Numerical simulation of the horseshoe driven cyclotron maser instability attributed to Auroral Kilometric Radiation

Auroral Kilometric Radiation (AKR) comprises a spectrum of narrowband (~lkHz) discrete electromagnetic emissions centred around a frequency of ~300kHz and sourced at high altitudes within the polar terrestrial magnetosphere. The emissions are generated by an electron cyclotron-maser instability driven by a component of the precipitating auroral electron flux having a horseshoe shaped velocity distribution. The conservation of magnetic moment in the increasing magnetic field of 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 emissions were observed for cyclotron frequencies of 4.42GHz and 11.7GHz resonating with near cut-off TE_0,1 and TE_0.3 modes, respectively. In the current context, we present a comparison of the experimental measurements with numerical predictions from the 3D PiC code KARAT. The 3D simulations accurately predict the radiation modes and frequencies observed from the experiment [3], with a predicted RF conversion efficiency of ~1% which is consistent with geophysical observations and the results of quasi-linear theoretical analysis. We also present the results of unbounded, 2D numerical simulations investigating the horseshoe-maser instability in the presence of a background Maxwellian plasma of variable density.