The Impact and Solar Wind Proxy of the 2017 September ICME Event at Mars

We study a large interplanetary coronal mass ejection event impacting Mars in mid‐September 2017 numerically. During this time period, MAVEN remained inside the Martian bow shock and therefore could not measure the solar wind directly. We first simulate the event using three steady state cases with estimated solar wind conditions and find that these cases were able to reproduce the general features observed by MAVEN. However, these time‐stationary runs cannot capture the response of the system to large variations in the solar wind associated with the event. To address this problem, we derive a solar wind proxy based on MAVEN observations in the sheath region and their comparison with steady state magnetohydrodynamic model results. The derived solar wind proxy is then used to drive a time‐dependent magnetohydrodynamic model, and we find that the data‐model comparison is greatly improved, especially in the magnetosheath. We are able to reproduce some detailed structures observed by MAVEN during the period, despite the lack of a direct measurement of the solar wind, indicating that the derived solar wind conditions are reliable. Finally, we examine in detail the impact of the event on the Martian system: including variations of the three typical plasma boundaries and the ion loss rates. Our results show that these plasma boundary locations varied drastically during the event, and the total ion loss rate was enhanced by more than an order of magnitude. Plain Language Summary A large interplanetary coronal mass ejection event impacted on Mars in mid‐September 2017. We use a numerical model to study in detail about the effect of the interplanetary coronal mass ejection on the Mars plasma environments. The model results are in good agreement with MAVEN observations, despite the lack of a direct measurement of the solar wind. We also examine in detail variations of typical plasma boundaries and the ion loss rates and found that the plasma boundary locations varied drastically during the event, and the total ion loss rate was enhanced by more than an order of magnitude. Key Points A solar wind proxy method is developed and validated for the 2017 September ICME event Time‐dependent MHD model reproduces detailed structures observed by MAVEN for the ICME event Model predicts drastic variation of plasma boundaries and large enhancement of ion loss rates during the event