JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

P (Space and Planetary Sciences ) » P-CG Complex & General

[P-CG25] Planetary Magnetosphere, Ionosphere, and Atmosphere

convener:Kanako Seki(Graduate School of Science, University of Tokyo), Takeshi Imamura(Graduate School of Frontier Sciences, The University of Tokyo), Hiroyuki Maezawa(Department of Physical Science Osaka Prefecture University), Naoki Terada(Graduate School of Science, Tohoku University)

[PCG25-07] Study of Molecular Ion Contribution to the Polar Plume from Mars Based on MAVEN Observations

*Sakakura Kotaro1, Kanako Seki1, Shotaro Sakai1, David A Brain2, James P McFadden3, James S Halekas4, Gina A Dibraccio5, Bruce M Jakosky2 (1.Department of Earth and Planetary Science, Graduate School of Science, The university of Tokyo, 2.Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA., 3.Space Sciences Laboratory, University of California, Berkeley, California, USA., 4.Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA., 5.Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA.)

Keywords:Mars, MAVEN, atmospheric escape, carbon dioxide, polar plume

Mars once had water on its surface about 4 billion years ago, but there is no liquid water on the surface at present. Escape of the atmosphere to space is considered as the main cause of this climate change. However, the mechanism of the large amount of atmospheric loss is far from understood. Ion escape is one of the important candidates of the loss mechanism. There are three channels of the ion escape, namely, tailward escape, pickup ion, and polar plume. Polar plume ions are accelerated by solar wind electric field and escape to positive E hemisphere of the Mars-Sun-Electric field (MSE) coordinates. It is estimated by Dong et al. [2017] that the escape rate of O+ plume is 20-30% of the total O+ escape depending on the solar EUV radiation. The rate is not negligible in order to understand ion escape from Mars. Molecular ions in the polar plume should also be studied since it is shown that main escape species is O2+ for tailward escape by Inui et al. [2019]. We recently reported a CO2+-rich plume event on August 28, 2015. Peak flux of CO2+ was 3.6x106 cm-2s-1, which is about one order of magnitude higher than the average flux of O+ in the polar plume reported by Dong et al. [2017]. Such high escape flux is unexpected because CO2+ is minor component at high-altitude ionosphere due to its small scale height. To fully understand the mechanism of polar plume, it is important to study the composition of the plumes.

In order to investigate relationship between CO2+-rich plume event and solar wind conditions, we conducted a statistical study. We analyzed data obtained by STATIC (Supra Thermal and Thermal Ion Composition), MAG (magnetometer) and SWIA (Solar Wind Ion Analyzer) onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) from Dec. 11, 2014 to Feb. 23, 2019. STATIC can measure ion distribution functions with mass discrimination. In order to derive CO2+ number density, we use the fitting method invented by Inui et al. [2018]. By fitting a log-normal distribution to O2+ count data, we remove O2+ contamination in the CO2+ mass range. We defined CO2+ plume event as the time period when the observed maximum flux of CO2+ in an MAVEN orbit is higher than 2.5x105 cm-2s-1. In order to remove false CO2+ plume events due to high O2+ counts, we set another requirement that CO2+ has evident peak which is at least comparable to O2+ contamination in the CO2+ mass range. The results show that the CO2+ plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. This result is consistent with hypothesis that CO2+ plume is caused by deep penetration of the solar wind electric field due to the high solar wind dynamic pressure. On one hand, detection probability of O2+ plume events don't show such dependences on the solar wind parameters. This is probably because O2+ is abundant near the ionopause enough to create dense (>2.5x105 cm-2s-1) O2+ plumes regardless of the solar wind conditions. On the other hand, the escape flux of O2+ due to the polar plume has weak positive correlation with the solar wind dynamic pressure. This result might reflect high density of O2+ near the ionopause under the high solar wind dynamic pressure conditions.

Dong, Y., et al. (2017), J. Geophys. Res. Space Phys., 122, 4009–4022, doi:10.1002/2016JA023517
Inui, S., et al. (2018), Geophys. Res. Lett., 45, 5283–5289, doi:10.1029/2018GL077584
Inui, S., et al. (2019). J. Geophys. Res. Space Phys., 124, 5482–5497. doi:10.1029/2018JA026452