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

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, utilising 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. Analysing 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 (E_SW). 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.