11:00 〜 11:15
[SIT21-02] Energetic solar activity from the first million years to present: Insights from noble gases in planetary materials
★Invited Papers
キーワード：太陽高エネルギー粒子、希ガス同位体、原始太陽、LIMAS
Solar storms are energetic solar events that are often associated with significant ejection of plasma mass from the solar corona. Solar particles are implanted into planetary materials, and stop at different depths from the material surface corresponding to their kinetic energies. Thus, a depth profile of implanted solar particles directly reflects the particle fluence spectrum. Noble gases are sensitive tracers of solar particles because they are depleted in planetary minerals. Recently, we identified a large excess of high-energy component from depth profiles of implanted solar wind He in a regolith particle from the asteroid Itokawa using a secondary neutral mass spectrometer, LIMAS, at Hokkaido University (Obase et al. 2025). The depth profiles suggest that the particle was exposed to an intense solar storm at some point in the last few million years. The event was approximately 40 times larger than the Halloween solar storms of 2003, which is one of the largest solar particle ejection events observed since space-based measurements began. The magnitude of the event is similar to the largest events inferred from the cosmogenic isotope records in polar ice cores and tree rings (Koldobskiy et al. 2023). Our result provides direct evidence of an extreme solar particle ejection event from the present-day Sun.
In the early solar system, the active young Sun has been inferred from observational studies of pre-main-sequence stars of near solar mass, which has a long-term average flux of solar energetic particles 105 times larger than that of the present Sun (Feigelson et al. 2002). Irradiation of the solar nebula with such a large flux of energetic particles could have played a role in amorphization of crystalline materials and production of some short-lived radionuclides in the early solar system (Trappitsch & Ciesla 2015).
Recently, we discovered a unique He-rich clast from a primitive CR chondrite MIL 090657 by isotope imaging using LIMAS. The He-rich clast is ~10 μm in diameter and embedded in a fine-grained matrix. The clast mainly consists of a porous aggregate of iron sulfide nanocrystals. Schreibersite crystals (maximum 2 μm) are embedded in the nanocrystalline aggregate. Euhedral pyrrhotite crystals are present at the periphery of the clast. A high concentration of 4He was detected from the schreibersite crystals (4×1019 atoms/cm3 or ~40 ppm), but not in the nanocrystalline aggregate and pyrrhotite. High-resolution 4He imaging using a small spot size (~300 nm) primary beam revealed that 4He is uniformly distributed in the 2 μm schreibersite crystal. The porous texture and mineral composition suggest that the nanocrystalline aggregate was formed by decomposition of tochilinite. The tochilinite and the euhedral pyrrhotite are likely formed by aqueous alteration that occurred at 4–13 Myr after the CAI formation (Jilly-Rehak et al. 2017). The fact that 4He is concentrated only in schreibersite indicates that the incorporation of 4He occurred before the aqueous alteration. At this time, the Sun was likely in its pre-main-sequence phase, which lasted ~10 Myr after the formation. The uniform distribution of 4He suggests that the schreibersite crystal was exposed to an extremely large fluence of solar energetic particles with energies higher than 1 MeV. The He-rich schreibersite is evidence for the active young Sun.
In the main-sequence phase, a secular decreasing trend in solar particle flux has been suggested from observational studies, where the flux 4 Gyr ago was ~100 times larger than the present (Wood et al. 2004). However, the past solar particle flux estimated from noble gases in regolith breccia meteorites was similar to the present value (Obase & Nakashima 2023). This may indicate that the ancient energetic solar activity was not so strong. Quantitative estimation of the timing of solar particle acquisition in meteorites is needed for further understanding of the secular evolution of energetic solar activity.
In the early solar system, the active young Sun has been inferred from observational studies of pre-main-sequence stars of near solar mass, which has a long-term average flux of solar energetic particles 105 times larger than that of the present Sun (Feigelson et al. 2002). Irradiation of the solar nebula with such a large flux of energetic particles could have played a role in amorphization of crystalline materials and production of some short-lived radionuclides in the early solar system (Trappitsch & Ciesla 2015).
Recently, we discovered a unique He-rich clast from a primitive CR chondrite MIL 090657 by isotope imaging using LIMAS. The He-rich clast is ~10 μm in diameter and embedded in a fine-grained matrix. The clast mainly consists of a porous aggregate of iron sulfide nanocrystals. Schreibersite crystals (maximum 2 μm) are embedded in the nanocrystalline aggregate. Euhedral pyrrhotite crystals are present at the periphery of the clast. A high concentration of 4He was detected from the schreibersite crystals (4×1019 atoms/cm3 or ~40 ppm), but not in the nanocrystalline aggregate and pyrrhotite. High-resolution 4He imaging using a small spot size (~300 nm) primary beam revealed that 4He is uniformly distributed in the 2 μm schreibersite crystal. The porous texture and mineral composition suggest that the nanocrystalline aggregate was formed by decomposition of tochilinite. The tochilinite and the euhedral pyrrhotite are likely formed by aqueous alteration that occurred at 4–13 Myr after the CAI formation (Jilly-Rehak et al. 2017). The fact that 4He is concentrated only in schreibersite indicates that the incorporation of 4He occurred before the aqueous alteration. At this time, the Sun was likely in its pre-main-sequence phase, which lasted ~10 Myr after the formation. The uniform distribution of 4He suggests that the schreibersite crystal was exposed to an extremely large fluence of solar energetic particles with energies higher than 1 MeV. The He-rich schreibersite is evidence for the active young Sun.
In the main-sequence phase, a secular decreasing trend in solar particle flux has been suggested from observational studies, where the flux 4 Gyr ago was ~100 times larger than the present (Wood et al. 2004). However, the past solar particle flux estimated from noble gases in regolith breccia meteorites was similar to the present value (Obase & Nakashima 2023). This may indicate that the ancient energetic solar activity was not so strong. Quantitative estimation of the timing of solar particle acquisition in meteorites is needed for further understanding of the secular evolution of energetic solar activity.