[P-PS05] Mercury Science and Exploration

[PPS05-08] Effects of the solar wind conditions on the cusp location (Na source region) in Mercury’s magnetosphere

*Kanako Seki¹, Manabu Yagi² (1.Graduate School of Science, University of Tokyo, 2.Previously at R-CCS, RIKEN)

Keywords:Mercury, magnetosphere, Na, exosphere

Mercury has no collisional thick atmosphere but possesses a weak global intrinsic magnetic field [e.g., Anderson et al., Science, 2011]. As a result, Mercury’s magnetosphere formed by interaction between the solar wind and planetary magnetic field, where Na+ ions originated from the solid planet are one of important plasma sources [e.g., Yagi et al., JGR, 2010; 2017; Zurbuchen et al., Science, 2011]. Since Mercury revolves around Sun at the radial distance of 0.31-0.47 AU, the solar wind density (~50 1/cc) and interplanetary magnetic field (IMF) strength (~35 nT) at the Mercury’s orbit is much larger than that at Earth. Previous studies indicate that configuration of the Mercury’s magnetosphere largely depends on the solar wind conditions as well as the surface conductivity of the solid planet [e.g., Seki et al., JGR, 2013]. The configuration change can cause variations of the cusp location in the magnetosphere, i.e., the source region of exospheric Na [e.g., Raines et al., JGR, 2022]. On one hand, exospheric Na observations by the ground-based telescope show that the spatial distribution of Na emission is variable in time and can be categorized in 8 types. Comparison with in-situ IMF observations by MAG/MESSENGER indicate that the Na emission patterns have complicated dependence on IMF [Mangano et al., PSS, 2015]. However, mechanisms to cause the IMF dependence are far from understood. Thus, investigation of the solar wind and IMF effects on the cusp location is important to understand planetary ion source in the Mercury’s magnetosphere.
In this study, we investigate how the cusp location change with solar wind and IMF conditions based on global MHD simulations. The CIP-based divB-free MHD model, which solves the vector potential instead of the magnetic flux [Yagi et al., JGR, 2017] are used for the simulations. A systematic simulation runs (24 cases) under Parker spiral IMF configurations with some Bz components added is conducted for three solar wind dynamic pressure conditions. The results show that locations of the high-pressure region in dayside (cusp-like region) systematically change with the IMF Bz and solar wind dynamic pressure under background Parker spiral IMF conditions. The simulation results are categorized into 8 types of spatial distributions proposed by Mangano et al. [PSS, 2015] and it is revealed that 2P patterns including 2PN and 2PS are dominant patterns. Different By polarity cause opposite longitudinal twist of the cusp location. As the solar wind dynamic pressure increases, WP or EP patterns can appear especially during negative Bz conditions. The results suggest that exospheric Na distribution can systematically change with the solar wind conditions.