Ionization Efficiency in the Dayside Ionosphere of Mars: Structure and Variability. Issue 12 (6th December 2021)

• Record Type: Journal Article
• Authors: Lillis, Robert J.; Xu, Shaosui; Mitchell, David; Thiemann, Ed; Eparvier, Frank; Benna, Mehdi; Elrod, Meredith
• Is Part Of: Journal of geophysical research. Volume 126:Issue 12(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-12-06
• Subjects: Mars -- ionization -- efficiency -- ionosphere -- dayside -- electron; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE006923
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Ionization efficiency, the ratio between photoelectron impact ionization and primary photoionization, is an important quantity from the perspective of ionospheric energy balance and as a quantity to be parameterized for use in global simulations. We investigate ionization efficiency using MAVEN in situ measurements of solar extreme ultraviolet (EUV) flux, suprathermal electron flux, and neutral density. Its behavior with respect to pressure and solar zenith angle (SZA) is explained by ionization cross‐sections and photoabsorption, plus photoelectron transport near the terminator. We find similar ionization efficiencies between the species CO2, O, N2, and CO, with Argon ∼40% higher, explained largely by its higher ionization potential. Efficiency depends positively on solar activity, increasing by ∼50–100% (depending on season) from low to moderate solar EUV levels, before flattening at higher EUV levels. EUV spectral hardening appears to be responsible for only ∼15% of this increase, the remainder best explained by variability in neutral composition. We find a negligible dependence of ionization efficiency on the strength and direction of crustal magnetic fields. Efficiencies agree poorly with the theoretical model of Nicholson et al. (2009), https://doi.org/10.1111/j.1365-2966.2009.15463.x, possibly reflecting differences in model versus measured solar EUV spectrum and neutral densities. We fit the data to the empirical parameterization of Mendillo et al. (2011), https://doi.org/10.1029/2011ja016865 but find that a single fit cannot capture ionization efficiency behavior across all dayside SZA values. We believe that an empirical model of Mars ionization efficiency will require MAVEN data and validated transport modeling to reach to sufficiently high pressures to capture Mars' photochemical peaks M1 and M2. Plain Language Summary: Solar extreme ultraviolet (EUV) photons ionize neutral atoms and molecules in the upper atmosphere of Mars, forming a layer known as the ionosphere. Photoionization (PI) (i.e., direct ionization by photons) is augmented by further ionization caused by impact on neutral particles of the electrons freed by the PI. Ionization efficiency is the ratio of PI to this electron impact ionization. We investigate ionization efficiency using data from the MAVEN spacecraft and find that it varies between 0.1 and >1000, with its basic structure explained by wavelength‐dependent absorption of EUV radiation, as well as photoelectron transport from the day to night sides. This efficiency is similar for the primary gases CO2, O, N2, and CO but higher for the noble gas Argon. Efficiency can vary by up to a factor of 2 over the course of the Martian seasons and as the sun's ultraviolet output naturally varies. These patterns of variability will enable modelers to better simulate the global behavior of the Martian ionosphere, knowledge of which is important for understanding atmospheric escape and climate evolution and for communication and navigation for future human explorers of Mars. Key Points: Ionization efficiency dependence on solar zenith angle and pressure follows expectations, but no dependence on crustal fields is found Argon is found to have ∼40% higher efficiency than the other primary atmospheric constituents, which are within 10% of each other Efficiency varies by up to a factor of two with both season and solar activity, driven largely by variability in relative species abundance