Characteristics of the Magnetotail Current Density from Cluster Observations

We present observations of the inferred current density in the magnetotail current sheet from Cluster magnetometer measurements during summer 2001. In particular, the average directions and magnitudes of the current density have been examined for approximately 25 current sheet encounters between July and October 2001. Considerable variation is observed in the current density magnitudes. The current sheet encounters have been organized in local time and we find that the cross-tail current density is not strictly organized along Y_GSM across the entire magnetotail. Large deviations from Y_GSM are observed at the flanks of the magnetotail. The current sheet encounters have also been organized by solar wind conditions and geomagnetic activity indices. We find that the current sheet becomes more disordered during intervals of increased geomagnetic activity and often the Y-component of the current density is not the dominant component. We will use substorm onset times determined from ground magnetometer observations to gauge changes of the tail current sheet direction and magnitude during substorms. This behavior will be explored for individual current sheet events and also in a superposed epoch analysis.