Cluster multispacecraft determination of AKR angular beaming

Abstract

[1] 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°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. The observations are inconsistent with either filled or hollow cone beaming models.

Backward Wave Cyclotron-Maser Emission in the Auroral Magnetosphere

“…Cairns et al [27] have demonstrated how the radiation, generated just below the electron cyclotron frequency, may successfully couple onto the upper branch of the plasma dispersion relation and escape. This model assumes near field-aligned R mode polarization prior to escape, as observed by the Cluster satellite mission [28]. However, it is well known that the radiation is generated near perpendicular to the magnetic field in the X mode [17] and will undergo strong resonant absorption when the radiation encounters the second harmonic layer [26].…”

Multipoint observations of plasma phenomena made in space by Cluster

“…9, allows for simultaneous observation of the same AKR burst from slightly different vantage points. This has enabled both an accurate determination of the source location and led to a comprehensive characterization of the AKR beaming pattern (Mutel et al 2003(Mutel et al , 2004(Mutel et al , 2008.…”

“…Because the Cluster constellation provides six independent baselines for calculating propagation time differences according to (2.1), the AKR source location (with only three unknowns) is over-determined and can be localized to within a volume by minimizing the signed sum to within the timing measurement uncertainties (Mutel et al 2003(Mutel et al , 2004. Using this technique Mutel et al (2004Mutel et al ( , 2008 determined the intrinsic properties of AKR sources and beaming characteristics. They found that AKR is generally emitted from individual sources located in the nightside auroral zone (see Fig.…”

“…The debate hinges upon the AKR emission process and the plasma structure surrounding the source. Using the fact that to obtain a valid source location, at least three (angularly separated) spacecraft need to be illuminated by the same AKR beam, Mutel et al (2008) determined that AKR emission beams are intrinsically narrow in latitude. In particular, they concluded that an AKR emission beam is confined to a plane (to within ∼ ± 20 • ) containing the active auroral field line threading its source and tangent to the latitudinal circle at the source (Fig.…”

Probing the magnetosphere of the M8.5 dwarf TVLM 513−46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?

“…The AKR angular beaming follows such laminar source model. After being emitted and passing through high-density regions, the radiation is refracted upwards (Mutel, Christopher & Pickett 2008;Menietti et al 2011). …”