[P-CG21] 惑星大気圏・電磁圏

[PCG21-06] Effects of the magnetic field structure on the Martian diffuse aurora based on Monte Carlo simulations and MAVEN observations

*沖山 太心¹、関 華奈子¹、中村 勇貴¹、Lillis Robert²、Rahmati Ali²、Larson Davin²、DiBraccio Gina³、Schneider Nicholas⁴、Jain Sonal⁴、Curry Shannon⁴ (1.東京大学大学院理学系研究科、2.Space Sciences Laboratory, University of California, Berkeley, California, USA、3.Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA、4.The University of Colorado at Boulder, Boulder, USA)

キーワード：火星、オーロラ、太陽高エネルギー粒子、誘導磁気圏

Martian diffuse auroras are global ultraviolet emissions including CO₂⁺ ultraviolet doublet (UVD) on nightside, caused by solar energetic particles including electrons and protons (Schneider et al., 2015; Schneider et al., 2018; Nakamura et al., 2022). The auroral emissions caused by the electrons are expected to vary with nightside magnetic fields around Mars because Larmor radiuses of electrons are smaller than those of protons. Mars does not have the global dipole magnetic field, and there are crustal magnetic field regions in parts of southern hemisphere. In this environment, solar wind magnetic fields drape around Mars and form induced magnetosphere, whose nightside structures during extreme solar events are not well known. The relationships between Martian diffuse auroras and magnetic fields are far from understood. If their relationships are revealed, the observational diffuse auroral emissions will be useful to infer the unknown nightside induced magnetic field structures during extreme even. It is thus important to understand effects of the magnetic fields on diffuse auroral emission profiles.

We here estimate the magnetic field effects based on a newly developed Monte Carlo model including electron cyclotron motions and MAVEN observations. The model was applied to the most prominent diffuse auroral event in September 2017 using MAVEN SEP electron observations and neutral atmospheric profile from Mars Climate Database as inputs. Comparison between horizontal and vertical magnetic field structures indicate that the horizontal dip angle results in the broader limb-integrated auroral altitude profiles than the vertical case, and the broader limb-integrated auroral altitude profiles enhance the auroral intensities at high altitudes (>75km). We also compare the auroral emissions between model results and the observational results. Our model results are consistent with the observational results. The observational results also infer the effects of magnetic fields on auroral emissions. The narrow auroral altitude profiles are consistent with medium crustal magnetic field regions, where the magnetic field direction will be roughly vertical. These results imply that magnetic field structures in the nightside Martian ionosphere can affect the auroral emissions, and nightside magnetic field structures will be inferred from observations of the limb-integrated diffuse aurora emissions.


References:
Schneider et al. (2015). Discovery of diffuse aurora on Mars. Science, 350(6261), aad0313. https://doi.org/10.1126/science.aad0313

Schneider et al. (2018). Global aurora on Mars during the September 2017 space weather event. Geophysical Research Letters, 45, 7391–7398. https://doi.org/10.1029/ 2018GL077772

Nakamura et al. (2022). Modeling of diffuse auroral emission at Mars: Contribution of MeV protons. Journal of Geophysical Research: Space Physics, 127, e2021JA029914. https://doi. org/10.1029/2021JA029914