Auroral magnetospheric cyclotron emission processes: numerical and experimental simulations

Satellites have observed powerful radio waves (up to 1 GW peaked at 300 kHz) radiating from regions of reduced plasma density about 3200 km above the Earth's surface in the polar magnetosphere. The emission is associated with the observation of horseshoe or 'shell' distributions in the velocity space of the Earthbound flux of electrons, arising from magnetic compression. It has been postulated that this distribution holds the free energy required to explain the radiation emission. To verify this proposition, a series of numerical and experimental simulations of the mechanism scaled to microwave frequencies have been conducted in concert with a theoretical analysis of the growth rate and propagation of the resultant radiation. The numerical simulations were conducted with electron beams of 75 keV and currents of 18 and 34 A gyrating in a magnetic field of 0.18 T, and 85 keV and 18 A in a magnetic field of 0.48 T. The simulations predicted that the radiation would be emitted close to the cyclotron frequency (4.42 and 11.7 GHz) in near cut-off TE modes (TE0,1 and TE0,3/TE2,3, respectively) with efficiencies of 2% and 1.3%. The experimental measurements demonstrated mode content very close to that predicted by the simulations with output powers of 19 kW and 35 kW from electron beams of 75 keV energy and 12 A and 34 A of current, respectively, at 4.42 GHz. Powers of 9.4 kW and 30 kW from 85 keV electron beams of 12 and 30 A were observed at 11.7 GHz. The spectral, polarization and propagation properties of the radiation are similar to those observed in the magnetosphere. A scheme will be outlined for introducing a background plasma into the apparatus, with ωpe < ωce/10 to enhance the comparison with the auroral magnetosphere.