Reproduction of auroral cyclotron emission mechanisms in laboratory experiments and 3D simulations

Efficient (~1%) electron cyclotron radio emissions are known to originate in the X-mode from regions of locally depleted plasma in the Earth's polar magnetosphere. These emissions are commonly referred to as the Auroral Kilometric Radiation (AKR). Two populations of electrons exist with rotational kinetic energy to contribute to this effect. The downward propagating auroral electron flux which acquires transverse momentum and a horseshoe or half shell distribution in electron velocity space, due to conservation of the magnetic moment, as it experiences increasing magnetic field and the mirrored component of this flux. It is now thought that the transverse momentum in the descending distribution can give rise to a cyclotron maser instability. KARAT 2D & 3D particle in cell (PiC) simulations were used to enhance the understanding of results from a laboratory experiment built to reproduce the mechanisms of AKR generation. In these experiments the kilometric radiation was scaled to microwave frequencies by increasing the magnetic field strength. Results from the laboratory experiment demonstrated excitation of the TE0,1 mode of a cylindrical waveguide at 4.42GHz and the TE₀,₃ mode at 11.7GHz, consistent with the 2D PiC code simulations. 3D simulations represent a significant extension to the previous work, as a two dimensional cylindrically symmetric simulation cannot account for waveguide modes with azimuthal structure. 3D simulations, as presented in this thesis, were therefore able to provide a representation of the full interaction, which more accurately describes the laboratory experiment. 3D PiC codes have been used to successfully simulate the interaction between these complex electron beams and electromagnetic radiation. These simulations have proven accurate in predicting the radiation modes and frequencies, polarisation and propagation behaviour. The simulations predicted wave excitation with efficiencies of ~2-3%, whilst the experiment measured conversion efficiencies of ~1-3%. They predicted excitation of near-to-cut-off TE modes (TE₀,₁ at 4.42GHz and TE₀,₃ at 11.7GHz) consistent with the experiment and with the wave propagation and polarisation observed by satellites in the magnetosphere. Simulations were conducted and experimental investigation extended to investigate the potential for excitation of modes away from perpendicular propagation. These showed that at small increases of cyclotron frequency above resonance with a perpendicular wave mode yielded a preference for emission in a slightly backwards propagation regime, at some ~3% below the cyclotron frequency. Inclusion of a reflector for the backward wave raised efficiency to ~7%-11%, significantly above that observed in the absence of the reflector. This may have important implications suggesting AKR emissions may be able to avoid absorption in the upper hybrid stop-band. R-X type emission was examined, showing efficient (up to 3%) emission into waves propagating at 55° from the waveguide axis and polarised in dipole-like waves at very close to, but slightly below, the cyclotron frequency.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2013

Author dc:creator

• Gillespie, Karen Margaret

DOI

10.48730/4ny5-ja57

Identifier

T13901

OAI identifier oai:identifier

oai:strathclyde:v692t628j