Demonstration of auroral radio emission mechanisms by laboratory experiment

Abstract

Auroral kilometric radiation occurs in regions of depleted plasma density in the polar magnetosphere. These emissions are close to the electron cyclotron frequency and appear to be connected to the formation of high pitch angle electron populations due to the conservation of the magnetic moment. This results in a horseshoe type distribution function being formed in velocity space where electrons are magnetically compressed as they descend towards the Earth's atmosphere. Satellites have observed that radio emissions occur in conjunction with the formation of this distribution and show the radiation to have propagation and polarization characteristics of the extraordinary (X-mode) plasma mode with emission efficiency observed at ∼1–2%. To investigate this phenomenon a laboratory experiment, scaled to microwave frequencies and lab dimensions by increasing the cyclotron frequency, was constructed whereby an electron beam propagated through a region of increasing magnetic field created by five independently variable solenoids. Results are presented for two experimental regimes of resonant coupling, 11.7 and 4.42 GHz, achieved by varying the peak magnetic field. Measurements of the experimental radiation frequency, power and efficiency were undertaken as a function of the magnetic compression. Results showed the radiation to be polarized in the near cut-off transverse electric radiation modes, with efficiency of emission ∼1–2%, peak power outputs of ∼19–30 kW and frequency close to the cyclotron frequency. This represented close correlation between the laboratory radiation efficiency, spectra, polarization and propagation with that of numerical predictions and the magnetospheric observations.