Energy transport along the auroral field lines, Cluster results.

Cluster spacecrafts, during their perigeum passages, cross the auroral field lines at about 4 Re distance that is well above the main auroral acceleration region which is at a distance of about 2-3 Re. Therefore, Cluster is well suited to study such important questions as how is energy transported to the auroral acceleration region and what is the potential structure of upward current regions at high altitudes. Thanks to the Cluster flying formation of "pearls on the string" during auroral zone crossings we can estimate the temporal variations of the current sheets as well as their velocity. This information is used to make corrected potential structure estimates using E field data from EFW instrument. Then we discuss the energy source of the auroral acceleration based on comparisons between Poynting flux and particle energy flux. This is especially important in the light of opposite predictions of such important auroral acceleration models as traditional U-shape and S-shape model, as well as the recently proposed O-shape and field line resonance models.