齋藤 義文

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 In this study, we investigated terrestrial-origin O⁺ ions below 1 keV around the Moon using data from the Kaguya satellite between December 2007 and June 2009. These terrestrial-origin low-energy O⁺ ions were identified based on three parameters: the periodicity of O⁺ ion count enhancement corresponding to Kaguya’s 2-h orbital period, the count ratio of O⁺ ions to Na⁺ and Al⁺ ions, and the direction of ion bulk velocity in the Sun–Earth direction. We identified three intervals that included such O⁺ ions: 14:30–20:30 UT on June 19, 2008, 19:00 UT on July 16, 2008 to 03:00 UT on July 17, 2008, and 14:00–24:00 UT on June 7, 2009. These intervals were found in the dawn sector, the dusk sector, and the midnight to dawn sector within the magnetotail, respectively. We examined the relation between geomagnetic storm conditions and increases in terrestrial-origin O⁺ ion counts and found that all three intervals occurred during the late recovery phase of moderate/weak magnetic storms. Since moderately/weakly disturbed conditions (Dst = –40 nT to –20 nT) account for approximately 21% of the total time between 1957 and 2016, we suggest that low-energy O⁺ ions from the Earth have a non-negligible impact on the ion composition and the ion mass density in the lunar plasma environment. Graphical abstract

Abstract The structure of the magnetic reconnection boundary, particularly the presence of slow-mode shocks in the near-Earth magnetotail was studied by using magnetospheric multiscale (MMS) observations and 2.5D hybrid simulations. A total of 51 crossings of MMS from 2017 to 2021 were analyzed. We found that the detection percentage of slow-mode shocks in the near-Earth magnetotail is 41%–55%. Previous studies have only reported one slow-mode shock event in the near-Earth magnetotail and a slow-mode shock detection percentage of 10% or lower in the mid-to-distant magnetotail. It was observed that if the high-energy beam region data is removed from the slow-mode shock downstream observations then the detection of slow-mode shocks reduces, implying that the kinetic effects play an important role in the detection of slow-mode shocks. For the crossings where the interface was not identified as a slow-mode shock, it was found that the turbulence in those crossings can change the mass flux values and disrupt the detection of slow-mode shock. However, the macroscopic slow-mode shock-like structure stably exists around the magnetic reconnection interface, as most of the conditions for slow-mode shocks were satisfied. This result suggests that slow-mode shocks are a general feature of magnetic reconnection geometry. We find that the lack of detection of slow-mode shocks in previous observations and simulations can be explained by taking into account the kinetic structure of slow-mode shocks and the presence of turbulence.

Abstract Although solar wind‐driven convection is expected to dominate magnetospheric circulation at Mercury, its exact pattern remains poorly characterized by observations. Here we present BepiColombo Mio observations during the third Mercury flyby indicative of convection‐driven transport of low‐energy dense ions into the deep magnetosphere. During the flyby, Mio observed an energy‐dispersed ion population from the duskside magnetopause to the deep region of the midnight magnetosphere. A comparison of the observations with backward test particle simulations suggests that the observed energy dispersion structure can be explained in terms of energy‐selective transport by convection from the duskside tail magnetopause. We also discuss the properties and origins of more energetic ions observed in the more dipole‐like field regions of the magnetosphere in comparison to previously reported populations of the plasma sheet horn and ring current ions. Additionally, forward test particle simulations predict that most of the observed ions on the nightside will precipitate onto relatively low‐latitude regions of the nightside surface of Mercury for a typical convection case. The presented observations and simulation results reveal the critical role of magnetospheric convection in determining the structure of Mercury's magnetospheric plasma. The upstream driver dependence of magnetospheric convection and its effects on other magnetospheric processes and plasma‐surface interactions should be further investigated by in‐orbit BepiColombo observations.

Abstract The Martian atmospheric Ne may reflect recent gas supply from its mantle via volcanic degassing, due to its short (∼100 Myr) escape timescale. The isotopic ratio of the Martian atmospheric Ne would therefore provide insights into that of the Martian mantle, further suggesting the origin of Mars volatiles during planetary formation. Mass spectrometric analysis of the Martian atmospheric Ne, however, has faced challenges from interference between ²⁰Ne⁺ and ⁴⁰Ar⁺⁺. Previous studies using a polyimide membrane for ²⁰Ne/⁴⁰Ar separation were limited by the drawbacks of elastomeric O-rings to support the membrane, such as low-temperature intolerance, outgassing, and the need to endure environmental conditions during the launch and before/after landing on Mars. This study proposes a new method employing a metal C-ring to secure a 100 μm polyimide sheet within vacuum flanges. Environmental tests, including vibration, shock, extreme temperatures, and radiation exposure, were conducted on the gas separation flanges. Pre- and post-test analyses for He, Ne, and Ar demonstrated the membrane-flange system’s resilience. Gas permeation measurements using terrestrial air effectively permeated ⁴He and ²⁰Ne, while reducing ⁴⁰Ar by more than six orders of magnitude. This study achieved a <3% accuracy in determining the ²⁰Ne/²²Ne ratio, sufficient for assessing the origins of Ne in the Martian mantle. Furthermore, experiments with a 590 Pa gas mixture simulating the Martian atmosphere achieved a 10% accuracy for the ²⁰Ne/²²Ne isotope ratio, with gas abundances consistent with numerical predictions based on individual partial pressures. These results validate the suitability of the developed polyimide membrane assembly for in situ Martian Ne analyses.

Context. The Mercury electron analyzer (MEA) obtained new electron observations during the first three Mercury flybys by BepiColombo on October 1, 2021 (MFB1), June 23 , 2022 (MFB2), and June 19, 2023 (MFB3). BepiColombo entered the dusk side magnetotail from the flank magnetosheath in the northern hemisphere, crossed the Mercury solar orbital equator around midnight in the magnetotail, traveled from midnight to dawn in the southern hemisphere near the closest approach, and exited from the post-dawn magnetosphere into the dayside magnetosheath. Aims. We aim to identify the magnetospheric boundaries and describe the structure and dynamics of the electron populations observed in the various regions explored along the flyby trajectories. Methods. We derive 4s time resolution electron densities and temperatures from MEA observations. We compare and contrast our new BepiColombo electron observations with those obtained from the Mariner 10 scanning electron spectrometer (SES) 49 yr ago. Results. A comparison to the averaged magnetospheric boundary crossings of MESSENGER indicates that the magnetosphere of Mercury was compressed during MFB1, close to its average state during MFB2, and highly compressed during MFB3. Our new MEA observations reveal the presence of a wake effect very close behind Mercury when BepiColombo entered the shadow region, a significant dusk-dawn asymmetry in electron fluxes in the nightside magnetosphere, and strongly fluctuating electrons with energies above 100s eV in the dawnside magnetosphere. Magnetospheric electron densities and temperatures are in the range of 10–30 cm⁻³ and above a few 100s eV in the pre-midnight-sector, and in the range of 1–100 cm⁻³ and well below 100 eV in the post-midnight sector, respectively. Conclusions. The MEA electron observations of different solar wind properties encountered during the first three Mercury flybys reveal the highly dynamic response and variability of the solar wind-magnetosphere interactions at Mercury. A good match is found between the electron plasma parameters derived by MEA in the various regions of the Hermean environment and similar ones derived in a few cases from other instruments on board BepiColombo.

Abstract On 10 August 2021, the Mercury-bound BepiColombo spacecraft performed its second fly-by of Venus and provided a short-lived observation of its induced magnetosphere. Here we report results recorded by the Mass Spectrum Analyzer on board Mio, which reveal the presence of cold O⁺ and C⁺ with an average total flux of ~4 ± 1 × 10⁴ cm⁻² s⁻¹ at a distance of about six planetary radii in a region that has never been explored before. The ratio of escaping C⁺ to O⁺ is at most 0.31 ± 0.2, implying that, in addition to atomic O⁺ ions, CO group ions or water group ions may be a source of the observed O⁺. Simultaneous magnetometer observations suggest that these planetary ions were in the magnetosheath flank in the vicinity of the magnetic pileup boundary downstream. These results have important implications regarding the evolution of Venus’s atmosphere and, in particular, the evolution of water on the surface of the planet.

Abstract We have developed a low-energy particle experiment that alternately measures ions and electrons in space. The ability to switch between ion and electron measurements is achieved by simply adding ultra-thin carbon foils and positive and negative outputs to a conventional top-hat electrostatic analyzer and a high-voltage power supply, respectively. The advantage of this experiment is that it can perform both ion and electron measurements using only one MCP-based detector for electrons, since it detects secondary electrons emitted from the carbon foils. For the SS520-3 sounding rocket program, we prepared two identical energy analyzers, one for ions and the other for electrons to demonstrate this technique. Laboratory tests confirmed that the performance of the two analyzers was comparable to that of conventional analyzers for ion and electrons. The SS520-3 rocket experiment in the high latitude auroral region yielded observations that captured typical features of ions and electrons, which were similar to previous observations. Graphical Abstract

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.

Among nearly 300 near-Mercury tail current sheet crossings performed by the MESSENGER spacecraft, we identified 37 traversals of an asymmetric current sheet, wherein the lobe densities on opposite sides differ by a factor of three or more. These asymmetric current sheet crossings primarily occur on the dawnside. A global magnetohydrodynamic (MHD) simulation was found to be in excellent agreement with the observations. The results suggest that the north–south density asymmetry is caused by solar wind entering via an upstream-connected window in one hemisphere. Furthermore, the Parker spiral interplanetary magnetic field (IMF) controls the near-tail density asymmetries, whereas Mercury's offset dipole magnetic field controls those in mid- or distant-tail regions. We propose that hemispheric asymmetries in Mercury's magnetospheric convection occur under strong IMF conditions.

Abstract During the first flyby of the BepiColombo composite spacecraft at Mercury in October 2021 ion spectrometers observed two intense spectral lines with energies between 10 and 70 eV. The spectral lines persisted also at larger distances from Mercury and were observed again at lower intensity during cruise phase in March 2022 and at the second and third Mercury flyby as a single band. The ion composition indicates that water is the dominant gas source. The outgassing causes the composite spacecraft to charge up to a negative potential of up to −50 V. The distribution and intensity of the lower energy signal depends on the intensity of low energy electron fluxes around the spacecraft which again depend on the magnetic field orientation. We interpret the observation as being caused by water outgassing from different source locations on the spacecraft being ionized in two different regions of the surrounding potential. The interpretation is confirmed by two dimensional particle‐in‐cell simulations.

Mass spectrometers have been widely employed as payloads for planetary exploration missions. The instruments measure the atomic and molecular compositions in planetary samples and are widely applicable to quantitative/qualitative analysis. Unlike remote sensing, mass spectrometers need to be in close to the target bodies for in-situ analysis, thus developing small mass spectrometers is an important issue in expanding the scope of analytical targets. There are additional possibilities for small mass spectrometers, namely piggyback or secondary payloads for missions. The small mass spectrometers may accompany sample return missions and provide complementary observations (e.g., multi-point; time continuity).In this study, we are developing an ultra-small neutral mass spectrometer of which ion optics fits into a volume of 1U (10×10×10 cm3). The mass spectrometer being developed is based on Electron Impact (EI) ionization method and time-of-flight (TOF) mass analyzing technique. The EI ion source is newly developed to achieve low power consumption (~0.5 W), high electron emission (>1.0 mA), and high ionization efficiency (~104 nA/Pa for N2 gas) with small size (~7×3×3 cm3). For the mass analysis part, ion trap electrodes are employed to increase TOF while reducing volume. The ion trap electrodes allow ions to pass around the same optics at specific frequency for each m/z, resulting in a small optics size and long TOF. So far, we developed a test model with an overall optics volume of ~7×7×8 cm3 and a weight of ~0.3 kg. As for performance with a single-lap, the mass resolution was m/Δm ~ 50, the sensitivity was ~1×10-6 (counts/s)/(particle/cc), and the detection limit (i.e., S/N=1) was ~1×104 particle/cc, respectively. In addition, we have completed a design with an improved mass resolution to obtain mass spectra for more than 2 laps, using a numerical simulation. According to the results of the simulation, the improved mass resolution (m/Δm > 1,000) will be achieved with > 100 laps.It should be noted that the new model does not require high voltage pulses; the instrument is expected to be small enough even if the flight-qualified electrical system is included. The timing of ion trap can be arbitrarily determined, making it possible to provide multiple observation modes that take into account the fundamental trade-off between resolution and sensitivity in TOF instruments. Additionally, the new ion source with low power consumption and high emission current is one of the achievements of this work and can be applied to other mass spectrometers as a stand-alone system; the ion source is planned to be used for JAXA/ISRO LUPEX mission.

We derive electron density and temperature from observations obtained by the Mercury Electron Analyzer on board Mio during the cruise phase of BepiColombo while the spacecraft is in a stacked configuration. In order to remove the secondary electron emission contribution, we first fit the core electron population of the solar wind with a Maxwellian distribution. We then subtract the resulting distribution from the complete electron spectrum, and suppress the residual count rates observed at low energies. Hence, our corrected count rates consist of the sum of the fitted Maxwellian core electron population with a contribution at higher energies. We finally estimate the electron density and temperature from the corrected count rates using a classical integration method. We illustrate the results of our derivation for two case studies, including the second Venus flyby of BepiColombo when the Solar Orbiter spacecraft was located nearby, and for a statistical study using observations obtained to date for distances to the Sun ranging from 0.3 to 0.9 A.U. When compared either to measurements of Solar Orbiter or to measurements obtained by HELIOS and Parker Solar Probe, our method leads to a good estimation of the electron density and temperature. Hence, despite the strong limitations arising from the stacked configuration of BepiColombo during its cruise phase, we illustrate how we can retrieve reasonable estimates for the electron density and temperature for timescales from days down to several seconds.

The spacecraft Geotail surveyed the near-Earth plasma sheet from XGSM = −10 to −31 RE and YGSM = −20 to +20 RE during the period from 1994 to 2022. It observed 243 magnetic reconnection events and 785 tailward flow events under various solar wind conditions during plasma sheet residence time of over 23,000 hr. Magnetic reconnections associated with the onset of magnetospheric substorms occur mostly in the range XGSM = −23 to −31 RE. When the solar wind is intense and high substorm activities continue, magnetic reconnection can occur closer to the Earth. The YGSM locations of magnetic reconnections depend on the solar wind conditions and on previous substorm activity. Under normal solar wind conditions, magnetic reconnection occurs preferentially in the pre-midnight plasma sheet. Under conditions with intense (weak) solar wind energy input, however, magnetic reconnection can occur in the post-midnight (duskside) plasma sheet. Continuous substorm activity tends to shift the magnetic reconnection site duskward. The plasma sheet thinning proceeds faster under intense solar wind conditions, and the loading process that provides the preconditions for magnetic reconnection becomes shorter. When magnetic flux piles up during a prolonged period with a strongly northward-oriented interplanetary magnetic field (IMF) Bz, the time necessary to provide the preconditions for magnetic reconnection becomes longer. Although the solar wind conditions are the primary factors that control the location and timing of magnetic reconnections, the plasma sheet conditions created by preceding substorm activity or the strongly northward IMF Bz can modify the solar wind control.

Short-period magnetic enhancements were detected by the MAP-LMAG magnetometer onboard Kaguya orbiting the moon in the solar wind at an altitude of 100 km. The duration was typically 10 s, which corresponds to 0.5 degrees in latitude along the Kaguya orbit and a scale size of 15 km. The magnitude of the magnetic field was enhanced up to 1.5–3.6 times as large as that of the preceding quiet periods. No such magnetic enhancements were found in the upstream solar wind magnetic field. The short-period magnetic enhancements were categorized into 2 groups. One is the sub-ion-gyro-scale limb compression detected at the terminator region of the moon in a nearly constant solar wind magnetic field. The magnetic field flared away from the moon consistently with the previously known limb compressions. The scale size deduced from the duration was 11 km, 85 times as small as that of previously reported limb compressions. It is significantly smaller than the typical proton gyroradius 50–100 km in the solar wind at 1AU. The other types of magnetic enhancements appeared at crossings of magnetic discontinuities of the solar wind. Some of them were found on the nightside of the moon. A possible explanation is that they were magnetic fields compressed by the solar wind ions reflected at the moon channeled back along the current sheet of an interplanetary tangential discontinuity, similar to the hot flow anomalies observed at the Earth’s bow shock. The reflected ions themselves were not detected on the nightside of the moon, while the magnetic field compressed by the expanding region can penetrate through the moon to be detected as magnetic field enhancements on the nightside of the moon. Graphical Abstract: [Figure not available: see fulltext.].

We carried out a statistical study of equatorial dipolarization fronts (DFs) detected by the Magnetospheric Multiscale mission during the full 2017 Earth's magnetotail season. We found that two DF classes are distinguished: class I (74.4%) corresponds to the standard DF properties and energy dissipation and a new class II (25.6%). This new class includes the six DF discussed in Alqeeq et al. (2022, https://doi.org/10.1063/5.0069432) and corresponds to a bump of the magnetic field associated with a minimum in the ion and electron pressures and a reversal of the energy conversion process. The possible origin of this second class is discussed. Both DF classes show that the energy conversion process in the spacecraft frame is driven by the diamagnetic current dominated by the ion pressure gradient. In the fluid frame, it is driven by the electron pressure gradient. In addition, we have shown that the energy conversion processes are not homogeneous at the electron scale mostly due to the variations of the electric fields for both DF classes.

Abstract Mercury’s magnetosphere is known to involve fundamental processes releasing particles and energy like at Earth due to the solar wind interaction. The resulting cycle is however much faster and involves acceleration, transport, loss, and recycling of plasma. Direct experimental evidence for the roles of electrons during this cycle is however missing. Here we show that in-situ plasma observations obtained during BepiColombo’s first Mercury flyby reveal a compressed magnetosphere hosts of quasi-periodic fluctuations, including the original observation of dynamic phenomena in the post-midnight, southern magnetosphere. The energy-time dispersed electron enhancements support the occurrence of substorm-related, multiple, impulsive injections of electrons that ultimately precipitate onto its surface and induce X-ray fluorescence. These observations reveal that electron injections and subsequent energy-dependent drift now observed throughout Solar System is a universal mechanism that generates aurorae despite the differences in structure and dynamics of the planetary magnetospheres.

Abstract Remotely sensed interplanetary scintillation (IPS) data from the Institute for Space-Earth Environmental Research (ISEE), Japan, allows a determination of solar-wind parameters throughout the inner heliosphere. We show the 3D analysis technique developed for these data sets that forecast plasma velocity, density, and component magnetic fields at Earth, as well at the other inner heliospheric planets and spacecraft. One excellent coronal mass ejection (CME) example that occurred on the 10 March 2022 was viewed not only in the ISEE IPS analyses, but also by the spacecraft near Earth that measured the CME arrival at one AU. Solar Orbiter, that was nearly aligned along the Earth radial at 0.45 AU, also measured the CME in plasma density, velocity, and magnetic field. BepiColombo at 0.42 AU was also aligned with the STEREO A spacecraft, and viewed this CME. The instruments used here from BepiColombo include: 1) the European-Space-Agency Mercury-Planetary-Orbiter magnetic field measurements; 2) the Japan Aerospace Exploration Agency Mio spacecraft Solar Particle Monitor that viewed the CME Forbush decrease, and the Mercury Plasma Experiment/Mercury Electron Analyzer instruments that measured particles and solar-wind density from below the spacecraft protective sunshield covering. This article summarizes the analysis using ISEE, Japan real-time data for these forecasts: it provides a synopsis of the results and confirmation of the CME event morphology after its arrival, and discusses how future IPS analyses can augment these results.

Abstract The second Venus flyby of the BepiColombo mission offer a unique opportunity to make a complete tour of one of the few gas-dynamics dominated interaction regions between the supersonic solar wind and a Solar System object. The spacecraft pass through the full Venusian magnetosheath following the plasma streamlines, and cross the subsolar stagnation region during very stable solar wind conditions as observed upstream by the neighboring Solar Orbiter mission. These rare multipoint synergistic observations and stable conditions experimentally confirm what was previously predicted for the barely-explored stagnation region close to solar minimum. Here, we show that this region has a large extend, up to an altitude of 1900 km, and the estimated low energy transfer near the subsolar point confirm that the atmosphere of Venus, despite being non-magnetized and less conductive due to lower ultraviolet flux at solar minimum, is capable of withstanding the solar wind under low dynamic pressure.

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

Wave–particle interactions are fundamental processes in space plasma, and some plasma waves, including electrostatic solitary waves (ESWs), are recognised as broadband noises (BBNs) in the electric field spectral data. Spacecraft observations in recent decades have detected BBNs around the Moon, but the generation mechanism of the BBNs is not fully understood. Here, we study a wake boundary traversal with BBNs observed by Kaguya, which includes an ESW event previously reported by Hashimoto et al. Geophys Res Lett 37:L19204 https://doi.org/10.1029/2010GL044529 (2010). Focusing on the relation between BBNs and electron pitch-angle distribution functions, we show that upward electron beams from the nightside lunar surface are effective for the generation of BBNs, in contrast to the original interpretation by Hashimoto et al. Geophys Res Lett 37:L19204 https://doi.org/10.1029/2010GL044529 (2010) that high-energy electrons accelerated by strong ambipolar electric fields excite ESWs in the region far from the Moon. When the BBNs were observed by the Kaguya spacecraft in the wake boundary, the spacecraft’s location was magnetically connected to the nightside lunar surface, and bi-streaming electron distributions of downward-going solar wind strahl component and upward-going field-aligned beams (at ∼ 124 eV) were detected. The interplanetary magnetic field was dominated by a positive BZ (i.e. the northward component), and strahl electrons travelled in the antiparallel direction to the interplanetary magnetic field (i.e. southward), which enabled the strahl electrons to precipitate onto the nightside lunar surface directly. The incident solar wind electrons cause negative charging of the nightside lunar surface, which generates downward electric fields that accelerate electrons from the nightside surface toward higher altitudes along the magnetic field. The bidirectional electron distribution is not a sufficient condition for the BBN generation, and the distribution of upward electron beams seems to be correlated with the BBNs. Ambipolar electric fields in the wake boundary should also contribute to the electron acceleration toward higher altitudes and further intrusion of the solar wind ions into the deeper wake. We suggest that solar wind ion intrusion into the wake boundary is also an important factor that controls the BBN generation by facilitating the influx of solar wind electrons there. Graphical Abstract: [Figure not available: see fulltext.]

Abstract Electromagnetic whistler-mode waves in space plasmas play critical roles in collisionless energy transfer between the electrons and the electromagnetic field. Although resonant interactions have been considered as the likely generation process of the waves, observational identification has been extremely difficult due to the short time scale of resonant electron dynamics. Here we show strong nongyrotropy, which rotate with the wave, of cyclotron resonant electrons as direct evidence for the locally ongoing secular energy transfer from the resonant electrons to the whistler-mode waves using ultra-high temporal resolution data obtained by NASA’s Magnetospheric Multiscale (MMS) mission in the magnetosheath. The nongyrotropic electrons carry a resonant current, which is the energy source of the wave as predicted by the nonlinear wave growth theory. This result proves the nonlinear wave growth theory, and furthermore demonstrates that the degree of nongyrotropy, which cannot be predicted even by that nonlinear theory, can be studied by observations.

Ion sources using electron impact ionization (EI) methods have been widely accepted in mass spectrometry for planetary exploration missions because of their simplicity. Previous space-borne mass spectrometers were primarily designed with the EI method using rhenium tungsten alloy filaments, enabling up to 100-200 ?mu A emission in typical cases. The emission level is desired to be enhanced because the sensitivity of mass spectrometers is a critical requirement for the future in situ mass spectrometry related to the measurement of trace components in planetary samples. In this study, we developed a new high-emission EI ion source using a Y2O3-coated iridium filament, which has a lower work function than rhenium tungsten alloy. The size of the ion source was 30 ?mm ?x ?26 ?mm ?x ?70 ?mm, and its weight was similar to 70 ?g. We confirmed that when consuming similar to 3.0 ?W power, the ion source records 1-2 ?mA electrons, which is 10 times greater than the conventional models' electron emission level. We verified the linearity of ionization efficiency and the electron current in the range of 0.1-1 ?mA, which indicates our new model increased the ionization efficiency. We conducted performance tests on the prototype with the 3.0 ?W heating condition, confirming a high ionization efficiency (similar to 10(4) ?nA/Pa). In addition, we conducted endurance tests of the ion source and demonstrated the persistence of the ionization efficiency for 30 ?min ?x ?100 cycles.

We present initial results of low-energy ion observations from BepiColombo's first Mercury flyby. Unprecedentedly high time resolution measurements of low energy ions at Mercury by BepiColombo Mio reveal rapid (a few seconds) and large (1–2 orders of magnitude) fluctuations of ion flux around the magnetopause and within the magnetosphere. Around the magnetic equator in the pre-midnight magnetotail, Mio observed plasma sheet ions consistent with previous observations. In the midnight magnetotail near the closest approach, Mio observed the co-existence of high-energy (∼keV/q) and low-energy (<∼300 eV/q) ion components. The low-energy component is inferred to be cold with a temperature well below 100 eV and have a major contribution to the total density as opposed to previously reported cold tenuous ions. Future observations by Mio will provide insights into the sources, transport, and acceleration of the newly identified ion components.

The fundamental processes responsible for energy exchange between large-scale electromagnetic fields and plasma are well understood theoretically, but in practice these theories have not been tested. These processes are ubiquitous in all plasmas, especially at the interface between high and low beta plasmas in planetary magnetospheres and other magnetic environments. Although such boundaries pervade the plasma Universe, the processes responsible for the release of the stored magnetic and thermal plasma energy have not been fully identified and the importance of the relative impact of each process is unknown. Despite advances in understanding energy release through the conversion of magnetic to kinetic energy in magnetic reconnection, how the extreme pressures in the regions between stretched and more relaxed field lines in the transition region are balanced and released through adiabatic convection of plasma and fields is still a mystery. Recent theoretical advances and the predictions of large-scale instabilities must be tested. In essence, the processes responsible remain poorly understood and the problem unresolved. The aim of the White Paper submitted to ESA's Voyage 2050 call, and the contents of this paper, is to highlight three outstanding open science questions that are of clear international interest: (i) the interplay of local and global plasma physics processes: (ii) the partitioning during energy conversion between electromagnetic and plasma energy: and (iii) what processes drive the coupling between low and high beta plasmas. We present a discussion of the new measurements and technological advances required from current state-of-the-art, and several candidate mission profiles with which these international high-priority science goals could be significantly advanced.

At Mercury, several processes can release ions and neutrals out of the planet's surface. Here we present enhancements of planetary ions (Na+-group ions) in Mercury's northern magnetospheric cusp during flux transfer event (FTE) "showers." FTE showers are intervals of intense dayside magnetopause reconnection, during which FTEs are observed in quick succession, that is, only separated by a few seconds. This study identifies 1953 FTE shower intervals and 1795 Non-FTE shower intervals. During the shower intervals, this study shows that the FTEs form a solar wind entry layer equatorward of the northern magnetospheric cusp. In this entry layer, solar wind ions are accelerated and move downward (i.e., planetward) toward the cusp, which sputter upward-moving planetary ions with a particle flux of 1 x 10(11) m(-2) s(-1) within 1 min. The precipitation rate is estimated to increase by an order of magnitude during FTE showers, to 2 x 10(25) s(-1), and the neutral density of the exosphere could vary by >10% in response to this FTE-driven sputtering. Such rapid large-scale variations driven by dayside reconnection may explain the minute-to-minute changes in Mercury's exosphere, especially on the high latitudes, observed by ground-based telescopes on Earth. Our MESSENGER in situ observation of enhanced planetary ions in the entry layer likely corresponds to an escape channel for Mercury's planetary ions. Comprehensive, future multipoint measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury's dynamic exosphere and magnetosphere.

This study analyzes the flux transfer event (FTE)-type flux ropes and magnetic reconnection around the day-side magnetopause during BepiColombo's Earth flyby. The magnetosheath has a high plasma beta (similar to 8), and the interplanetary magnetic field (IMF) has a significant radial component. Six flux ropes are identified around the magnetopause. The motion of flux ropes together with the maximum magnetic shear model suggests that the reconnection X-line possibly swipes BepiColombo near the magnetic equator due to an increase in the radial component of the IMF. The flux rope with the highest flux content contains a clear coalescence signature, i.e., two smaller flux ropes merge, supporting theoretical predictions that the flux contents of flux ropes can grow through coalescence. The coalescence of the two FTE-type flux ropes takes place through secondary reconnection at the point of contact between the two flux ropes. The BepiColombo measurements indicate a large normalized guide field and a reconnection rate comparable to that measured at the magnetopause (similar to 0.1).

The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2023. The Ganymede Laser Altimeter (GALA) is an instrument onboard JUICE, whose main scientific goals are to understand ice tectonics based on topographic data, the subsurface structure by measuring tidal response, and small-scale roughness and albedo of the surface. In addition, from the perspective of astrobiology, it is imperative to study the subsurface ocean scientifically. The development of GALA has proceeded through an international collaboration between Germany (the lead), Japan, Switzerland, and Spain. Within this framework, the Japanese team (GALA-J) is responsible for developing three receiver modules: the Backend Optics (BEO), the Focal Plane Assembly (FPA), and the Analog Electronics Module (AEM). Like the German team, GALA-J also developed software to simulate the performance of the entire GALA system (performance model). In July 2020, the Proto-Flight Models of BEO, FPA, and AEM were delivered from Japan to Germany. This paper presents an overview of JUICE/GALA and its scientific objectives and describes the instrumentation, mainly focusing on Japan's contribution.

We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm's law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the electromagnetic field to the plasma (J · E > 0) ahead or at the DF, whereas it is the opposite (J · E < 0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J · E ′ < 0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.

The solar wind particles reflected by the lunar magnetic field are the major energy source of electromagnetic wave activities, such as the 100 s magnetohydrodynamic waves and the 1 Hz whistler-mode waves generated by protons and the non-monochromatic whistler-mode waves generated by mirror-reflected electrons. Kaguya found a new type of whistler-mode waves at 100 km altitude above the polar regions of the Moon with a broad frequency range of 1–16 Hz. The waves appear diffuse in both the time and frequency domains, and their occurrence is less sensitive to the magnetic connection to the lunar surface. The polarization is right-handed with respect to the background magnetic field, and the wave number vector is nearly parallel to the magnetic field perpendicular to the solar wind flow. The diffuse waves are thought to be generated by the solar wind ions reflected by the lunar magnetic field through cyclotron resonance. The resonant ions are expected to have a velocity component parallel to the magnetic field larger than the solar wind bulk speed; however, such ions were not always simultaneously detected by Kaguya. The waves may have been generated above the dayside of the Moon and then propagated along the magnetic field being convected by the solar wind to reach the polar regions to be detected by Kaguya.

BepiColombo is a joint mission between the European Space Agency, ESA, and the Japanese Aerospace Exploration Agency, JAXA, to perform a comprehensive exploration of Mercury. Launched on 20 th October 2018 from the European spaceport in Kourou, French Guiana, the spacecraft is now en route to Mercury. Two orbiters have been sent to Mercury and will be put into dedicated, polar orbits around the planet to study the planet and its environment. One orbiter, Mio, is provided by JAXA, and one orbiter, MPO, is provided by ESA. The scientific payload of both spacecraft will provide detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. Mercury is the planet closest to the Sun, the only terrestrial planet besides Earth with a self-sustained magnetic field, and the smallest planet in our Solar System. It is a key planet for understanding the evolutionary history of our Solar System and therefore also for the question of how the Earth and our Planetary System were formed. The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere, and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein’s theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload such that individual measurements can be interrelated and complement each other.

The Advanced Small Analyzer for Neutrals (ASAN) on board the Chang'E-4 Yutu-2 rover first detected energetic neutral atoms (ENAs) originating from the lunar surface at various lunar local times on the lunar farside. In this work, we examine the ENA energy spectra, obtained in the first 23 lunar days from 2019 January 11 to 2020 October 12, and find a higher ENA differential flux on the lunar dawnside than on the duskside. Combined with Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) data, we analyze the correlation between the ENA differential flux and solar wind parameters, such as flux, density, dynamic pressure, and velocity, for each ASAN energy channel on the dawnside and duskside. The results show that ENA differential flux is positively correlated with solar wind flux, density, and dynamic pressure and relatively lower on the duskside than on the dawnside. To determine the relationship between solar wind energy and ENA energy, we analyze the correlation between solar wind energy and ENA cutoff energy and temperature on the dawnside and duskside. The results show that the ENA cutoff energy and temperature are lower on the duskside than on the dawnside at the same solar wind energy. The difference between the ENA-solar wind observation on the dawnside and duskside is possibly caused by solar wind deflection and deceleration on the duskside, which can be attributed to the interaction between solar wind and the lunar magnetic anomalies located nearby in the northwestern direction of the Chang'E-4 landing site.

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. [Figure not available: see fulltext.]

Out of the two Venus flybys that BepiColombo uses as a gravity assist manoeuvre to finally arrive at Mercury, the first took place on 15 October 2020. After passing the bow shock, the spacecraft travelled along the induced magnetotail, crossing it mainly in the YVSO direction. In this paper, the BepiColombo Mercury Planetary Orbiter Magnetometer (MPO-MAG) data are discussed, with support from three other plasma instruments: the Planetary Ion Camera (SERENA-PICAM) of the SERENA suite, the Mercury Electron Analyser (MEA), and the BepiColombo Radiation Monitor (BERM). Behind the bow shock crossing, the magnetic field showed a draping pattern consistent with field lines connected to the interplanetary magnetic field wrapping around the planet. This flyby showed a highly active magnetotail, with e.g. strong flapping motions at a period of ĝ1/47ĝmin. This activity was driven by solar wind conditions. Just before this flyby, Venus's induced magnetosphere was impacted by a stealth coronal mass ejection, of which the trailing side was still interacting with it during the flyby. This flyby is a unique opportunity to study the full length and structure of the induced magnetotail of Venus, indicating that the tail was most likely still present at about 48 Venus radii.

The Moon drives observable perturbations in the upstream solar wind in a similar manner to the terrestrial foreshock. Recent observations suggested that lunar dayside electrostatic waves can arise from two different driving mechanisms, both involving reflected particles from lunar crustal magnetic fields. However, their association with the global distribution of lunar magnetic anomalies have not been fully characterized. Here we exploit polar orbiting Kaguya to generate first global maps of electrostatic waves and solar wind electron modification above the day side of the Moon. The maps clearly demonstrate that the two signatures are correlated with lunar crustal magnetic fields. Additionally, we observe different characteristics of electron modification for different interplanetary magnetic field orientations. The lunar crustal magnetic fields cause a wide range of reflected electron and ion intensities, thereby serving as a test bed to investigate the relative roles of reflected particles on wave excitation and particle heating.

A structure of polarization of magnetic field variation in the frequency range from 0.01 to 0.3 Hz was found by Kaguya in the lunar wake. In a dawnward-directed magnetic field, the polarization was left-handed in the southern hemisphere and right-handed in the northern hemisphere of the lunar wake. The sense of rotation was consistent with vortices behind an obstacle in a fast stream, although no such vortices were detected by Kaguya. The direction of magnetic field variation was predominantly perpendicular to the background magnetic field, suggesting that the waves were of shear Alfvén mode. The same polarization structure was found in rotated magnetic fields. In a selenocentric coordinate system with x axis antiparallel to the solar wind flow and the magnetic field in x-z plane, the polarization was dependent only on y position of the spacecraft. The polarization structure was stable and unchanged in various solar wind conditions showing no preferences. It was observed in the magnetosheath as well as in the solar wind, but not in the terrestrial magnetosphere. The power density showed preferences to high-temperature, high-speed solar wind. The power density was higher at the wake boundary than in the center of the wake, suggesting that the magnetic fluctuations were generated at the wake boundary by ions refilling the wake, and propagate into the deep wake as shear Alfvén waves.

BepiColombo Mio (previously called MMO: Mercury Magnetospheric Orbiter) was successfully launched by Ariane 5 from Kourou, French Guiana on October 20, 2018. The Mercury Plasma/Particle Experiment (MPPE) is a comprehensive instrument package onboard Mio spacecraft used for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors including two Mercury Electron Analyzers (MEA1 and MEA2), Mercury Ion Analyzer (MIA), Mass Spectrum Analyzer (MSA), High Energy Particle instrument for electron (HEP-ele), High Energy Particle instrument for ion (HEP-ion), and Energetic Neutrals Analyzer (ENA). Significant efforts were made pre-flight to calibrate all of the MPPE sensors at the appropriate facilities on the ground. High voltage commissioning of MPPE analyzers was successfully performed between June and August 2019 and in February 2020 following the completion of the low voltage commissioning in November 2018. Although all of the MPPE analyzers are now ready to begin observation, the full service performance has been delayed until Mio’s arrival at Mercury. Most of the fields of view (FOVs) of the MPPE analyzers are blocked by the thermal shield surrounding the Mio spacecraft during the cruising phase. Together with other instruments on Mio including Magnetic Field Investigation (MGF) and Plasma Wave Investigation (PWI) that measure plasma field parameters, MPPE will contribute to the comprehensive understanding of the plasma environment around Mercury when BepiColombo/Mio begins observation after arriving at the planet Mercury in December 2025.

An energy spectrum of electrons from 180 to 550 keV precipitating into the dayside polar ionosphere was observed under a geomagnetically quiet condition (AE ≤ 100 nT, Kp = 1-). The observation was carried out at 73–184 km altitudes by the HEP instrument onboard the RockSat-XN sounding rocket that has been launched from Andøya, Norway. The observed energy spectrum of precipitating electrons follows a power law of −4.9 ± 0.4 and the electron flux does not vary much over the observation period (∼274.4 s). A nearby ground-based VLF receiver observation at Lovozero, Russia shows the presence of whistler-mode wave activities during the rocket observation. A few minutes before the RockSat-XN observation, POES18/MEPED observed precipitating electrons, which also suggest whistler-mode chorus wave activities at the location close to the rocket trajectory. A test-particle simulation for wave-particle interactions was carried out using the data of the Arase satellite as the initial condition which was located on the duskside. The result of the simulation shows that whistler-mode waves can resonate with sub-relativistic electrons at high latitudes. These results suggest that the precipitation observed by RockSat-XN is likely to be caused by the wave-particle interactions between whistler-mode waves and sub-relativistic electrons.

The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror-mode type structures. We apply the Wave-Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst-mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time.

The dual spacecraft mission BepiColombo is the first joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) to explore the planet Mercury. BepiColombo was launched from Kourou (French Guiana) on October 20th, 2018, in its packed configuration including two spacecraft, a transfer module, and a sunshield. BepiColombo cruise trajectory is a long journey into the inner heliosphere, and it includes one flyby of the Earth (in April 2020), two of Venus (in October 2020 and August 2021), and six of Mercury (starting from 2021), before orbit insertion in December 2025. A big part of the mission instruments will be fully operational during the mission cruise phase, allowing unprecedented investigation of the different environments that will encounter during the 7-years long cruise. The present paper reviews all the planetary flybys and some interesting cruise configurations. Additional scientific research that will emerge in the coming years is also discussed, including the instruments that can contribute.

© 2021. The American Astronomical Society. Understanding the sources of lunar water is crucial for studying the history of lunar evolution, as well as the interaction of solar wind with the Moon and other airless bodies. Recent orbital spectral observations revealed that the solar wind is a significant exogenous driver of lunar surficial hydration. However, the solar wind is shielded over a period of 3-5 days per month as the Moon passes through the Earth's magnetosphere, during which a significant loss of hydration is expected. Here we report the temporal and spatial distribution of polar surficial OH/H2O abundance, using Chandrayaan-1 Moon Mineralogy Mapper (M3) data, which covers the regions inside/ outside the Earth's magnetosphere. The data shows that polar surficial OH/H2O abundance increases with latitude, and that the probability of polar surficial OH/H2O abundance remains at the same level when in the solar wind and in the magnetosphere by controlling latitude, composition, and lunar local time. This indicates that the OH/H2O abundance in the polar regions may be saturated, or supplemented from other possible sources, such as Earth wind (particles from the magnetosphere, distinct from the solar wind), which may compensate for thermal diffusion losses while the Moon lies within the Earth's magnetosphere. This work provides some clues for studies of planet- moon systems, whereby the planetary wind serves as a bridge connecting the planet with its moons.

This White Paper outlines the importance of addressing the fundamental science theme “How are charged particles energized in space plasmas” through a future ESA mission. The White Paper presents five compelling science questions related to particle energization by shocks, reconnection, waves and turbulence, jets and their combinations. Answering these questions requires resolving scale coupling, nonlinearity, and nonstationarity, which cannot be done with existing multi-point observations. In situ measurements from a multi-point, multi-scale L-class Plasma Observatory consisting of at least seven spacecraft covering fluid, ion, and electron scales are needed. The Plasma Observatory will enable a paradigm shift in our comprehension of particle energization and space plasma physics in general, with a very important impact on solar and astrophysical plasmas. It will be the next logical step following Cluster, THEMIS, and MMS for the very large and active European space plasmas community. Being one of the cornerstone missions of the future ESA Voyage 2050 science programme, it would further strengthen the European scientific and technical leadership in this important field.