14:30 〜 14:45
[PCG19-04] Carbon and nitrogen ion escape from Mars based on 11-species multifluid MHD simulation
キーワード：火星、大気散逸、MHDシミュレーション、炭素進化、窒素進化
The escape of ionized atmospheric particles into space, i.e., ion escape, has contributed to atmospheric loss and significant climate change on early Mars. Numerous studies have investigated ion escape from Mars based on spacecraft observations and numerical simulations. However, most of these studies primarily focused on oxygen ions (O+ and O2+) since they are the most dominant species in the ionosphere and ion outflows. Carbon and nitrogen are also abundant on Mars and may play important roles in atmospheric evolution and the potential for life. While some recent research investigated the distributions and outflows of some carbon and nitrogen ions based on MAVEN observations, the details of the escape process, such as dependencies on external conditions, remain poorly understood.
This study aims to investigate the escape of carbon and nitrogen ions from Mars. We employed a three-dimensional global multifluid MHD model MAESTRO (Sakata et al., 2024). This model was developed for five ion species (planetary H+, O+, O2+, CO2+, and solar wind H+); however, we incorporated chemical reactions and collision frequencies for six ion species (C+, CO+, HCO+, N+, N2+, and NO+) to conduct 11-species multifluid simulations. We used the neutral atmosphere profiles during the solar minimum, which included nine neutral species (H, O, O2, CO2, C, CO, N, N2, and NO). The solar wind was set to nominal conditions with a number density of 4 cm-3, a velocity of 400 km s-1, and a Parker spiral interplanetary magnetic field strength of 2.5 nT. The crustal magnetic field was included so that the strong crustal magnetic field region was at noon. For comparison, we conducted a 5-species multifluid simulation under the same neutral, upstream, and crustal magnetic field conditions. Only four neutral species (H, O, O2, and CO2) were considered in the 5-species simulation case.
The two simulation cases show nearly identical locations of the plasma boundaries (the bow shock and the magnetic pileup boundary). The density profiles of the major species (O+, O2+, and CO2+) along the subsolar line also remain unchanged in the 11-species case. Among the additional ion species, NO+ and C+ are abundant at lower and higher altitudes, respectively. In the nightside ionosphere, NO+ and HCO+ are the dominant species in addition to O2+, consistent with previous MAVEN observations. Including six ion species increased the total escape rate of oxygen by only 6%. On the other hand, the total escape rate of carbon is increased from 1.2×1023 s-1 in the 5-species case to 4.0×1023 s-1 in the 11-species case as the C+ escape rate is comparable to the CO2+ escape rate. Additionally, the 11-species case estimates the total escape rate of nitrogen as 2.1×1023 s-1, nearly half the carbon escape rate.
These results suggest that 5-species simulations cover enough species and reactions to understand the global configuration of interactions with solar wind flow and oxygen ion escape. However, 11-species simulations are necessary to investigate carbon and nitrogen ion escape. The addition of carbon and nitrogen ions is particularly critical for early Mars, when ion escape surpasses other processes, such as photochemical escape and sputtering.
References
Sakata, R., Seki, K., Terada, N., Sakai, S. & Shinagawa, H. (2024). Effects of an intrinsic magnetic field on ion escape from ancient Mars based on MAESTRO multifluid MHD simulations. Journal of Geophysical Research: Space Physics, 129, e2023JA032320. https://doi.org/10.1029/2023JA032320
This study aims to investigate the escape of carbon and nitrogen ions from Mars. We employed a three-dimensional global multifluid MHD model MAESTRO (Sakata et al., 2024). This model was developed for five ion species (planetary H+, O+, O2+, CO2+, and solar wind H+); however, we incorporated chemical reactions and collision frequencies for six ion species (C+, CO+, HCO+, N+, N2+, and NO+) to conduct 11-species multifluid simulations. We used the neutral atmosphere profiles during the solar minimum, which included nine neutral species (H, O, O2, CO2, C, CO, N, N2, and NO). The solar wind was set to nominal conditions with a number density of 4 cm-3, a velocity of 400 km s-1, and a Parker spiral interplanetary magnetic field strength of 2.5 nT. The crustal magnetic field was included so that the strong crustal magnetic field region was at noon. For comparison, we conducted a 5-species multifluid simulation under the same neutral, upstream, and crustal magnetic field conditions. Only four neutral species (H, O, O2, and CO2) were considered in the 5-species simulation case.
The two simulation cases show nearly identical locations of the plasma boundaries (the bow shock and the magnetic pileup boundary). The density profiles of the major species (O+, O2+, and CO2+) along the subsolar line also remain unchanged in the 11-species case. Among the additional ion species, NO+ and C+ are abundant at lower and higher altitudes, respectively. In the nightside ionosphere, NO+ and HCO+ are the dominant species in addition to O2+, consistent with previous MAVEN observations. Including six ion species increased the total escape rate of oxygen by only 6%. On the other hand, the total escape rate of carbon is increased from 1.2×1023 s-1 in the 5-species case to 4.0×1023 s-1 in the 11-species case as the C+ escape rate is comparable to the CO2+ escape rate. Additionally, the 11-species case estimates the total escape rate of nitrogen as 2.1×1023 s-1, nearly half the carbon escape rate.
These results suggest that 5-species simulations cover enough species and reactions to understand the global configuration of interactions with solar wind flow and oxygen ion escape. However, 11-species simulations are necessary to investigate carbon and nitrogen ion escape. The addition of carbon and nitrogen ions is particularly critical for early Mars, when ion escape surpasses other processes, such as photochemical escape and sputtering.
References
Sakata, R., Seki, K., Terada, N., Sakai, S. & Shinagawa, H. (2024). Effects of an intrinsic magnetic field on ion escape from ancient Mars based on MAESTRO multifluid MHD simulations. Journal of Geophysical Research: Space Physics, 129, e2023JA032320. https://doi.org/10.1029/2023JA032320