[P-CG19] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG19-04] Statistical study of effects of dust storms on the ion density variations in the Martian ionosphere based on MAVEN observations

*Akira Kurosu¹, Kanako Seki¹, Takuya Hara², Christopher Fowler³, Shannon Curry², James P. McFadden², Gwen Hanley² (1.Graduate School of Science, The University of Tokyo , 2.Space Sciences Laboratory, University of California, Berkeley, 3.Department of Physics and Astronomy, West Virginia University)

Keywords:Mars, Ionosphere, crustal magnetic fields, dust storm, MAVEN, STATIC

Mars currently has a tenuous atmosphere, and the dust storm is an important phenomenon to understand the vertical coupling of the Martian atmosphere and the resultant water escape to space (e.g., Chaffin et al., 2021). The dust storms of various spatial scales occur in the Martian atmosphere, and in particular, global dust storms have been shown to affect the composition of the exosphere (Liu et al., 2018, Girazian et al., 2019). On the other hand, Withers et al. (2019) showed that variations in ionospheric ion density are affected by Martian crustal magnetic fields. However, effects of dust storms on the density variations of individual ion species are far from understood. In this study, we aim to reveal the effects of dust storms on the variations in the density of major ion species (H⁺, O⁺, O₂⁺, and CO₂⁺) in the Martian ionosphere based on observations by the MAVEN spacecraft.
In the statistical analysis, we primarily use Level 3 data from the ion mass spectrometer STATIC onboard MAVEN (Fowler et al., 2022) and data from the magnetic field instrument MAG, which were acquired over a period including more than two Mars years from February 2016 to December 2020. From these data, we first selected ionospheric observation data and derived the median altitude distribution of the density for each ion species at every 15 km altitude. Next, dependence of these median altitude distributions on solar zenith angle (SZA) was examined, and we decided to use data of SZA <=105 degrees and altitudes from 225 km to 325 km, where little dependence was found. For the selected data, the ratio of the individual density data to the median altitude distribution (R) was then calculated for each ion species The dependence of R on the magnetic field dip angle and the geographic distribution were compared between seasons with and without large dust storms to investigate the effects of crustal magnetic fields and dust storms on the density variations of each ion species.
The results show that (1) molecular ions (O₂⁺ and CO₂⁺) densities tend to increase in regions with strong crustal magnetic fields, (2) Dust storms have little effects on O⁺, (3) Densities of H⁺, O₂⁺ and CO₂⁺ tend to increase with dust storms by 2.1, 1.7, and 2.6 times, respectively and (4) the density increase due to dust storms is more remarkable at high altitudes (225-275 km) for H⁺ and at low altitudes (275-325 km) for CO₂⁺. The dependence on crustal magnetic fields is consistent with previous study by Withers et al. (2019). The seasonal dependences can be explicable with changes in production and loss of each ion species caused by increases of CO₂ and H in the neutral atmosphere during dust storms. The observed increase in H⁺ ion density at the high altitudes during dust storms indicates that the H⁺ ion escape from the ionosphere may also increase. As for the escape of atmospheric oxygen from Mars, the results suggest that the contribution of molecular ions to the ionospheric ion escape increases during dust storms.

References:
Chaffin, M.S., Kass, D.M., Aoki, S. et al. (2021), Nature Astron., 5, 1036–1042, doi:10.1038/s41550-021-01425-w.
Fowler, C. M., McFadden, J., Hanley, K. G. et al. (2022), J. Geophys. Res., 127, e2022JA030352, doi:10.1029/2022JA030352.
Girazian, Z., et al. (2019), J. Geophys. Res., 124, e2019JE006092, doi:10.1029/2019JE006092.
Liu, G., et al. (2018), Geophys. Res. Lett., 45, 2901–2910, doi:10.1002/2018GL077525.
Withers, P., et al. (2019), J. Geophys. Res., 124, 3100–3109, doi:10.1029/2018JA026266.