MESSENGER observations of Mercury's planetary ion escape rates and their dependence on true anomaly angle
arxiv(2024)
摘要
This study investigates the escape of Mercury's sodium-group ions (Na+-group,
including ions with m/q from 21 to 30 amu/e) and their dependence on true
anomaly angle (TAA), i.e., Mercury's orbital phase around the Sun, using
measurements from MESSENGER. The measurements are categorized into solar wind,
magnetosheath, and magnetosphere, and further divided into four TAA intervals.
Na+-group ions form escape plumes in the solar wind and magnetosheath, with
higher fluxes along the solar wind's motional electric field. The total escape
rates vary from 0.2 to 1 times 10^25 atoms/s with the magnetosheath being the
main escaping region. These rates exhibit a TAA dependence, peaking near the
perihelion and similar during Mercury's remaining orbit. Despite Mercury's
tenuous exosphere, Na+-group ions escape rate is comparable to other inner
planets. This can be attributed to several processes, including that Na+-group
ions may include several ion species, efficient photoionization frequency for
elements within Na+-group, etc.
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