A laboratory experiment to investigate auroral kilometric radiation emission mechanisms

Speirs, D. C.; Vorgul, I.; Ronald, K.; Bingham, R.; Cairns, R. A.; Phelps, A. D. R.; Kellett, B. J.; Cross, A. W.; Whyte, C.G.; Robertson, C. W.

doi:10.1017/s0022377804003459

Cited by 29 publications

(34 citation statements)

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“…5). Figure 6 shows the results of numerical simulations with the PIC code KARAT, 24 where the horseshoe formation evident as the beam moves into the stronger magnetic field, and its subsequent saturation after the emissions occur can be seen. For near perpendicular propagation the resonant electrons, determined by the cyclotron resonance condition (1), have almost constant energy and if the parameters are such that they lie along the inside of the horseshoe the system is unstable to cyclotron instability.…”

Section: Satellite Observations and The Horseshoe Shaped Momentumentioning

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Cyclotron maser emission: Stars, planets, and laboratory

et al. 2011

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This paper is a review of results by the group over the past decade on auroral kilometric radiation and similar cyclotron emissions from stars and planets. These emissions are often attributed to a horseshoe or crescent shaped momentum distribution of energetic electrons moving into the convergent magnetic field which exists around polar regions of dipole-type stars and planets. We have established a laboratory-based facility that has verified many of the details of our original theoretical description and agrees well with numerical simulations. The experiment has demonstrated that the horseshoe distribution does indeed produce cyclotron emission at a frequency just below the local cyclotron frequency, with polarization close to X-mode and propagating nearly perpendicularly to the beam motion. We discuss recent developments in the theory and simulation of the instability including addressing a radiation escape problem and the effect of competing instabilities, relating these to the laboratory, space, and astrophysical observations.

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Section: Satellite Observations and The Horseshoe Shaped Momentumentioning

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Cyclotron maser emission: Stars, planets, and laboratory

et al. 2011

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“…In all cases, the cyclotron-maser instability is presumed to be driven by an electron horseshoe or modified horseshoe-ring distribution. Scaled laboratory experiments have demonstrated the potential for such horseshoe distributions to generate narrowband cyclotron-maser emission at GHz frequencies over distances of a few meters [21][22][23][24][25].Although the generation mechanism is well established, there remain important questions with regard to how the radiation may propagate within the parent auroral density cavity and ultimately escape [26]. Cairns et al [27] have demonstrated how the radiation, generated just below the electron cyclotron frequency, may successfully couple onto the upper branch of the plasma dispersion relation and escape.…”

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Backward Wave Cyclotron-Maser Emission in the Auroral Magnetosphere

et al. 2014

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In this Letter, we present theory and particle-in-cell simulations describing cyclotron radio emission from Earth's auroral region and similar phenomena in other astrophysical environments. In particular, we find that the radiation, generated by a down-going electron horseshoe distribution is due to a backwardwave cyclotron-maser emission process. The backward wave nature of the radiation contributes to upward refraction of the radiation that is also enhanced by a density inhomogeneity. We also show that the radiation is preferentially amplified along the auroral oval rather than transversely. The results are in agreement with recent Cluster observations. DOI: 10.1103/PhysRevLett.113.155002 PACS numbers: 94.05.Dd, 94.30.Aa, 96.12.Wx, 98.62.Nx The electron cyclotron-maser instability plays a significant role in the generation of highly nonthermal, polarized radio emission from the auroral region of planets and stars having a dipolelike magnetic field [1,2]. More recently, this radiation model has been extended to a variety of astrophysical environments including Blazar jets, collisionless shocks, and brown dwarfs [3][4][5][6][7]. Planetary auroral radio emission has been a topic of interest for many years with several mechanisms having been proposed [8][9][10][11][12][13][14][15]. As the dispersion relation describing the electromagnetic wave growth only depends on the factor by which the magnetic field increases and on the ratio of plasma and cyclotron frequencies, the mechanism can scale effectively over many orders of magnitude both in wavelength and spatial scale. Blazar jets, in particular, represent a prime example of this scalability [3]. Generated perpendicular to the accretion disk of super massive black holes, they can extend over many thousands of light years and have been observed to generate highly nonthermal radio emission at frequencies in the hundreds of GHz range. It has been suggested that small scale magnetic mirrors or convergent flux tubes may be formed within the jet via hydrodynamic instabilities or shocks, providing the means of generating the required electron velocity distribution Bing2003, Begel2005. In Earth's auroral region, observations by the Viking spacecraft [11,16] and the Fast Auroral Snapshot (FAST) satellite [14] led to the suggestion that the free energy source for AKR is a population of downward accelerated electrons having a large perpendicular velocity produced by a combination of parallel accelerating electric fields and converging magnetic field lines. The electron distribution takes the form of a horseshoe or crescent shape due to the first adiabatic invariance as the electrons move into the stronger magnetic field. The FAST satellite [13,14,17] clearly demonstrates a strong correlation between the electromagnetic wave emission and the occurrence of the horseshoe distribution [13].Further observations by FAST of localized double-layer structures within the auroral density cavity have combined our understanding of auroral particle acceleration with the auroral kilome...

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“…The main research themes include gyro-devices with particular emphasis on gyro-TWAs [1,2] and gyro-BWOs, and natural sources of radiation such as auroral kilometric radiation (AKR) [3][4][5][6], novel Cherenkov mm-wave sources [7,8] and pseudospark-driven sub-THz sources [9][10][11].…”

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Progress in microwave to sub-THz sources at Strathclyde

Phelps

2017

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Research progress during recent years at the University of Strathclyde is presented. The main research themes include gyro-devices with particular emphasis on gyro-TWAs [1,2] and gyro-BWOs, and natural sources of radiation such as auroral kilometric radiation (AKR) [3][4][5][6], novel Cherenkov mm-wave sources [7,8] and pseudospark-driven sub-THz sources [9][10][11].While there has been some theoretical work, the majority of the research has involved either laboratory experiments, or numerical simulations. Gyro-devicesMost of the work on gyro-devices has involved helical waveguide interaction structures [1,2]. This interest began in the 1990's stimulated by collaborative research with IAP. The early experiments were in the 10GHz frequency range, whereas our more recent experiments have been in the 90 to 100 GHz range. The applications for these mm-wave amplifiers include DNP-NMR, cloud radar and communications, etc. Fig. 1. Gyro-TWA, 90 -100 GHzThe gyro-TWA shown in Fig. 1 has produced 3.4kW output power. The limit on the output power is presently the maximum power that is available at the input to the amplifier. Natural sources of EM radiationPrevious research on laboratory gyro-devices and electron cyclotron masers has provided the background for investigations of the mechanism of some naturally occurring sources of electromagnetic radiation. A research programme stretching over more than 10 years has explored the detailed mechanism of auroral kilometric radiation (AKR) and similar emissions emanating from other astrophysical bodies. Laboratory experiments to simulate the natural phenomena, combined with theoretical analysis and numerical modelling have been compared with natural observations. Using all four approaches it has been possible to understand how AKR arises and the mechanism by which the energy in electron streams can be converted with 1% or 2% efficiency into AKR. The mechanism depends upon the existence of an anisotropy in velocity space. This anisotropy is created when electrons are spiralling down the magnetospheric field lines. The magnetic moment is an adiabatic invariant of the electron motion and as the electrons penetrate further into the increasing magnetic field the electrons increase the perpendicular component of their velocity and decrease the component of their velocity parallel to the magnetic field lines. This results in a horseshoe shaped particle distribution in velocity space. The positive value of the gradient with respect to the perpendicular velocity component of the distribution function drives this kinetic instability that is now known as the "horseshoe instability". The theoretical analysis, the PiC simulations, the laboratory experiment and the space observations all agree. Fig. 2. Cylindrical over-sized Cherenkov structure Cherenkov sourcesOne of the aims of the research on Cherenkov sources is to retain reasonably high output power levels as the frequency increases. Traditionally Cherenkov sources have followed the trend that as the frequency increases and the wavelength...

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A laboratory experiment to investigate auroral kilometric radiation emission mechanisms

Cited by 29 publications

References 9 publications

Cyclotron maser emission: Stars, planets, and laboratory

Cyclotron maser emission: Stars, planets, and laboratory

Backward Wave Cyclotron-Maser Emission in the Auroral Magnetosphere

Progress in microwave to sub-THz sources at Strathclyde

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