solar wind, where the energy spectra are minimally affected by surface charging. Meanwhile, the energy spectra obtained in the geomagnetic tail can be significantly altered by lunar surface potentials. We show that it is difficult to determine a unique combination of the scaling factor and the lunar surface potential with the ARTEMIS energy resolution because of a strong parameter degeneracy. Nevertheless, for a fixed scaling factor, a strong correlation is identified between the lunar surface potentials inferred from the shifts of the energy spectra and those from the upward photoelectron beam energies, providing a proof of concept for the use of the photo‐emitted electrons as a new remote sensing tool of the lunar surface potential. We advocate for future observations of lunar electrons with a high energy resolution. This article is protected by copyright. All rights reserved.

Abstract The density of the solar wind plasma near the Earth’s magnetosphere sometimes decreases to only several per cent of the usual value, and such density extrema result in a significant reduction of the dynamic pressure and Alfvén Mach number ($$M_A$$) of the solar wind flow. While a symmetric expansion of the Earth’s magnetosphere by the low dynamic pressure was assumed in previous studies, a global magnetohydrodynamic (MHD) simulation study predicted a remarkable dawn-dusk asymmetry of the magnetospheric shape under low-density solar wind and Parker-spiral interplanetary magnetic field (IMF) configuration. Here, we present observations consistent with the asymmetric deformation of the magnetosphere under low-$$M_A$$ solar wind and Parker-spiral IMF conditions, focusing on the significant expansion of the dawn-flank magnetosphere detected by the Geotail spacecraft. A global MHD simulation reproduced the dawnward expansion of the near-Earth magnetosphere, which was consistent with the observation by Geotail. The solar wind flow had a non-negligible dusk-to-dawn component and partly affected the dawnward expansion of the magnetosphere. Local, roughly Alfvénic sunward acceleration of magnetosheath ions at the dawn flank magnetopause suggests magnetosheath plasma entry into the magnetosphere through open field lines generated by magnetic reconnection at the dayside magnetopause. At the same time, Cluster 1 and 3, located near the southern polar cusp, also detected continuous antisunward ion jets and occasional sunward jets, which are consistent with the occurrence of magnetic reconnection near the southern cusp. These observations suggest that enhanced plasma acceleration at the dayside magnetopause operates under the low-$$M_A$$ solar wind and Parker spiral IMF conditions and that plasma influx across the dawnside magnetopause is at work under such a low-$$M_A$$ condition. These results can be helpful in understanding interactions between low-$$M_A$$ solar/stellar winds and celestial objects, such as inner planets and exoplanets. Graphic Abstract

The Moon and Mercury are airless bodies, thus they are directly exposed to the ambient plasma (ions and electrons), to photons mostly from the Sun from infrared range all the way to X-rays, and to meteoroid fluxes. Direct exposure to these exogenic sources has important consequences for the formation and evolution of planetary surfaces, including altering their chemical makeup and optical properties, and generating neutral gas exosphere. The formation of a thin atmosphere, more specifically a surface bound exosphere, the relevant physical processes for the particle release, particle loss, and the drivers behind these processes are discussed in this review.

<title>Abstract</title>The mass spectrum analyzer (MSA) will perform in situ observations of ions and magnetic fields around Phobos as part of the Martian Moons eXploration (MMX) mission to investigate the origin of the Martian moons and physical processes in the Martian environment. MSA consists of an ion energy mass spectrometer and two magnetometers which will measure velocity distribution functions and mass/charge distributions of low-energy ions and magnetic field vectors, respectively. For the MMX scientific objectives, MSA will observe solar wind ions, those scattered at the Phobos surface, water-related ions generated in the predicted Martian gas torus, secondary ions sputtered from Phobos, and escaping ions from the Martian atmosphere, while monitoring the surrounding magnetic field. MSA will be developed from previous instruments for space plasma missions such as Kaguya, Arase, and BepiColombo/Mio to contribute to the MMX scientific objectives.

Volatiles and refractories represent the two end-members in the volatility range of species in any surface-bounded exosphere. Volatiles include elements that do not interact strongly with the surface, such as neon (detected on the Moon) and helium (detected both on the Moon and at Mercury), but also argon, a noble gas (detected on the Moon) that surprisingly adsorbs at the cold lunar nighttime surface. Refractories include species such as calcium, magnesium, iron, and aluminum, all of which have very strong bonds with the lunar surface and thus need energetic processes to be ejected into the exosphere. Here we focus on the properties of species that have been detected in the exospheres of inner Solar System bodies, specifically the Moon and Mercury, and how they provide important information to understand source and loss processes of these exospheres, as well as their dependence on variations in external drivers.