Abstract
When a mainly rectilinear electron beam is subject to significant magnetic compression, conservation of magnetic moment results in the formation of a horseshoe shaped velocity distribution. It has been shown that such a distribution is unstable to cyclotron emission and may be responsible for the generation of Auroral Kilometric Radiation (AKR) -an intense rf emission sourced at high altitudes in the terrestrial auroral magnetosphere. PiC code simulations have been undertaken to investigate the dynamics of the cyclotron emission process in the absence of cavity boundaries with particular consideration of the spatial growth rate, spectral output and rf conversion efficiency. Computations reveal that a well-defined cyclotron emission process occurs albeit with a low spatial growth rate compared to waveguide bounded simulations. The rf output is near perpendicular to the electron beam with a slight backward-wave character reflected in the spectral output with a well defined peak at 2.68GHz, just below the relativistic electron cyclotron frequency. The corresponding rf conversion efficiency of 1.1% is comparable to waveguide bounded simulations and consistent with the predictions of kinetic theory that suggest efficient, spectrally well defined radiation emission can be obtained from an electron horseshoe distribution in the absence of radiation boundaries.
Highlights
Astrophysical radio emissions in association with plasmas having non-uniform magnetic fields have been the subject of particular interest and debate over the last thirty years [1]
If one considers the electron beam distribution in figure 1a with finite pitch factor and energy spread impingent on an increasing axial magnetic flux density, conservation of magnetic moment μ results in the conversion of axial velocity into perpendicular velocity
For the purpose of simulating the unbounded interaction geometry, a 44cm radius region with radially increasing conductivity was defined around the beam propagation path
Summary
SUPA, Department of Physics, University of Strathclyde, Glasgow, G4 0NG, UK 2 School of Mathematics and Statistics, University of St Andrews, Fife, KY16 9SS, UK 3 STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
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