Experimental and numerical investigation of auroral cyclotron maser processes

When an electron beam with initial spread in rotational velocity 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 [1] 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. In a scaled laboratory reproduction of this process, a 75-85keV electron beam of 5-40A was magnetically compressed by a system of solenoids and electromagnetic emissions observed for cyclotron frequencies of 4.42GHz and 11.7GHz [2]. A comparison of these experimental measurements with the results of 2D and 3D numerical simulations will be presented, showing the effect of cyclotron-wave detuning on the efficiency of forward and backward wave coupling. The experiment presently differs from the astrophysical case in that it has a well defined radiation boundary. PiC code calculations have been undertaken to investigate the dynamics of the cyclotron emission process in the absence of such metallic boundaries and incorporating a background plasma of variable density. Computations reveal that the cyclotron emission process persists although its spatial growth is reduced. A quenching of the instability is also apparent as the ratio $\omega _{ce}$ / $\omega _{p}$ is reduced to be $<$ 1. This is consistent with the predictions of theory [3] and satellite observations that suggest AKR emission is localized within a region of plasma depletion where $\omega _{ce}$ / $\omega _{p} \quad >$ 1. \\[4pt] [1] D. Gurnett, ``Waves in Space Plasmas'', 50th Annual meeting of the APS Division of Plasma Physics, 2008. \\[0pt] [2] K. Ronald et al, Physics of Plasmas, 15, 056503, 2008. \\[0pt] [3] R. Bingham and R. A. Cairns, Physics of Plasmas, 7, 3089, 2000.