The effect of ion multi-scales on magnetic reconnection in earth's magnetotail -Cluster and MMS observations and comparisons with simulations

The O⁺ content of the plasma sheet prior to substorm onset has important ramifications in the substorm growth phase, onset time and expansion phase. A recent statistical study using Cluster observations showed that a higher O⁺ content during the substorm growth phase makes the substorm more difficult to trigger. In contrast with the predictions, the magnetotail unloading rate is faster when the O⁺ content is higher [Liu et al, 2013]. We have used magnetotail reconnection events observed by Cluster and MMS to deduce the local reconnection rate for different O⁺ densities. Initial results indicate that while the normalized reconnection rate is independent of the mass density in the plasma sheet, the rate of reconnected flux does increase with the density of O⁺. This may be because the higher O⁺ density leads to greater substorm loading, and this more stressed configuration drives faster reconnection. These results will be compared with the estimation of the reconnection rate using fully kinetic and hybrid particle-in-cell simulations that model reconnection in the presence of O⁺ in both local geometry and in a model magnetotail equilibrium.