Mercury’s exospheric He I 58.4 nm emission: Dependence on the orbital phase

Aims. Emission from helium atoms in Mercury's exosphere at 58.4 nm was observed by the Mariner 10 spacecraft in the 1970s. This emission is due to resonant scattering of solar radiation. Since Mercury's orbit is ellipsoidal, solar radiation and wind fluxes on Mercury vary along its orbit. Furthermore, the flux of the solar He I 58.4 nm emission observed on Mercury varies with Mercury's radial velocity with the Sun. Using model simulations, we demonstrate how Mercury's exospheric He I 58.4 nm emission varies due to the periodic changes in solar radiation and wind. Methods. The simulation is based on our Monte-Carlo model that accounts for changes in fluxes of the solar wind and radiation. Results. The model results indicate that the He I 58.4 nm brightness varies by between one and three orders of magnitude along the planet's orbit. These findings are strongly dependent on the intrinsic line width of the solar He I 58.4 nm emission. Conclusions. Although this variation has never been observed because Mariner 10 only sampled emission near Mercury's aphelion, we expect this variation to be observable by new missions, such as BepiColombo and Hisaki. Our results are also important for the characterization of exoplanets with ultraviolet space missions.