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The ion composition experiment on Cluster measures 3-D distributions in onespin of the spacecraft (4 s). These distributions often measurefield-aligned ion beams (H+, He+ and O+) accelerated out of theionosphere. The standard model of these beams relies on a quasi-staticU-shaped potential model. The beams contain important information about thestructure and distribution of the U-shaped potential structures. For example,a simple beam with a narrow velocity range tells us that the particles areaccelerated going through a quasi-static U-shaped potential structurelocalized in space. A more complex beam with a large range of velocitiesvarying smoothly (a few tens of kilometers per second to> 100 km s−1) tells us that the potential structure is extended anddistributed along the magnetic field. The Cluster experiment has now revealednew features about the beams. Some beams are broken into many individualstructures each with their own velocity. The U-shaped potential model wouldinterpret the new features in terms of particles accelerated by narrowisolated potential structures maintained over an extended region of themagnetic field. Another interpretation is that these features arise asCluster traverses toward the center of a small-scale U-shaped potentialregion detecting particles accelerated on different equipotential contours.The estimate of the distance of the adjacent contours is ~ 590–610 mat a Cluster height of ~ 3.5 RE. The observed dimensionsmap to ~ 295–305 m in the ionosphere, suggesting Cluster has measuredthe potential structure of an auroral arc

Relating field-aligned beams to inverted-V structures and visible auroras