DIRECT-DETECTION OF THE CURRENT IN A MAGNETOTAIL FLUX ROPE

The field-aligned current densities in the core of a magnetotail flux rope were directly detected with plasma instrumentation on board the G eotail spacecraft at a downstream distance of about 96 R(E) (Earth rad ii). The axis of the flux rope and the core magnetic fields were direc ted cross-tail, and the magnitudes of these core magnetic fields were similar to that in the magnetotail lobe at these distances. The radius of the core cross-section was about 0.7 R(E), and the total current w as 500,000 A. Together with a recently reported, similar measurement w ith the Galileo spacecraft these direct detections of current greatly strengthen the previous identifications in favor of flux ropes that we re solely based on their magnetic signatures. The detection of the flu x rope with the Geotail spacecraft occurred during a period of relativ e magnetic quiescence.