Cluster observations of O + escape in the magnetotail due to shock compression effects during the initial phase of the magnetic storm on 17 August 2001

We study the O + outflow into the magnetotail during the initial phase of the 17 August 2001 magnetic storm. Solar wind and magnetospheric data are used from instruments onboard ACE, WIND, and Cluster spacecraft. The interplanetary shock arrival at Earth caused a large compression of the magnetosphere. During this event, the Cluster spacecraft were crossing the plasma sheet boundary layer of the magnetotail. Thus this case study constitutes a unique opportunity to observe the magnetotail response to a shock driven storm with multiple S/C data. We observed a field aligned oxygen (O + ) outflow in the plasma sheet boundary layer and a bidirectional O + flux in the central plasma sheet. Using the Cluster multiple spacecraft data, we derived the O + gradient and the escape rate into the magnetotail during the initial phase of the magnetic storm. We found an escape rate of 4.0 × 10 24 O + ions s −1 in the plasma sheet boundary layer of the magnetotail, which is about 20 times the O + inflow into the ring current during an intense magnetic storm.