[P-CG19] 惑星大気圏・電磁圏

[PCG19-06] Crustal magnetic field effects on the Martian ionosphere and ion escape: Ion species dependence based on MAVEN observations

*関 華奈子¹、原 拓也²、Dave Brain³、Christopher Fowler⁴、James McFadden²、Jasper Halekas⁵、DiBraccio Gina⁶ (1.東京大学大学院理学系研究科、2.Space Science Laboratory, University of California, Berkeley, USA、3.LASP, University of Colorado Boulder, USA、4.Department of Physics and Astronomy, West Virginia University, USA、5.Department of Physics and Astronomy, University of Iowa, USA、6.NASA Goddard Space Flight Center, USA)

キーワード：火星、地殻磁場、電離圏、大気散逸

In order to explain a drastic climate change at ancient Mars, removal of H₂O and CO₂ from the surface environment is necessary. The atmospheric escape from Mars to the interplanetary space is considered to have played an important role in the removal. On the other hand, mechanisms to achieve substantial rapid escape of heavy species such as carbon and oxygen is far from understood. The solar wind induced ion escape process is one of the promising candidates to achieve significant loss of heavy ion species especially under strong solar X-ray and EUV (XUV) radiation and solar wind conditions. Among solar wind induced escape, how ion outflows from the Martian ionosphere depend on the solar wind variations is one of key questions. In this study, we report on their dependence on the solar wind variations with a focus on the difference and similarity between major ion species based on observations by NASA’s MAVEN (Mars Atmosphere and Volatile EvolutioN) satellite.

There are two main escape channels of the ionospheric ion outflows, i.e., tailward escape [Inui et al., JGR, 2019] and polar plume [Sakakura et al., 2022]. In order to investigate their responses to the solar wind variations, we conducted three sets of statistical data analysis for the tailward escape, polar plume, and Martian ionosphere. As for the tailward escape, MAVEN data from July 2015 to December 2017 obtained in the optical shadow within induced magnetotail are used. The escape rate ratio (O⁺, O₂⁺, and CO₂⁺) through the tailward escape is 29, 68, and 4. The escape rate and mechanisms depend on the solar wind electric field as well as the location in the induced magnetotail. As to the polar plumes, we analyzed MAVEN data from November 2014 to October 2019 obtained either in the magnetosheath or upstream solar wind region. The escape rate ratio (O⁺, O₂⁺, and CO₂⁺) through the polar plume is approximately 45, 53, and 3. The escape rates of the molecular ion polar plumes depend on the solar wind dynamic pressure.

In order to separate effects of the solar wind variations on change in the source region (ionosphere) and response of each escape mechanism, we also conducted a statistical survey of the Martian ionosphere. MAVEN data obtained in the dayside ionosphere in the altitude ranges from 225 to 325 km are analyzed for the period from February 2016 to December 2020. The density variation ratio to the median altitude profile of each ion species is investigated. While the ionospheric density distribution mostly depends on the magnetic field and/or dust storms, there are intermittent density enhancement by the solar energetic particles. Comparing the results of the three sets of statistical data analysis, we will discuss how we can apply the obtained knowledge to infer the responses of ion escape from Mars to the characteristic solar wind structures such as CME (coronal mass ejection) and CIR (corotation interaction region).