金星超高層における酸素イオン流出とイオン加速メカニズムの研究

要約のみTohoku University寺田直樹課

Reflectivity of Venus’s Dayside Disk During the 2020 Observation Campaign: Outcomes and Future Perspectives

We performed a unique Venus observation campaign to measure the disk brightness of Venus over a broad range of wavelengths in 2020 August and September. The primary goal of the campaign was to investigate the absorption properties of the unknown absorber in the clouds. The secondary goal was to extract a disk mean SO2 gas abundance, whose absorption spectral feature is entangled with that of the unknown absorber at ultraviolet wavelengths. A total of three spacecraft and six ground-based telescopes participated in this campaign, covering the 52–1700 nm wavelength range. After careful evaluation of the observational data, we focused on the data sets acquired by four facilities. We accomplished our primary goal by analyzing the reflectivity spectrum of the Venus disk over the 283–800 nm wavelengths. Considerable absorption is present in the 350–450 nm range, for which we retrieved the corresponding optical depth of the unknown absorber. The result shows the consistent wavelength dependence of the relative optical depth with that at low latitudes, during the Venus flyby by MESSENGER in 2007, which was expected because the overall disk reflectivity is dominated by low latitudes. Last, we summarize the experience that we obtained during this first campaign, which should enable us to accomplish our second goal in future campaigns

Global Venus-Solar wind coupling and oxygen ion escape

The present‐day Venusian atmosphere is dry, yet, in its earlier history a significant amount of water evidently existed. One important water loss process comes from the energy and momentum transfer from the solar wind to the atmospheric particles. Here, we used measurements from the Ion Mass Analyzer onboard Venus Express to derive a relation between the power in the upstream solar wind and the power leaving the atmosphere through oxygen ion escape in the Venusian magnetotail. We find that on average 0.01% of the available power is transferred, and that the percentage decreases as the available energy increases. For Mars the trend is similar, but the efficiency is higher. At Earth, the ion escape does not behave similarly, as the ion escape only increases after a threshold in the available energy is reached. These results indicate that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind.Originally included in thesis in manuscript form.</p

Seasonal and dust‐related variations in the dayside thermospheric and ionospheric compositions of Mars observed by MAVEN/NGIMS

International audienceWe report seasonal and dust-related variations in neutral and ion species (CO2, O, and N2, and CO2+, O2+, O+, and N+, respectively) in the dayside Martian upper atmosphere between altitudes of ∼150 and ∼250 km observed by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile Evolution spacecraft. The sinusoidal seasonal variations in CO2+ and O2+ densities are clearly identified, while that of O+ is less discernible. These observed variations in ion densities are well reproduced by a photochemical equilibrium model for CO2+ and O+ densities when we combine them with solar cycle variations. Furthermore, we find a decrease in O, O+, and O2+ densities in the whole altitude range at Ls = 342 - 346 in MY 33 during a regional dust event. The decrease in O density would lead to decreases in O+ and O2+ densities in the ionosphere through ion-neutral reactions. Observed variations in ion and neutral species associated with the season and a regional dust storm are also confirmed in pressure coordinates. Observations show that the CO2+/O+ ratio at a given pressure level in the ionosphere varies by a factor of ∼3, which can modify the composition of ion outflow from the Martian atmosphere

The impact of factor Xa inhibitors on bleeding risk in patients with respiratory diseases

Abstract It is unclear which factor Xa (FXa) inhibitors are associated with higher bleeding risk in patients with respiratory diseases, and there are no studies on the association between prothrombin time–international normalized ratio (PT–INR) and bleeding risk. We conducted a retrospective cohort study comparing 1-year-outcomes and PT–INR between patients with respiratory diseases treated with rivaroxaban (R group, n = 82) or edoxaban (E group, n = 138) for atrial fibrillation or venous thromboembolism from 2013 to 2021. The most frequent event of all bleeding discontinuations was respiratory bleeding in both groups (7.3 and 4.3%, respectively). The cumulative incidence of bleeding discontinuation was significantly higher in the R group (25.6%) than in the E group (14.4%) (hazard ratio [HR], 2.29; 95% confidence interval [CI] 1.13–4.64; P = 0.023). PT–INR after initiation of therapy significantly increased and was higher in the R group than in the E group (median value, 1.4 and 1.2, respectively; P < 0.001). Multivariate analysis using Cox proportional hazards and Fine-Gray models revealed that PT–INR after initiation of therapy was an independent risk factor of bleeding discontinuation events (HR = 4.37, 95% CI 2.57–7.41: P < 0.001). Respiratory bleeding occasionally occurs in patients receiving FXa inhibitors, and monitoring the PT–INR may need to ensure safety

Flow directions of low-energy ions in and around the diamagnetic cavity of comet 67P

The flow direction of low-energy ions around comet 67P/Churyumov–Gerasimenko has previously been difficult to constrain due to the influence of the spacecraft potential. The Ion Composition Analyzer of the Rosetta Plasma Consortium (RPC-ICA) on Rosetta measured the distribution function of positive ions with energies down to just a few eV/q throughout the escort phase ofthe mission. Unfortunately, the substantial negative spacecraft potential distorted the directional information of the low-energy data. In this work, we present the flow directions of low-energy ions around comet 67P, corrected for the spacecraft potential using Particle-In-Cell simulation results. We focus on the region in and around the diamagnetic cavity, where low-energy ions are especially important for the dynamics. We separate between slightly accelerated ‘burst’ features and a more constant ‘band’ of low-energy ions visible in the data. The ‘bursts’ are flowing radially outwards from the nucleus with an antisunward component while the ‘band’ is predominantly streaming back towards the comet. This provides evidence of counter-streaming ions, which has implications for the overall expansion velocity of the ions. The backstreaming ions are present also at times when the diamagnetic cavity was not detected, indicating that the process accelerating the ions back towards the comet is not connected to the cavity boundary

Detection and dynamics of Martian plasma boundaries

International audienc