Species-dependent solar rotation effects on the Martian ionosphere.

Atmospheric escape is a central issue in understanding how Mars lost the habitability and it is critically controlled by the link between the atmosphere and the interplanetary space, i.e. the ionosphere. The predominant driver of the Martian ionospheric variability is the solar extreme ultraviolet radiation. To date, how the ionospheric electrons respond to this radiation has been well investigated, but the responses of various ion species are to be understood. Based on a multi-instrument data set from the Mars Atmosphere and Volatile Evolution, we report direct observations of quasi-periodical variations of photoelectrons and ions, with a common period of ≈27.2 d and almost in exact phase with the solar rotation. These diverse variations present remarkably different amplitudes. The ion density variation has a much larger amplitude than the solar flux variation and the electron density variation has a smaller amplitude. For comparison, the amplitude of the photoelectron intensity variation is nearly identical to that of the solar flux variation. The species-dependent solar rotation effects provide a good diagnostic of the upper atmospheric and ionospheric chemistry, urging reconsiderations of the solar-driven composition and variability of any planetary ionosphere. [ABSTRACT FROM AUTHOR]