Composition of Mars's Dayside Ionosphere Under Changing Solar Irradiance Conditions.

The response of the Mars ionosphere to changes in solar irradiance is an important aspect of how conditions on Mars are shaped by our dynamic Sun. Changes in the composition of ionospheric plasma with changes in solar irradiance will affect how the ionosphere mediates the interaction of the neutral atmosphere with the surrounding space environment. Here we use MAVEN ion and neutral density measurements acquired at low (Deep Dip 8) and high (Deep Dip 2) solar irradiance conditions to determine how ion and neutral densities change when the ionizing solar irradiance doubles. We find that the neutral composition does not change significantly when examined at fixed total neutral number density. Furthermore, we find that CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ and O+ densities increase by a factor of 2, but O2+ ${\mathrm{O}}_{2}^{+}$ densities increase by a factor of 1.5. The relative abundance of O2+ ${\mathrm{O}}_{2}^{+}$ decreases as solar irradiance increases. These results are explained by straightforward theoretical considerations of changes in ion production and loss rates. However, a photochemical model fails to reproduce these results with its default set of inputs. Consistent with previous modeling efforts, CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ densities are over‐predicted. Acceptable model–data agreement requires significant adjustments to important model inputs, such as reduction in irradiance by 15%, reduction in CO2 density by a factor of 2, and increase in O density by a factor of 2. These large adjustments are suggestive of the need for improvements to the state of Mars ionosphere models based on MAVEN inputs rather than issues with the underlying data. Key Points: The composition of Mars's ionosphere changes with changes in solar irradianceThese compositional changes vary with respect to altitude but are remarkably constant with respect to total neutral number densityPhotochemical modeling illuminates the photochemical pathways responsible for the observed changes [ABSTRACT FROM AUTHOR]