Laboratory Experimental Investigations of Auroral Cyclotron Emissions

Abstract

Magnetic field lines at the Earth's poles create regions of increasing field which magnetically compresses particles as they descend towards the ionosphere. This produces a horseshoe shaped distribution function in velocity space which has been observed by satellites [1]. Research has shown this distribution to be unstable to a cyclotron maser type instability [2]. The emitted radiation is observed to be polarised in the extraordinary mode. We present results of experimental and computational investigations of radiation emissions from an electron beam with a horseshoe distribution in velocity space to compare with the astrophysical observations. Results are presented using an electron beam of energy 75keV and a cyclotron frequency of 4.45GHz [3], compressed using magnetic coils to mimic the naturallv occurring phenomenon. An FFT spectrometer was used to measure the radiation output from the experiment which was observed to be close to the cyclotron frequency. Electron transport measurements confirmed that the horseshoe distribution function was obtained. I Experimental measurements of the antenna pattern radiated from the output window allowed the TE mode structures excited by the beam to be analysed demonstrating the radiation to be polarised and propagating perpendicular to the static magnetic field. The efficiency was estimated to be ~2%. These results were in close agreement to the predictions of the 21) PiC code KARAT. The efficiency is also comparable with estimates of the astrophysical phenomenon. Mode structures are also consistent with 3D simulations of the instability.