[P-EM09] Space Weather and Space Climate

[PEM09-25] Numerical prediction of the ozone depletion in the Martian atmosphere during solar energetic particle events

*中村 勇貴^1,2,3、Leblanc Francois²、寺田 直樹¹、晝場 清乃¹、村田 功¹、中川 広務¹、堺 正太朗^1,4、青木 翔平^5,6、Piccialli Arianna⁶、Willame Yannick⁶、Neary Lori⁶、Vandaele Ann Carine⁶、村瀬 清華^7,8、片岡 龍峰^7,8 (1.東北大学 大学院理学研究科地球物理学専攻、2.LATMOS/CNRS, Sorbonne Université、3.東京大学 大学院理学系研究科地球惑星科学専攻、4.東北大学 大学院理学研究科 惑星プラズマ・大気研究センター、5.東京大学 大学院新領域創成科学研究科、6.Royal Belgian Institute for Space Aeronomy, BIRA-IASB、7.国立極地研究所、8.総合研究大学院大学)

キーワード：火星、太陽高エネルギー粒子、光化学反応、オゾン

Owing to the absence of a global intrinsic magnetic field and thin atmosphere on Mars, solar energetic particles (SEPs), high energy charged particles emitted from the Sun associated with solar flares and coronal mass ejections, can easily precipitate into the Martian atmosphere globally and deeply. In contrast to the terrestrial atmosphere, where depletion of ozone in the polar mesosphere has been studied by observations and models for decades, there have been no studies on the effects of SEPs on the neutral chemical composition of Mars’ present-day atmosphere. We investigated the effects of SEPs on the atmospheric composition in the Martian atmosphere by coupling a Monte Carlo model PTRIP (Particle TRansport In Planetary atmospheres) and a photochemical model PROTEUS (Photochemical and RadiatiOn Transport model for Extensive USe). We found that an enhancement of the HOx density and a depletion of the ozone density by a factor of 10 occur in the altitude range 20-60 km, at which altitudes correspond to the penetration of SEP protons with energy 5-50 MeV during a Halloween-class SEP event. Variations of the ozone and HOx densities converge in 5 hours during a Halloween-class SEP event due to the short chemical production and loss time scales, while the NOx density continues increasing during the SEP event due to its long chemical timescale. We compared our results with the detection limit of the Nadir and Occultation for MArs Discovery (NOMAD) instrument on board the Trace Gas Orbiter (TGO) spacecraft. The depletion of the ozone density is expected to be detectable by TGO/NOMAD, while the variations of other species such as HO₂, NO₂, H₂CO, and N₂O are not expected to be detected due to the small abundances below the detection limit of TGO/NOMAD. We discussed the frequency of SEP events that cause ozone depletion and found that a 75% depletion of the ozone density at 40 km altitude and an 8-10% depletion of the column ozone density can be expected during SEP events occurring once a year on average.