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

Abstract

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 normalCnormalO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ 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, normalCnormalO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ 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.

Troubles in Mars Ionosphere Modeling

“…Values from Hensley et al. (2022) apply to results from both Deep Dip 2 and Deep Dip 8 (16–23 October 2017) (Stone et al., 2018). Values from Mayyasi et al.…”

“…Such a large error in ionizing irradiance seems unrealistic. Hensley et al (2022) compared MAVEN EUV measurements to empirical representations of ionizing irradiance based on well-validated TIMED SEE measurements, and found that they agreed to within 15%. Wu et al (2021) did not explore the effects of rescaling the neutral CO 2 density, which could also modify their model output.…”

“…(2021), and Hensley et al. (2022). Second, we asssess the “photochemistry and transport” models of Mayyasi et al.…”

“…Section 7 compares data and model output for Hensley et al. (2022). Section 8 compares data and model output for Mayyasi et al.…”

“…The chemical schemes adopted in Xu et al (2018) and Yoshida et al (2021) include fewer than 10 chemical reactions, making the system sufficiently simple that ion densities can be determined from algebraic equations. Approximately 10 chemical reactions are included in Hensley et al (2022). The other models adopted more comprehensive approaches to ion chemistry.…”