11:00 〜 11:15
[PCG19-07] Effects of magnetic field structure on the Martian diffuse aurora based on Monte Carlo simulations and MAVEN observations
キーワード：火星、オーロラ、太陽高エネルギー粒子、モンテカルロ、誘導磁気圏
The diffuse aurora emission at Mars consists of significant CO2+ ultraviolet doublet (UVD) emission and having peak below 100 km altitude. Schneider et al. (2018) showed that the time variation of the auroral emission does not always correlate with the variation of the upstream SEP electron flux. The emission correlates also with SEP protons in some events. The cause of the time variations of the auroral emission is far from understood. The horizontal magnetic field structure is formed when interplanetary magnetic field is draped around the Mars, and the magnetic field structure can change the flux of the penetrating SEP electrons. The purpose of this study is to investigate effects of magnetic field structure on the emission profile of Martian diffuse aurora based on Monte Carlo simulations and MAVEN observations.
We have developed a Monte Carlo model that calculates the vertical emission profile of CO2+ UVD. Our model used similar methods to the model by Bhardwaj & Jain (2009), which calculates the energy degradation of electrons below 1000 eV through collisions between CO2 and electrons. The energy range of our models is expanded up to hundreds of keV by including the cross sections for collisional reactions between electrons and neutral atmosphere used in the model by Gérard et al. (2017), which reproduces vertical emission profiles of Martian diffuse aurora. A difference of our model from the previous models (e.g., Schneider et al., 2015, Gérard et al., 2017, and Nakamura et al., 2022) is to trace the trajectory of each electron in the given magnetic field structure including its cyclotron motion to investigate the effect of the draped magnetic field. We use MAVEN observational data, such as electron flux and magnetic fields, during the diffuse auroral event as inputs to our model. The model results show that effects of the elevation angle of the magnetic field from the horizontal direction are greater than those of the magnetic field strength. The effect of elevation angle on auroral mean intensity increases with increasing magnetic field strength in the case of isotropic downward electron flux, and this trend disappears if electron pitch angle distribution is strongly field aligned. However, elevation angle effects on auroral mean intensity are only about 10% because of our uniform magnetic field assumptions. The observational results show that the increasing of observational mean auroral intensity between 50-100 km altitude corresponds to the increasing of magnetic field intensity around 300 km altitude. The results suggest that non-uniform magnetic field structure in the vicinity of the planet is one of the important factors to cause variations of the Martian diffuse aurora.
References:
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
Bhardwaj & Jain (2009). Monte Carlo model of electron energy degradation in a CO2 atmosphere. Journal of Geophysical Research, 114, A11309. https://doi.org/10.1029/2009JA014298
Gérard et al. (2017). The Mars diffuse aurora: A model of ultraviolet and visible emissions. Icarus 288 (2017) 284–294. http://dx.doi.org/10.1016/j.icarus.2017.01.037
Schneider et al. (2015). Discovery of diffuse aurora on Mars. Science, 350(6261), aad0313. https://doi.org/10.1126/science.aad0313
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
We have developed a Monte Carlo model that calculates the vertical emission profile of CO2+ UVD. Our model used similar methods to the model by Bhardwaj & Jain (2009), which calculates the energy degradation of electrons below 1000 eV through collisions between CO2 and electrons. The energy range of our models is expanded up to hundreds of keV by including the cross sections for collisional reactions between electrons and neutral atmosphere used in the model by Gérard et al. (2017), which reproduces vertical emission profiles of Martian diffuse aurora. A difference of our model from the previous models (e.g., Schneider et al., 2015, Gérard et al., 2017, and Nakamura et al., 2022) is to trace the trajectory of each electron in the given magnetic field structure including its cyclotron motion to investigate the effect of the draped magnetic field. We use MAVEN observational data, such as electron flux and magnetic fields, during the diffuse auroral event as inputs to our model. The model results show that effects of the elevation angle of the magnetic field from the horizontal direction are greater than those of the magnetic field strength. The effect of elevation angle on auroral mean intensity increases with increasing magnetic field strength in the case of isotropic downward electron flux, and this trend disappears if electron pitch angle distribution is strongly field aligned. However, elevation angle effects on auroral mean intensity are only about 10% because of our uniform magnetic field assumptions. The observational results show that the increasing of observational mean auroral intensity between 50-100 km altitude corresponds to the increasing of magnetic field intensity around 300 km altitude. The results suggest that non-uniform magnetic field structure in the vicinity of the planet is one of the important factors to cause variations of the Martian diffuse aurora.
References:
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
Bhardwaj & Jain (2009). Monte Carlo model of electron energy degradation in a CO2 atmosphere. Journal of Geophysical Research, 114, A11309. https://doi.org/10.1029/2009JA014298
Gérard et al. (2017). The Mars diffuse aurora: A model of ultraviolet and visible emissions. Icarus 288 (2017) 284–294. http://dx.doi.org/10.1016/j.icarus.2017.01.037
Schneider et al. (2015). Discovery of diffuse aurora on Mars. Science, 350(6261), aad0313. https://doi.org/10.1126/science.aad0313
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