Role of Cold Ions in Parallel Current and Hall Field Enhancement along Separatrices in Magnetopause Reconnection

Magnetic reconnection at the magnetopause involves complex multiscale dynamics, in which most particles do not traverse the electron or ion diffusion regions directly but are instead energized along the separatrices and within the reconnection outflow. Multiple ion populations, i.e., hot and cold ions strongly influence current systems and Hall physics. Cold ions, which have small gyroradii, remain magnetized longer than hot ions and follow the E x B drift along the separatrices together with electrons, whereas hotter ions decouple at larger spatial scales. This difference modifies the Hall physics.

In a previous study using a 2.5D fully kinetic particle-in-cell simulation setup with and without cold ions, it was shown that the delayed demagnetization of cold ions near the separatrices reduces the perpendicular ion current. Using the same simulations, we further find that the presence of cold ions enhances both the parallel electron current and the Hall magnetic field. These results provide a framework for future studies of energy dissipation during magnetopause magnetic reconnection in the presence of cold ions.