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

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.