The North-south Structure of a Flux Rope From Cluster Observations

The encounter of a flux rope (FR) in the magnetotail by the four Cluster spacecraft during a substorm is investigated in detail. The FR was aligned with its principal axis closely along the dawn-dusk direction. Based on the observed x-component of the magnetic field, the four spacecraft traversed the FR at different distances from the neutral sheet. This fortuitous situation reveals the north-south structure of the FR, showing the existence of a velocity shear. The relationship between the strength of the embedded current density from the curlometer method and the velocity shear within the FR will be discussed.