Laboratory Reproduction of Auroral Magnetospheric Radio Wave Sources

Auroral Kilometric Radiation, AKR, occurs naturally in the polar regions of the Earth's magnetosphere where electrons are accelerated by electric fields into the increasing planetary magnetic dipole. Here conservation of the magnetic moment converts axial to rotational momentum forming a horseshoe distribution in velocity phase space. This distribution is unstable to cyclotron emissions and radiation is emitted in the X-mode. In the laboratory a 75-85kV electron beam of 5-40A was magnetically compressed by a system of solenoids. Results are presented for an electron beam gyrating at cyclotron frequencies of 4.42GHz and 11.7GHz resonating with near cut-off TE01 and TE03 modes respectively. Measurements of the electron transport combined with numerical simulations demonstrated that a horseshoe distribution function was formed in electron velocity space. Analysis of the experimental measurements allowed the inference of the I D number density as a function of the electron beam pitch angle. The total power emitted experimentally was similar to 19-35kW with a maximum RF emission efficiency of similar to 2%. These results were compared to those obtained numerically using a 2D PiC code KARAT with a maximum efficiency of 2% predicted for the same mode and frequency, consistent with astrophysical and theoretical results.