  Simultaneous observations of AKR emission using the four-spacecraft Cluster
array were used to make the first direct measurements of the angular beaming
patterns of individual bursts. By comparing the spacecraft locations and AKR
burst locations, the angular beaming pattern was found to be narrowly confined
to a plane containing the magnetic field vector at the source and tangent to a
circle of constant latitude. Most rays paths are confined within 15 deg of this
tangent plane, consistent with numerical simulations of AKR k-vector
orientation at maximum growth rate. The emission is also strongly directed
upward in the tangent plane, which we interpret as refraction of the rays as
they leave the auroral cavity. The narrow beaming pattern implies that an
observer located above the polar cap can detect AKR emission only from a small
fraction of the auroral oval at a given location. This has important
consequences for interpreting AKR visibility at a given location. It also helps
re-interpret previously published Cluster VLBI studies of AKR source locations,
which are now seen to be only a subset of all possible source locations. These
observations are inconsistent with either filled or hollow cone beaming models.
