Study of a dayside magnetopause reconnection event detected by MMS related to a large-scale solar wind perturbation and magnetospheric cold ions

Magnetic reconnection is a fundamental process that is ubiquitous in the universe and allows the conversion of magnetic field energy into heating and acceleration of plasma. It is responsible for the dominant transport of plasma, momentum, and energy across the magnetopause from the solar wind into the Earth's magnetosphere. The present study reports on a magnetic reconnection event detected by the Magnetospheric Multiscale mission (MMS) on 21 October 2015 around 04:40 UT far from the diffusion regions and related to a large-scale solar wind (SW) perturbation impacting the Earth’s magnetosphere. Based on OMNI data, the event impacting the Earth’s magnetosphere is ahead of weak Stream Interacting Region (SIR) (SW beta≈7 and Alfvénic Mach number≈15) where the averaged density of solar wind is about ~20 cm^-3 (compared with average SW density ~3-10 cm^-3). On one hand, the magnetosheath (MSH) density measured by MMS just after the crossing of the magnetosphere separatrix layer (identified by the large decrease of energetic electrons fluxes) is very large ~95 cm^-3 (compared with average MSH density ~20 cm^-3). In such a condition, we show that the current density at this separatrix is dominated by the ion diamagnetic current. On the other hand, cold ions are detected close to the magnetic reconnection separatrix layer on the magnetosphere side. Their origin and impact on the ongoing reconnection process are investigated/discussed. The drifting cold ions and the presence of a guide field have significant effects on the orientation of the electric field normal to the magnetopause.