Solar and Magnetic Control of Minor Ion Peaks in the Dayside Martian Ionosphere

Abstract

The Neutral Gas and Ion Mass Spectrometer of the Mars Atmosphere and Volatile Evolution provides a large data set to explore the ion composition and structure of the Martian ionosphere. Here, the dayside measurements are used to investigate the minor ion density profiles with distinctive peaks above 150 km, revealing a systematic trend of decreasing peak altitude with increasing ion mass. We specifically focus on a subset of species including O+, N 2+/CO+, C+, N+, He+, and O++, all of which are mainly produced via direct photoionization of parent neutrals. Our analysis reveals weak or no variation with solar zenith angle (SZA) in both peak density and altitude, which is an expected result because these ion peaks are located within the optically thin regions subject to the same level of solar irradiance independent of SZA. In contrast, the solar cycle variations of peak density and altitude increase considerably with increasing solar activity, as a result of enhanced photoionization frequency and atmospheric expansion at high solar activities. He+ serves as an exception in that its peak density increases toward large SZA and meanwhile shows no systematic variation with solar activity. The thermospheric He distribution on Mars should play an important role in determining these observed variations. Finally, the peak altitudes for all species are elevated by at least several km within the weakly magnetized regions, possibly attributable to the suppression of vertical diffusion by preferentially horizontal magnetic fields in these regions.

Empirical Models of Ion Density Distribution in the Dayside Martian Ionosphere

“…For instance, the clear manifestation of a distinctive layer structure for some species such as CO + / 𝐴𝐴 N + 2 , N + , and O + , as well as the more or less fuzzy appearance of the layer structure for some other species such as C + , are well reproduced by the empirical models. In general, the SZA and solar cycle variations of the ion density distribution in terms of peak density and peak altitude for the one-layer category, as reported by Huang et al (2020) and Withers et al (2019), are favorably seen in our model distributions. The models also reveal extra features such as the variation of the layer width for the one-layer species and the variation of the topside density scale height for the no-layer species (e.g., Wu et al, 2019).…”

“…In previous studies, ion density distribution in the dayside Martian upper atmosphere for the one‐layer category has been explored in terms of the peak density and altitude (e.g., Huang et al., 2020; Withers et al., 2019), which are reflected by N i ,0 and z i ,0 in Table 1. Under the subsolar condition, with horizontal magnetic fields, and for a fixed solar EUV energy flux of 0.8 mW m −2 , the peak density covers a broad range from $$\approx 6$$ cm −3 for Ar + to $$\approx 660$$ cm −3 for O + , whereas the peak altitude ranges from $$\approx 147$$ km for HCO + to $$\approx 228$$ km for N + .…”

“…ΔH in the sigmoid function characterizes the altitude range near z i,m over which H i varies rapidly. The above parameterizations are motivated by the conclusion made in Huang et al (2020) that the NGIMS ion densities tend to be significantly higher than those predicted by the idealized Chapman theory both above and below the peak, essentially indicating that the actual width of the ion peak is broader than the width of a Chapman peak.…”

“…Existing studies have revealed significant SZA and solar cycle variations in ion density distribution (e.g., Huang et al., 2020; Withers et al., 2019). To properly represent these variations and meanwhile to facilitate data‐model comparison, we divide all available ion density profiles into a sequence of subsamples covering the SZA range of 5°–75° with an increment of 10°, and the EUVI range of 0.7–2.7 with an increment of 0.2.…”

Species‐dependent Response of the Martian Ionosphere to the 2018 Global Dust Event

“…We have made no attempt to guarantee that a quantitatively consistent enhancement or depletion factor is reached during the GDS between the NGIMS observation and photochemical equilibrium computation. In addition, our findings reported here are referred to a representative altitude under photochemical equilibrium, whereas the compositional variation of the Martian ionosphere over the full available altitude range (e.g., Huang et al, 2020) should provide more insights into the consequences of dust storms. The above issues are beyond the scope of this preliminary study and will be attempted in a future investigation including both ionospheric chemistry and transport.…”