Modification of the Hall physics in magnetic reconnection due to cold ions at the Earth's magnetopause

Magnetic reconnection is a process permitting mass and energy exchange across plasma boundaries and converting magnetic energy into particle kinetic energy. Strong electric fields and currents due to the Hall effect are set up in narrow regions where ions are demagnetized but electrons remain magnetized. It has been recently shown that cold (few eV) ions are abundant or even dominate the Earth's magnetosphere, but most of the previous reconnection studies do not account for this population. These cold ions remain magnetized down to smaller length scales than the hot ions, introducing a new scale into the system. In addition, the Hall currents are partially canceled by magnetized cold ions. We use observations by the Cluster spacecraft at the Earth's magnetopause to verify that when a mixture of ions of very different temperatures is present in reconnecting plasmas, the microphysics related to the Hall effect is significantly modified.