O+ behavior in magnetopause reconnection and its possible effect on the reconnection rate: Cluster observations

It is still an open question whether and how heavy ions (e.g., O⁺) are involved in magnetopause reconnection. From the fluid perspective, the O⁺ increases the ion mass density, slowing the Alfvén speed and therefore slowing the reconnection rate. However, because of the large O⁺ gyroradius, O⁺ is often not fully entrained in the flow, and its effect depends on kinetic processes. In this study, we analyze magnetopause reconnection events when Cluster crosses the diffusion region or the outflow region. The velocity distribution functions (VDF) provide evidence that O⁺ ions are accelerated in the reconnection process. Using the results from a three-species (O⁺, H⁺ and e^-) symmetric PIC simulation, we first evaluate methods to calculate the reconnection rate to determine how well they work in different regions in the reconnection geometry. We apply these methods to the magnetopause reconnection events and evaluate their usefulness in this region, where the asymmetric structure and the magnetopause motion need to be taken into account, so that the more solid methods can be used in future studies. The goal is to understand the behavior of the heavy ions and determine whether they affect the reconnection rate.