Figure 4: Observations indicating the CME arrival at Mars.

(a) At Mars Express, the CME is observed by the Electron Spectrometer (ELS) as an increase in the electron magnetosheath and solar wind differential energy flux (colour coded) starting late on 10 January, with clear enhancements on early 11 January to late 12 January. The horizontal arrow bar delimits the interval of enhanced high-energy electrons (30–400 eV) on 11–12 January. (b) Counts of energetic particles per second by the RAD experiment on the surface of Mars onboard Mars Science Laboratory’s Curiosity Rover. The high-energy solar energetic particle event stems from an eruption on 6 January. The CME of our study was launched from the Sun on 7 January and its shock hit Mars on 10 January 22:30 UT ±1 h, as indicated by the onset of a Forbush decrease of the cosmic ray flux.