Solar-Planetary System Science

Solar-Planetary System Science

Planets in the solar system, including the Earth on which we live, are constantly influenced by the sun and the outer space surrounding the planets. From the sun, the solar radiation and plasma flows called as the solar wind are continuously emitted, and the latter significantly varies with the solar activity. In our research group, we investigate the sunspot variation, solar flares, and coronal heating problems, which are the energy sources of the solar-planetary system, by making full use of a large-scale simulation using supercomputers. We combine numerical simulations with satellite and ground-based observations in order to understand underlying physics in the space weather phenomena such as the aurora, radiation belt variations, and the geospace storms.
Planets have different characteristics in various aspects, such as distance from the sun, size, intrinsic magnetic field, and atmosphere. Although the solar-planetary system is a complex system in which these interact intricately, studies of other planets with different conditions give us important insights on understand the influence of specific elements. For example, by studying Mars and Venus which do not have a strong global intrinsic magnetic field like the Earth, we can clarify the influence of the intrinsic magnetic field of the planet on the variation of the solar-planetary system and evolution of the planetary surface environment. In our research group, we conduct our studies in close collaboration with related satellite missions such as Hinode (solar observation), Hisaki (planetary telescope), MAVEN (Mars), and BepiColombo (Mercury).
Below, we will introduce the main research topics.

Solar physics

The Sun is our nearest star and has a long history of its research. There remains, however, many unsolved issues. Flares, the most energetic phenomena in the present solar system, are a manifestation of rapid release of magnetic energy in the solar atmosphere, while its main engine, the magnetic reconnection process needs further research. Solar corona, outer most atmospheric layer, has a temperature of 1 MK that is much higher than that of its inner layers. This means a necessity of some mechanical energy transport, which is yet unclear. All these phenomena originate from the magnetic energy. Generation and maintenance of magnetic field in the Sun are one of the most exciting field of research.

Terrestrial magnetosphere and space weather researches

The Earth’s magnetosphere is formed by the interactions between the Earth’s magnetic field and the solar wind, filled with plasma of both solar wind and atmospheric origins. Energy of the solar wind is transported and stored in the magnetosphere. Variations of the solar wind cause global, dynamic phenomena of plasma and electromagnetic fields; for example, aurora activity associated with magnetic reconnection and high-energy particle acceleration in the radiation belts. We are conducting observational and theoretical researches to better understand energy input from the solar wind and plasma transport/acceleration in the magnetosphere. International collaborations are also in progress, with in-situ observations of radiation belt electrons and ring current ions made by the Arase (ERG) spacecraft launched in FY2016, in close coordination with ground-based observations and numerical simulations. Our group plays an important role in spacecraft observations and numerical simulations in the ERG project. These studies also make a significant contribution to understanding of geospace and development of social infrastructures in space.

Comparative planetary environments: Their diversity and universality

Earth is a water planet covered by the sea, about 70% of the planetary surface, and the atmosphere including moderate atmospheric pressure and greenhouse gases keep its surface environment a habitable one for the terrestrial-type life. It is essential to consider the role of the atmosphere in understanding the planetary surface environment, whether the planet can have a habitable environment. Planets in the solar system have evolved under constant exposure to both electromagnetic radiation (solar radiation) and mass emission (solar wind) from the sun. For example, Mars is considered to have lost a habitable warm and humid climate probably due to the significant atmospheric loss to space. However, the mechanism to cause the atmospheric loss is not well understood. How has the solar planetary system responded and evolved with the evolution of the sun, the central star? What is the reason why the surface environments of the terrestrial planets in our solar system, i.e., Mercury, Venus, Earth, and Mars, have reached to significantly different surface environment in the present day? In our research group, we are conducting researches aiming at elucidating the diversity and universality of the solar planetary system by combining data analysis and numerical simulations by participating in planetary explorations such as JAXA’s HISAKI and NASA’s MAVEN missions. We also study how to apply our knowledge of the solar system planets to exoplanets, through such studies on the role of intrinsic magnetic field strength on atmospheric loss from planets.

Study of Outer Planets

Outer planets like Jupiter and Saturn have surrounding strong magnetic field. The electromagnetic process as extremely bright UV aurora, and particle acceleration (up to ultra-relativistic energy) are seen in these magnetic regions. Our group is approaching for those targets through computer modeling and data analysis of satellite missions like Galileo (NASA), Cassini (NASA), HST (NASA), and Hisaki (JAXA). We are also developing the instrument onboard the spacecraft for the future missions.