Title:  "The Impact of 0+ on Magnetotail Dynamics"

Abstract:Using measurements from the Cluster satellites, we assess the role of O+ in the dynamics of the magnetotail.  We discuss two aspects: first, the role O+ plays in determining the degree of loading, and the unloading rate of substorms, and second, the role that O+ plays in triggering sawtooth events.  During the substorm growth phase, magnetic flux is accumulated in the tail lobes until the magnetotail reaches an unstable state.  A near-earth neutral line then forms, and this excess flux is reconnected.  The increase and decrease of the lobe magnetic flux can be monitored by observing the increase and decrease in the magnetotail pressure.  Our results show that both the maximum accumulated flux and the unloading rate increase with increased O+ density and O+/H+ ratio.  This indicates that the additional O+ makes it more difficult to trigger the substorm onset, but once it is triggered, the unloading is faster.  Sawtooth events are a series of substorms that occur with a regular period of ~ 3 hours.  It has been suggested that O+ outflow may play a key role in triggering these events.  In order to study the relationship between sawtooth events and the plasma composition of the plasmasheet, we perform a superposed epoch study of O+ concentration inside the near-Earth plasmasheet during sawtooth events and substorms sorted by different geomagnetic storm phases. The statistical results show that the O⁺ content increases during the sawtooth growth phase, regardless of the storm phase, while it is relatively constant during isolated substorms. During storm main phase, the plasma sheet O+ concentration during both sawtooth events and isolated substorm events is high. However, during storm recovery phase and non-storm times, there is significantly more O+ within the plasma sheet during sawtooth events than during isolated substorm events. Finally, despite the association between sawtooth events and high O+/H+ ratio, there are times when the O+/H+ density ratio is high in the plasma sheet but no sawtooth event is observed, and there are sawtooth events when the O+/H+ ratio is low.  This indicates that enhanced O+ may play a role in triggering sawtooth events, but there are other factors that are also important.