Cluster Multi-spacecraft Determination of AKR Angular Beaming

TL;DR

This study used four-spacecraft observations to directly measure the angular beaming patterns of AKR bursts, revealing a narrow, plane-confined emission pattern that refines understanding of AKR visibility and source localization.

Contribution

First direct measurement of AKR angular beaming patterns using multi-spacecraft data, showing a narrow, plane-confined emission pattern inconsistent with traditional cone models.

Findings

AKR emission is confined within 15 degrees of a tangent plane containing the magnetic field.

Most rays are directed upward and refracted, indicating a narrow beaming pattern.

Implications for AKR visibility and source localization are significant, affecting interpretation of previous studies.

Abstract

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.