He⁺ ions in the vicinity of mercury observed by the MESSENGER and BepiColombo spacecraft

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched in 2004, and between March 2011 and April 2015 it was the first spacecraft in orbit around Mercury. The Fast Imaging Plasma Spectrometer (FIPS) instrument on board MESSENGER measured the ion composition in the vicinity of Mercury and in the inner solar system. We aim to determine the origin of He⁺ ions in the inner solar system and in the environment of Mercury, continuing earlier work by Gershman et al., (2013). We have analyzed measurements of He⁺ and He²⁺ ions made by the FIPS instrument during the interplanetary cruise phase of MESSENGER and its entire orbital mission at Mercury. We determined the spatial distributions of He⁺ ions in the regions sampled by MESSENGER during that period and compare the spectra to a similar observation by the Mass Spectrum Analyzer instrument which is part of the Mercury Plasma Particle Experiment (MPPE-MSA) onboard BepiColombo. We consider two possible sources of He⁺: (1) interstellar neutral helium ionized close to Mercury and (2) solar He²⁺ ions converted close to or at the surface of Mercury. We also compare the observed densities with a simple model of the ionization of the interstellar helium flow. The FIPS data show a continuous evolution of the He⁺ energy spectra from solar wind towards Mercury - changing from a shape typical for pick-up ions to a thermalized spectrum. This could mean that interstellar He⁺ is concentrated at Mercury by increased electron impact ionization close to the planet. We find a remarkably similar high mean ratio of He⁺/He²⁺ ions in the upstream solar wind and in the inner magnetosphere, while the ratio is reduced in the magnetosheath. Outside Mercury's magnetosphere the source of He⁺ ions is interstellar helium, while inside the planet's magnetosheath and the magnetosphere both interstellar helium and solar wind helium may be of similar magnitude. The observed median upstream He⁺ densities are in good quantitative agreement with a simple model — though the existence of an upstream density crescent cannot yet be confirmed.