論文

Abstract The asymmetric distribution of Kelvin‐Helmholtz (KH) instability at Venusian ionopause is investigated using a multifluid model. Results show that KH instability distributes asymmetrically, preferentially developing in the −E (anti‐parallel to the interplanetary electric field) hemisphere. In the magnetosheath, solar wind H + ions deflect toward both ±E hemispheres. Within the ionosphere, however, ionospheric ions are accelerated in the +E direction by the convective electric field. In addition, the ionopause is elevated in the −E hemisphere. These effects lead to a stronger velocity shear at the ionopause of the −E hemispheric, thereby promoting the KH growth there. The magnetic field variations in the KH region correlate with the H + density but anti‐correlate with ionospheric ion density. The period of KH instability gradually decreases as it propagates downstream. During its evolution, plasma clouds form and take away ionospheric (especially O + ) ions effectively, resulting in considerable ion escape.

Abstract Due to the lack of a significant planetary magnetic field, the upper atmospheres of Venus‐like planets interact directly with the stellar wind. Therefore, thermal atomic oxygen in the thermosphere and hot oxygen in the corona produced by non‐thermal processes in the upper atmosphere act as a source of ion pickup loss. Here, we investigate the effects of hot oxygen corona on ion escape under different X‐ray and EUV (XUV) environments and stellar wind conditions. A Monte Carlo model is developed to simulate the transport of hot oxygen in the thermosphere. A simulation under present‐day Venus conditions successfully reproduces the observed oxygen density in the corona. The calculated hot oxygen distributions under different XUV environments are then used as input for a multispecies magnetohydrodynamic (MHD) model to simulate the stellar wind interaction with the upper atmosphere. The model results suggest that ion escape is controlled by flow patterns that depend on the pressure balance between ionospheric thermal and stellar wind dynamic pressures. This finding may account for the observed ion loss rate dependence on stellar wind dynamic pressure, as pointed out by previous studies on present‐day Mars and Venus. Under high XUV radiation, enhanced thermospheric heating causes the thermospheric component to dominate over the hot oxygen corona. Under low‐density stellar wind conditions, reduced stellar wind density suppresses ionization processes via charge exchange and electron impact ionization. Consequently, the contribution of hot oxygen corona to the total escape rate decreases under high XUV radiation or low‐density stellar wind.

Abstract The enrichment of heavy isotopes of volatile elements in the Martian atmosphere indicates that Mars lost a large portion of its atmosphere through escape to space. Recent atmospheric measurements by the ExoMars Trace Gas Orbiter (TGO) have suggested that the vertical profiles of oxygen isotopic compositions are influenced by chemical reactions involving isotopic fractionation. However, their quantitative impacts have not yet been fully evaluated. In this study, we develop a 1D photochemical model that incorporates oxygen isotopic fractionation associated with CO 2 photolysis and O 3 formation to investigate the vertical profiles of oxygen isotopic compositions. Our calculations show that CO is depleted in heavy oxygen isotopes relative to CO 2 , reaching δ 18 O ∼ −25‰ and δ 17 O ∼ −15‰, primarily due to isotopic fractionation during CO 2 photolysis. The vertical profiles of oxygen and carbon isotopic compositions are in good agreement between our model and the TGO measurements. O 3 is strongly enriched in 18 O and 17 O, reaching δ 18 O ∼ 100‰ and δ 17 O ∼ 50‰ as a consequence of the isotopic fractionation during its formation, whereas atomic oxygen is highly depleted in the heavy oxygen isotopes with δ 18 O ≲ −100‰ and δ 17 O ≲ −50‰ so as to compensate for their enrichment in O 3 . These chemical fractionation processes can deplete the heavy oxygen isotopes in species that escape from the upper atmosphere and thereby enhance the isotopic fractionation associated with oxygen escape to space. Such fractionated isotopic compositions of escaping oxygen may be detectable by the Martian Moons eXploration (MMX) mission.

Abstract The ionospheres of Jupiter's icy moons have been observed by in situ plasma measurements and radio science. However, their spatial structures have not yet been fully characterized. To address this, we developed a new ray tracing method for modeling the radio occultation of the ionospheres using Jovian auroral radio sources. Applying our method to Jovian auroral radio observations with the Galileo spacecraft, we derived the electron density of the ionosphere of Ganymede and Callisto. For Ganymede's ionosphere, we found that the maximum electron density on the surface was 76.5–288.5 cm−3 in the open magnetic field line regions and 5.0–20.5 cm−3 in the closed magnetic field line region during the Galileo Ganymede 01 flyby. The difference in the electron density distribution was correlated with the accessibility of Jovian magnetospheric plasma to the atmosphere and surface of the moons. These results indicated that electron‐impact ionization of the Ganymede exosphere and sputtering of the surface water ice were effective for the producing Ganymede's ionosphere. For Callisto's ionosphere, we found that the densities were approximately 350 and 12.5 cm−3 on the night side hemisphere during Callisto 09 and 30 flybys, respectively. These results combined with previous observations indicated that atmospheric production through sublimation controlled the ionospheric density of Callisto. This method is also applicable to upcoming Jovian radio observation data from the Jupiter Icy Moon Explorer, JUICE.

Abstract The mass spectrum analyzer (MSA) is one of the instruments onboard MMX and observes the interaction between the Martian moons (Phobos and Deimos) and the solar wind as well as the material transport between Mars and its moons. MSA consists of an ion mass spectrum analyzer and a magnetometer. The objective of the magnetometer, MSA-MG, is to measure the magnetic field at the MMX position to trace the motion of the ions. We defined the requirements for the performance of the MSA-MG and designed the instrument to meet them. It is confirmed that MSA-MG as a unit has the required characteristics by the ground performance test and calibration. One of the essential calibration parameters, artificial bias in the data, must be determined by analyzing the flight data. To improve the accuracy of the determined bias, efforts to remove the magnetic noise from other components onboard MMX are essential.

Abstract An ion energy mass spectrum analyzer was developed for the Martian Moons eXploration (MMX) mission to measure the three-dimensional velocity distribution function and mass profile of low-energy ions around the Mars-Moon system. The hemispheric field-of-view (FOV) is acquired by a pair of angular scanning deflectors, and the energy/charge and mass/charge are determined for each ion by an electrostatic analyzer and a linear-electric-field (LEF) time-of-flight (TOF) analyzer, respectively, with an enhanced mass resolution of $$m/\Delta m\sim 100$$ . The ion analyzer, together with magnetometers, constitutes the mass spectrum analyzer (MSA), one of the scientific instruments on board the MMX spacecraft. This paper describes the instrumentation of the ion analyzer, and results of the performance tests of its flight model (FM).

Abstract Terrestrial planets currently in the habitable zone around M dwarfs are estimated to have been in runaway greenhouse conditions for up to ∼1 Gyr due to the long-term pre-main-sequence phase of M dwarfs. These planets likely lose a significant portion of water during the pre-main-sequence phase owing to H2O photolysis followed by hydrogen and oxygen loss to space. However, the effects of H2O reproduction reactions and UV shielding by chemical products that reduce photolysis-induced water loss have yet to be estimated. Here, we apply a 1D photochemical model to a H2O-dominated atmosphere of an Earth-like planet around a pre-main-sequence M dwarf to estimate these effects. We find that water loss is suppressed by efficient H2O reproduction reactions and by UV shielding due to O2. The water loss rate decreases by several to several hundred times compared to that in previous studies, with the assumption that the water loss rate is limited by stellar X-ray and extreme-ultraviolet flux or hydrogen diffusion through the atmosphere. Our results imply that terrestrial planets currently in the habitable zone around M dwarfs are more likely to retain surface water than previously estimated.

Abstract The isotopic signature of atmospheric carbon offers a unique tracer for the history of the Martian atmosphere and the origin of organic matter on Mars. The photolysis of CO2 is known to induce strong isotopic fractionation of the carbon between CO2 and CO. However, its effects on the carbon isotopic compositions in the Martian atmosphere remain uncertain. Here, we develop a 1D photochemical model to consider the isotopic fractionation via photolysis of CO2, to estimate the vertical profiles of the carbon isotopic compositions of CO and CO2 in the Martian atmosphere. We find that CO is depleted in 13C compared with CO2 at each altitude, due to the fractionation via CO2 photolysis: the minimum value of the δ13C in CO is about −170‰ under the standard eddy diffusion setting. This result supports the hypothesis that fractionated atmospheric CO is responsible for the production of the 13C-depleted organic carbon in the Martian sediments detected by the Curiosity Rover, through the conversion of CO into organic materials and their deposition on the surface. The photolysis and transport-induced fractionation of CO that we report here leads to a ∼15% decrease in the amount of inferred atmospheric loss when combined with the present-day fractionation of the atmosphere and previous studies of carbon escape to space. The fractionated isotopic composition of CO in the Martian atmosphere may be observed by ExoMars Trace Gas Orbiter and ground-based telescopes, and the escaping ion species produced by the fractionated carbon-bearing species may be detected by the Martian Moons eXploration mission in the future.

Abstract TOI‐700 d is the first Earth‐sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite (TESS). Here, we assess whether a Venus‐like exoplanet at the TOI‐700 d location could retain an atmosphere for a time comparable to the age of the host star based on multispecies magnetohydrodynamics (MHD) simulations. We investigate the effects of X‐ray and EUV (XUV) radiation from the host star, the interplanetary magnetic field (IMF) orientation, and the planetary intrinsic magnetic field. In unmagnetized cases, major ion loss is caused by $\rm{O^+}$ escape through a ring‐shaped region by the mass loading process after the ionization of the extended oxygen corona. As the IMF Parker spiral angle increases, the escape flux in the magnetotail shows stronger enhancement around the meridional current sheet, and the escape rate of molecular ions ($\rm{O_2^+}$ and $\rm{CO_2^+}$) increases by an order of magnitude due to acceleration in the ionosphere by magnetic tension forces. In magnetized cases, the intrinsic magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind and preventing ion pickup while promoting cusp‐origin escape from the lower ionosphere. These results suggest that the unmagnetized exoplanet would have difficulty retaining its atmosphere over a few billion years under extreme conditions where XUV is 30 times stronger than at the current Earth. However, the dipole intrinsic magnetic field of 1000 nT at the equatorial surface reduces the escape rate and would help the exoplanet to retain its atmosphere even under strong XUV conditions.

Abstract Dust storms and the atmospheric waves can play a significant role in the dynamics in the upper neutral atmosphere of Mars. Recent observations found that a periodic variation of neutral H and O exists in the upper atmosphere, which is likely associated with atmospheric waves that occurred during the regional dust storm on 4 September 2016. However, such periodic variations accompanying the dust storm are far from understood in terms of the Martian ionized particles (i.e., ionosphere). Here we investigated the periodic variation of the ion density in the Martian ionosphere during the regional dust storm in September 2016 based on the MAVEN/STATIC observations. Assuming a simple Chapman layer model, we implemented numerical fitting for the ion density altitude profile to retrieve the peak ion densities in the Martian ionosphere. We then applied periodogram analysis to these peak ion densities in order to identify the peak periodicities together with their confidence levels. We identified several distinct peak periodicities with a scale from ∼1 day up to ∼20 days. The peak periodicities around 6–9 days are comparable to those seen in the Martian neutral atmosphere. In addition, the other peak periodicities likely correspond to the periodic crossings of the local crustal fields and periodic variation of the upstream solar wind, indicating a strong regional coupling between the lower atmosphere and the upstream solar wind.