Journal Article

Asymmetrical Looping Magnetic Fields and Marsward Flows on the Nightside of Mars

Song, Yihui, Wang, Jianxuan, Ip, Wing‐Huen, Chen, Nihan, Zhang, Xiaoxin, Cui, Jun, Lu, Haoyu, Cao, Jinbin, Li, Shibang, Wild, James A.

2024

Overview

As the interplanetary magnetic field (IMF) carried by the solar wind encounters the martian atmosphere, it tends to pile up and drape around the planet, forming looping magnetic fields and inducing marsward ion flows on the nightside. Previous statistical observations revealed asymmetrical distribution features within this morphology; however, the underlying physical mechanism remains unclear. In this study, utilizing a three‐dimensional multi‐fluid magnetohydrodynamic simulation model, we successfully reproduce the asymmetrical distributions of the looping magnetic fields and corresponding marsward flows on the martian nightside. Analyzing the magnetic forces resulting from the bending of the IMF over the polar area, we find that the asymmetry is guided by the orientation of the solar wind motional electric field (ESW). A higher solar wind velocity leads to enhanced magnetic forces, resulting in more tightly wrapped magnetic fields with an increased efficiency in accelerating flows as they approach closer to Mars. Plain Language Summary As incident solar wind interacts with Mars, the entrained interplanetary magnetic field drapes and stretches around the planet. The draped fields are orientated in opposite directions on the east and west sides of Mars, resulting in magnetic reconnection, the formation of looping magnetic fields and induced marsward plasma flows on the nightside. Statistical observations suggest an asymmetric distribution of these structures, however, the underlying physical mechanisms for this asymmetry have not been determined due to limited spatial coverage offered by the single spacecraft investigations. In this study, by taking advantage of a Hall‐MHD model, we have successfully reproduced the asymmetrical distributions and elucidated the underlying mechanisms governing these morphologies. Simulation results demonstrate that magnetic forces tend to emerge around the polar region, which are determined by the orientation of the ESW, shifting the plasma flows and magnetic fields in the direction opposite to the motional electric field. A higher solar wind velocity condition will induce amplified magnetic forces, resulting in more tightly wrapped magnetic fields and an increase in the velocity of marsward flows closer to the martian nightside. These findings provide valuable insights into the influence of the ESW and solar wind velocity on the martian induced magnetosphere. Key Points The asymmetrical distributions of looping magnetic fields and associated marsward flows can be well reproduced by a multi‐fluid magnetohydrodynamic (MHD) model The impacts of the motional electric field direction and the solar wind velocity on the asymmetries are investigated, respectively Magnetic forces shift the magnetic fields and marsward flows toward the direction opposite to the motional electric field

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Asymmetry / Deformation / Electric field / Electric fields / Interplanetary magnetic field / Magnetic field / Magnetic fields / Magnetic reconnection / Magnetohydrodynamic simulation / Mars / Mars atmosphere / Morphology / Planetary magnetic fields / Planetary magnetospheres / Polar environments / Polar regions / Simulation models / Skewed distributions / Solar wind / Solar wind velocity / Spacecraft / Statistics / Velocity / Wind speed / Wind velocities