The relationship between sawtooth events and O + in the plasma sheet

In order to study the relationship between sawtooth events and the composition of the plasma sheet, we perform a superposed epoch analysis (SEA) of the O + concentration inside the near‐Earth plasma sheet during sawtooth events and substorms sorted by different geomagnetic storm phases, using Cluster/Composition Distribution Function data. The SEA shows that the O + content increases during sawtooth growth phase, regardless of storm phase, and reaches 20% around the onset of dipolarization. For storm main phase events, the plasma sheet O + concentration during sawtooth events is only slightly higher than that observed during substorm events. However, for storm recovery phase and nonstorm time events, there is significantly more O + within the plasma sheet during sawtooth events than during substorm events. No difference is found in the comparison between the O + /H + density ratio changes during the first tooth and the subsequent teeth in a series of a sawtooth interval. Hence, there is no evidence to support the hypothesis that due to the higher O + inside the plasma sheet, subsequent teeth will lead to a closer near‐Earth X line and then a wider magnetic local time response. 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 + is neither a necessary nor a sufficient condition but is likely one of many factors that play a role in triggering sawtooth events.