Formation Mechanisms of the Molecular Ion Polar Plume and Its Contribution to Ion Escape From Mars
Kotaro Sakakura, K. Seki, Shotaro Sakai, Ryoya Sakata, Hiroyuki Shinagawa, D. A. Brain, J. P. McFadden, J. S. Halekas, G. A. DiBraccio, B. M. Jakosky, Naoki Terada, Takashi Tanaka
Abstract
Abstract We investigated the formation mechanism of a molecular ion plume and its contribution to ion escape based on Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from November 2014 to October 2019 and numerical models. Here, we report a CO 2 + ‐rich plume event and a statistical study of the molecular ion plume. MAVEN observed a CO 2 + ‐rich plume event, in which the maximum CO 2 + escape flux is approximately 4.2 × 10 6 cm −2 s −1 , on 28 August 2015 under strong solar wind dynamic pressure conditions. A numerical simulation using strong solar wind dynamic pressure conditions from the event suggested that the molecular ion plume is formed by deep penetration of the solar wind‐induced electric field, which is caused by strong solar wind dynamic pressure. A statistical study showed that CO 2 + plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. This tendency is consistent with the formation mechanism of the molecular ion plume suggested by the event study. The O 2 + plume does not show the same tendency. This is because O 2 + ions are abundant in the high‐altitude ionosphere, and O 2 + plumes can be formed even under weak solar wind conditions. The subsolar crustal magnetic fields tend to prevent the formation of the molecular ion plume by shielding the ionosphere from the solar wind. The escape rate ratio is approximately 45:53:3 during the whole statistical survey period, suggesting that a molecular ion plume from the ionosphere is a non negligible ion escape channel from Mars.