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Current knowledge of the Uranian system is limited to observations from the flyby of Voyager 2 and limited remote observations. However, Uranus remains a highly compelling scientific target due to the unique properties of many aspects of the planet itself and its system. Future exploration of Uranus must focus on cross-disciplinary science that spans the range of research areas from the planet’s interior, atmosphere, and magnetosphere to the its rings and satellites, as well as the interactions between them. Detailed study of Uranus by an orbiter is crucial not only for valuable insights into the formation and evolution of our solar system but also for providing ground truths for the understanding of exoplanets. As such, exploration of Uranus will not only enhance our understanding of the ice giant planets themselves but also extend to planetary dynamics throughout our solar system and beyond. The timeliness of exploring Uranus is great, as the community hopes to return in time to image unseen portions of the satellites and magnetospheric configurations. This urgency motivates evaluation of what science can be achieved with a lower-cost, potentially faster-turnaround mission, such as a New Frontiers–class orbiter mission. This paper outlines the scientific case for and the technological and design considerations that must be addressed by future studies to enable a New Frontiers–class Uranus orbiter with balanced cross-disciplinary science objectives. In particular, studies that trade scientific scope and instrumentation and operational capabilities against simpler and cheaper options must be fundamental to the mission formulation.

著者人数: 25名, Accepted: 2019-09-18, 資料番号: SA1190125000

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 $20^{\mathrm{th}}$ 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.

著者人数: 19名, Accepted: 2020-01-31, 資料番号: SA1190220000

著者人数: 20名, Accepted: 2018-05-15, 資料番号: SA1180065000

Stable auroral red (SAR) arcs are optical events with dominant 630.0-nm emission caused by low-energy electron heat flux into the topside ionosphere from the inner magnetosphere. SAR arcs are observed at subauroral latitudes and often occur during the recovery phase of magnetic storms and substorms. Past studies concluded that these low-energy electrons were generated in the spatial overlap region between the outer plasmasphere and ring-current ions and suggested that Coulomb collisions between plasmaspheric electrons and ring-current ions are more feasible for the SAR-arc generation mechanism rather than Landau damping by electromagnetic ion cyclotron waves or kinetic Alfvén waves. This work studies three separate SAR-arc events with conjunctions, using all-sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes [RBSP]) during non-storm-time substorms on December 19, 2012 (event 1), January 17, 2015 (event 2), and November 4, 2019 (event 3). We evaluated for the first time the heat flux via Coulomb collision using full-energy-range ion data obtained by the satellites. The electron heat fluxes due to Coulomb collisions reached ∼109 eV/cm2/s for events 1 and 2, indicating that Coulomb collisions could have caused the SAR arcs. RBSP-A also observed local enhancements of 7–20-mHz electromagnetic wave power above the SAR arc in event 2. The heat flux for the freshly detached SAR arc in event 3 reached ∼108 eV/cm2/s, which is insufficient to have caused the SAR arc. In event 3, local flux enhancement of electrons (

Auroral kilometric radiation (AKR) is known to be transient emissions generated by rapidly accelerated electrons together with sudden auroral activation in the polar magnetosphere. In contrast, the characteristics and relationship with the auroral acceleration of rather continuous AKR emissions are not well understood. We examine the emission using long-term data and report that the continuous AKR emission frequency changes with universal time (UT) as the Earth rotates, indicating that the Earth is a spin-modulated variable radio source. The observed UT variation of AKR frequency means that the acceleration altitude changes periodically with planetary rotation. The observations indicate that the diurnal wobble of the tilted geomagnetic field in the solar wind flow alters the magnetosphere-ionosphere (M-I) coupling state in the polar magnetosphere, giving rise to periodic variation of auroral particle acceleration altitude. These observations of planetary radio wave properties provide insight into the physics of planetary particle acceleration.

International audience; A one-dimensional full-particle model of the Martian upper thermosphere-exosphere has been developed, where the Direct Simulation Monte Carlo (DSMC) method is applied to both thermal and non-thermal components. Our full-particle model can self-consistently solve the transition from collisional to collisionless domains in the upper thermosphere, so that the energy deposition from non-thermal energetic components to thermal components in the transition region is properly described. For the solar EUV condition during the Viking-1 measurement (1 EUV case), computed density profiles are in good agreement with those observed by Viking 1 and with the conventional model. For a solar EUV flux six times the Viking-1 condition (6 EUV case), the computed heating efficiency is essentially the same as 1 EUV case but slightly increases by about 10 % below the exobase, and temperature deviates from the conventional model in and above the transition region. This result suggests that the conventional heating efficiency of 0.18 is a good approximation for low (1 EUV case) to moderately strong (6 EUV case) solar EUV conditions but would be inappropriate for an extremely strong solar EUV (up to ~100 times stronger flux) environment. We also find that applying different models of the CO2-O collisional energy transfer rate results in a difference in the calculated exobase temperature by 150 K for 6 EUV case.

We have analysed thermal emission spectra obtained from Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEX) for Martian Year (MY) 28 in presence and absence of dust storm at low latitude. A radiative transfer model for dusty atmosphere of Mars is developed to estimate the thermal emission spectra at latitude range 0-10oS, 10-20oS and 20-30oS. These calculations are made at Ls=240o, 280o, 300o, and 320o between wave numbers 250-1400 cm-1. We have also retrieved brightness temperatures from thermal emission spectra by inverting the Planck function. The model reproduces the observed features at wave numbers 600-750 cm-1 and 900-1200 cm-1 due to absorptions by CO2 and dust respectively. In presence of dust storm thermal emission spectra and brightness temperature are reduced by a factor of ~ 2 between wave numbers 900-1200 cm-1. The altitude profiles of dust concentration are also estimated for different aerosol particles of sizes 0.2 to 3 micron. The best fit to the PFS measurements is obtained in presence of aerosol particle of size 0.2 micron.

著者人数：27名, Accepted: 2005-05-15, 資料番号: SA1000450000

'Solar-Sail Project', which has been examined by ISAS/JAXA as an engineering mission, has a possibility of a small probe into the Jovian orbit. This paper summarizes the basic design of Jovian magnetospheric and auroral studies by this small but valuable 'chance'. The small orbiter aims to establish the feasibility of future outer planet missions by JAXA. The 'a large-scale Jovian mission' has been a hope since the 1970s, when the examinations of planetary exploration were started in Japan. In the one of plans, 'the largest planet in the solar system' would be solved by two main objectives: (1) Structure of a gas planet: the internal & atmospheric structures of the giant gas planet which could not become a star (following the objectives of 'Planet-C' to Venus and 'BepiColombo' to Mercury). (2) Jovian-type magnetosphere: the structure and processes of the strongest magnetospheric activities in the solar system, as a 'pulsar-like' magnetosphere driven by its own rotational energy (following the objectives of 'BepiColombo' to Mercury and a multi-spacecraft project 'SCOPE (Spacecraft Camera experiment for Observation of Planets and the Earth)'). The small orbiter in 'Solar-Sail Project' aims the former target in its limited resources., 資料番号: AA0063353025

The spacecraft potential has been used to derive the electron density surrounding the spacecraft in the magnetosphere and solar wind. The previous studies have examined the relationship between the spacecraft potential and the electron density in the distant tail regions and obtained an empirical formula to show their relation. However the electron density obtained by the empirical formula is often overestimated in the near tail regions with high electron temperature, In this study, we investigate the relationship between the Geotail spacecraft potential and the electron density/temperature in the near tail regions during the period from November 1994 to February 1997, and improve the empirical formula considering the electron temperature. Then we discuss on the investigation of distribution of low energy plasma in the near tail region by comparing the electron density obtained by the improved empirical formula with that measured by the low-energy particle instrument onboard the Geotail spacecraft., 資料番号: AA0049206025, レポート番号: JAXA-SP-05-001E

In the Earth's magnetosphere where the spacecraft potential is usually positive, photoelectrons emitted from the spacecraft surface are attracted back to the spacecraft and some of them are detected by electron analyzer onboard. Then photoelectrons are unnecessary contamination. By analyzing such a component detected by LEP/EA-e onboard GEOTAIL spacecraft, we examined velocity/energy distribution functions of photoelectrons, and their relationship to the spacecraft potential. We found that the ratio of the duskward photoelectron flux to the dawnward flux increases when the spacecraft potential is large, and decreases when it is small. Further analysis revealed, by plotting the ratio as a function of the photoelectron energy normalized by the spacecraft potential (E/V(sub sc)), that it is the largest when E/V(sub sec) is about one third. This result implies the existence of azimuthal component of electric field as well as the radial component which is regarded to be dominant in an ordinary case around the spacecraft surface, although it is not clear why such an electric field is generated around GEOTAIL., 資料番号: AA0049206021, レポート番号: JAXA-SP-05-001E

著者人数：12名, Accepted: 2000-08-04, 資料番号: SA1002495000

Accepted: 2003-03-04, 資料番号: SA1003832000