Heavy ion effects on substorm loading and unloading in the Earth's magnetotail

Abstract

We have performed a statistical study investigating how substorm triggering and unloading is affected by the heavy ion content of the magnetotail plasma sheet. 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 increased lobe magnetic flux during the substorm growth phase increases the magnetopause flaring angle. As a result, a greater fraction of the solar‐wind dynamic pressure is observed in the tail lobes and plasma sheet. Therefore, the increase and decrease of the lobe magnetic flux can be monitored by observing the increase and decrease in the magnetotail pressure. Using Cluster data from 2001 to 2004, we have determined how the maximum pressure (or flaring angle) and the rate of change of pressure (or flaring angle) during substorms depend on the O+ content of the plasma sheet. In addition, we have estimated the maximum magnetic flux, and rate of change of the magnetic flux. Our results show that both the maximum tail pressure and the rate of change in the pressure are positively correlated with the amount of O+ in the plasma sheet. When the measurements are normalized to account for the external solar‐wind pressure and the different Cluster locations in the tail, the maximum accumulated flux and the unloading rate still correlate positively with the O+ density and O+/H+ ratio. This suggests that the additional O+ makes it more difficult to trigger the substorm onset, but once it is triggered, the unloading is faster. This could either indicate that the presence of O+ increases the reconnection rate, or that it initiates reconnection over a broader width of the tail.