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Electron cyclotron maser emission mode coupling to the z-mode on a longitudinal density gradient in the context of solar type III bursts  

Roman Pechhacker   Submitted: 2013-01-16 05:54

A beam of super-thermal, hot electrons was injected into maxwellian plasma with a density gradient along a magnetic field line. 1.5D particle-in-cell simulations were carried out which established that the EM emission is produced by the perpendicular component of the beam injection momentum. The beam has a positive slope in the distribution function in perpendicular momentum phase space, which is the characteristic feature of a cyclotron maser. The cyclotron maser in the overdense plasma generates emission at the electron cyclotron frequency. The frequencies of generated waves were too low to propagate away from the injection region, hence the wavelet transform shows a pulsating wave generation and decay process. The intensity pulsation frequency is twice the relativistic cyclotron frequency. Eventually, a stable wave packet formed and could mode couple on the density gradient to reach frequencies of the order of the plasma frequency that allowed for propagation. The emitted wave is likely to be a z-mode wave. The total electromagnetic energy generated is of the order of 0.1% of the initial beam kinetic energy. The proposed mechanism is of relevance to solar type III radio bursts, as well as other situations, when the injected electron beam has a non-zero perpendicular momentum, e.g., magnetron.

Authors: Roman Pechhacker and David Tsiklauri
Projects: None

Publication Status: published
Last Modified: 2013-01-16 07:36
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The effect of electron beam pitch angle and density gradient on solar type III radio bursts  

Roman Pechhacker   Submitted: 2013-01-16 05:52

1.5D particle-in-cell simulations of a hot, low density electron beam injected into magnetized, maxwellian plasma were used to further explore the alternative non-gyrotropic beam driven electromagnetic (EM) emission mechanism, first studied in Tsiklauri, Phys. Plasmas 18, 052903 (2011). Variation of beam injection angle and background density gradient showed that the emission process is caused by the perpendicular component of the beam injection current, whereas the parallel component only produces Langmuir waves, which play no role in the generation of EM waves in our mechanism. Particular emphasis was put on the case, where the beam is injected perpendicularly to the background magnetic field, as this turned off any electrostatic wave generation along the field and left a purely electromagnetic signal in the perpendicular components. The simulations establish the following key findings: (i) Initially, waves at a few ωce/γ are excited, mode converted and emitted at ~ωpe (ii) The emission intensity along the beam axis is proportional to the respective component of the kinetic energy of the beam; (iii) The frequency of the escaping EM emission is independent of the injection angle; (iv) A stronger background density gradient causes earlier emission; (v) The beam electron distribution function in phase space shows harmonic oscillation in the perpendicular components at the relativistic gyrofrequency; (vi) The requirement for cyclotron maser emission, ∂f∂v⊥>0, is fulfilled; (vii) The degree of linear polarization of the emission is strongly dependent on the beam injection angle; (viii) The generated electromagnetic emission is left-hand elliptically polarized as the pitch angle tends to 90? and (ix) The generated electromagnetic energy is of the order of 0.1% of the initial beam kinetic energy.

Authors: Roman Pechhacker and David Tsiklauri
Projects: None

Publication Status: published
Last Modified: 2013-01-16 07:36
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Electron cyclotron maser emission mode coupling to the z-mode on a longitudinal density gradient in the context of solar type III bursts
The effect of electron beam pitch angle and density gradient on solar type III radio bursts

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